GIFT F MICHAEL REESE TUNNELING: A PRACTICAL TREATISE BY CHARLES PRELINI, C. E, WITH ADDITIONS BY CHARLES S. HILL, C.E. ASSOCIATE EDITOR "ENGINEERING NEWS" DIAGRAMS AND ILLUSTRATIONS NEW YORK: D. VAN NOSTRAND COMPANY 23 MURRAY AND 27 WARREN STS. 1901 COPYRIGHT, IQOI, D. VAN NOSTRAND COMPANY. TYPOGRAPHY BY C. J. PETERS & SON, BOSTON, MASS. U.S.A. PREFACE IN his work at Manhattan College the writer found himself confronted by the fact that there were but two books on Tun- neling in the English language, neither of which he could rec- ommend as text books for his pupils. Drinker's tunneling is a splendid reference book, and may be consulted with advan- tage by any engineer, but it is too voluminous and expensive to be suitable for the beginner. Simins's Practical Tunnel- ing is a magnificent exposition of the English method of tunneling, but it is too old for anyone who looks for the most modern methods of tunneling, as the art has progressed greatly since Mr. Simms's death. The additions introduced by Mr. D. K. Clarke, although they convey an excellent idea of the manner of excavating long tunnels like the Mont Cenis and St. Gothard, fail in what may be called real practical value, viz., to explain to engineers and contractors the vaiiou's meth- ods of driving tunnels of ordinary dimensions through different soils. Having thus felt the want of a book of convenient size and moderate price, the author began to enlarge the notes of his lectures for publication. The general purpose of the book which has resulted is to explain all the operations that are re- quired hi tunneling, and then illustrate by suitable examples the actual application of these methods in practice. Formulas and difficult calculations have been avoided, the book being simply descriptive, and the text well illustrated, so that it can be easily understood by students and others unfamiliar with tunneling work. This work of preparation has been very diffi- cult to the writer owing to the fact that, being a foreigner, the iii IV PREFACE language was not fully mastered, and, having been but a few years in this country, he was not familiar with what had been accomplished here in previous years. The latter fault was remedied as far as possible by a careful consultation of the Transactions of the American Society of Civil Engineers, the volumes of Engineering News, and of other periodicals, with all of which the writer made very free use. The writer has received much assistance from his friends in the preparation of the manuscript, and takes this opportunity to thank them for their trouble and encouragement. It is his wish, however, to give special thanks to Mr. Charles S. Hill, Associate Editor Engineering News, whose suggestions and criticisms led to many changes and additions, and extended the scope of the book. CHAKLES PRELINI. February, 1901. CONTENTS PAGE INTRODUCTION THE HISTORICAL DEVELOPMENT OF TUNNEL BUILD- ING ix CHAPTER I. PRELIMINARY CONSIDERATIONS ; CHOICE BETWEEN A TUNNEL AND AN OPEN CUT ; METHOD AND PURPOSE OF GEOLOGICAL SURVEYS . 1 II. METHODS OF DETERMINING THE CENTER LINE AND FORMS AND DIMENSIONS OF CROSS-SECTION 9 1 III. EXCAVATING MACHINES AND ROCK DRILLS ; EXPLOSIVES AND BLASTING 19 IV. GENERAL METHODS OF EXCAVATION ; SHAFTS ; CLASSIFICATION OF TUNNELS . * 32 V. METHODS OF TIMBERING OR STRUTTING TUNNELS 43 VI. METHODS OF HAULING IN TUNNELS 55 VII. TYPES OF CENTERS AND MOLDS EMPLOYED IN CONSTRUCTING TUNNEL LININGS OF MASONRY 62 VIII. METHODS OF LINING TUNNELS . . .- . . -. . * . ... 68 IX. TUNNELS THROUGH HARD ROCK ; GENERAL DISCUSSION ; EXCA- VATION BY DRIFTS; MONT CENIS TUNNEL 79 X. TUNNELS THROUGH HARD ROCK (continued) ; THE SIMPLON TUNNEL ..*.."... 94 XI. TUNNELS THROUGH HARD ROCK (continued) ; EXCAVATION BY DRIFTS ; ST. GOTHARD TUNNEL ; BUSK TUNNEL 114 XII. REPRESENTATIVE MECHANICAL INSTALLATIONS FOR TUNNEL WORK .,.-....*.. 124 XIII. EXCAVATING TUNNELS THROUGH SOFT GROUND ; GENERAL DIS- CUSSION ; THE BELGIAN METHOD 133 XIV. THE GERMAN METHOD OF EXCAVATING TUNNELS THROUGH SOFT GROUND ; BALTIMORE BELT-LINE TUNNEL 145 XV. THE FULL-SECTION METHOD OF TUNNELING ; ENGLISH METHOD, AUSTRIAN METHOD 156 v VI CONTENTS CHAPTER PAGE XVI. SPECIAL TREACHEROUS GROUND METHOD ; ITALIAN METHOD ; QUICKSAND TUNNELING ; PILOT METHOD 167 XVII. OPEN- CUT TUNNELING METHODS; TUNNELS UNDER CITY STREETS ; B.OSTON SUBWAY AND NEW YORK RAPID TRANSIT, 180 XVIII. SUBMARINE TUNNELING ; GENERAL DISCUSSION ; THE SEVERN TUNNEL 201 XIX. SUBMARINE TUNNELING (continued) ; THE EAST RIVER GAS TUNNEL ; THE VAN BUREN STREET TUNNEL, CHICAGO . . 208 XX. SUBMARINE TUNNELING (continued) ; THE MILWAUKEE WATER- WORKS TUNNEL 280 XXI. SUBMARINE TUNNELING (continued) ; THE SHIELD SYSTEM . . 242 XXII. ACCIDENTS AND REPAIRS IN TUNNELS DURING AND AFTER CON- STRUCTION 266 XXIII. RELINING TIMBER-LINED TUNNELS WITH MASONRY .... 280 XXIV. THE VENTILATION AND LIGHTING or TUNNELS DURING CON- STRUCTION . . . 290 XXV. THE COST OF TUNNEL EXCAVATION, AND THE TIME REQUIRED FOR THE WORK 300 INDEX . 309 IWTEODUOTIOU" THE HISTORICAL DEVELOPMENT OF TUNNEL BUILDING. A TUNNEL, defined as an engineering structure, is an artificial gallery, passage, or roadway beneath the ground, under the bed of a stream, or through a hill or mountain. The art of tunnel- ing has been known to man since very ancient times. A The- ban king on ascending the throne began at once to drive the long, narrow passage or tunnel leading to the inner chamber or sepulcher of the rock-cut tomb which was to form his final resting-place. Some of these rock-cut galleries of the ancient Egyptian kings were over 750 ft. long. Similar rock-cut tun- neling work was performed by the Nubians and Indians in building their temples, by the Aztecs in America, and in fact by most of the ancient civilized peoples. The first built-up tunnels of which there are any existing records were those constructed by the Assyrians. The vaulted drain or passage under the southeast palace of Nimrud, built by Shalmaneser II. (860-824 B.C.), is in all essentials a true soft- ground tunnel, with a masonry lining. A much better exam- ple, however, is the tunnel under the Euphrates River, which may quite accurately be claimed as the first submarine tunnel of which there exists any record. It was, however, built under the dry bed of the river, the waters of which were temporarily diverted, and then turned back into their normal channel after the tunnel work was completed, thus making it a true sub- marine tunnel only when finished. The Euphrates River tun- nel was built through soft ground, and was lined with brick ix X INTRODUCTION masonry, having interior dimensions of 12 ft. in width and 15 ft. in height. Only hand labor was employed by these ancient peoples in their tunnel work. In soft ground the tools used were the pick and shovels, or scoops. For rock work they possessed a greater range of appliances. Research has shown that among the Egyptians, by whom the art of quarrying was highly de- veloped, use was made of tube drills and saws provided with cutting edges of corundum or other hard, gritty material. The usual tools for rock work were, however, the hammer, the chisel, and wedges ; and the excellence and magnitude of the works accomplished by these limited appliances attest the unlimited time and labor which must have been available for their ac- complishment. The Romans should doubtless rank as the greatest tunnel builders of antiquity, in the number, magnitude, and useful character of their works, and in the improvements which they devised in the methods of tunnel building. They introduced fire as an agent for hastening the breaking down of the rock, and also developed the familiar principle of prosecuting the work at several points at once by means of shafts. In their use of fire the Romans simply took practical advantage of the familiar fact that when a heated rock is suddenly cooled it cracks and breaks so that its excavation becomes comparatively easy. Their method of operation was simply to build large fires in front of the rock to be broken down, and when it had reached a high temperature to cool it suddenly by throwing water upon the hot surface. The Romans were also aware that vinegar affected calcareous rock, and in excavating tunnels through this material it was a common practice with them to substitute vinegar for water as the cooling agent, and thus to attack the rock both chemically and mechanically. It is hardly necessary to say that this method of excavation was very severe on the workmen because of the heat and foul gases generated. This was, however, a matter of small concern to the builders* INTRODUCTION XL since the work was usually performed by slaves and prisoners of war, who perished by thousands. To be sentenced to labor on Roman tunnel works was thus one of the severest penalties to which a slave or prisoner could be condemned. They* were places of suffering and death as are to-day the Spanish mercury mines. Besides their use of fire as an excavating agent, the Romans possessed a very perfect knowledge of the use of vertical shafts in order to prosecute the excavation at several different points simultaneously. Pliny is authority * for the statement that in the excavation of the tunnel for the drainage of Lake Fucino forty shafts and a number of inclined galleries were sunk along its length of 3^ miles, some of the shafts being 400 ft. in depth. The spoil was hoisted out of these shafts in copper pails of about ten gallons' capacity by windlasses. The Roman tunnels were designed for public utility. Among those which are most notable in this respect, as well as for being fine examples of tunnel work, may be mentioned the nu- merous conduits driven through the calcareous rock between Subiaco and Tivoli to carry to Rome the pure water from the mountains above Subiaco. This work was done under the Consul Marcius. The longest of the Roman tunnels is the one built to drain Lake Fucino, as mentioned above. This tunnel was designed to have a section of 6 ft. X 10 ft. ; but its actual dimensions are not uniform. It was driven through calcareous rock, and it is stated that 30,000 men were employed for eleven years in its construction. The tunnels which have been men- tioned, being designed for conduits, were of small section ; but the Romans also built tunnels of larger sections at numerous points along their magnificent roads. One of the most notable of these is that which gives the road between Naples and Poz- zuoli passage through the Posilipo hills. It is excavated through volcanic tufa, and is about 3000 ft. long and 25 ft. wide, with a section of the form of a pointed arch. In order * " Tunneling," Encly. Brit., 1889, vol. xxiii., p. 623. Xll INTRODUCTION" to facilitate the illumination of this tunnel, its floor and roof were made gradually converging from the ends toward the middle ; at the entrances the section was 75 ft. high, while at the center it was only 22 ft. high. This double funnel-like construction caused the rays of light entering the tunnel to concentrate as they approached the center, and thus to improve the natural illumination. The tunnel is on a grade. It was probably excavated during the time of Augustus, although some authorities place its construction at an earlier date. During the Middle Ages the art of tunnel building was practiced for military purposes, but seldom for the public need and comfort. Mention is made of the fact that in 1450 Anne of Lusignan commenced the construction of a road tunnel under the Col di Tenda in the Piedmontese Alps to afford better communication between Nice and Genoa ; but on account of its many difficulties the work was never completed, although it was several times abandoned and resumed. For the most part, therefore, the tunnel work of the Middle Ages was in- tended for the purposes and necessities of war. Every castle had its private underground passage from the central tower or keep to some distant concealed place to permit the escape of the family and its retainers in case of the victory of the enemy, and, during the defense, to allow of sorties and the entrance of supplies. The tunnel builders of the Middle Ages added little to the knowledge of their art. Indeed, until the 17th century and the invention of gunpowder no practical improvement was made in the tunneling methods of the Romans. Engravings of mining operations in that century show that underground excavation was accomplished by the pick or the hammer and chisel, and that wood fires were lighted at the ends of the headings to split and soften the rocks in advance. Although gunpowder had been previously employed in mining, the first important use of it in tunnel work was at Malpas, France, in 1679-81, in the tunnel for the Languedoc Canal. This INTRODUCTION Xlll tunnel was 510 ft. long, 22 ft. wide, and 29 ft. high, and was excavated through tufa. It was left unlined for seven years, and then was lined with masonry. With the advent of gunpowder and canal building the first strong impetus was given to tunnel building, in its modern sense, as a commercial and public utilitarian construction, since the days of the Roman Empire. Canal tunnels of notable size were excavated in France and England during the last half of the 17th century. These were all rock or hard-ground tunnels. Indeed, previous to 1800 the soft-ground tunnel was beyond the courage of engineer except in sections of such small size that the work better deserves to be called a drift or heading than a tunnel. In 1803, however, a tunnel 24 ft. wide was excavated through soft soil for the St. Quentin Canal in France. Timbering or strutting was employed to support the walls and roof of the excavation as fast as the earth was removed, and the masonry lining was built closely following it. From the experience gained in this tunnel were developed the various systems of soft-ground subterrannean tunneling since employed. It was by the development of the steam railway > however, that the art of tunneling was to be brought into its present prominence. In 1820-26 two tunnels were built on the Liver- pool & Manchester Ry. in England. This was the beginning of the rapid development which has made the tunnel one of the most familiar of engineering structures. The first railway tunnel in the United States was built on the Alleghany & Portage R.R. in Pennsylvania in 1831-33 ; and the first canal tunnel had been completed about 13 years previously (1818-21) by the Schuylkill Navigation Co., near Auburn, Pa. It would be interesting and instructive in many respects to follow the rise and progress of tunnel construction in detail since the con- struction of these earlier examples, but all that may be said here is that it was identical with that of the railway. The art of tunneling entered its last and greatest phase XIV INTRODUCTION with the construction of the Mont Cenis tunnel in Europe and the Hoosac tunnel in America, which works established the utility of machine rock-drills and high explosives. The Mont Cenis tunnel was built to facilitate railway communication between Italy and France, or more properly between Pied- mont and Savoy, the two parts of the kingdom of Victor Emmanuel II., separated by the Alps. It is 7.6 miles long, and passes under the Col di Frejus near Mont Cenis. Som- meiller, Grattoni, and Grandis were the engineers of this great undertaking, which was begun in 1857, and finished in 1872. It was from the close study of the various difficulties, the great length of the tunnel, and the desire of the engineers to finish it quickly, that all the different improvements were developed which marked this work as a notable step in the advance of the art of tunneling. Thus the first power-drill ever used in tunnel work was devised by Sommeiller. In addition, com- pressed air as a motive power for drills, aspirators to suck the foul air from the excavation, air compressors, turbines, etc., found at Mont Cenis their first application to tunnel construc- tion. This important role played by the Mont Cenis tunnel in Europe in introducing modern methods had its counterpart in America in the Hoosac tunnel completed in 1875. In this work there were used for the first time in America power rock- drills, air compressors, nitro-glycerine, electricity for firing blasts, etc. There remains now to be noted only the final development in the art of soft-ground submarine tunneling, namely, the use of the shield and metal lining. The shield was invented and first used by Sir Isambard Brunei in excavating the tunnel under the River Thames at London, which was begun in 1825, and finished in 1841. In 1869 Peter William Barlow used an iron lining in connection with a shield in driving the second tunnel under the Thames at London. From these inventions has grown up one of the most notable systems of tunneling now practiced, which is commonly known as the shield system. INTRODUCTION XV In closing this brief review of the development of modern methods of tunneling, to the presentation of which the re- mainder of this book is devoted, mention should be made of a form of motive power which promises many opportunities for development in tunnel construction. Electricity has long been employed for blasting and illuminating purposes in tunnel work. It remains to be extended to other uses. For hauling and for operating certain classes of hoisting and excavating machinery it is one of the most convenient forms of power available to the engineer. Its successful application to rock- drills is another promising field. For operating ventilating fans it promises unusual usefulness. TUNNELING CHAPTER I PRELIMINARY CONSIDERATIONS. CHOICE BE- TWEEN A TUNNEL AND OPEN CUT. GEOLOGICAL SURVEYS CHOICE BETWEEN A TUNNEL AND AN OPEN CUT WHEN a railway line is to be carried across a range of mountains or hills, the first question which arises is whether it is better to construct a tunnel or to make such a detour as will enable the obstruction to be passed with ordinary surface construction. The answer to this question depends upon the comparative cost of construction and maintenance, and upon the relative commercial and structural advantages and disad- vantages of the two methods. In favor of the open road there are its smaller cost and the decreased time required in its con- struction. These mean that less capital Avill be required, and that the road will sooner be able to earn something for its builders. Against the open road there are : its greater length and consequently its heavier running expenses; the greater amount of rolling-stock required to operate it ; the heavy ex- pense of maintaining a mountain road ; and the necessity of employing larger locomotives, with the increased expenses which they entail. In favor of the tunnel there are : the shortening of the road, with the consequent decrease in the operating expenses and amount of rolling-stock required ; the smaller cost .84746 2 TUNNELING of maintenance, owing to the protection of the track from snow and rain and other natural influences causing deterioration ; and the decreased cost of hauling due to the lighter grades. Against the tunnel, there are its enormous cost as compared with an open road and the great length of time required to construct it. To determine in any particular case whether a tunnel or an open road is best, requires a careful integration of all the factors mentioned. It may be asserted in a general way, however, that the enormous advance made in the art of tunnel building has done much to lessen the strength of the principal objections to tunnels, namely, their great cost and the length of time required for their construction. Where the choice lies between a tunnel or a long detour with heavy grades it is sooner or later almost always decided in favor of a tunnel. When, however, the con- ditions are such that the choice lies between a tunnel or a heavy open cut with the same grades the problem of deciding between the two solutions is a more difficult one. It is generally assumed that when the cut required will have a vertical depth exceeding 60 ft. it is less expensive to build a tunnel unless the excavated material is needed for a nearby embankment or fill. This rule is not absolute, but varies according to local conditions. For instance, in materials of rigid and unyielding character, such as rock, the practical limit to the depth of a cut goes far beyond that point at which a tunnel would be more economical according to the above rule. In soils of a yielding character, on the other hand, the very flat slope required for stability adds greatly to the cost of making a cut. It may be noted in closing that the same rule may be em- ployed in determining the location of the ends of the tunnel, for assuming that it is more convenient to excavate a tun- nel than an open cut when the depth exceeds 60 ft., then the open cut approaches should extend into the mountain- or hill-sides only to the points where the surface is 60 ft. above CHOICE BETWEEN A TUNNEL AND AN OPEN CUT grade, and there the tunnel should begin. If, therefore, we draw on the longitudinal profile of the tunnel a line parallel to the plane of the tracks, and 60 ft. above it, this line will cut the surface at the points where the open-cut approaches should cease and the tunnel begin. This is a rule-of-thumb determi- nation at the best, and requires judgment in its use. Should the ground surface, for example, rise only a few feet above the 60 ft. line for any distance, it is obviously better to continue the open cut than to tunnel. THE METHOD AND PURPOSE OF GEOLOGICAL SURVEYS When it has been decided to build a tunnel, the first duty of the engineer is to make an accurate geological survey of the locality. From this survey the material penetrated, the form of section and kind of strutting to be used, the best form of lining to be adopted, the cost of excavation, and various other facts, are to be deduced. In small tunnels the geological knowledge of the engineer should enable him to construct a geological map of the locality, or this knowledge may be had in many cases by consulting the geological maps issued by the State or general government surveys. When, however, the tunnel is to be of great length, it may be necessary to call in the assistance of a professional geologist in order to reconstruct accurately the interior of the mountain and thereby to ascer- tain beforehand the different strata and materials to be excavated, thus obtaining the data for calculating both the time and cost of excavating the tunnel. The geological survey should enable the engineer to deter- mine, (1) the character of the material and its force of cohe- sion, (2) the inclination of the different strata, and (3) the presence of water. Character of Material. The character of the material through which the proposed tunnel will penetrate is best ascertained by means of diamond rock-drills. These machines bore an 4 TUNNELING annular hole, and take away a core for the whole depth of the boring, thus giving a perfect geological section showing the character, succession, and exact thickness of the strata. By making such borings at different points along the center line of the projected tunnel, and comparing the relative sequence and thickness of the different strata shown by the cores, the geological formation of the mountain may be determined quite exactly. Where it is difficult or impracticable to make dia- mond drill borings on account of the depth of the mountain above the tunnel, or because of its inaccessibility, the engineer must resort to other methods of observation. The present forms of mountains or hills are due to- weathering, or the action of the destructive atmospheric influ- ences upon the original material. From the manner in which the mountain or hill has resisted weathering, therefore, may be deduced in a general way both the nature and consistency of the materials of which it is composed. Thus we shall gener- ally find mountains or hills of rounded outlines to consist of soft rocks or loose soils, while under very steep and crested mountains hard rock usually exists. To the general knowl- edge of the nature of its interior thus afforded by the ex- terior form of the mountain, the engineer must add such information as the surface outcroppings and other local evi- dences permit. For the purposes of the tunnel builder we may first classify all materials as either, (1) hard rock, (2) soft rock, or (3). soft soil. Hard rocks are those having sufficient cohesion to stand vertically when cut to any depth. Many of the primary rocks r like granite, gneiss, feldspar, and basalt, belong to this class, but others of the same group are affected by the atmosphere, moisture, and frost, which gradually disintegrate them. They are also often found interspersed with pyrites, whose well- known tendency to disintegrate upon exposure to air intro- duces another destructive agency. For these reasons we may CHOICE BETWEEN A TUNNEL AND AN OPEN CUT i> divide hard rocks into two sub-classes ; viz., hard rocks un- affected by the atmosphere, and those affected by it. This distinction is chiefly important in tunneling as determining whether or not a lining will be required. Soft rocks, as the term implies, are those in whicti the force of cohesion is less than in hard rocks, and which in consequence offer less resistance to attacks tending to break down their original structure. They are always affected by the atmosphere. Sandstones, laminated clay shales, mica-schists, and all schistose stones, chalk and some volcanic rocks, can be classified in this group. Soft rocks require "to be supported by timbering during excavation, and need to be protected by a strong lining to exclude the air, and to support the vertical pressures, and prevent the fall of fragments. Soft soils are composed of detrital materials, having so little cohesion that they may be excavated without the use of explosives. Tunnels excavated through these soils must be strongly timbered during excavation to support the verti- cal pressure and prevent caving ; and they also always require a strong lining. Gravel, sand, shale, clay, quicksand, and peat are the soft soils generally encountered in the excavation of tunnels. Gravels and dry sand are the strongest and firmest ; shales are very firm, but they possess the great defect of being liable to swell in the presence of water or merely by exposure to the air, to such an extent that they have been known to crush the timbering built to support them. Quicksand and peat are proverbially treacherous materials. Clays are some- times firm and tenacious, but when laminated and in the presence of water are among the most treacherous soils. Laminated clays may be described as ordinary clays altered by chemical and mechanical agencies, and several modifications of the same structure are often found in the same locality. They are composed of laminae of lenticular form separated by smooth surfaces and easily detached from each other. Lami- nated clays generally have a dark color, red, ocher or greenish 6 TUNNELING blue, and are very often found alternating with strata of stiatites or calcareous material. For purposes of construction they have been divided into three varieties. Laminated clays of the first variety are those which 'alter- nate with calcareous strata and are not so greatly altered as to lose their original stratification. Laminated clays of the second variety are those in which the calcareous strata are broken and reduced to small pieces, but in which the former structure is not completely destroyed ; the clay is not reduced to a humid state. Laminated clays of the third variety are those in which the clay by the force of continued disturb- ance, and in the presence of water, has become plastic. Laminated clays are very treacherous soils ; quicksand and peat may be classed, as regards their treacherous nature, among the laminated clays of the third variety. Inclination of Strata Knowing the inclination of the strata, or the angle which they make with the horizon, it is easy to determine where they intersect the vertical plane of the tunnel passing through the center line, thus giving to a certain extent a knowledge of the different strata which will be met in the excavation,. On the inclination of the strata depend : (1) The cost of the excavation ; the blasting, for instance, will be more efficient if the rocks are attacked perpendicular to the stratification; (2) The character of the timbering or strut- ting ; the tendency of the rock to fall is greater if the strata are horizontal than if they are vertical ; (3) The character and thickness of the lining; horizontal strata are in the weakest position to resist the vertical pressure from the load above when deprived of the supporting rock below, while vertical strata, when penetrated, act as a sort of arch to support the pressure of the load above. The foregoing remarks apply only to hard or soft rock materials. In detrital formations the inclination of the strata is an important consideration, because of the unsymmetrical pres- sures developed. In excavating a tunnel through soft soil CHOICE BETWEEN A TUNNEL AND AN OPEN CUT 7 whose strata are inclined at 30 to the horizon, for instance, the tunnel will cut these strata at an angle of 30. By the excavation the natural equilibrium of the soil is disturbed, and while the earth tends to fall and settle on bath sides at an angle depending upon the friction and cohesion of the material, this angle will be much greater on one side than on the other because of the inclination of the strata; and hence the prism of falling earth on one side is greater than on the other, and consequently the pressures are different, or in other words, they are unsymmetrical. These unsymmetrical pressures are usually easily taken care of as far as the lining is concerned, but they may cause serious cave-ins and badly distort the strutting. Caving-iii during excavation may be prevented by cutting the materials according to their natural slope ; but the distortion of the strutting is a more serious problem to handle, and one which oftentimes requires the utmost vigilance and care to prevent serious trouble. Presence of Water. An idea of the likelihood of finding water in the tunnel may be obtained by studying the hydro- graphic basin of the locality. From it the source and direction of the springs, creeks, ravines, etc., can be traced, and from the geological map it can be seen where the strata bearing these waters meet the center line. Not only ought the surface water to be attentively studied, but underground springs, which are frequently encountered in the excavation of tunnels, re- quire careful attention. Both the surface and underground waters follow the pervious strata, and are diverted by im- pervious strata. Rocks generally may be classed as im- pervious ; but they contain crevices and faults, which often allow water to pass through them ; and it is, therefore, not uncommon to encounter large quantities of water in excavating tunnels through rock. As a rule, water will be found under high mountains, which comes from the melted ice and snow percolating through the rock crevices. Some detrital soils, like gravel and sand, are pervious, and 8 TUNNELING others, like clay and shale, are impervious. Detrital soils lying above clay are almost certain to carry water just above the clay stratum. In tunnel work, therefore, when the exca- vation keeps well within the clay stratum, little trouble is likely to be had from water ; should, however, the excavation cut the clay surface and enter the pervious material above, water is quite certain to be encountered. The quantity of water encountered in any case depends upon the presence of high mountains near by, and upon other circumstances which will attract the attention of the engineer. A knowledge of the pressure of the water is desirable. This may be obtained by observing closely its source and the character of the strata through which it passes. Water coming to the excavation through rock crevices will lose little of its pressure by friction, while that which has passed some distance through sand will have lost a great deal of its pressure by friction. Water bearing sand, and, in fact, any water bearing detrital material, has its fluidity increased by water pressure ; and when this reaches the point where flow results, trouble ensues. The streams of water met in the construction of the St. Gothard tunnel had sufficient pressure to carry away timber and materials. DETERMINING THE CENT Ell LINE CHAPTER II. METHODS OF DETERMINING THE CENTER LINE AND FORMS AND DIMENSIONS OF CROSS-SECTION. DETERMINING THE CENTER LINE. TUNNELS may be either curvilinear or rectilinear, but the latter form is the more common. In either case the first task of the engineer, after the ends of the tunnel have been definitely fixed, is to locate the center line exactly. This is done on the surface of the ground; and its purpose is to find the exact length of the tunnel, and to furnish a reference line by which the excavation is directed. Rectilinear Tunnels. In short tunnels the center line may be accurately enough located for all practical purposes by means of a common theodolite. The work is performed on a calm, cleat day, so as to have the instrument and observations sub- jected to as little atmospheric disturbance as possible. Wooden stakes are employed to mark the various located points of the center line temporarily. The observations are usually repeated once at least to check the errors, and the stakes are altered as the corrections dictate ; and after the line is finally decided to be correctly fixed, they are replaced by permanent monu- ments of stone accurately marked. The method of checking the observations is described by Mr. \V. D. Haskoll * as follows : 44 Let the theodolite be carefully set up over one of the stakes, with the nail driven into it, selecting one that will command the best position so as to range backwards and forwards over the whole length of line, and also obtain a view of the two distant points that range with the center line ; this being done, * " Prnotfrni Tunneling." %y F. "W. Simnis. 10 TUNNELING let the centers of every stake ... be carefully verified. If this be carefully done, and the centers be found correct, and thoroughly in one visual 1'r.e as seen through the telescope, there will be no fear but that a perfectly straight line has been obtained. The center line which has thus been located on the ground surface has to be transposed to the inside of the tunnel to direct the excavation. To do this let A and B be the entrances and a and b be the two distinct fixed points which have been ranged in with the center line located on the ground surface over the hill Af B, Fig. 1. The instrument is set up at V, any point on the line A a produced, and a bearing secured by observation on the center line marked on the surface. This bearing is then carried into the tunnel by plunging the tele- scope, and setting pegs in the roof of the heading. Lamps "A v FIG. 1. Diagram Showing Manner of Lining in Rectilinear Tunnels. hung from these pegs furnish the necessary sighting points. This same operation is repeated on the ' opposite side of the hill to direct the excavation from that end of the tunnel. These operations serve to locate only the first few points inside the tunnel. As the excavation penetrates farther into the hill, it becomes impossible to continue to locate the line from the outside point, and the line has to be run from the points marked on the roof of the heading. Great accuracy is required in all these observations, since a very small error at the begin- ning becomes greater and greater as the excavation advances. In very long tunnels excavated under high mountains more elaborate methods have to be adopted for locating the center line. The theodolites employed must be of large size ; in ran- ging the center line of the St. Gothard tunnel, the theodolite used had an object glass eight inches in diameter.* Instead of * See also Simplon Tunnel, Chapter IX. DETERMINING THE CENTER LINE 11 the ordinary mounting a masonry pedestal with a perfectly level top is employed to support the instrument during the observations. The location is made by means of triangulation. The various operations must be performed with the greatest accuracy, and repeated several times in such a way as to reduce the errors to a minimum, since the final meeting of the head- ings depends upon their elimination. The St. Gothard tunnel furnishes perhaps the best illus- tration of careful work in locating the center line of long recti- linear tunnels of any tunnel ever built. The length of this tunnel is 9.25 miles, and the height of the mountain above it is very great. The center line was located by triangulation by .Stabbiefto FIG. 2. Triangulation System for Establishing the Center Line of the St. Gothard TunneL two different astronomers using different sets of triangles, and working at different times. The set or system of triangles used by Dr. Koppe, one of the observers, is shown by Fig. 2 ; it con- sists of very large and quite small triangles combined, the latter being required because the entrances both at Airolo and Goeschenen were so low as to permit only of a short sight being taken. The apices of the triangles were located by means- of the contour maps of the Swiss Alpine Club. Each angle was read ten times, the instrument was collimated four times for each reading, and was afterwards turned off 5 or 10 to avoid errors of graduation. The average of the errors in read- ing was about one second of arc. The triangulation was compen- 12 TUNNELING Wire sated according to the method of least squares. The probable iror in the fixed direction was calculated to be 0.8" of arc at Goeschenen and 0.7" of arc at Airolo. From this it was assumed that the probable deviation from the true center would be about two inches at the middle of the tunnel, but when the headings finally met this deviation was found to reach eleven inches. Comparatively few tunnels are driven by working from the entrances alone, the excavation being usually prosecuted at several points at once by means of shafts. In these cases, in order to direct the excavation cor- rectly, it is necessary to fix the center line on the bottom of the shaft. This is accomplished in two ways, one being employed when the shaft is located directly over the center line, and the other when the shaft is located to one side of the center line. When the shaft is located on the center line two small pillars are placed on opposite edges of the shaft and collimating with the center . line as shown by Fig. 3. On these two pillars the points' corresponding to the center line are correctly marked, and con- nected by a wire stretched between them. To this wire two plumb bobs are fastened as far apart as possible. These plumb bobs mark two points on the center line at the bottom of the shaft, and from them the line is extended into the headings as the work advances. Compass readings are employed to check the transit lines ranged on the plumb bobs. Where there are rocks containing iron ore a miner's transit should be employed for making the compass reading. When the shaft is placed at one side of the tunnel the FlG. 3. Method of Transferring the Center Line down Center Shafts. DETERMINING THE CENTER LINE IS pillars or bench marks are placed normal to the center line on. the edges of the shaft as shown by Fig. 4. Between the points A and B a wire is stretched, and from it two plumb bobs are suspended, as described in the preceding case ; these plumb ? A bobs establish a vertical plane normal to the axis of the tun- nel. The excavation of the side tunnel is carried along the line, BW until it intersects the - z line of the main tunnel, whose center line is determined by ~'Cefiter~ ' -0 "~Ifn~' measuring off underground a ~]W~ distance equal tO the distance FIG. 4 -Method of Transferring the Center Line down Side Shafts. B on the surface. By setting the instrument over the under-ground point 0, and turning off a right angle from the line B 0, the center line of the tunnel is extended into the headings. Curvilinear Tunnels. There are various methods of locating the center line of curvilinear tunnels, but the method of tangent offsets is the one most commonly employed. It p consists in finding the length of an ordinate DC, Fig. 5, perpendicular to the tangent AX, at a point D taken at a known distance AD = d from the point of tangent A, being the center of the arc AB and OA being the radius. F FIG. 5. - Diagram From draw OZ parallel to the tangent AX, of^De^^ng and P roduce the perpendicular DC until it in- Tangent offsets tersects the line OZ at E. Join and (7. From the right-angle triangle OCE, OE == AD = d CO = r EC= V^^^ ........... (1) ED = OA = r DC = r - EC = y . . .... . (2) 14 TUNNELING TIG. C. Diagram Showing Method of Determining Tangent Off sets for Arcs of over 90. Substituting these values in equations (1) and (2) we have, y = r Vr 2 d 2 . When the arc AB is greater than a quadrant, as in Fig. 6, the projection AF of the arc becomes equal to its radius, and for any value of d between this pro- jection and that of the chord AK there are thus two values of y, viz., ?/! and / 2 , both deduced from the formula, y = r Vr 2 cP. Assuming the value of d A G-, to locate the point H, GrN = y v r Vr 2 (P, and to locate the point J, e employed in horizontal as well as vertical holes, which was, of course, not possible in its liquid state. Dynamite must contain at least 50 $ of nitroglycerine. Some manufacturers, instead of diatomaceous earth, use other absorbents which develop gases upon explosion and increase the force of the explosion. These mixtures are classed under the general name of false dyna- mites. A great many varieties of dynamite are manufactured, EXCAVATING MACHINES AND ROCK DRILLS 27 and each manufacturer usually makes a number of grades to which he gives special names. Dynamite for railway work, tunneling, and mining contains about 50 % of nitroglycerine ; for quarrying about 35 f /, and for blasting soft rocks aboufc 30 $. It is sold in cylindrical cartridges covered with paper. Storage of Explosives In driving tunnels through rock large quantities of explosives must be used, and it is necessary to have some safe place for storing them. In many States there are special laws governing the transportation and storage of explosives ; where there is no regulation by law the engineer should take suitable precautions of his own devising. It is best to build a special house or hut in one of the most con- cealed portions of the work and away from the tunnel, and protect it with a lightning-rod and from fire. Strict orders should be given to the watchman in charge not to allow persons inside with lamps or fire in any form, and smoking should be prohibited. The use of hammers for opening the boxes should be prohibited ; and dynamite, gunpowder, and fulminate of mercury should not be stored together in the same room. A quantity of dynamite for two or three days' consumption may be stored near the entrance of the tunnel in a locked box, the keys of which are kept by the foreman of the work. When dynamite has been frozen the engineer should provide some arrangement by which it may be heated to a temperature not exceeding 120 F., and absolutely forbid it being thawed out on a stove or by an open fire. Fuses When gunpowder is used in tunneling it is ignited by the Blickford match. This match, or fuse as it is more commonly called, consists of a small rope of yarn or cotton having as a core a small continuous thread of fine gunpowder. To protect the outside of the fuse from moisture it is coated with tar or some other impervious substance. These fuses are so well made that they burn very uniformly at the rate of about 1 ft. in 20 seconds, hence the moment of explosion can be pretty accurately fixed beforehand. Blickford matches 28 TUNNELING have the objection for tunnel work of burning with a bad odor, especially when they are coated with tar, and to remedy this many others have been invented. Those of Rzika and Franzl are the best known of these. The former has many advantages, but it burns too quickly, about 3 ft. per second, and is expensive ; the latter consists of a small hollow rope filled with dynamite. Blickford matches cannot be used to explode dynamite, the use of a cartridge being required. These cartridges are small copper cylinders containing fulminate of mercury. They may be attached to the end of the Blickford match, which being ignited the spark travels along its length until it reaches the copper cylinder, where it explodes the fulminate of mercury, which in turn explodes the dynamite. Blasts may also be fired by electricity, which, in fact, is the most common and the preferable method, because several blasts can be fired simulta- neously, and because the current is turned on at a great dis- tance, thus affording greater safety to the workmen. The method of electric firing generally employed in America is known as the connecting series method, and consists in firing- several mines simultaneously. The ends of the wires are scraped bare, and the wire of the first hole of the series is twisted together with the wire of the second hole, and so on ; finally the two odd wires of the first and last holes are connected to two wires of a single cable or to two separate cables extend- ing to some safe place to which the men can retreat. Here the two cable wires are connected by binding screws to the poles of a battery, or sometimes to a frictional electric machine. The cur- rent passes through the wires, making a spark at each break, and so fires the fulminate of mercury, which explodes the dynamite. Simultaneous firing by electricity by utilizing the united strength of the blasts at the same instant secures about 10$ greater efficiency from the explosives. Another advantage of electric firing is that in case of a missfire of any one of the holes there is slight possibility of explosion afterwards, and the place can be approached at once to discover the cause. EXCAVATING MACHINES AND HOCK DRILLS 29 Tamping. Tamping is the material placed in the hole above the explosive to prevent the gases of explosion from escaping into the air. Tamping generally consists of clay. When gun- powder is used the clay must be well rammed with a wooden tool, and paper, cotton, or some other dry material must be placed between the moist clay and the powder. When dyna- mite is used it is not necessary to ram the tamping, since the suddenness of the explosion shatters the rock before the clay can be driven from the hole. A few experienced men should be appointed to fire the blasts. These men should give ample warning previous to the blast in order that all machinery and tools which might be injured by flying fragments may be removed out of danger, and so that the workmen may seek safety. When all is ready they should fire the blasts, keeping accurate count of the explosions to ensure that no holes have missed fire, and should call the workmen back when all danger is over. In case any hole has missed fire it should be marked by a red lamp or flag. Nature of Explosions. When the explosives are ignited a sudden development of gases results, producing a sudden and violent increase of pressure, usually accompanied by a loud report. The energy of the explosion is exerted in all directions in the form of a sphere having its center at the point of explo- sion, and the waves of energy lose their force as the distance from this central point increases. The energy of the explosion at any point in the sphere of energy is, therefore, inversely proportional to the distance of this point from the center of explosion. In the vicinity of the center of explosion the gases have sufficient power to destroy the force of cohesion and shatter the rock ; further on, as they lose strength, they only destroy the elasticity of the material and produce cracks ; and still further away they only produce a shock, and do not affect the material. Within the sphere of energy there are, therefore, three other concentric spheres: the first one being where cohesion is destroyed, the second where elasticity is overcome, 30 and the third where the shock is transmitted by elasticity, When the latter sphere comes below the surface, the gases remain inside the rock; but when the surface intersects either of the other two spheres, the gases blow up the rock, forming a cone or crater, whose apex is at the point of explosion, and which is called the blasting-cone. The larger the blasting-cone is, the greater is the amount of rock broken up ; and the object of the engineer should, therefore, always be so to regulate the depth of the hole and the quantity of explosive as to secure the largest possible blasting cone in each case. Experiments are required to determine the most efficient depth of hole, and quantity of explosive to be employed, since these differ in different kinds of rock, with the position of the rock strata, etc. ; but in ordinary practice, the depths of the holes are made from 1^ ft. to 2 ft. in the heading and upper portion of the tunnel, when drilled by hand; and from 3 ft. to 5 ft. when drilled by power drills. In the lower portion of the profile, the holes are made deeper, from 3 ft. to 4 ft. when drilled by hand, and exceeding 5 ft. when drilled by power. The dis- tance of the holes apart should be about equal to the diameter of the blasting-cone ; as a general rule it is assumed that the base of the blasting-cone has a diameter equal to twice the depth of the hole. The following table gives the average number of holes required in each part of the excavation for the St. Gothard tunnel : NO. OF PABT* NAME OF PART NO. OF HOLES 1. Heading 6 to 9 2. Right wing of heading 3 to 5 3. Left wing of heading 3 to 5 4. Shallow trench with core 2 5. Deepening of trench to floor 6 to 9 6. Narrow mass of core to left 3 7. Greater mass of core to left 6 to 9 8. Culvert 1 Total section ..... 30 to 43 * The location of the parts numbered is shown by Fig. 15, p. 32. EXCAVATING MACHINES AND ROCK DRILLS 31 The quantity of explosives required for blasting depends upon the quality of the rock, since the force of the explosives must overcome the cohesion of the rock, winch varies with its nature, and often differs greatly in rocks of the same kind and composition. The quantity of explosives required to secure the greatest efficiency in blasting any particular rock may be determined experimentally, but in practice it is usually deduced by the following rules : (1) The blasting force is directly pro- portional to the weight of the explosives used, and (2) the bulk of the blasted rock is proportional to the cube of the depth of the holes. It is usually assumed, also, that the explosive should fill at least one-fourth the depth of the hole. 32 TUNNELING CHAPTER IV. GENERAL METHODS OF EXCAVATION: SHAFTS: CLASSIFICATION OF TUNNELS. A NUMBEII of different modes of procedure are followed in excavating tunnel?, and each of the more important of these will be considered in a separate chapter. There are, however, certain characteristics common to all of these methods, and these will be noted briefly here. Division of Section. It may be asserted at the outset that the whole area of the tunnel section is not ordinarily excavated at one time, but that it is removed in sections, and as each section is excavated it is thoroughly timbered or strutted. The order in which these different sections are excavated varies with the method of excavation, and it is clearly shown for each method in succeeding chap- ters. As a single example to illus- trate the proposition just made, the division of the section and the se- quence of excavation adopted at the 0-^1,1 i, t i . J /T^- ^t. (jrOthard tuilliel IS Selected (*lg. 15). The different parts of the section were excavated in the order numbered ; the names given to each part, and the number of holes employed in breaking it down, are given by the table on page 30. Whatever method is employed, the work always begins by driving a heading, which is the most difficult and expensive part of the excavation. All the other operations required in breaking down the remainder FIG. 15. Diagram Showing Sequence of Excavation lor St. Gothard CKNKKAL METHODS OF EXCAVATION 33 of the tunnel section are usually designated by the general term of enlargement of the profile. The various operations of excavation may, therefore, be classified either as excavation of the heading or enlargement of the profile. Excavation of the Heading. - There is considerable confusion among the different authorities regarding the exact definition of the term "heading" as it is used in tunnel work. Some authorities call a small passage driven at the top of the profile a heading, and a similar passage driven at the bottom of the profile a drift ; others call any passage driven parallel to the tunnel axis, whether at the top or at the bottom of the profile, a drift; and still others give the name "heading" to all such passages. For the sake of distinctness of terminology it seems preferable to call the passage a heading when it is located at the top of the profile, and a drift when it is located near the bottom. Headings and drifts are driven in advance of the general excavation for the following purposes: (1) To fix correctly the axis of the tunnel; (2) to allow the work to go on at different points without the gangs of laborers interfering with each other ; (3) to detect the nature of material to be dealt with and to l>e ready in any contingency to overcome any trouble caused by a change in the soil ; and (4) to collect the water. The dimensions of headings in actual practice vary according to the nature of the soil through which they are driven. As a general rule they should not be less than 7 ft. in height, so as to allow the men to work standing, and have room left for the roof strutting. The width should not be less than 6 ft., to allow two men to work at the front, and to give room for the material cars without interfering with the wall strutting. Usually headings are made 8 ft. wide. The length of headings in practice varies according to circumstances. In very long tunnels through hard rock the headings are sometimes ex- cavated from 1000 ft; to 2000 ft. in advance, in order that they may meet as soon as possible and the ranging of the center line 34 TUNNELING be verified, and so that as great an area of rock as possible may be attacked at the same time in the work of enlarging the profile. In short tunnels, where the ranging of the center line is less liable to error, shorter headings are employed, and in soft soils they are made shorter and shorter as the cohesion of the soil decreases. When the material has too little cohesion to stand alone, the tops and sides of the heading require to be supported by strutting. To prevent caving at the front of the heading, the face of the excavation is made inclined, the inclination following as near as may be the natural slope of the material. Enlargement of the Profile. The enlargement of the profile is accomplished by excavating in succession several small prisms parallel to the heading, and its full length, which are so located that as each one is taken out the cross-section of the original heading is enlarged. The number, location, and sequence of these prisms vary in different methods of excavation, and are explained in succeeding chapters where these methods are described. To direct the excava- Fio. 16. Diagram Showing Manner of Determining Correspondence of tioil SO as to keep it always Within Excavation to Sectional Profile. ,11 n < , the boundaries of the adopted pro- file, the engineer first marks the center line on the roof of the heading by wooden or metal pegs, or by some other suitable means by which a plumb line may be suspended. He next draws to a large scale a profile of the proposed section ; and beginning at the top of the vertical axis he draws horizontal lines at regular intervals, as shown by Fig. 16, until they inter- sect the boundary lines of the profile, and designates on each of these lines the distance between the vertical axis and the point where it intersects the profile. It is evident that if the foreman of excavation divides his plumb line in a manner corre- sponding to the engineer's drawing, and then measures horizon- GENERAL METHODS OF EXCAVATION 35 tally and at right angles to the vertical center plane of the tunnel the distance designated on the horizontal lines of the drawing, he will have located points on the profile of the sec- tion, or in other words have established the limits of excava- tion. In the excavation of the Croton Aqueduct for the water supply of New York city, an instrument called a polar pro- tractor was used for determining the location of the sectional FIG. 17. Polar Protractor for Determining Profile of Excavated Cross-Section. profile. This instrument consists of a circular disk graduated to degrees, and mounted on a tripod in such a manner that it may be leveled up, and also Lave a vertical motion and a motion about the vertical axis. The construction is shown clearly by Fig. 17. In use the device is mounted with its center at the axis of the tunnel. A light wooden measuring- rod tapering to a point, shod with brass and graduated to feet and hundredths of a foot, lies upon the wooden arm or rest, which revolves upon the face of the disk, and slides out to 36 TUNNELING a contact with the surface of the excavation at such points as are to be determined. If the only information desired is whether or not the excavation is sufficient or beyond the es- tablished lines, the rod is set to the proper radius, and if it swings clear the fact is determined. If a true copy of the actual cross-section is desired, the rod is brought into contact with the significant points in the cross-section, and the angles and distances are recorded. The general method of directing the excavation in enlarging the profile by referring all points of the profile to the vertical axis is the one usually employed in tunneling, and gives good results. It is considered better in actual practice to have the excavation exceed the profile somewhat than to have it fall short of it, since the voids can be more easily filled in with riprap than the encroaching rock can be excavated during the building of the masonry. In tunnels where strutting is neces- sary the excavation must be made enough larger than the finished section to provide the space for it. In sofkground tunnels it is also usual to enlarge the excavation to allow for the probable slight sinking of the masonry. The proper allow- ance for strutting is usually left to the judgment of the fore- man of excavation, but the allowance for settlement must be fixed by the engineer. SHAFTS. Shafts are vertical walls or passages sunk along the line of the tunnel at one or more points between the entrances, to permit the tunnel excavation to be attacked at several different points at once, thus greatly reducing the time required for excavation. Shafts may be located directly over the center of the tunnel or to one side of it, and, while usually vertical, are sometimes inclined. During the construction of the tunnel the shafts serve the same purpose as the entrances ; hence they must afford a passageway for the excavated materials, which GENERAL METHODS OF EXCAVATION 37 have to be hoisted out, and also for the construction tools and materials which have to be lowered down them. They must also afford a passageway for workmen, draft animals, and for pipes for ventilation, water, compressed air, etc. The character of this traffic indicates the dimensions required, but these de- pend also upon the method of hoisting employed. Thus, when a windlass or horse gin is used, and the materials are hoisted in buckets of small dimensions, the dimensions of the shaft may also be small; but when steam elevators are employed, and the material is carried on cars run on to the platform of the elevator, large dimensions must be given to the shaft. Generally the parts of the shaft used for different purposes are separated by partitions. The elevator for workmen and the various pipes are placed in one compartment, while the elevator for hoisting the excavated material and lowering construction material is placed in another. Shafts may be either temporary or permanent. They are temporary when they are filled in after the tunnel is completed, and permanent when they are left open to supply ventilation to the tunnel. Permanent shafts are usually made circular, and lined with brick, unless excavated in very hard and durable rock. When sunk for temporary use only, shafts are usually made rectangular with the greater dimension transverse to the tunnel. They are strutted with timber. A pump is generally located at the bottom of the shaft to collect the water which seeps in from the sides of the shaft and from the tunnel excavation. The dimensions of this pump will of course vary with the amount of water encountered, as will also the capacity of the pump for forcing it up and out of the shaft, which has always to be kept dry. The majority of engineers prefer to sink shafts directly over the center line of the tunnel. Side shafts are employed chiefly by French engineers. The chief advantage of the former method is the great facility which it affords for hoisting out the materials, while in favor of the latter method is the 38 TUNNELING non-interference of the shaft with the operations inside the tunnel. Were it not that the side shaft requires the intro- duction of a transverse gallery connecting it with the tunnel, it would be on the whole superior to the center shaft ; but the side gallery necessitates turning the cars at right angles, and consequently the use of a very sharp curve or a turntable to reach the shaft bottom, which is a disadvantage that may outweigh its advantages in some other respects. It is impos- sible to state absolutely which of these methods of locating shafts is the best ; both present advantages and disadvantages, and the use of one or the other is usually determined more by the local conditions than by any general superiority of either. When side shafts are employed they are sometimes made inclined instead of vertical. This form is used when the depth of the shaft is small. By it the hauling is greatly simplified, since the cars loaded at the front with excavated material can be hauled directly out of the shaft and to the dumping-place, surmounting the inclined shaft by means of continuous cables. The short galleries connecting the side shafts with the tunnel proper usually have a smaller section than the tunnel, but are excavated in exactly the same manner. Another form of side shaft sometimes used is one reaching to the surface when the tunnel runs close to the side of cliff, as is the case with some of the Alpine railway tunnels. CLASSIFICATION OF TUNNELS. Tunnels are classified in various ways, but the most logical method would appear to be a grouping according to the quality of the material through which they are driven ; and this method will be adopted here. By this method we have first the fol- lowing general classification : (1) Tunnels in hard rock ; (2) tunnels in ordinary loose soil; (3) tunnels in quicksand; ( 4) open-cut tunnels ; and (5) submarine tunnels. It is hardly necessary to say that this classification, like all others, is simply GENERAL, METHODS OF EXCAVATION 39 an arbitrary arrangement adopted for the sake of order and convenience in treating the subject. Tunnels in Hard Rock. With the numerous labor-saving methods and machines now available, hard rock is perhaps the safest and easiest of all materials through which to drive a tunnel. Tunnels through hard rock may be excavated, either by a drift or by a heading. The difference depends upon whether the advance gallery is located close to the floor or near the soffit of the section. Tunnels in Loose Soils. In driving tunnels through loose soils many different methods have been devised, which may be grouped as follows: (1) Tunnels excavated at the soffit Belgian method; (2) tunnels excavated along the perimeter German method; (3) tunnels excavated in the whole sec- tion English and Austrian methods ; (4) tunnels excavated in two halves independent of each other Italian method. (1) Excavating the tunnel by beginning at the soffit of the section, or by the Belgian method, is the method of tunnel- ing in loose soils most commonly employed in Europe at the present time. It consists in excavating the soffit of the section first ; then building the arch, which is supported upon the unexcavated ground ; and finally in excavating the lower portion of the section, and building the side walls and invert. (2) In excavating tunnels along the perimeter an annular excavation is made, following closely the outline of the sec- tional profile in which the lining masonry is built, after which the center core is excavated. In the German method two drifts are opened at each side of the tunnel near the bottom. Other drifts are excavated, one above the other, on each side to extend or heighten the first two until all the perimeter is open except across the bottom. The masonry lining is then built from the bottom upwards on each side to the crown of the arch, and then the center core is removed and the invert is built 40 TUNNELING (3) This method, as its name implies, consists in taking out short lengths of the whole sectional profile before begin- ning the building of the masonry. In the English method the lengths of section excavated vary from 10 ft. to 25 ft. The masonry invert is built first, then the side walls, and finally the arch. The excavators and the masons work alter- nately, the excavation being stopped while the masonry is being built, and vice versa. The Austrian method differs in two particulars from the English : the length of section opened is made great enough to allow the excavators to continue work ahead of the masons, and the side walls and roof are built before the invert. (4) The Italian method is very seldom employed on account of its expensiveness, but it can often be used where the other methods fail. It consists in excavating the lower half of the section, and building the invert and side walls, and then filling the space between the walls in again except for a narrow passageway for the cars ; next the upper part of the section is excavated, as in the Belgian method, and the arch is built ; and finally the soil in the lower part is permanently removed. Tunnels in Quicksand. - Tunnels through quicksand are driven by one of the ordinary soft-ground methods after drain- ing away the water, or else as submarine tunnels. Open-Cut Tunnels. Open-cut tunnels are those driven at such a small depth under the surface that it is more convenient to excavate an open cut, build the tunnel masonry inside it, and then refill the open spaces, than it is to carry on the work entirely underground. In firm soils the usual mode of opera- tion is to excavate first two parallel trenches for the side walls, then remove the core, and build the arch and the invert. In unstable soils, since the invert must be built first, it is usual to open up a single wide trench. In infrequent cases where a tunnel is desired in a place which is to be filled in, the masonry is built as a surface structure, which in due time is covered. GENERAL METHODS OF EXCAVATION 41 Submarine Tunnels. -- The mode of procedure followed in excavating submarine tunnels depends upon whether the mate- rial penetrated is pervious or impervious to water. In imper- vious material any of the ordinary methods of tunneling found suitable may be employed. In pervious material the excava- tion may be accomplished either by means of compressed air to keep the water out of the excavation, or by means of a shield closing the front of the excavation, or by a combination of these two methods. Tunnels on the river bed are built by means of coffer dams which inclose alternate portions of the work, or by sinking a continuous series of pneumatic caissons and opening communication between them. In hard rock, j In loose soil. < METHODS OF EXCAVATING - TUNNELS. In quicksand. Open-cut tunnels. Submarine tunnels. By drifts. By a heading. By upper half: the arch is built be- fore the side walls. By the perimeter: excavated and lined before the central nucleus is battered down. By whole section: the lining begins after the whole section is excavated. By halves: the lower half is ex- cavated, lined, and filled in again, fol- lowed by the work of the upper half. In resistant soils. In loose soils. Built up. At - TIMBERING OR STRUTTING TUNNELS 47 the heading is advanced. The poling-boards at the sides of the heading are placed in a similar manner to the roof poling- boards. A second method of using inclined poling-boards is shown by Fig. 25. Here the poling-boards run transversely, and are supported by the arrangement of timbering shown. The chief advantage of using these inclined poling-boards, particularly in the manner shown by Fig. 24, is that the excavators work .under cover at all times, and are thus safe from falling fragments or sudden cavings. Box Strutting. In very treacherous soils, such as quick- sand, peat, and laminated clay, box strutting is commonly em- ployed. The method of building this strutting is to set up at the face of the work a rectangular frame^ and use it as a guide in driving a lagging or boxing of horizontal planks into the soft soil ahead. These planks have sharp edges, and are driven to a distance of 2 ft. or 3 ft. into the face of the heading, so as to inclose a rectangular body of earth. This earth is excavated nearly to the ends of the planks, and then another frame is inserted close up against'the new face of the excavation, which supports the planks so that the remainder of the earth included by them may be removed. These two frames, with their plank lagging, constitute a " box ; " and a series of these boxes, one succeeding another, form the strutting of the heading. Strutting the Face. In some cases it is found necessary to strut the face of the heading in order to prevent it from caving in. This is generally done by setting plank vertically, and bracing them up by means of inclined props whose feet abut against the sill of the nearest cross frame. This strutting is erected while the workmen are placing the side and roof strutting, and is removed to permit excavation. Full Section Timber Strutting. For strutting the full section two forms of timbering are employed, known as the polygonal system and the longitudinal system. Longitudinal strutting consists of a timber structure so arranged as to have all the principal members supporting the 48 TUNNELING poling-boards parallel to the axis of the tunnel. This system of strutting is peculiar to the English method of tunneling. The longitudinal timbers rest on this finished masonry at one end, and are carried on a cross frame or by props at the other end. At intermediate points the longitudinals are braced apart by struts in planes transverse to the tunnel axis. This construction makes a very strong strutting framework, since the transverse struts act as arch ribs to stiffen the longitu- dinals; but the use of transverse poling-boards requires the excavation of a larger cross-section than is necessary when longi- tudinal poling-boards are employed, and this increases the cost both for the amount of earth excavated and the greater quantity of filling required. In polygonal strutting the main members are in a plane normal to the axis of the tunnel. They form a polygon whose sides follow closely the sectional profile of the excavation. These polygonal frames are placed at more or less short inter- vals apart, and are braced together by short longitudinal struts lying close to the sides of the excavation, and running from one frame to the next, and also by longer longitudinal members which extend over several frames. The polygonal system of strutting is peculiar to the Austrian method of tunneling, and is fully described in a succeeding chapter. One of its distinc- tive characteristics is that the poling-boards are in- serted parallel to the tunnel axis. Polygonal strutting is generally held to be stronger than longitudinal strutting under uniform loads, but it is more liable to distortion when the loads are unsym metrical. Strutting of Shafts. Tunnel shafts are strutted both to prevent the caving-in of the sides and to divide them into Fin. 26. Shaft with Single Transverse Strutting. TIMBERING OK STRUTTING TUNNELS 49 compartments. When the material penetrated is very compact, and caving is not likely, a single series of transverse struts, one above the other, running from the top to the bottom of the shaft, as shown by Fig. 26, is used to divide it into two com- partments. In softer material, where the sides of the shaft require support, Fig. 27 shows a form of strutting commonly employed. It consists of vertical corner posts braced apart at inter- vals by four horizontal struts placed close to the walls of the shaft. The longer side FIG. 27. Rectangular Frame Strutting for Shafts. struts are also braced apart at the center by a middle strut which divides the shaft into two compartments. A lagging of vertical plank is placed between the walls of the shaft and the horizontal side struts. In very loose soils the form of strutting shown by Fig. 28 is employed. This is practically the same construction as is shown by Fig. 27, with the addition of an interior polygonal horizontal bracing in each half of the shaft. Referring to Fig. 28, the timbers #, a, etc., are vertical and con- tinuous from the top to the bottom of the shaft; and the horizontal timbers, ft, ft, etc., are spaced at more or less close intervals verti- cally. The lagging plank may be laid with spaces between them, or close together, or, in case of very loose material, with their edges overlapping. The manner of constructing the strutting is also governed by the stability of the soil. In firm soils it is possible to sink the shaft quite a depth without timbering, and the timbering can FIG. 28. Reinforced Rectangular Frame Strut- ting for Shafts in Treacherous Materials. 50 TUNNELING be erected in sections of considerable length, which is always an advantage, but in loose soils the timbering has to follow closely the excavation. The solid wall shaft struttings which have been described are discontinued at the point where the shaft intersects the tunnel excavation ; and from this point to the floor of the tunnel an open timbering is employed, whose only duty is to support the weight of the solid strutting above. This timber- ing is made in various forms, but the most common is a timber truss or arch construction which spans the tunnel section. Quantity of Timber. The quantity of timber employed in strutting a tunnel varies with the character of the material through which the tunnel is excavated : it is small for solid- rock tunnels, and large for soft-ground tunnels. In the Bel- gian method of excavation a smaller quantity of timber is used than in any of the other ordinary methods. For single- track tunnels excavated by this method there will be needed on an average about 3 to 3J cu. yds. of timber per lineal foot of tunnel. Practical experience shows that about four-fifths of the timber once used can be employed for the second time. In any of the methods in which the whole tunnel section is excavated at once, the average amount of timber required per lineal foot is about 8.7 cu. yds. Of this amount about two- thirds can be used a second time. In the Italian method, in which the upper half and the lower half are excavated separately, about 5 cu. yds. of timber are required per lineal foot of tunnel, about one-half of which can be employed a second time. For quicksand tunnels the amount of timbering required per lineal foot varies from 3 to 5 cubic yds. Shaft strutting requires from 1 to 1-j- cu. yds. of timber per lineal foot. Dimensions of Timber. The dimensions of the principal mem- bers composing the strutting of headings, full section, and shafts, are given in Table I. The planks used for lagging or the poling-boards are usually from 4 ins. to 6 ins. wide, with a length depending upon the method of strutting employed. TIMBERING OB STRUTTING TUNNELS TABLE I. 51 Showing Sizes of Various Timbers Used in Strutting Tunnels Driven Through Different Materials. Headings : Cap-pieces and vertical struts ROCK. SOFT SOILS. 1 ins. 6 5 6 12 10 8 10 6 4.5 8 10 12 12 10 6 8 8 8 6 !_ ins. 8 5 4.5 14 12 8 12 8 4 10 12 14 14 10 4.5 8 8 8 6 4.5 5 "S st ins. 10 8 6 3 14 14 10 14 10 3 12 14 16 16 12 10 4 10 8 10 8 4 I >> c * g > JS Inir 14 12 8 2.6 16 18 24 24 18 12 3 14 12 12 8 2.6 ins. 12 10 7 2.6 14 16 20 20 16 12 3 12 10 12 8 3 Sills Distance apart of the frames in feet . . . Strutting of the tunnel, longitudinal strutting : Props vertical or inclined supporting the crown Sills Cap-pieces or saddles Struts to stiffen the structure Distance apart of the frames (in feet) . . . Polygonal strutting : Vertical struts on top Vertical struts below .... Intermediate sills Rakinf props Distance apart of the frames (in feet) . . . Shafts : Horizontal beams forming the frame .... Transverse beams Vertical struts between the frames Struts to reenforce the frame Distance apart of the strutting (in feet) . . . IRON STRUTTING. In 1862 Mr. Rziha employed old iron railway rails for strutting the Naenesn tunnel, and his example was successfully followed in several tunnels built later where timber was scarce 52 TUNNELING and expensive. The advantages which iron strutting is claimed to possess over the more common wooden structure are : its greater strength ; the smaller amount of space which it takes up ; and the fact that it does not wear out, and may, therefore, be used over and over again. Iron Strutting in Headings. In strutting the headings the cross frames have a crown bar consisting of a section of old railway rail carried either by wood or iron side posts. When wooden side posts are used their upper ends have a dovetail mor- tise, and are bound with an iron band, as shown by Fig. 29. The base of the rail crown bar is set into the dovetail mortise and fastened by wedges. When iron FIG. 29. -Strut- Slde P StS are em P lo y ed the J FIG. 30. - Strutting- ting of Timber usually consist of sections of rail- made entirel y of PostsandRail- ., , . Railway Rails, way Rail Caps. wa J rails and the crown bar 1S attached to them by fish-plate connections, as shown by Fig. 30. The iron cross frames are set up as the heading advances, and carry the plank lagging or poling-boards, exactly in the same manner as the timber cross frames previ- ously described. Full Section Iron Strutting-. The iron strutting devised by Mr. Rziha for full section work is shown by Fig. 31. Briefly described, it consists of voussoir-shaped cast-iron segments, which are built up in arch form. Fig. 32 shows the construc- tion of one of the segments, all of which are alike, with the exception of the crown segment, which has a mortise and tenon joint which is kept open by filling the mortise with sand. The segments are bolted together by means of suitable bolt- holes in the vertical flanges, and when fully connected form an arch rib of cast iron. This arch rib, A, Fig. 31, carries a series of angle or T-iron frames bent into approximately voussoir shape, as shown at B, Fig. 31. Above these frames are inserted TIMBERING OR STRUTTING TUNNELS 53 FIG. 31. Rziha's Combined Strutting and Centering of Cast Iron. the poling-boards, running longitudinally, and spanning the distance between consecutive arch ribs. By removing the bent iron frames the cast-iron rib forms a center upon which to con- struct the masonry. Fi- nally, to remove the cast- iron rib itself, the sand is drawn out of the mor- tise and tenon joint in the crown segment, which allows the joint to close, and loosen the segments so that they are easily unbutted. The illustration, Fig. 31, shows longitudinal poling-boards; more often longitudinal crown bars of railway rails span the space between connective arch ribs, and support transverse poling-boards. In building the masonry, work is begun at the bottom on each side, the bent iron frames being removed one after another to give room for the masonry. As each frame is removed, it is replaced with a sort of screw jack- screw to support the poling-boards until the masonry is sufficiently completed to allow their removal. The interior bracing of the arch rib shown at a a and b b consists of railway rails carried by brack- ets cast on to the segments. A similar bracing of rails connects the successive arch ribs. These lines of bracing serve to carry the scaffolding upon which the masons work in building the lining. Iron Shaft Strutting. In soft-ground shaft work, the use of an iron strutting, consisting of consecutive cast-iron rings, has Pro. 32. Cast-iron Segment of Rziha's Strutting and Centering. TUNNELING sometimes been employed to advantage. Fig. 33 shows the construction of one of these rings, which, it will be seen, is com- posed of four segments connected to each other by means of bolted flanges. The holes shown in the circumferential web of the ring are to allow for the seepage from the earth side walls. The method of placing this cylindrical strutting is to start with a ring having a cutting-edge. By means of excavation inside the ring, and by ramming, the ring is sunk into the ground a distance equal to its height. Another ring is then fastened by special hooks on top of the first one, and the sinking continued until the second ring is down flush with the surface. A third ring is then added, and so on until the entire shaft is excavated and strutted. As in timber shaft strutting, the solid iron ring strutting is carried down only to the top of the tunnel section, and below this point there is an open timber or iron supporting framework. FIG. 33. Cast-iron Segmental Strutting for Shafts. METHODS OF HAULING IN TUNNELS 55 CHAPTER VI. METHODS OF HAULING IN TUNNELS. THE transportation from one point to another within the tunnel and its shafts of any material, whether it is excavated spoil or construction material, is defined as hauling. In all engineering construction, the transportation of excavated materials, and materials for construction, constitutes a very important part of the expense of the work; but hauling in tunnels where the room is very limited, and where work is constantly in progress over and at the sides of the track, is a particularly expensive process. Hauling in tunnels may be done either by way of the entrances, or by way of the shafts, or by way of both the entrances and shafts. Hauling by Way of Entrances. When the hauling is done by the way of the entrances, the materials to be hauled are taken directly from the point of construction to the en- trances, or in the opposite di- rection, by means of special cars of different patterns. For general purposes, these differ- ent patterns of cars may be grouped into three classes, platform-cars, dump-cars, and box-cars. Representative ex- FlG . ^ _ P ^ tform Car for Tunnel Work . amples of these several classes of cars are shown in Figs. 34 to 37 * inclusive, but it will be readily understood that there are many other forms. Briefly described, platform-cars (Fig. 34) consist of a * Reproduced from catalogue of Arthur Koppel, New York. 56 TUNNELING FIG. 35. Iron Dump-Car for Tunnel Work. wooden platform mounted on tracks, and they are usually em- ployed for the transportation of timber, ties, etc. Dump-cars are used in greater numbers in tunnel work than any other form. Fig. 35 shows a dump-car of metal construction, and Fig. 36 one constructed with a metal under-frame t*nd wooden box. These cars are made to run on narrow-gauge tracks, and usually have a capacity of about one to one and one-half cubic yards. Box-cars are more extensively employed in Europe for tunnel work than in America. Fig. 37 shows a typical European box-car for tunnel work. It is made either to run on narrow-gauge or standard- gauge tracks. It is usually desirable in tunnel work to employ cars of different forms for different parts of the work. In rock tunnels it is a common practice to use narrow-gauge cars of small size in the headings, arid larger, broad-gauge cars for the enlargement of the profile. Where narrow-gauge cars are employed for all purposes, it will also be found more convenient to use platform-cars for handling the construction material, and dump-cars for removing the spoil. The extent to which it is desir- able tO USe Cars Of different forms FIG. 36. Wooden Dump-Car for Tunnel will depend upon the character and conditions of the work, and particularly upon how far it is possible to install the permanent track. As a general rule, it is considered preferable to lay the permanent tracks at once, and do all the hauling upon them, so that as soon as the tunnel is completed, trains may pass METHODS OF HAULING IX TUNNELS 57 through without delay. To what extent this may be done, or whether it can be done at all or not, depends upon the method of excavation and other local conditions. In soft-ground tunnels excavated by the English or Austrian methods, it is quite possible to lay the permanent tracks at first, since the whole section is excavated at once, and the excavation is kept bat a little ahead of the complete/1 tunnel. In rock tunnels, where the heading is driven far ahead of the com- pleted section, it is, of course, impossible to keep the perma- nent track close to the advance work, and narrow-gauge tracks must be laid in the heading. The same thing is true in soft- ground tunnels driven by successive headings and drifts. In these cases, therefore, where narrow-gauge tracks have to be used for some portions of the work anyway, the question comes up whether it is preferable FIG. 37 Box-Car for Tunnel Work. to use temporary narrow-gauge tracks throughout, or to lay the permanent track as far ahead as possible, and then extend narrow-gauge tracks to the advance excavation. In the latter case it will, of course, be necessary to trans-ship each load from the narrow-gauge to the standard-gauge cars, or vice versa, which means extra cost and trouble. To avoid this, the method is sometimes adopted of laying a third rail between the standard-gauge rails, so that either standard- or narrow-gauge cars may be transported over the line. Whatever form the local conditions may require the system of construction tracks to assume, it may be set down as a general rule that the permanent tracks should be kept as far advanced as possible, and temporary tracks employed only where the permanent tracks are impracticable. The motive power employed for hauling in tunnels may be furnished by animals or by mechanical motors. Animal power 58 TUNNELING is generally employed in short tunnels and in the advance headings and galleries. In long tunnels, or where the exca- vated material has to be transported some distance away from the tunnel, mechanical power is preferable, for obvious reasons. The motors most used are small steam locomotives, special compressed-air locomotives, and electric motors. Compressed air and electric locomotives are built in various forms, and are particularly well adapted for tunnel work because of their small dimensions, and freedom from smoke and heat. Hauling by Way of Shafts. When the excavated material and materials of construction are handled through shafts, the operation of hauling may be divided into three processes : the transportation of the materials along the floor of the tunnel, the hoisting of them through the shaft, and the sur- face transportation from and to the mouth of the shaft. These three operations should be arranged to work in harmony with each other, so as to avoid waste of time and unnecessary han- dling of the materials. An endeavor should be made to avoid, if possible, breaking or trans-shipping the load from the time it starts at the heading until it is dumped at the spoil bank. This can be accomplished in two ways. One way is to hoist the boxes of the cars from their trucks at the bottom of the shaft, and place them on similar trucks running on the surface tracks. The other way is to run the loaded cars on to the ele- vator platform at the bottom, hoist them, and then run them on to the surface tracks. If the first method is employed, the car box is usually made of metal, and is provided at its top edges with hooks or ears to which to attach the hoisting cables. When the second method is used, the elevator platform has tracks laid on it which connect with the tracks on the tunnel floor, and also with those on the surface. Hoisting Machinery. The machines most commonly em- ployed for hoisting purposes in tunnel shafts are steam hoisting engines, horse gins, and windlasses operated by hand. Wind- lasses and horse gins are rather crude machines for hoisting- METHODS OF HAULING IN TUNNELS 59 loads, and are used only in special circumstances, where the shaft is of small depth, when the amount of material to be hoisted is small, or where for any reason the use of hoisting engines is precluded. The steam hoisting engine is the stan- dard machine for the rapid lifting of heavy vertical loads. Recently oil engines and electric hoists have also come to be used to some extent, and under certain conditions these ma- chines possess notable advantages. The construction of hand windlasses is familiar to every one. In tunnel work this device is located directly over the shaft, with its axis a little more than half a man's height, so that the crank handle does not rise above the shoulder line. To develop its greatest efficiency the hoisting rope is passed around the windlass drum so that the two ends hang down the shaft, and as one end descends the other ascends. A skip, or bucket, is attached to each of the rope ends ; and by loading the descend- ing skip with construction materials and the ascending skip with spoil, the two skip loads tend to balance each other, thus increasing the capacity of the windlass, and decreasing the manual labor required to operate it. Skips varying from 0.3 cu. yd. to 0.5 cu. yd. are used. The horse gin consists of a vertical cylinder or drum provided with radial arms to which the horses are hitched, which revolve the cylinder by walking around it in a circle. The hoisting rope is rove around the drum so that the two ends extend down the shaft with skips attached, as described in speaking of the hand windlass. The power of the horse gin is, of course, much greater than that of a windlass operated by hand, skips of 1 cu. yd. capacity being commonly used. Horse gins are no longer economical hoisting machines, according to one prominent authority, when V (H+20) > 5000, where V equals the volume of material to be hoisted, and H equals the height of the hoist, the weight of the excavated material being 2100 Ibs. per cu. yd. As a gen- eral rule, however, it is assumed that it is not economical to employ horse gins with a depth of shaft exceeding 150 ft. 60 TUNNELING As already stated, the most efficient and most commonly used device for hoisting at tunnel shafts is the steam hoisting engine. There are numerous builders of hoisting engines, each of which manufactures several patterns and sizes of engines. In each case, however, the apparatus consists of a boiler supply- ing steam to. a horizontal engine which operates one or more rope drums. The engines are always reversible. They may be employed to hoist the skips directly, or to operate elevators upon which the skips or cars are loaded. In either case the hoisting ropes pass from the engine drum to and around ver- tical sheaves situated directly over the shaft so as to secure the necessary vertical travel of the ropes down the shaft. Where the shaft is divided into two compartments, each having an ele- vator or hoist, double-drum engines are employed, one drum being used for the operations in one compartment, and the other for the operations in the other compartment. Where the work is to be of considerable duration, or when it is done in cold weather, more or less elaborate shelters or engine houses are built to cover and protect the machinery. Choice between the method of hoisting the skips directly, and the method of using elevators, depends upon the extent and character of the work. Where large quantities of material are to be hoisted rapidly, it is generally considered preferable to employ elevators instead of hoisting the skips directly. In direct hoisting at high speed, oscillations are likely to be pro- duced which may dash the skips against the sides of the shaft and cause accidents. The loads which can be carried in single skips are also smaller than those possible where elevators are used ; and this, combined with the slower hoisting speed required, reduces the capacity of this method, as compared with the use of elevators. Where elevators are employed, however, the plant required is much more extensive and costly ; it comprising not only the elevator cars with their safety devices, etc., but the construction of a guiding framework for these cars in the tun- nel shaft. For these various reasons the elevator becomes the METHODS OF HAULING IN TUNNELS 61 preferable hoisting device where the quantity of % material to be handled is large, where the shafts are deep, and where the work will extend over a long period of time ; but when the contrary conditions are the case, direct hoisting of the skips is generally the cheaper. The engineer has to integrate the various factors- in each individual case, and determine which method will best fulfill his purpose, which is to handle the material at the least cost within the given time and conditions. The construction of ele- vators for tunnel work is simple. The elevator car consists usually of an open framework box of timber and iron, having a plank floor on which car tracks are laid, and its roof arranged for connecting the hoisting cable O O (Fig. 38 *). Rigid construc- tion is necessary to resist the hoisting strains. The sides of the car are usually de- signed to slide against tim- ber guides on the shaft walls. Some form of safety device, of which there are several kinds, should be employed to pre- vent the fall of the elevator, in case the hoisting rope breaks, or some mishap occurs to the hoisting machinery, which en- dangers the fall of the car. Speaking tubes and electric-bell signals should also be provided to secure communication be- tween the top and bottom of the shaft. * Reproduced from the catalogue of the Ledgerwood Manufacturing Company, New FIG. 38. Elevator Car for Tunnel Shafts. York. 62 TUNNELING CHAPTER VII. TYPES OF CENTERS AND MOLDS EMPLOYED IN CONSTRUCTING TUNNEL LININGS OF MASONRY. THE masonry lining of a tunnel may be described as con- sisting of two or more segments of circular arches combined so as to form a continuous solid ring of masonry. To direct the operations of the masons in constructing this masonry ring, templates or patterns are provided which show the exact dimensions and form of the sectional profile which it is de- sired to secure. These patterns or templates will vary in number and construction with the form of lining and the method of excavation adopted. Where the excavation is fully lined on all four sides, the masonry work is usually divided into three parts, the invert or floor masonry, the side-wall masonry, and the roof-arch masonry. At least one separate pattern has to be employed in constructing each of these parts of the lining; and they are known respectively as ground molds, leading frames, and arch centers, or simply centers. In the following paragraphs the form and construction usually employed for each of these three kinds of patterns is de- scribed. Ground Molds. Ground molds are employed in building the tunnel invert. They are generally constructed of 3-inch plank cut exactly to the form and dimensions of the invert masonry as shown in Fig. 39. To permit of convenience of handling in a restricted space, they are generally made in two parts, which are joined at the middle by means of iron fish-plates and bolts. Either one or two ground molds may be employed. Where two TYPES OF CENTERS AND MOLDS 63 molds are used they are set up a short distance apart, and cords are stretched from one to the other parallel to the axis of the tunnel, by which the masons are guided in their work. Ex- treme care has to be taken in setting the molds to ensure that they are fixed at the proper grade, and are in a plane normal FIG. 39. Ground Moid for constructing , - , , , -fTTi Tunnel Invert Masonry. to the axis of the tunnel. Where only one ground mold is employed, the finished masonry is depended upon to supply the place of the second mold, cords being stretched from it to the single mold placed the requisite distance ahead. The leveling and centering of the molds is ac- complished by means of transit and level. Two modifications of the form of ground mold shown by Fig. 39 are employed. The first modification is peculiar to the English method of excavation, and consists in combining the ground mold with the leading frame for the lower part of the side walls, as shown by Fig. 40. The second modification is FIG. 40. -Combined Grotmd Mold and Leading Frame enir> l ove( J w h er p the two for Invert and Side Wall Masonry. mpioye halves or sides of the invert are built separately, and it consists simply in using one- half of the mold shown by Fig. 39. When the last method of constructing the invert masonry is resorted to, extreme care has to be observed in setting the half-mold in order to avoid error. Leading Frames. As already stated, leading frames are the patterns, or molds, used in building the side walls of the lining. Like the ground mold they are usually built of plank ; one side being cut to the curve of the profile, and the other being made parallel to the vertical axis of the tunnel section. The vertical side usually has some arrangement by which a plumb 64 TUNNELING bob can be attached, as shown by Fig. 41, to guide the work- men in erecting the frame. The combined leading frame and ground mold shown in Fig. 40 has already been described. The use of this frame is possible only where the- masonry is begun at the invert and carried up on each side at the same time. This mode of con- struction is peculiar to the English method of tunneling ; in all other methods the form of sep- arate ground frame shown by Fig. 41 is employed. FIG. 4i. Lead- ^Q ground frames are lined in and leveled up by ing Frame for _ * constructing transit and level; and, as in setting the ground frames, care must be taken to secure accuracy in both direction and elevation. Arch Centers. The template or form upon which the roof arch is built is called a center. Unlike the ground molds and leading frames, the arch centers have to support the weight of the masonry and the roof pressures during the construction of the lining, and they, therefore, require to be made strong. Owing to the fact that the pressures are indeterminate, it is impossible to design a rational center, and resort is had to those constructions which past experience has shown to work satis- factorily under similar conditions. In a general way it can always be assumed that the construction should be as simple as possible, that the center should be so designed that it can be set up and removed with the least possible labor, and that the different pieces of the framework and lagging should be as short as possible, for convenience in handling. Tunnel centers are usually composed of two parts, a mold or curved surface upon which the masonry rests, and a frame- work which supports the mold. The curved surface or mold consists of a lagging of narrow boards running parallel to the tunnel axis, which rests upon the arched top members of two or more consecutive supporting frames. The supporting frame is built in the form of a truss, and must be made strong enough to withstand the heavy superimposed loads, consisting of the TYPES OF CENTERS AND MOLDS 65 arch masonry during construction, and of the roof pressures which are transferred to the center' when the strutting is removed to allow the masonry to be placed. The framework of the center is supported either by posts resting upon the floor of the excavation, or upon the invert masonry when this is built first, as in the English and Austrian methods, or it may be supported directly upon the ground where the arch masonry is built first, as in the Belgian method of tunneling. In describing the various methods of tunneling in succeed- ing chapters, the center construction and method of supporting the center peculiar to each will be fully explained, and only a few general remarks are necessary here. Centers may be classi- fied according to their construction and composition into plank centers, truss centers, and iron centers. One of the most common forms of plank centers is shown by Fig. 42. It consists of two half-polygons whose sides consist of 15 in. X 4 ft. planks having radial end-joints. These two half- polygons are laid one upon the other so that they break joints, as Shown by the figure, and the ex- FIG. 42. Plank Center for Construct- ing the Roof Arch. trados 01 the frame is cut to the true curve of the roof arch. The planks commonly used for making these centers are 4 ins. thick, making the total thick- ness of the center 8 ins. Plank centers of the construction described are suitable only for work where the pressures to be resisted are small, as in tunnels through a fairly firm rock, al- though there have been instances of their successful use in soft- ground tunnels. Where heavy loads have to be carried, trussed centers are generally employed, the trusses being composed of heavy square beams with scarfed and tenoned joints, reinforced by iron plates. Different forms of trusses are employed in each of the differ- ent methods of tunneling, and each of these is described in sue- 66 TUNNELING FIG. 43. Trussel Center for Constructing the Hoof Arch. ceeding chapters ; but they are generally either of the king-post or queen-post type, or so memodification of them. The king- post truss may be used alone, with or with- out the tie-beam, as shown by Fig. 43 ; but generally a queen-post truss is made to form the base of support for a smaller king post truss mounted on its top. This arrange- ment gives a trapezoidal form to the center, which approaches closely to the arch pro- file. Owing to the character of the pres- sures transmitted to the center, the usual tension members can be made very light. The combined center and strutting system devised by Mr. Rziha has already been described in a previous chapter. In recent European tunnel work quite extensive use has also been made of iron centers consisting of several segments of curved I-beams, connected by fish-plate joints so as to form a semi- circular arch rib. The ends or feet of these I-beam ribs have bearing-plates or shoes made by riveting angles to the I-beams. Centers constructed in a similar manner, but made of sections of old railway rail, were used in carrying out the tunnel work on the Rhine River Railroad in Germany. The advantages claimed for iron centers are that they take up less room, and that they can be used over and over again. Setting Up Centers. According to the method of excava- tion followed in building the tunnel, the centers for building the roof arch may have to be supported by posts resting on the tunnel floor ; or where the arch is built first, as in the Belgian and Italian methods, they may be carried on blocking resting on the unexcavated earth below. Whichever method is em- ployed, an unyielding support is essential, and care must be taken that the centers are erected and maintained in a plane normal to the tunnel axis. To prevent deflection and twisting, the consecutive centers are usually braced together by longi- tudinal struts or by braces running to the adjacent strutting. TYPES OF CENTERS AND MOLDS 67 Only skilled and experienced workmen should be employed in erecting the centers ; and they should work under the immedi- ate direction of the engineer, who must establish the axis and level of each center by transit and level. Lagging. By the lagging is meant the covering of narrow longitudinal boards resting upon the upper curved chords of the centers, and spanning the opening between consecutive centers. This lagging forms the curved surface or mold upon which the arch masonry is laid. When the roof arch is of ashlar masonry the strips of lagging are seldom placed nearer together than the joints of the consecutive ring stones, but in brick arches they are laid close together. Besides the weight of the arch masonry, the lagging timbers support the short props which keep the poling-boards in place after the strutting is removed and until the arch masonry is completed. Striking the Centers. The centers are usually brought to the proper elevation by means of wooden wedges inserted be- tween the sill of the center and its support, or between the bottom of the posts carrying the center and the tunnel floor. These wedges are usually made of hard wood, and are about 6 ins. wide by 4 ins. thick by 18 ins. long. To strike the center after the arch masonry is completed, these wedges are with- drawn, thus allowing the center to fall clear of the masonry. Usually the center is not removed immediately after striking, so that if the arch masonry fails the ruins will remain upon the center. The method of striking the iron center devised by Mr. Rziha has been described in the previous chapter on strutting. 68 TUNNELING CHAPTER VIII. METHODS OF LINING TUNNELS. TUNNELS in soft soils and in loose rock, and rock liable to disintegration, are always provided with a lining to hold the walls and roof in place. This lining may cover the entire sectional profile of the tunnel, or only a part of it, and it may be constructed of timber, iron, iron and masonry, or, more commonly, of masonry alone. Timber Lining. Timber is seldom employed in lining tunnels except as a temporary expedient, and is replaced by masonry as soon as circumstances will permit. In the first construction of many American railways, the necessity for extreme economy in construction, and of getting the line open for traffic as soon as possible, caused the engineers to line many tunnels with timber, which was plentiful and cheap. Except for their small cost and the ease and rapidity with which they can be constructed, however, these timber linings possess few advantages. It is only the matter of a few years when the decay of the timber makes it necessary to rebuild them, and there is always the serious danger of fire. In several instances timber-lined tunnels in America have been burned out, causing serious delays in traffic, and necessitating complete reconstruction. Usually this reconstruction has con- sisted in substituting masonry in place of the original timber lining. In a succeeding chapter a description will be given of some of the methods employed in replacing timber tunnel linings with masonry. Various forms of timber lining are employed, of which Fig. 44 and the illustrations in the chapter METHODS OF LINING TUNNELS 69 discussing the methods of relining timber-lined tunnels with masonry are typical examples. Iron Lining. The use of iron lining for tunnels was^ intro- duced first on a large scale by Mr. Peter William Barlow in 1869, for the second tunnel under the River Thames at London, England, and it has greatly extended since that time. The lining of the second Thames tunnel consisted of cylindrical cast-iron rings 8 ft. in diameter, the abutting edges of the successive rings being flanged and provided with holes for bolt fastenings. Each ring was made up of four segments, Cross ^Section . Longitudinal, Section* FIG. 44. A Typical Form of Timber Lining for Tunnels. three of which were longer than quadrants, and one much smaller forming the " key-stone " or closing piece. These segments were connected to each other by flanges and bolts. To make the joints tight, strips of pine or cement and hemp yarn were inserted between the flanges. Since the construc- tion of the second Thames tunnel, iron lining has been em- ployed for a great many submarine tunnels in England, Continental Europe, and America, some of them having a section over 28 ft. in diameter. Where circular iron lining is employed, the bottom part of the section is leveled up with concrete or brick masonry to carry the tracks, and the whole 70 TUNNELING interior of the ring is covered with a cement plaster lining- deep enough thoroughly to embed the interior joint flanges. In the succeeding chapter describing the methods of driving tunnels by shields several forms of iron tunnel lining are fully described. Iron and Masonry Lining. During recent years a form of combined masonry and iron lining has been extensively em- ployed in constructing city underground railways in both Europe and America. Generally this form of lining is built with a rectangular section. Two types of construction are employed. In the first, masonry side walls carry a flat roof of girders and beams, which carry a trough flooring filled with concrete, or between which are sprung concrete or brick arches. Sometimes the roof framing consists of a series of parallel I-beams laid transversely across the tunnel, and in other cases- transverse plate girders carry longitudinal I-beams. In the second type of construction the roof girders are supported by columns embedded in the side walls. Where the tunnel pro- vides for two or four tracks, intermediate column supports are in some cases introduced between the side columns. In this construction the roofing consists of concrete filled troughs or of concrete or brick arches, as in the construction first described. Examples of combined masonry and iron tunnel lining are- illustrated in the succeeding chapter on tunneling under city streets. Masonry Lining. The form of tunnel lining most commonly employed is brick or stone masonry. Concrete masonry lining has been employed in several tunnels built in recent years. The masonry lining may inclose the whole section or only a part of it. The floor or invert is the part most commonly omitted; but sometimes also the side walls and invert are both omitted, and the lining is confined simply to an arch supporting the roof. The roof arch, the side walls, and the invert compose the tunnel lining; and all three may consist of stone or brick alone, or stone side walls may be employed with brick invert METHODS OF LINING TUNNELS 71 and roof arch. Rubble-stone masonry is usually employed, except at the entrances, where the masonry is exposed to view. Here ashlar masonry is usually used. The stone selected for tunnel lining should be of a durable quality which weathers well. Where bricks are used they should be of good qual- ity. Owing to the comparative ease with which brick arches can be built, they are generally used to form the roof arch, even where the side walls are of stone masonry. Masonry lining may be built in the form of a series of separate rings, or in the form of a continuous structure extending from one end of the tunnel to the other. The latter method of construction pro- duces a stronger structure ; but in case of failure by crush- ing, the damage done is likely to be more widespread than where separate rings are employed, one or two of which may fail without injury to the others adjacent to them. The construction is also somewhat simpler where separate rings are employed, since no provision has to be made for bonding the whole lining into a continuous structure. Where a series of separate rings is employed, the length of each ring runs from 5 ft. up to 20 ft., it depending upon the character of the material penetrated, and the method of construction employed. For the purpose of detailed discussion the construction of masonry lining may be divided into four parts, the side-wall foundations, the side walls themselves, the roof arch, and the invert. . Foundations. In tunnels through rock of a hard and dur- able character the foundations for the side walls are usually laid directly on the rock. In loose rock, or rock liable to dis- integration, this method of construction is not generally a safe one, and the foundation excavation should be sunk to a depth at which the atmospheric influences cannot affect the founda- tion bed. In either case the foundation masonry is made thicker than that of the side walls proper, so as to distribute the pressure over a greater area, and to afford more room for adjusting the side-wall masonry to the proper profile. In 72 TUNNELING yielding soils a special foundation bed has to be prepared for the foundation masonry. In some instances it is found suffi- cient to lay a course of planks upon which the masonry is con- structed, but a more solid construction is usually preferred. This is obtained by placing a concrete footing from 1 ft. to 2 ft. deep all along the bottom of the foundation trench, or in some cases by sinking wells at intervals along the trench and filling them with concrete, so as to form a series of supporting pillars. The form given to the foundation courses and lower portions of the side walls varies. Where a large bearing area is required, the back of the wall is carried up vertically as shown by the line AB, Fig. 45, otherwise the rear face of the wall follows the line of excavation A C. For similar reasons the front _ face of the wall may be made vertical, as at B C H 6 jvj, or inclined as at FSm The line FlG. 45. Diagram showing Forms indicates the shelf construction designed Adopted for Side- , . /> /. , -, -i . waii Foundations. * support the feet of the posts used to carry the arch centers during the construc- tion of the roof arch. Side Walls. The construction of the side walls above the foundation courses is carried out as any similar piece of masonry elsewhere would be built. To direct the work and insure that the inner faces of the walls follow accurately the curve of the chosen profile, leading frames previously described are employed. Roof Arch. For the construction of the roof arch, the centers previously described are employed. Beginning at the edges of the center on each side, the masonry is carried up a course at a time, care being taken to have it progress at the same rate on both sides, so that the load brought onto the centering is symmetrical. As soon as the centers are erected, the roof strutting is removed, and replaced by short props METHODS OF LINING TUNNELS 73 which rest on the lagging of the centers and support the poling- boards. These props are removed in succession as the arch masonry rises along the curve of the center, and the space between the top of the arch masonry and the ceiling ftf the excavation is filled with small stones packed closely. The key- stone section of the arch is built last, by inserting the stones or bricks from the front edge of the arch ring, there being no room to set them in from the top, as is the practice in ordinary open-arch construction. The keying of the arch is an espe- cially difficult operation, and only experienced men skilled in the work should be employed to perform it. The task becomes one of unusual difficulty when it becomes necessary to join the arches coming from opposite directions. Invert. In all but one or two methods of tunneling, the invert is the last portion of the lining to be built. In the English method of tunneling, the invert is the first portion of the lining to be built, and the same practice is sometimes neces- sary in soft soils where there is danger of the bottoms of the side walls being squeezed together by the lateral pressures unless the invert masonry is in place to hold them apart. The ground molds previously described are employed to direct the construction of the invert masonry. General Observations. In describing the construction of the roof arch, mention was made of the stone filling employed between the back of the masonry ring and the ceiling of the excavation. The spaces behind the side walls are filled in a similar manner. The object of this stone filling, which should be closely packed, is to distribute the vertical and lateral press- ures in the walls of the excavation uniformly over the lining masonry. As the masonry work progresses, the strutting employed previously to support the walls of the excavation has to be removed. This work requires care to prevent accident, and should be placed in charge of experienced mechanics who are familiar with its construction, and can remove it with the least damage to the timbers, so that they may be used again, 74 TUNNELING and without endangering the fall of the roof or the caving of the sides by removing too great a portion of the timbers at one time. Thickness of Lining Masonry. It is obvious, of course, that the masonry lining must be thick enough to support the press- ure of the earth which it sustains ; but, as it is impossible to estimate these pressures at all accurately, it is difficult to say definitely just what thickness is required in any individual case. Rankine gives the following formulas for determining the depths of keystone required in different soils : For firm soils, and for soft soils, d = where d = the depth of the crown in feet, r = the rise of the arch in feet, and s = the span of the arch in feet. Other writers, among them Professor Curioni, attempt to give rational methods for calculating the thickness of tunnel lining ; but they are all open to objection because of the amount of hypothesis required concerning pressures which are of necessity indetermi- nate. Therefore, to avoid tedious and uncertain calculations, the engineer adopts dimensions which experience has proven to be ample under similar conditions in the past. Thus we have all gradations in thickness, from hard-rock tunnels requiring no lining, and tunnels through rocks which simply require a thin shell to protect them from the atmosphere, to soft-ground tunnels where a masonry lining 3 ft. or more in thickness is employed. Table II. shows the thickness of masonry lining used in tunnels through soft soils of various kinds. The thickness of the masonry lining is seldom uniform at all points, as is indicated by Table II. Figs. 46 and 47 show common methods of varying the thickness of lining at different points, and are self-explanatory. METHODS OF LINING TUNNELS 75 Side Tunnels. When tunnels are excavated by shafts located at one side of the center line, short side tunnels or galleries are built to connect the bottoms of the shafts with the tunnel ^h proper. These side tunnels are usually from 30. ft to 40 ft. long, and are generally made from 12 ft. to 14 ft. high, and about 10 ft. wide. The excavation, strutting, and lining of these side tunnels are carried on exactly as they are in the main tunnel, with such exceptions as these short lengths make possible. Table III. gives the thickness of lining used for side tunnels, the figures being taken from European practice. FIGS. 46 and 47. Transverse Sections of Tunnels Showing Methods of Increasing the Thickness of the Lining at Different Points. Culverts, The purpose of culverts in tunnels is to collect the water which seeps into the tunnel from the walls and shafts. The culvert is usually located along the center line of the tunnel at the bottom. In soft-ground tunnels it is built of masonry, and forms a part of the invert, but in rock tunnels it is the common practice to cut a channel in the rock floor of the excavation. Both box and arch sections are employed for culverts. The dimensions of the section vary, of course, with the amount of water which has to be carried away. The fol- lowing are the dimensions commonly employed : 76 KIND OF CTTLVEBT. HEIGHT IN FEET. WIDTH IN FEET. THICKNESS OF WALLS IN FEET. THICKNESS OF COY EKING IN FEET. Box culvert .... Arch culvert .... 1 to 1.5 1 to 1.5 Itol 5 1 to 1.5 0.8 to 1.2 0.8 to 1.2 0.3 0.4 It should be understood that the dimensions given in the table are those for ordinary conditions of leakage ; where larger quantities of water are met with, the size of the culverts has, of course, to be enlarged. To permit the water to enter the culvert, openings are provided at intervals along its side ; and these openings are usually provided with screens of loose stones which check the current, and cause the suspended material to 1., FIG. 48. Refuge Niche in St. Gothard Tunnel. be deposited before it enters the culvert. In cases where springs are encountered in excavating the tunnel, it is necessary to make special provisions for confining their outflow and con- ducting it to the culvert. In all cases the culverts should be provided with catch basins at intervals of from 150 ft. to 300 ft., in which such suspended matter as enters the culverts is deposited, and removed through covered openings over each basin. At the ends of the tunnel the culvert is usually divided into two branches, one running to the drain on each side of the track. Niches. In short tunnels niches are employed simply as places of refuge for trackmen and others during the passing of trains, and are of small size. In long tunnels they are made METHODS OF LINING TUNNELS 77 larger, and are also employed as places for storing small tools and supplies employed in the maintenance of the tunnel. Niches are simply arched recesses built into the sides of the tunnel, and lined with masonry ; Fig. 48 shows this construc- tion quite clearly. Small refuge niches are usually built from 6 ft. to 9 ft. high, from 3 ft. to 6 ft. wide, and from 2 ft. to 3 ft. deep. Large niches designed for storing tools and supplies ai-e made from 10 ft. to 12 ft. high, from 8 ft. to 10 ft. wide, and from 18 ft to 24 ft. deep, and are provided with FIG. 49. East Portal of Hoosac Tunnel. doors. Refuge niches are usually spaced from 60 ft. to 100 ft. apart, while the larger storage niches may be located as far as 3000 ft. apart. The niche construction shown by Fig. 47 is that employed on the St. Gothard tunnel. Entrances. The entrances, or portals, of tunnels usually consist of more or less elaborate masonry structures, depending upon the nature of the material penetrated. In soft-ground tunnels extensive wing walls are often required to support the earth above and at the sides of the entrance ; while in tunnels 78 TUNNELING through rock, only a masonry portal is required, to give a finish to the work. Often the engineer indulges himself in an elabo- rate architectural design for the portal masonry. There is danger of carrying such designs too far for good taste unless care is employed ; and 011 this matter the writer can do no better than to quote the remarks of the late Mr. Frederick W. Simms in his well-known " Practical Tunneling " : " The designs for such constructions should be massive to be suitable as approaches to works presenting the appearance of gloom, solidity, and strength. A light and highly decorated structure, however elegant and well adapted for other purposes, would be very unsuitable in such a situation ; it is plainness combined with boldness, and massiveness without heaviness 1 that in a tunnel entrance constitutes elegance, and, at the same time, is the most economical." Fig. 49 is an engraving from a photograph of the east portal of the Hoosac tunnel, which is an especially good design. TABLE II. Showing Thickness of Masonry Lining for Tunnels through Soft Ground. CHARACTER OF MATERIAL. KEYSTONE. SPRINGERS. INVERT. Laminated clay, first variety . . Laminated clay, second variety . Laminated clay, third variety . Quicksand . Ft. 2.15 to 3 3 to 4.5 4.5 to 6.5 2 to 3.28 Ft. 2.75 to 3.5 3.5 to 5.5 5.5 to 8.1 2 to 4 1 Ft. 1.6 to 2.5 2.5 to 4 4 to 4. 5 1 33 to 2 5 TABLE III. Showing Thickness of Masonry Lining for Side Tunnels through Soft Ground. CHARACTER OF MATERIAL. KEYSTONE. SPRINGERS. INVERT. Laminated clay, first variety . . Laminated clay, second variety . Laminated clay, third variety Quicksand .... Ft. 1.6 to 2.3 2.3 to 3 3 to 4 1 6 to 2 5 Ft. 1.8 to 3 3 to 4.1 4.1 to 5 1 3 to 2 Ft. 1.5 to 2 2 to 2 6 2.6 to 3.29 1 3 to 2 TUNNELS THROUGH HARD ROCK 79 CHAPTER IX. TUNNELS THROUGH HARD ROCK ; GENERAL DISCUSSION; EXCAVATION BY DRIFTS. MONT CENIS TUNNEL. THE present high development of labor-saving machinery for excavating rock makes this material one of the safest and easiest to tunnel of any with which the engineer ordinarily has to deal. To operate this machinery requires, however, the development of a large amount of power, its transmission to considerable distances, and, finally, its economical application to the excavating tools. The standard rock excavating ma- chine is the power drill, which requires either air or hydraulic pressure for its operation according to the special type em- ployed. Under present conditions, therefore, the engineer is limited either to air or water under compression for the trans- mission of his power. Steam-power may be employed directly to operate percussion rock drills ; but owing to the neat and humidity which it generates in the confined space where the drills work, and because of other reasons, it is seldom employed directly. Electric transmission, which offers so many advan- tages to the tunnel builder, in most respects is largely excluded from use by the failure which has so far followed all attempts to apply it to the operation of rock drills. As matters stand, therefore, the tunnel engineer is practically limited to steam and falling water for the generation of power, and to com- pressed air and hydraulic pressure for its transmission. Whether the engineer should adopt water-power or steam to generate the power required for his excavating machinery de- pends upon their relative availability, cost, and suitability to the 80 TUNNELING conditions of work in each particular case. Where fuel is plen- tiful and cheap, and where water-power is not available at a comparatively reasonable cost, steam-power will nearly always prove the more economical ; where, however, the reverse con- ditions exist, which is usually the case in a mountainous country far from the coal regions, and inadequately supplied with transportation facilities, but rich in mountain torrents,, water-power will generally be the more economical. In a suc- ceeding chapter the power generating and transmission plants for a number of rock tunnels are described, and here only a general consideration of the subject will be presented. Steam-Power Plant. A steam-power plant for tunnel work should be much the same as a similar plant elsewhere, except that in designing it the temporary character of its work must be taken into consideration. This circumstance of its tempo- rary employment prompts the omission of all construction except that necessary to the economical working of the plant during the period when its operation is required. The power- house, the foundations for the machinery, and the general con- struction and arrangement, should be the least expensive which will satisfy the requirements of economical and safe operation for the time required. It will often be found more economical as a whole to operate the machinery with some loss of economy during the short time that it is in use than to go to much greater expense to secure better economy from the machinery by design and construction, which will be of no further use after the tunnel is completed. The longer the plant is to be required, the nearer the construction may economically approach that of a permanent plant. As regards the machinery itself, whose further usefulness is not limited by the duration of any single piece of work, true economy always dictates the purchase of the best quality. Speaking in a general way, a steam-power plant for tunnel work comprises a boiler plant, a plant of air compressors with their receivers, and an electric light dynamo. When hydraulic transmission of power is employed, the air TUNNELS THROUGH HAUD ROCK 81 compressors are replaced by high-pressure pumps ; and when electric hauling is employed, one or more dynamos may be re- quired to generate electricity for power purposes, as well as for lighting. In addition to the power generating machines proper, there must be the necessary piping and wiring for transmitting this power, and, of course, the equipment of drills and other machines for doing the actual excavating, hauling, etc. Reservoirs. When water-power is employed, a reservoir has to be formed by damming some near-by mountain stream at a point as high a practicable above the tunnel. The provision of a reservoir, instead of drawing the water directly from the stream, serves two important purposes. It insures a continuous supply and constant head of water in case of drought, and also permits the water to deposit its sediment before it is delivered to the turbines. The construction of these reservoirs may be of a temporary character, or they may be made permanent structures, and utilized after construction is completed to sup- ply power for ventilation and other necessary purposes. In the first case they are usually destroyed after construction is fin- ished. In either case, it is almost unnecessary to say, they should be built amply safe and strong according to good engi- neering practice in such works, for the duration of time which they are expected to exist. Canals and Pipe Lines. For conveying the water from the reservoirs to the turbines, canals or pipe lines are employed. The latter form of conduit is generally preferable, it being both less expensive and more easily constructed than the former. It is advisable also to have duplicate lines of pipe to reduce the possibility of delay by accident or while necessary repairs are being made to one of the pipes. The pipe lines terminate in a penstock leading into the turbine chamber, and provided with the necessary valves for controlling the admis- sion of water to the turbines. Turbines. There are numerous forms of turbines on the market, but they may all be classed either as impulse turbines 82 TUNNELING or as reaction turbines. Impulse turbines are those in which the whole available energy of the water is converted into kinetic energy before the water acts on the moving part of the turbine. Reaction turbines are those in which only a part of the available energy of the water is converted into kinetic energy before the water acts on the moving vanes. Impulse turbines give efficient results with any head and quantity of water, but they give better results when the quantity of water varies and the head remains constant. Reaction turbines, on the contrary, give better results when the quantity of water remains constant and the head varies. These observations indicate in a general way the form of turbine which will best meet the particular conditions in each case. The number of turbines required, and their dimensions, will be determined in each case by the number of horse-power required and the quantity of water available. The power of the turbines is transmitted to the air compressors or pumps by shafting and gearing. Air Compressors. An air compressor is a machine usually driven by steam, although any other power may be used by which air is compressed into a receiver from which it may be piped for use. For a detailed description of the various forms of air compressors the reader should consult the catalogues of the several makers and the various textbooks relating to air compression and compressed air. Air compressors, like other machines, suffer a loss of power by friction. The greatest loss of power, however, results from the heat of compression. When air is compressed, it is heated, and its relative volume is increased. Therefore, a cubic foot of hot air in the com- pressor cylinder, at say, 60 Ibs. pressure, does not make a cubic foot of air at 60 Ibs. pressure after cooling in the receiver. In other words, assuming pressure to be constant, a loss of volume results due to the extraction of the heat of compression after the air leaves the compressor cylinder. To reduce the amount of this loss, air compressors are designed with means TUNNELS THROUGH HARD ROCK 83 to extract the heat from the air before it leaves the com- pressor cylinder. Air compressors may first be divided into two classes, according to the means employed for cooling the air, as follows: (1) Wet compressors, and (2) dry comprtss- ors. A wet compressor is one which introduces water directly into the cylinder during compression, and a dry compressor is one which admits no water to the air during compression. Wet compressors may be subdivided into two classes : (1) Those which inject water in the form of spray into the cylinder during compression, and (2) those which use a water piston for forcing the air into confinement. The following brief discussion of these various types of compressors is based on the concise practical discussion of Mr. W. L. Saunders, M. Am. Soc. C. E., in " Compressed Air Production." The highest isothermal results are obtained by the injection of water into the cylinders, since it is plain that the injection of cold water, in the shape of a finely divided spray, directly into the air during compression will lower the temperature to a greater degree than simply to surround the cylinder and parts by water jackets which is the means of cool- ing adopted with dry compressors. A serious obstacle to water injection, and that which condemns this type of compressor, is the influence of the injected water upon the air cylinder and parts. Even when pure water is used, the cylinders wear to such an extent as to produce leakage and to require reboring. The limitation to the speed of a compressor is also an important objection. The chief claim for the water piston compressor is that its piston is also its cooling device, and that the heat of compression is absorbed by the water. Water is so poor a conductor of heat, however, that without the addition of sprays it is safe to say that this compressor has scarcely any cooling advantages at all so far as the cooling of the air during com- pression is concerned. The water piston compressor operates at slow speed and is expensive. Its only advantage is that it has no dead spaces. In the dry compressor a sacrifice is made 84 TUNNELING in the efficiency of the cooling device to obtain low first cost,, economy in space, light weight, higher speed, greater durability, and greater general availability. Air compressors are also distinguished as double acting and simple acting. They are simple acting when the cylinder is arranged to take in air at one stroke and force it out at the next, and they are double acting when they take in and force out air at each stroke. In form compressors may be simple or duplex. They are simple when they have but one cylinder, and duplex when they have two cylinders. A straight line or direct acting compressor is one in which the steam and air cylinders are set tandem. An indirect acting compressor is one in which the power is applied indirectly to the piston rod of the air cylinder through the medium of a crank. Mr. W. L, Saunders writes in regard to direct and indirect compression as follows : " The experience of American manufacturers, which has been more exten- sive than that of others, has proved the value of direct compression as distin- guished from indirect. By direct compression is meant the application of power to resistance through a single straight rod. The steam and air cylinders- are placed tandem. Such machines naturally show a low friction loss because of the direct application of power to resistance. This friction loss has been recorded as low as 5 %, while the best practice is about 10 % with the type which conveys the power through the angle of a crank shaft to a cylinder connected to the shaft through an additional rod." Receivers. Compressed air is stored in receivers which are simply iron tanks capable of withstanding a high internal pressure. The puipose of these tanks is to provide a reservoir of compressed air, and also to allow the air to deposit its moisture. From the receivers the air is conveyed to the work- ings through iron pipes, which decrease gradually in diameter from the receivers to the front. Rock Drills. The various forms of rock drills used in tun- neling have been described in Chapter III., and need not be considered in detail here except to say that American engi- TUNNELS THROUGH HARD ROCK \ 85 neers usually employ percussion drills, while European engineers also use rotary drills extensively. A comparison between these, two types of drills was made in excavating the Aarlberg tunnel in Austria, where the Brandt hydraulic rotary drill was used at one end, and the Ferroux percussion drill was used at the other end. The rock was a mica-schist. The average monthly progress was 412 ft., with a maximum of 646 ft., with the rotary drills, and an average of 454 ft. with the percussion drill. Excavation. Since considerable time is required to get the power plant established, the excavation of rock tunnels is often begun by hand, but hand work is usually continued for no longer a period than is necessary to get the power plant in operation. Generally speaking, the greatest difficulty is encountered in excavating the advanced drift or heading. Based on the mode of blasting employed, there are two methods of driving the advanced gallery, known as the circular cut and the center cut methods. In the first method a set of holes is first drilled near the center of the front in such a manner that they inclose a cone of rock ; the holes, starting at the perimeter of the base of the cone, converge toward a junction at its apex. Seldom more than four to six holes are comprised in this first set. Around these first holes are driven a ring of holes which inclose a cylinder of rock, and if necessary succeeding rings of holes are driven outside of the first ring. These holes are blasted in the order in which they are driven, the first set taking out a cone of rock, the second set enlarging this cone to a cylinder, and the other sets enlarging this cylinder. These holes are seldom driven deeper than 4 or 5 ft. In the center-cut method, which is the one commonly employed in America, the holes are arranged in vertical rows, and are driven from 15 ft. to 20 ft. deep. Figs. 50 to 53 inclusive show the arrangement of the holes and the method f blasting them. The two center rows of holes converge toward each other so as to take out a wedge of rock, but the 86 TUNNELING others are bored " straight " or parallel with the vertical plane of the tunnel. The width of the advanced gallery or heading depends upon the quality of the rock. In hard rock American engi- neers give it the full width of the tunnel section; but this cannot be done in loose or fissured rock, which has to be sup- ported, the headings here being usually made about 8 x 5 ft. The wider heading is always preferable, where it is possible, PlGS. 60 to 53. Sketches Illustrating American Center-Cut Method of Blasting Tunnels. since more room is available for removing the rock, and deeper holes can be bored and blasted. With the preceding general discussion of tunneling through rock we may proceed to a detailed consideration of the con- struction of typical examples of rock tunnels. For this pur- pose the Mont Cenis and Simplon tunnels are selected as examples of rock tunnels driven by a drift, and the St. Goth- ard and Busk tunnels as examples of rock tunnels driven by headings. TUNNELS THROUGH HARD ROCK 87 EXCAVATION BY DRIFTS: MONT CENIS TUNNEL. General Description. The method of tunneling tfirough hard rock by drifts is preferred by European engineers. Both the Mont Cenis tunnel built in 1857-70, and the great Simplon tunnel now under construction, are examples of tunneling by drifts. In this method the sequence of excavation is shown diagrammatically by Fig. 54. As soon as the top portion of the section has been opened, the roof arch is built with its feet resting on the tops of parts No. 4. These parts are removed by breaking down the outer portion between the sides of part No. 1 and the lines a b and a 1 b l first, and then by driving transverse cuts through to the sides of the section at intervals, and filling them with the masonry of the side walls. These short sections or pillars of masonry serve to carry the arch while the rock between them is being excavated and the remainder of the side walls built. In hard rock the successive parts Xos. 1 to 4 are driven several hundred feet in advance of each other. The drift is usually strutted by means of side posts carrying a cap-piece placed at intervals, and having a ceiling of longi- tudinal planks resting on the successive caps. In hard rock the roof of the section does not, as a rule, require regular strutting, occasional supports being placed at intervals to pre- vent the fail of isolated fragments. When the rock is disinte- grated or full of seams, a regular strutting may be necessary, and this may be either longitudinal or polygonal in type. When longitudinal strutting is employed, a sill is laid across the roof of the drift, and upon this are set up two struts con- a cr FIG. 54. Diagram Showing Se- quence of Excavations in Drift Method of Tunneling Kock. 88 TUNNELING verging toward the top and supporting a cap-piece close to the roof. On this cap-piece are placed the first longitudinal crown bars carrying transverse poling-boards. Additional props standing on the sill and radiating outward are inserted as parts No. 3 are excavated. These radial props carry longitudinal bars which in turn support transverse poling- boards. When polygonal strutting is used, it may have the construction described below as being employed in the Mont Cenis tunnel, or may take the form of three or five segment arches of heavy timbers. The roof arch, usually of brick masonry, is built before the side walls, which are generally of rubble masonry, with its feet supported temporarily by the unexcavated rock below. Plank centers are usually employed, since the pressures they carry are usually limited to the weight of the masonry. The method of underpinning the roof arch with the side walls is that pecu- liar to the Belgian method of tunneling. The drain is usually constructed of brick masonry, and may be located at the center or at one side of the tunnel floor. Tunnels excavated by drifts enable simple means of hauling to be employed, and this is one of the reasons why the method finds so much favor with European engineers. The tracks are laid along the floor of the drift, and carry all the spoil from parts Nos. 2, 3, and 4, as well as from the front of the drift itself. As fast as the full section is completed, this single track in the drift is replaced by two tracks running close to the sides of the tunnel, or by a broad-gauge track with a third rail. Mont Cenis Tunnel. The Mont Cenis tunnel -was the first of the great Alpine tunnels to be built. It is 7.9 miles long, and connects France and Italy by a double-track railway. Con- struction was begun in 1857, and the tunnel was opened for traffic in 1872. Material Penetrated. The material penetrated by the ex- cavation consisted chiefly of limestone, calcareous schist, gneiss, TUNNELS THROUGH HARD HOCK 89 and schistose sandstone. The stratification of the rock was nearly perpendicular to the axis of the tunnel, except at a few points where the strata intersected the axis at angles varying from 35 to 60. Excavation. The tunnel was driven exclusively from the -ends by means of a drift. The diagram, Fig. 55, shows the order of the excavation, which began by the drift No. 1, whose dimensions were 9.5 x 8.5 ft., its roof reaching the line of the springers of the arch, and its floor being about 3 ft. higher than that of the tunnel. With the excavations of the part No. 2, the drift was widened on each side, -except at the roof, and the floor of the tunnel was reached. Above the drift, the heading No. 3 was exca- vated, and when the parts No. 4 were battered down the excavation of the upper portion of the tunnel section was completed, and the ma- sonry arch of the lining built. The parts No. 5 were afterward re- moved, and the side walls built up from foundations, and the arch un- derpinned. In the middle of the floor, the part No. 6 was excavated, and the culvert built. The excavation of the Mont Cenis tunnel was carried on by hand labor up to the year 1861, when the first drilling machine was employed. The drift, when excavated by hand labor, was blasted by means of many charges placed in holes no more than l ft. deep, and very close together. When the per- forating machines were first used the same manner of boring numerous shaft-holes was followed, and the drift was excavated by a circular cut. Near the center 13 holes were driven, which formed the first round of blasting ; close to sides 16 holes were bored on each side for the second round ; 8 holes below and 13 above the circular cut formed the third round ; and close to the FIG. 55. Diagram Showing Se- quence of Excavation in Mont Cenis Tunnel. 90 TUNNELING floor 5 more holes were bored for the fourth round, by which the floor of the drift was reached. The total number of the holes bored at the front of the drift varied from 70 to 80 ; their depth was 3 ft. Three holes in the middle of those of the first round were made deeper so as to loosen the rock a little to facilitate the blasting of the succeeding rounds. Gunpowder was the only explosive used in the excavation of the Mont Cenis tunnel, both when the work was done by hand labor and by machines. The time required for boring the holes of the drift varied between 6 and 8 hours. From 1 to 2 hours were required for filling in the holes with explosives, and from 3 to 5 hours in removing the blasted rock, so that in 24 hours no more than two blasts were made at the front of the drift. The different excavations were made by various gangs following each other at an average distance of 900 ft. Power Plant. The mechanical installation consisted of the Sommeilier air compressors built near the portals. The Sommeilier compressors, Mr. W. L. Saunders says, were oper- ated as a ram, utilizing a natural head of water to force air at 80 Ibs. pressure into a receiver. The column of water con- tained in the long pipe on the side of the hill was started and stopped automatically by valves controlled by engines. The weight and momentum of the water forced a volume of air with such a shock against the discharge valve that it was opened, and the air was discharged into the tank ; the valve was then closed, the water checked ; a portion of it was allowed to dis- charge, and the space was filled with air, which was in turn forced into the tank. Only 73 % of the power of the water was available, 27 % being lost by the friction of the water in the pipes, valves, bends, etc. Of the 73 % of net work, 49.4 was consumed in the perforators, and 23.6 in a dummy engine for working the valves of the compressors and for special ventilation. The compressed air was conveyed from each end through a TUNNELS THROUGH HARD ROCK 91 cast-iron pipe 7| in. in diameter, up to the front of the excava- tion. The joints of the pipes were made with turned faces,, grooved to receive a ring of oakum which was tightly screwed and -compressed into the joint. To ascertain the amount of leakage of the pipes, they and the tanks were filled with air compressed to 6 atmospheres, and the machines stopped ; after 12 hours the pressure was reduced to 5.7 atmospheres, or to 95 % of the original pressure. Sommeilier's percussion drilling machines were used in the excavation of this tunnel. They were provided with 8 or 10- drills acting at the same time, and mounted on carriages running on tracks. These were withdrawn to a safe place during the blasting, and advanced again after the broken rock was removed from the front and the new tracks laid. Machine shops were built at both ends of the tunnel for building and repairing the drilling machines, bits, tools, etc. A gas factory was built at each end for lighting purpose. Strutting. The roof of the drift was strutted by means of longitudinal planks supported by cap-pieces laid across the line of the tunnel and resting on vertical props close to the sides of the excavation. This strutting was necessitated in order to prevent the fall of the rock from the upper part of the section. For the upper portion of the profile no continuous strutting was required, but at places where the rock was fissured or disintegrated a polygonal strutting was employed. This consisted of a sill laid across the axis of the tunnel and just above the roof of the drift On this sill two inclined props were placed supporting a cap-piece. Close to the feet of these two inclined props other props were inserted abutting against wooden blocks close to the faces of the excavation. These blocks were of trapezoidal shape, the smaller side being near the excavation, while the longer ones abutted against the props. Between two consecutive wooden blocks small beams were inserted as close as possible to the excavation, and in such a manner as to assume the form of a polygon. Planks- 92 TUNNELING were stretched longitudinally between the beams forming the polygons of the consecutive timber structures. Masonry. After the upper portion of the tunnel section had been excavated, the arch was built with its feet resting upon heavy planks. For the construction of the arch light centers were used. The arch was made of brick, and rested on the unexcavated portions of the bench. When these were removed, pillars of rock from 6 to 8 ft. long were left at equal intervals between them. In the spaces left vacant, balks of timber were inserted in order to support the arch. In the space between the rock pillars the side walls were built up from foundation and the arch underpinned ; then the rock pillars were in their turn battered down, new timbers were inserted to support the arch, and the side walls were built and the arch underpinned. In this way the masonry of the lining was made continuous. At every 3,000 ft. large niches were built, while all along the line on both sides small sheltering niches were built 150 ft. apart. Hauling. In the Mont Cenis tunnel all the hauling was done by horses. On the floor of the drift small tracks were placed, upon which ran the cars that removed the broken rock produced by blasting at the front. At the end of the drift the small cars dumped the rock into larger cars running on the floor of the part No. 2 which was the tunnel floor. There a single track was laid, which was afterward switched into a double track where the full section of the tunnel was opened. The materials excavated from the upper portion of the profile, by means of openings left in the roof of the drift, were loaded directly on to the large cars running on the tunnel floor. Ventilation. Ventilation was at first obtained by the air discharged from the drills, which exhausted from 250,000 to 280,000 cu. ft. of fresh air every hour at the front. When this quantity was considered too small, a blower 2.5 ft. in diameter was employed. It was operated by a small compressed air motor, and the air was driven to the front through a 10 in. box TUNNELS THROUGH HARD ROCK 9S conduit of square section. When the work was well advanced this apparatus was deemed to be insufficient, and the exhaust- ing bells described in the Chapter Ventilation were used, and operated by a powerful turbine, whose motive-power was a stream of 75 gallons of water per second with a head of 60 ft. 94 TUNNELING CHAPTER X. TUNNELS THROUGH HARD ROCK (Continued), THE SIMPLON TUNNEL.* BEFORE entering upon a description of the constructive details of this, the longest railway tunnel in the world, it may be well to give a general idea of the undertaking. Many schemes for the connection of Italy and Switzerland by a rail- way near the Simplon Road Pass have been devised, including one involving no great length of underground work, the line mounting by steep gradients and sharp curves. The present scheme, put forward in 1881 by the Jura-Simplon Ry. Co., con- sists broadly of piercing the Alps between Brigue, the present railway terminus in the Rhone Valley, and Iselle, in the gorge of the Diveria, on the Italian side, from which village the railway will descend to the existing southern terminus at Domo d'Ossola, a distance of about 11 miles. In conjunction with this scheme a second tunnel is pro- posed, to pierce the Bernese Alps under the Lotschen Pass from Mittholz to a point near Turtman in the Rhone Valley ; and thus, instead of the long detour by Lausanne and the Lake of Geneva, there will be an almost direct line from Berne to Milan via Thun, Brigue, and Domo d'Ossola. Starting from Brigue, the new line, running gently up the valley for 1J miles, will, on account of the proximity of the Rhone, which has already been slightly diverted, enter the tunnels on a curve to the right, of 1,050 ft. radius. At a distance of 153 yards from the entrance, the straight portion * Abstract from a paper read before the Institution of Civil Engineers by Charles B. Tox, Jan. 26, 1900. TUNNELS THROUGH HARD ROCK 95 of the tunnel commences, and extends for 12 miles. The line then curves to the left with a radius of 1,311 ft before emerging on the left bank of the Diveria. Commencing at the northern entrance, a gradient of 1 in 500 (the minimum for efficient drainage) rises for a length of 5^ miles to a level length of 550 yards in the center, and then a gradient of 1 in 143 de- scends to the Italian side. On the way to Domo d'Ossola one helical tunnel will be necessary, as has been carried out on the St. Gothard. There will be eventually two parallel tunnels, having their centers 56 ft. apart, each carrying one line of way; but at the present time only one heading, that known as No. 1, is being excavated to full size, No. 2 being left, masonry lined where necessary, for future developments. By means of cross headings every 220 yds. the problems of transport and ventila- tion are greatly facilitated, as will be seen later. As both entrances are on curves, a small "gallery of direction" is necessary, to allow corrections of alinement to be made direct from the two observatories on the axis of the tunnel. The outside installations are as nearly in duplicate as cir- cumstances will allow, and consist of the necessary offices, workshops, engine-sheds, power-houses, smithies, and the nu- merous buildings entailed by an important engineering scheme. Great care is taken that the miners and men working in the tunnel shall not suffer from the sudden change from the warm headings to the cold Alpine air outside ; and for this purpose a large building is in course of erection, where they will be able to take off their damp working clothes, have a hot and cold douche, put on a warm dry suit, and obtain refreshments at a moderate cost before returning to their homes. Instead of each man having a locker in which to stow his clothes, a perfect forest of cords hangs down from the wooden ceiling, 25 ft. above floor-level, each cord passing over its own pulleys and down the wall to a numbered belaying-pin. Each cord supports three hooks and a soap-dish, which, when loaded with their owner's property, are hauled up to the ceiling out of the 96 TUNNELING way. There are 2,000 of these cords, spaced 1 ft. 6 ins. apart r one to each man. The engineers and foremen are more priv- ileged, being provided with dressing-rooms and baths, partitioned off from the two main halls. An extensive clothes washing and drying plant has been laid down, and also a large restau- rant and canteen. At Iselle, a magazine holding 2,200 Ibs. of dynamite is surrounded and divided into two separate parts by earth-banks, 16 ft. high. The two wooden houses, in which the explosive is stored, are warmed by hot-water pipes to a temperature between 61 F. and 77 F., and are watched by a military patrol; but at Brigue a dynamite manufactory, started by an enterprising company at the time of the com- mencement of the works, supplies this commodity at frequent intervals, thereby avoiding the necessity of storing in such large quantities. This dynamite factory has been largely in- creased, and supplies dynamite to nearly all the mining and tunneling enterprises in Switzerland. Geological Conditions. Before the Simplon tunnel was au- thorized, expert evidence was taken as to the feasibility of the project. The forecasts of the three engineers chosen, in reference to the rock to be encountered and its probable temperature, have, as far as the galleries have gone (an ag- gregate distance of nearly 2j miles), generally been found correct. At the north end, a dark argillaceous schist veined with quartz was met with, and from time to time beds of gypsum and dolomite have been traversed, the dip of the strata being on the whole favorable to progress, though timber- ing is resorted to at dangerous places. Water was plentiful at the commencement ; in fact, one inrush has not been stopped, and is still flowing down the heading. The total quantity of water flowing from the tunnel mouth is 16 gallons per second, of which 2 gallons per second are accounted for by the drilling machines. At Iselle, however, a very hard antigorio gneiss obtains, and is likely to extend for 4 miles. Very dry and very compact, it requires no timbering, and presents no great TUNNELS THROUGH HARD ROCK 97 difficulty to the powerful Brandt rock-drills, which work under a head of 3,280 ft. of water. The temperature of the rock depends not only on the depth from the surface, but largely upon the general form of that sur- face combined with the conductivity of the rock. Taking these points into consideration with the experience gained from the construction of the St. Gothard tunnel, 95 F. was esti- mated as the probable maximum temperature, owing to the height of Monte Leone (11,660 ft), which lies almost directly over the tunnel axis. Survey After having determined upon the general position of the tunnels, taking into consideration the necessary gra- dients, the temperature of the rock, and a large bed of trouble- some gypsum on the north side, two fixed points on the proposed center line were taken, one at each entrance of tunnel No. 1, and the bearings of these two points, with reference to a triangulation survey made in 1876, were calculated sufficiently accurately to determine, for the time being, the direction of the tunnel. In 1898, a new triangulation survey was made, taking in eleven summits, Monte Leone holding the central position. This survey was tied into that of the Wasenhorn and Faulhorn, made by the Swiss Government, and the accuracy was such that the probable error in the meeting of the two headings is only 6 cms. or 2^- ins. On the top of each summit is placed a signal, consisting of a small pillar of masonry founded on rock, and capped with a sharp pointed cone of zinc, 1 ft. 6 ins. high. An observatory was built at each end of the tunnel in such a position that three of the summits could be seen, a condition very difficult to fulfill on the south side owing to the depth of the gorge, the moun- tains on either side being over 7,000 ft. high. Having taken the angles to and from each visible signal, and therefrom having calculated the direction of the tunnel, it was necessary to fix, with extreme accuracy, sighting-points on the axis of the tunnel, in order to avoid sighting on to the surrounding peaks for each 98 TUNNELING subsequent correction of the alinement of the galleries. To do this, a theodolite 24 ins. long and 2f ins. in diameter, with a magnifying power of 40 times, was set up in the observ- atory, and about 100 readings were taken of the angles between the surrounding signals and the required sighting-points. In this manner the error likely to occur was diminished to less than 1'. Thus at the north end two points were found about 550 yds. before and behind the observatory, while on the south side, owing to the narrowness of the gorge, the points could only be placed at 82 yds. and 126 yds. in front. One of these sighting-points consists of a fine scratch ruled on a piece of glass fixed in an iron frame, behind which is placed an acetylene lamp, corrections of alinement are always done by night, the whole being rigidly fixed into a niche cut in the rock and protected from climatic and other disturbing agencies by an iron plate. Method of Checking Alinement. The direction of heading No. 1 is checked by experts from the Government Survey De- partment at Lausanne about three times a year, and for this purpose a transit instrument is set up in the observatory. A number of three-legged iron tables are placed at intervals of 1 mile or 2 miles along the axis of tunnel No. 1, and upon each of these is placed a horizontal plane, movable by means of an adjusting screw, in a direction at right angles to the axis, along a graduated scale. On this plane are small sockets, into which the legs of an acetylene lamp and screen, or of the transit instrument, can be quickly and accurately placed. The screen has a vertical slit, 3 ins. in height, and variable between |f in. and ^\ in. in breadth, according to the state of the atmos- phere, and at a distance shows a fine thread of light. The instrument, having first been sighted on to the illuminated scratch of the sighting-point, is directed up the tunnel, where a thread of light is shown from the first table. With the aid of a telephone this light is adjusted so that its image is exactly coincident with the cross hairs, and the reading on the gradu- TUNNELS THROUGH HARD BOCK 99 ated scale is noted. This is done four or five times, the aver- age of these readings being taken as correct, and the plane is clamped to that average. The instrument is then taken to the first table and is placed quickly and accurately over the jflpint just found (by means of the sockets), and the lamp is carried to the observatory. After first sighting back, a second point is given on the second table, and so on. These points are marked either temporarily in the roof of the heading by a short piece of cord hanging down, or permanently by a brass point held by a small steel cylinder, 8 ins. long and 3 ins. in diameter, em- bedded in concrete in the rock floor, and protected by a circular casting, also sunk in cement concrete, holding an iron cover resembling that of a small manhole. From time to time the alinement is checked from these points by the engineers, and after each blast the general direction is given by the hand from the temporary points. To check the results of the triangula- tion survey, astronomical observations have been taken simul- taneously at each end. With regard to the levels, those given on the excellent Government surveys have been taken as cor- rect, but they have also been checked over the pass. Details of Tunnels. In cross-section, tunnel No. 1 is 13 ft. 7 ins. wide at formation level, increasing to 16 ft. 5 ins., with a total height of 18 ft. above rail-level, and a cross-sectional area of about 250 sq. ft. This large section will allow of small repairs being executed in the roof without interruption of the traffic, and will also allow of strengthening the walls by additional masonry on the inside. The thickness of the lining, never wholly absent, and the material of which it is composed, depend upon the pressure to be resisted, and only in the worst case is an invert resorted to. The side drain, to which the rock floor is made to slope, will be composed of half-pipes of 7 to 1 cement concrete. The roof is constructed of radial stones. Tunnel No. 2, being left as a heading, is driven on that side nearest to No. 1, to minimize the length of the cross-headings, and measures 10 ft 2 ins. wide by 6 ft. 7 ins. high. Masonry 100 TUNNELING is used only where necessary, and in that case is so built as to form part of the lining of the tunnel when eventually com- pleted. Concrete is put in to form a foundation for the side wall, and a water channel. The cross-headings, connecting the two parallel headings, occur every 220 yds., and are placed at an angle of 56 to the axis of the tunnel, to avoid sharp curves in the contractors' railway lines. They will eventually be used as much as possible for refuges, chambers for storing the tools and equipment of the platelayers, and signal-cabins. The ref- uges, 6 ft. 7 ins.wide by 6 ft. 7 ins. high and 3 ft. 3 ins. deep, occur every 110 yards, every tenth being enlarged to 9 ft. 10 ins. wide by 9 ft. 10 ins. deep and 10 ft. 2 ins. high, still larger chambers being constructed at greater intervals. Method of Excavation. The work at each end of the tunnel is carried on quite independently, consequently, though similar in principle, the methods vary in detail, apart from the fact that different geological strata require different treatment. Broadly speaking,' the two parallel headings, each 59 sq. ft. in section, are first driven by means of drilling-machines and the use of dynamite, this work being carried on day and night, seven days. in the week; No. 1 heading is then enlarged -to full size by hand-drilling and dynamite. On the Italian side, where the rock is hard and compact, breakups are made at intervals of 50 yds., and a top gallery is driven in both directions, but, for ventilation reasons, is never allowed to get more than 4 yds. ahead of the breakup, which is gradually lengthened and widened to the required section. No timbering is required, except to facilitate the excavation and the construction of the side walls. Steel centers are employed for the arch ; they entail fewer supports, give more room, and are capable of being used over again more frequently, without damage. They consist of two I-beams bent to a template and riveted together at the crown, resting at either side on scaffolding at intervals of 6 ft. ; longitudinals, 12 ft. by 4 ins. by 4 ins., support the roof. Hand rock-drilling is carried out in the ordinary way, one man holding TUNNELS THROUGH HARD ROCK 101 the tool and a second striking ; measure- ments of excavation are taken every 2 or 3 yds., a plumb-line is suspended from the center of the roof, and at every half-meter (20 ins.) of height horizontal measure- ments are taken to each side. At the Brigue end a softer rock is en- countered, necessitate ing at times heavy timbering in the head- ing, and especially in the final excavation to full size, Fig. 56. The bottom heading, 6 ft. 6 in. high, is driven in the center, and the heading is then widened to the full extent and tim- bered ; the concrete forming the water channel and the foun- dation for one side wall is put in ; the side walls are built to a height of 6 ft. 6 ins., and the tunnel is fully excavated to a further height of 6 ft. 6 ins. from the first staging. The side walls are then continued up for the second 6 ft. 6 ins., and from the second floor a third height of FlO. 56. Sketches Showing Sequence of Work in Excavating and Lining the Simplon Tunnel. 102 TUNNELING 6 ft. 6 ins. is excavated and timbered. Finally the crown is cleared out, heavy wooden centers are put in, the arch is turned, and all timbers are withdrawn except the top poling-boards, supporting the loose rock. The masonry for the side walls is obtained either from the tunnel itself or from a neighboring quarry, and varies in char- acter according to the pressure ; but the face of the arch is al- ways of cut or artificial stones, the latter being of 7 to 1 cement concrete. Where the alinement heading, or the "gallery of direction," joins the curving portion of tunnel No. 1, the section is very much greater, and necessitates special timbering. Transport (Italian Side). A small line of railway, 2 ft. 7j ins. gauge, with 40-lb. rails, enters all three portals ; but since the construction of a wooden bridge over the Diveria, the route through the "gallery of direction," across heading No. 2, to tunnel No. 1, is used exclusively; this railway leads to the face in both headings, and, where convenient, from one heading to the other by the cross-galleries. Different types of wagons are in use ; but in general they are four-wheeled, non-tipping box wagons, supplied with brakes and holding 2 cu. yds. of debris. A special type of locomotive is used, designed to pass round curves of 50 ft. radius, and supplied with a specially large boiler to avoid firing in the tunnel. Method of Working. The drilling-machines employed are of the Brandt type, Fig. 57, and are mounted in the following manner: A small four-wheeled carriage supports at its center a beam, the shorter arm of which carries the boring mechanism and the longer a counterpoise ; near its center is the distributor. In the short arm is a clamp holding the rack-bar or butting column, which is a wrought-iron cylinder with a plunger con- stituting a ram, and is jammed by hydraulic pressure between the walls of the heading, thus forming a rigid support, for the boring-machine, and an efficient abutment against the i eaction of the drill. This rack-bar can be rotated on its cl inp in a plane parallel to the axis of the beam. Three or four separate TUNNELS THROUGH HARD ROCK 103 boring-machines can be mounted on the rack-bar, and can be adjusted in any reasonable position. The boring-machine performs the double function of con- tinually pressing the drill into the rock by means of a hallow ram (1), and of imparting to the drill and ram a uniform rotary motion. This rotary motion is given by a twin cylinder single- acting hydraulic motor (^), the two pistons, of 2| ins. stroke, acting reciprocally as valves. The cranks are fixed at an angle of 90 to each other on the shaft, which carries a worm, gearing with a worm-wheel ($) mounted upon the shell (jR) of the FIG. 57. General Details of the Brandt Rotary Drills Employed at the Simplon Tunnel. hollow ram (1), and this shell in turn engages the ram by a long feather, leaving it free to slide axially to or from the face of the rock. The average speed of the motor is 150 revolutions to 200 revolutions per minute, the maximum speed being 300 revolutions per minute. The loss of power between the worm and worm-wheel is only 15 f at the most; the worm being of hardened steel and the wheel of gun-metal, the two surfaces in contact acquire a high degree of polish, resulting in little wear- ing or heating. Taking into consideration all other sources of loss, 70 % of the total power is utilized. The pressure on the 104 TUNNELING drill is exerted by a cylinder and hollow ram (J), which revolves about the differential piston ($), which is fixed to the envelope holding the shell (.#). This envelope is rigidly connected to the bed-plate of the motor, and, by means of the vertical hinge and pin (T), is held by the clamp (F') embracing the rack-bar. When water is admitted to the space in front of the differential piston the ram carrying the drilling-tool is thrust forward, and when admitted to the annular space behind the piston, the ram recedes, withdrawing the tool from the blast-hole. The drill proper is a hollow tube of tough steel 2| ins. in external diame- ter, armed with three or four sharp and hardened teeth, and makes from five to ten revolutions per minute, according to the nature of the rock. When the ram has reached the end of its stroke of 2 ft. 2^ ins., the tool is quickly withdrawn from the hole and unscrewed from the ram; an extension rod is then screwed into the tool and into the ram, and the boring is con- tinued, additional lengths being added as the tool grinds for- ward; each change of tool or rod takes about 15 sees, to 25 sees, to perform. The extension rods are forged steel tubes, fitted with four-threaded screws, and having the same external diameter as the drill. They are made in standard lengths of 2 ft. 8 ins., 1 ft. 10 ins., and 11| ins. The total weight of the drilling-machine is 264 Ibs., and that of the rack-bar when full of water is 308 Ibs. The exhaust water from the two motor cylinders escapes through a tube in the center of the ram and along the bore of the extension rods and drill, thereby scouring away the debris and keeping the drill cool ; any superfluous water finds an exit through a hose below the motors and thence away down the heading. The distributor, already mentioned, supplies each boring-machine and the rack-bar with hydraulic pressure from the mains, with which connection is effected by means of flexible or articulated pipe connections, allowing free- dom in all directions. The area of the piston for advancing the tool is 15^ sq. ins., which under a pressure of 1470 Ibs. per sq. in. gives a pressure of over 10 tons on the tool, while for TUNNELS THROUGH HARD ROCK 105 i withdrawing the tool 2 tons is available. In the rock found at Iselle, namely, antigorio gneiss, a hole 2 ins. in diameter and 3 ft 8 ins. in length is drilled, normally, in 12 mins. to 25 mins. ; a daily rate of advance of 18 ft. to 19 ft. 6 ins. is made* in a heading having a minimum cross-section of 59 sq. ft. ; the time taken to drill ten to twelve holes, 4 ft. 7 ins. deep, is 2J hrs. When the debris resulting from one operation has been sufficiently cleared away, a steel flooring, which is provided near the face to enable shoveling to be more easily done, and to give an even floor for the wheels of the drilling-carriage, is laid bare at the head of the line of rails, and the drilling- machines are brought up on their carriage by eight or ten men. When advanced sufficiently close to the face, the rack- bar is slewed round across the gallery and is wedged up against the rock sides ; connection is made between the distributor and the hydraulic main, by means of the flexible pipe, and pressure is supplied by a small copper tube to the rack-bar ram, thereby rigidly holding the machine. Next, connections are made between the three drilling-machines and the distributor, and in 20 mins. from the time the machine was brought up all three drills are hard at work, water pouring from the holes. The noise of the motors and grinding- tools is sufficient to drown all but shouts ; and where the extension rods do not fit tightly, small jets of water play in all directions, necessitating the wearing of tarpaulins by the men directing the tools. Lighting is done wholly by small oil-lamps, provided with a hook to facilitate fixing in any crack in the rock ; electricity will probably be used to light that portion of the tunnel which is completed. Two men are allotted to each drill, one to drive the motor, the other to direct and replenish the tool, one foreman and two men in reserve completing the gang. A small hammer is freely used to loosen the screw joints of the extension rods and drill. A hole is usually commenced by a two-edged flat-pointed tool, until a sufficient depth is reached to prevent the circular tool 106 TUNNELING from wandering over the face of the rock, but in many instances the hole is commenced with a circular tool. The exhaust water during this period flows away by the hose underneath the motor. In the antigorio gneiss, ten to twelve holes are drilled for each attack, three to four in the center to a depth of 3 ft. 3 ins., the remainder, disposed round the outside of the face, having a depth of 4 ft. 7 in. The average time taken to complete the holes is If hr. to 2-j- hrs. Instead of pulverizing the rock, as do the diamond drills, it is found that the rock is crushed, and that headway is gained somewhat in the manner of a circular saw through wood. The core of rock inside the tool breaks up into small pieces, and can be taken out if necessary when the drill requires lengthening. The lowest holes, inclined down-wards, are full of water ; consequently two detonators and two fuses are inserted, but apart from this, water has little effect on the charge. The fuses of the central holes are brought together and cut off shorter than those of the outer holes, in order that they may explode first to increase the effect of the outer charges. All portable objects, such as drills, pipe connections, tools, etc., have meanwhile been carried back ; the steel flooring is covered over with a layer of debris to prevent injury from falling rock, and to the end of the hydraulic main is screwed a brass plug pierced by five holes ; and immediately the explosions occur a valve is opened in the tunnel, and five jets of water play upon the rock, laying the dust and clearing the air. The necessity for this was shown on one occasion when this nozzle was broken by the explosion and the water had to be turned off immediately to avoid useless waste ; on reaching the face, the atmosphere was found to be so highly charged with dust and smoke that it was impossible to distinguish the stones at the feet, although a lamp had been placed on the ground; and despite the fact that the air tube was in full blast, the men ex- perienced great difficulty in breathing. A truck is now brought up, and four men clear a passage in front, through the heap of TUNNELS THROUGH HARD BOCK 107 debris, two with picks and two with shovels, while on either side and behind are as many men as space will permit. The stone is thrown either to the sides of the heading or into the wagon, shoveling being greatly aided by the steel flooring, which, before the explosion, had been laid over the rails for nearly 10 yds. down the tunnel to receive the falling rock. These steel plates are taken up when cleared, and the wagon is pushed forward until the drilling-machine can be brought up again, leaving the remaining debris at the sides to be handled at leisure during the next attack. The roof and side walls are, of course, carefully examined with the pick, to discover and detach any loose or hanging rock. The times taken for each portion of the attack in this particular antigorio gneiss are as follows : Bringing up and adjustment of drills, 20 mins. ; drill- ing, between If hr. and 2^- hrs. ; charging and firing, 15 mins. ; clearing away debris, 2 hrs. ; or for one whole attack, between 4 hrs. and 54^ hrs., resulting in an advance of 3 ft 9 in., or a daily advance of nearly 18 ft. From this it appears that the time spent in clearing away the debris equals that taken up in drilling, and it is in this clear- ing that a saving of time is likely to be effected rather than in the process of drilling. Many schemes have been tried, such as- a mechanical plow for making a passage ; at Brigue, " marin- age," or clearing by means of powerful high-pressure water-jets, directed down the tunnel, was tried, but the idea is not yet sufficiently developed. Another series of experiments has been tried at Brigue with regard to the utilization of liquid air as an explosive agent instead of dynamite ; and for this purpose a plant has been laid down, consisting of one ammonia-compressor, two air-com- pressors, and two refrigerators, furnishing T V gallon of liquid air per hour at an expenditure of 17 H.P. The system used is that of Professor Linde, who himself directs the experiments. The great difficulty experienced is that of shortening the interval of time that must elapse between the manufacture of th& 108 TUNNELING cartridge and its explosion. The liquid oxygen, with which the cartridge, containing kieselguhr (silicious earth) and paraffin, is saturated, evaporates very readily, losing power every moment ; hence the effect of each cartridge cannot be guaranteed, and though it is an exceedingly powerful explosive when used immediately after manufacture, no practical result has yet been obtained. Power Station. Water is abundant at either end, and there- fore hydraulic power is the motive force employed. On the Italian side, a dam 5 ft. high has been thrown across the Diveria at a point near the Swiss frontier, about 3 miles above the site of the installations. A portion of the water thus held back enters, through regulating doors and gratings, a masonry channel leading to two parallel settling tanks, each 111 ft. by 16 ft., whence, after dropping all its sand and solid matter, the now pure water passes into the water-house, and, after flowing over a dam, through a grating and past the admission doors, enters a metallic conduit of 3-ft. pipes. Each of the settling tanks and the approach canal are provided with doors at the lower end leading direct to the river, through which all the sand and solid matter deposited can be scoured naturally by allowing the river-water to rush freely through. For this pur- pose the floor of the basins is on an average gradient of 1 in 30. For a similar reason the river-bed just outside the entrance to the approach canal is lined with wooden planks, from which the stones collecting behind the dam can be scoured by allow- ing an iron flap, hinged at the bottom, to change its position from the vertical to the horizontal in a gap left purposely in the dam, so causing a rushing torrent to sweep it clean. The chief levels are : Level of water at dam 794.00 meters above sea level. " in water-house 703.70 " " " " " at turbines 618.50 " " " " giving a total fall of 175.20 ms. or 570 ft., and a pressure of 17.52 atmospheres. TUNNELS THROUGH HARD ROCK The quantity of water capable of being taken from the Diveria in winter, when the rivers which are dependent upon the mountain snows for their supply are at their lowest, ia calculated to be 352 gallons per second. Thus, taking" the fall to be diminished by friction, etc., to 440 ft., and the use- ful effect at 70 %, there is obtained 2,000 H.P. on the turbine shaft. The metallic conduit varies in material according to the pressure ; thus cast-iron pipes 3 ft. in diameter and j| in. thick are used up to a pressure of '2 atmospheres, from which point they are of wrought-iron. The cast-iron portion has of late caused a good deal of trouble, owing to settlement of the piers causing occasional bursts, consequently a masonry pier has been placed under each joint of this portion. The follow- ing table gives the thicknesses and diameters, varying with the pressure : WATER PRESSURE. THICKNESS. DIAMETER. WEIGHT PER YARD. Head in Feet. Milli- meters. Inch. Feet. Inches. Lbs. . 246 6 1 3 326 311 7 3 383 300 8 3 431 393 9 3 483 426 10 3 556 476 12 3 651 590 16 1 3 3i 977 This pipe is supported every 30 ft. on small masonry piers, on the top of which is placed a block of wood hollowed out to receive the pipe, thus allowing any movement due to the con- traction and expansion of the conduit. However, to prevent this movement becoming excessive, the pipe is passed at intervals of 300 yds. to 500 yds. through a cubical block of masonry of 13 ft. side, strengthened by longitudinal tie-bars. Five bands of angle-bar riveted round the pipe, with their 110 TUNNELING flanges embedded in the masonry, constitute a rigid fixed point. Straw mats are thrown over the pipe where it is exposed to the sun. The temperature of the conduit is not, however, found to vary greatly, since the pipe is kept full of water. To supply the rock-drills with water at a maximum pressure of 100 atmospheres, or 1,470 Ibs. per sq. in., a plant of four pairs of high-pressure pumps has been laid down, and a still larger addition is in course of erection. At present, two Pelton turbines of 250 H.P. each, running at 170 revolutions per minute, drive the pumps, by means of toothed gearing, at 63 revolutions per minute. These pumps are of very simple but strong construction, single suction and double delivery, entail- ing one suction and one delivery-valve, both heavy and both of small lift. The larger portion of the plunger has exactly double the cross-sectional area of the smaller portion, so that in the forward stroke half of the water taken in at the last admission is pumped into the high-pressure mains, and at the same time a fresh supply of water is sucked in. During the backward stroke half of this new supply is pumped into the mains, and the remainder enters the second chamber, to be pumped during the next forward stroke. Thus the work done in the two strokes is practically the same. The pumps are in pairs, and are set at an angle of 90, to insure uniform pressure and uniform delivery in the mains. Their size varies ; but at Iselle there are three pairs, with a stroke of 2 ft. 2^ ins., and the plungers of 2 |^ in. and If ins. (approximately) in diameter, supplying 1.32 gallons per second. To avoid injury to the valves, the water to be pumped is taken from a stream up the mountain side, and is passed through filter screens. The high-pressure water, after passing an accumulator, enters the tunnel in solid drawn wrought-iron tubes, 3^ ins. in internal diameter, T \ in. thick, and in lengths of 26 ft. The diameter of these mains varies with their length, so as to avoid loss of pressure. With the 1,250 yds. of tunnel now driven 10 atmospheres are lost. TUNNELS THROUGH HARD ROCK 111 At Brigue the installations are, as far as possible, identical. The Rhone water, however, before reaching the water-house, is carried from the filter basins, a distance of 2 miles, in an armored canal built upon the Hennebique system,* the walls and supporting beams, of cement concrete, being strengthened by internal tie-bars of steel. The concrete struts, resembling balks of timber at a distance, are occasionally 35 ft. high and 1 ft. 7 ins. square. The metallic conduit is 5 ft. in diameter, with a minimum flow of 176 cu. ft. per second and a total fall of 185 ft. In case water-power should be unavailable, three semi-portable steam engines, two of 80 H.P. and one of 60 H.P., are always kept in readiness at each end of the tunnel, and are geared by belts to the turbine shaft! Ventilation. In tunneling, one of the most important prob- lems to be solved is that of ventilation, and it is for this reason that the Simplon tunnel consists of two parallel headings with cross cuts at intervals of 220 yds. At Brigue, a shaft 164 ft. deep was sunk through the overlying rock until the " gallery of direction" was encountered. Up this chimney the foul air is drawn by wood fires, the fresh air a volume of 19,000,000 cu. ft. per day, or 13,200 cu. ft. per minute entering by heading No. 2, penetrating up to the last cross gallery, and returning by tunnel No. 1. The entrances of No. 1 and the " gallery of direction," besides those of all the intermediate cross galleries, are closed by doors. By this arrangement, how- ever, fresh air does not reach the working faces ; therefore a pipe, 8 ins. in diameter, is led from the fresh air in No. 2 to within 15 yds. of the face of each heading, and up this pipe a draft of air is induced by means of a jet of water, the volume to each face being 800 cu. ft. per minute. One single jet of water from the high-pressure mains, with a diameter of T V in., is capable of supplying over 1,000 cu. ft. of air per minute at the end of 160 yds. of pipe, and during the attack the men at the drills are in a constant breeze with the thermometer stand- * Network of steel rods embedded in concrete. 112 TUNNELING ing at 70 F. At Iselle, air is blown into the entrance of headirg No. 2 at the rate of 14,100 cu. ft. per minute by two fans driven from the turbine shaft. This air travels from the fans along a pipe, 18 ins. in diameter, till a point 15 yds. up the tunnel is reached, where beyond a door the pipe narrows to form a nozzle 10 ins. in diameter. This door is kept open to allow the outside air to be induced up the tunnel, as the head- ings are at present only 2,500 yds. long, giving a resistance of not quite sufficient power to cause the air to return. The fresh air then travels up No. 2, crossing over the top of the " gallery of direction," from which it is shut off by doors, to the last cross gallery, returning by No. 1, and finally leaving either by the " gallery of direction " or by No. 1. A system of cooling the air and driving it on by means of a large number of water- jets will be installed in No. 2 where that heading crosses over the " gallery of direction," but at present there is no need for it. The average temperature at the face is 73 F. during the drilling operation, 76 F. after firing the charges, and a max- imum of 80 F., lately attaining to 86 F. on the south side, with 80 F. and 85 F. before and after firing. The tempera- ture of the rock is taken at every 110 yds. in holes 5 ft. deep, and shows a gradual increase according to the depth of over- laying rock, to the conductivity of the rock, and to the form of the mountain surface. The maximum hitherto reached on the north side is 68 F., while on the south side, although a smaller distance has been traversed, it attains to 79 F., due to the more rapid increase in depth. Moreover, the temperature of the rock is observed at the permanent stations, 550 yds. from the entrances, in its relation to that of the tunnel and outside air, and though on the north side that of the rock varies almost as quickly as that of the tunnel air, on the south it is influenced very much less. A few statistics may be of interest with regard to the prog- ress of the last three months (taken from the trimes trial report TUNNELS THROUGH HARD ROCK 113 of January, 1900). At Brigue, where there are three drilling- machines in No. 1 and two in the parallel heading, the total length excavated was 995 yds. or 6,409 cu. yds. in 89 working days, the average cross-sectional area being 57 sq. ft. This re- quired 507 attacks and 3,06(5 holes, which had a total depth of 26,600 ft., and 14,700 re-sharpenings of the drilling-tool, with 44,000 Ibs. of dynamite. The average time occupied in drilling was '2 hrs. 45 mins., while charging, firing, and clearing away the debris took 6 hrs., 35 mins. At Brigue 648 men and 29 horses were employed at one time in the tunnel. At Iselle the numbers were 496 men and 16 horses, working in shifts of 8 hrs. Outside the tunnel, in the shops, forges, etc., the men work 8 hrs. to 11 hrs. per day, the total being 541 men at Biigue and 346 men at Iselle. On the Italian side, where the rock is very much harder, there were three drilling-machines in each heading ; the total length excavated, with a cross-sectional area of 62 sq. ft., was 960 yds. or 6,700 cu. yds. in 91 working days. This required 61,293 re-sharpened tools, 758 attacks, 7,940 holes with a total depth of 33,000 ft,, and 56,000 Ibs. of dynamite. The average time spent in drilling was 2 hrs. 55 mins., and in charging and clear ing 2 hrs. 36 mins. Thus, in the hard gneiss, to excavate 1 cu. yd. of rock required 8 Ibs. of dynamite, and each tool pierced 6 ins. of rock before it required re-sharpening. Up to January 1, 1900, the total length of heading on the north side was 2,515 yds., and on the south side 1,720 yds., or a total of 4,235 yds. out of 21,575 yds., the full length of the tunnel. Allowing for unavoidable and unforeseen occurrences, such as strikes, war, etc., the contractors expect to complete tunnel No. 1 and the parallel heading by May, 1904. 114 TUNNELING CHAPTER XL TUNNELS THROUGH HARD ROCK (Continued). EXCAVATION BY DRIFTS. ST. GOTHARD TUNNEL. BUSK TUNNEL. THE more common method of tunneling through hard rock is to begin the work by a heading, instead of by a drift. This heading may be of small dimensions, and the remainder of the section may also be removed in successive small parts, or it may be the full width of the section, and the enlargement of the section be made in one other cut. General Discussion. When the tunnel is excavated by means of several cuts, which is the method usually employed in Europe, the sequence of work is as indicated by Fig. 5*b Work is begun by driving the center top heading No 1, whose floor is at the level of the bottom of the roof arch, and which is usually excavated by the circular cut method. This heading is widened by removing parts No. 2 until the top part of the sec- tion is removed, when the roof arch is built with its feet rest- ing on the unexcavated rock below. The lower portion of the section or bench is removed by first sinking the trench No. 3, after which part No. 4 is taken out, and then part No. 5, and the side walls built. Part No. 6 for the culvert is finally opened. The heading is, as a rule, driven far in advance, but the excavation of each of the other parts follows the preceding one at a distance behind of about 300 ft. The strutting, when any is required, is usually the typical radial strutting of the Belgian method of tunneling. The masonry lining is constructed practically the same as in tunnels excavated by a drift. The hauling is done on a single track laid in the heading No. 1, which separates into double tracks TUNNELS THROUGH HARD ROCK 115 where the full top section has been excavated by the removal of parts No. 2. These two tracks are again combined and form a single track along the top of part No. 5, which has been left wider than part No. 4 for this particular purpose. Whenpart No. 3 is excavated a standard-gauge track is laid on its floor ; and as the full section of the tunnel is completed by taking out parts Nos. 4 and 5, this single track is replaced by two standard- gauge tracks, into which it switches. Spoil is transferred from the narrow-gauge tracks on the upper level, to the standard- gauge tracks on the tunnel floor, by means of chutes, and build- ing material is transferred in the opposite direction by means of hoisting apparatus. When the excavation is made by a single wide heading, and a single other cut for removing the bench, which is the method preferred by American engineers, the work begins by removing a top heading the full width of the section. This heading is usually made 7 ft. or 8 ft. high, and is excavated by the center cut method. The method of strutting usually employed, is to erect successive three- or five-segment timber arches, whose feet rest on the top of the bench ; when the bench is removed, posts are inserted under the feet of each arch. These arches are covered with a lagging of plank. In America it has often been the practice to let this strutting serve as a temporary lining, and to replace it only after some time, often after years, with a perma- nent lining of masonry. In a succeeding chapter, some of the methods adopted in relining timber-lined arches with masonry are described. The hauling is done by a narrow-gauge track laid on the bottom of the heading, and by either narrow or broad gauge tracks laid on the floor of 1 the completed section below. A device called a bench carriage is often employed to enable the cars running on the heading tracks to dump their loads into the cars below, without interfering with the work on the bench front. This device consists of a wide platform carried on trucks, running on rails at the sides of the tunnel floor, so that it is level with the floor of the heading. The 116 TUNNELING front of this platform carries a hinged leaf which may be raised and lowered, and which forms a sort of gang-plank reaching to the floor of the heading. By running the heading cars out on to this traveling platform, they can be dumped into the cars below entirely clear of the work in progress on the bench front. For the purpose of illustrating the two methods of driving tunnels by a heading, which have been briefly described, the St. Gothard and the Busk tunnels have been selected. The St. Gothard tunnel is selected, as being the longest tunnel in the world, and because it was excavated by a number of small parts ; and the Busk tunnel, as being a single-track tunnel, driven by a heading, and bench, and having a timber lining. St. Gothard Tunnel. The St. Gothard tunnel penetrates the Alps between Italy and France, and is 9^ miles long. It was constructed in 1872-82. Material Penetrated, The St. Gothard tunnel was excavated through rock, consisting chiefly of gneiss, mica-schist, serpen- tine, and hornblend, the strata having an inch' nation of from 45 to 90. At many points the rock was fissured, and disin- tegrated easily, and water was en- countered in large quantities, caus- ing much trouble. Excavation. The sequence of excavation is shown by Fig. 15, p. 32. First the top center head- ing, No. 1, whose dimensions varied frqm 8.25 x 8.6 ft. to 8.5 x 9 ft., according to the quality of the rock r was driven never less than 1,000 ft. and sometimes over 3,000 ft. in advance of parts No. 2. The exca- vation of parts No. 2 opened up the full top section, and parts Nos. 3, 4, 5, 6, and 7, were removed in the order numbered. Strutting. Where regular strutting was required, the con- struction shown in Fig. 58 was adopted. FIG. 58. Diagram Showing Se- quence of Excavation in Heading Method of Tunneling Rock. TUNNELS THROUGH HARD HOCK 11 7 Masonry. The St. Gothard tunnel is lined throughout with masonry. After the upper portion of the section was fully excavated, the roof arch was built with its feet resting upon short planks on the top of the bench. Plank centers were used in constructing the arch. For the arch brick masonry was employed, but the side walls were built of rubble masonry. Shelter niches, about 3 ft. deep, were built into the side walls at intervals, and about every 3,000 ft. storage niches about 10 ft. deep, and closed with a door, were constructed. The cul- vert was of brick masonry. Mechanical Installation. Water-power was used exclusively in driving the St Gothard tunnel. At the north end, the Reuss, and at the south end, the Tessin and the Tremola, rivers or torrents were dammed, and their waters conducted to tur- bine plants at the opposite ends of the tunnel. The power thus furnished by the Reuss was about 1,500 H.P., and the power furnished by the combined supply of the Tessin and Tremola was 1,220 H.P. The turbine plant at both ends at first con- sisted of four horizontal impulse turbines, but later, two more turbines were added at the south end. Each of the two sets of four turbines first installed drove five groups of three compres- sors each, and the two supplementary turbines drove two groups of four compressors each. The compressors were of the Colladon type with water injection, and four groups of three compressors each were capable of furnishing 1,000 cu. yds. of air compressed to between seven and eight atmospheres every hour, or about 100 H.P. per hour, delivered to the drills at the front. This air when exhausted provided about 8,000 cu. yds. of fresh air per hour for ventilation. The compressors at each entrance discharged into a group of four cylindrical receivers of wrought-iron each 5.3 ft. in diameter by 29.5 ft. long, and having a capacity of 593 cu. ft. The cylinders were placed horizontally, the first one receiving the air at one end and discharging it at the other end into the next cylinder, and so on. By this arrangement the air was 118 TUNNELING drained of its moisture, and the discharge from the end receiver into the tunnel delivery pipes was not affected by the pulsations of the compressors. The delivery pipe decreased from 8 in. in diameter at the receiver to 4 ins. in diameter, and finally to 2^ ins. in diameter, at the front. The drills employed were of various patterns. The first one employed was the Dubois & Frangois " perforator," in which the drill-bit was fed forward by hand. This was replaced by Fer- roux drills having an automatic feed. Jules McKean's " perfo- rator " was employed at the north end of the tunnel. All of these drills were of the percussion type, and were mounted on carriages running on tracks. Their comparative efficiency was officially tested in drilling granitic gneiss with an operating air pressure of 5.5 atmospheres with the following results : NAME OF DRILL. PENETRATION INS. PER MIN. Ferroux . 1.6 McKean 1.4 Dubois & Frangois 1.04 Souinmelier 0.85 The heading was excavated by the circular cut method, the holes being driven as follows : Near the center of the heading three holes were first drilled, converging so as to inclose a pyramid with a triangular base. Around these center holes from 9 to 13 others were driven parallel to the tunnel axis. The center holes were blasted first, and then the surrounding holes. From 3 to 5 hours were required to drill the two sets of holes, and from three to four hours were required to remove the blasted rock. The number of holes drilled in removing each of the various parts was as follows : Part No. 1 6 to 9 Part No. 2 6 to 10 Part No. 3 2 Part No. 4 6 to 9 Part No. 5 3 Part No. 6 6 to 9 Part No. 7 . 1 Total for full section 36 to 40 TUNNELS THROUGH HARD ROCK 119 Hauling. Two different systems were employed for haul- ing the spoil and construction material in the St. Gothard tunnel. To remove the spoil from parts Nos. 1 and 2 a narrow- gauge track was laid on the floor of the heading, and tLe cars were hauled by horses, the grade being descending from the fronts. These narrow-gauge cars were dumped into larger broad-gauge cars running on the track laid on the floor of the completed section and hauled by compressed air locomotives (Fig. 59). To raise the incoming structural material from the broad-gauge cars to the narrow-gauge cars running on the level above, hoisting devices were employed. Method of Strutting Roof, St Gothard Tunnel. Sketch Showing Arrangement of Car Tracks, St. Gothanl Tunnel. FIG. 59. Busk Tunnel. The Busk tunnel, 9,094 ft long, was built between Busk and Ivanhoe stations, on the Colorado Midland R.R. in Colorado. Fig. 60 is a trans verse section of the tunnel ; it is for a single track, and is 15 ft. wide and 21 ft. high. Material Penetrated. The material through which the tunnel was driven was a gray granite of irregular character. In some places the rock was found extremely hard to drill and blast, and stood perfectly upon exposure to the air, while in other places, where it seemed at first equally as hard and firm, it dis- integrated upon exposure, and it was found necessary to timber 120 TUNNELING the excavation. In other places, where no disinte- gration was apparent, the rock was full of seams and faults, and it was necessary to support the detached fragments by timbering. In a few places quite large cavi- ties were encountered, which were filled with liquid mud. In one place the inrush of liquid mud was so sudden and the stream so strong that the men barely escaped with their lives. Excavation. The excavation was made by a heading 7 ft. high and the full width of the section, and by a single bench excavation. - In driving the heading two sets of holes were OouWe Timtering in Heavy (around. FIG. 60. Transverse Section of Busk Tunnel Colorado Midland R. R., Colorado. TUNNELS THROUGH HARD ROCK 121 drilled. The first set of eight holes were driven in two rows from top to bottom, the holes being about 2 ft. apart on the surface, and converging toward the center of the t^innel. These holes were 12 ft. deep, and the action of the blast was to blow out a wedge-shaped cavity in the face. The holes of the second set were drilled at the sides of the front and parallel to the sides of the section, and the blast blew out the remainder of the rock into the wedge-shaped center cavity. The method of excavating the bench was nearly the same as that of excavating the heading. Mechanical Installation. The following machinery was employed in connection with the construction of the tunnel: at the Ivanhoe end, three 100 H. P. boilers; two 20 x 24 in. Ingersoll compressors, and one 20 X 24 in. Norwalk compres- sor ; a 10 H. P. engine driving an electric-light dynamo, and a 20 H. P. engine driving a No. 6 Baker blower, forcing fresh air into the tunnel through a 14-in. pipe. In the tunnel one No. 7 and one No. 9 Cameron pump, and a Deane duplex pump with a 10-in. stroke, were employed to keep the excava- tion clear of water, since ths grade descended uniformly from the Ivanhoe end, and the water followed the workings. At the Busk end the plant consisted of three 80 H. P. boilers, two 20 x 24-in. Ingersoll compressors, 10 H. P. and 20 H. P. en- gines respectively, for the electric light dynamo and the blower. Four 3^ in. Ingersoll eclipse drills were used in each heading, and two on each bench, making six drills at each end of the tunnel. Strutting and Lining. For about 78 % of its length the tunnel is lined with timber. The timbering consists of a five- segment arch for the roof, resting on a wall plate which is car- ried by vertical side posts. The segments of the arch, the wall plates, and the posts, are 12x1 2-inch timbers. The roof arches and the posts supporting the wall plates are spaced 4 ft. apart, center to center. Above the arches is laid a lagging of 2-inch longitudinal planks. The arches were set up as fast as the 122 TUNNELING heading was driven, and rested upon the bench until it was removed and the side posts inserted. Where mud pockets were met the plank lagging was inserted behind the side posts as well as above the roof-arch ribs, and when the pressures were unusually great a double lining was employed. Progress of Work. The rate of progress made in exca- vating the Busk tunnel was as follows : Total time consumed in driving the heading 1,118 days Average daily progress for both headings 8.4 feet Greatest progress in one month 337 " Average daily progress, one month, 31 days 10.87 u Greatest progress in one month (28 days) at one end . . 202.5 " Average progress in one month (28 days) at one end . . 7.23 " Greatest monthly progress on bench 218 " Average daily progress on bench 7.79 " Cost of Work. The cost of the tunnel was calculated on the assumption that the excavation per lineal foot was 10.19 cu. yds., and where the section was enlarged for timbering, 1379 cu. yds. The contractors' estimate for excavating and timbering the tunnel was as follows : Excavation of 9,393.66 lineal feet @ $62,50 .... $587,103.73 Enlargement for timbering 32,575 cubic yards . . . 81,437.50 Cost of timber 81,600.00 Cost of labor on timbering 2,723,000 ft. B. M. @ $12 . 32,676.00 Total 8782,817.25 This is an average cost per lineal foot of tunnel of $83.14,, which is very close to the average cost of single-track timber- lined tunnels in America, which is usually figured at $85 per lineal foot. COMPARISON OF METHODS. The differences between the drift and heading methods of excavating tunnels through rock, consist chiefly in the excava- tions, strutting, and hauling. When the drift method is em- ployed an advanced gallery is opened along the floor of the TUNNELS THROUGH HARD ROCK 12& tunnel before the upper part of the section is removed, and when the heading method is employed the upper part of the section is completely excavated and lined before any part of the section below is excavated. When the drift method of driving is employed polygonal strutting is usually used, and longitudinal strutting is employed with the heading method of driving. In the drift method the hauling is done by one system of tracks at the same level, while in the heading method two systems of tracks are employed at different levels. It is, perhaps, impossible to state without qualification, which method is the better. European engineers unanimously prefer excavation by a drift, especially for long tunnels. An advan- tage that this method affords in long tunnels is, that the water which is usually found in large quantities under high moun- tains is easily collected in the drift and conveyed to the culvert, while in the heading method the water from the advance gallery before being collected into the culvert built on the floor of the tunnel, must pass through all the workings. This may be a serious inconvenience when water is found in large quantities, as, for instance, was the case in the St Gothard tunnel, where the stream amounted to 57 gallons per second. The heading method has an advantage in tunneling loose rock, since it is the more economical in strutting. TUNNELING CHAPTER XII. REPRESENTATIVE MECHANICAL INSTALLA- TIONS FOR TUNNEL WORK. THE important role played by the power plant and other mechanical installations in constructing tunnels through rock has already been mentioned. In some methods of soft-ground tunneling, and particularly in soft-ground subaqueous tunnel- ing, it is also often necessary to employ a mechanical installa- tion but slightly inferior in size and cost to those used in tunneling rock. The general character of the mechanical plant required for tunnel work has been described in another chapter. It is proposed to describe very briefly here a few typical individual plants of this character, which will in some respects give a better idea of this phase of tunnel work than the more general descriptions. Rock Tunnels. The tunnels selected to illustrate the me- chanical installations employed in tunneling through rock ore : The Hoosac Tunnel, the Cascade Tunnel, the Niagara Falls Power Tunnel, the Palisades Tunnel, the Croton Aqueduct Tunnel, the Strickler Tunnel, in America, and the Graveholz Tunnel and the Sonnstein Tunnel in Europe. In addition there will be found in other chapters of this book a description of the mechanical installation at the Busk tunnel and at the St. Gothard and Simplon tunnels. Hoosac Tunnel. The Hoosac tunnel on the Fitchburg R.R. in Massachusetts is 25,000 ft. long, and the longest tunnel in America. The material through which the tunnel was driven was chiefly hard granitic gneiss, conglomerate, and mica-schist rock. The excavation was conducted from the entrances and MECHANICAL INSTALLATIONS FOR TU one shaft, the wide heading and single-beii^k^tKod being employed, with the center-cut system of blasting which was here used for the first time. The tunnel was begun in 1854, and. continued by hand until 1866, when the mechanical plant was installed. Most of the particular machines employed have now become obsolete, but as they were the first machines used for rock tunneling in America they deserve mention. The drills used were Burleigh percussion drills, operated by com- pressed air. Six of these drills were mounted on a single car- riage, and two carriages were used at each front. The air to operate these drills was supplied by air compressors operated by water-power at the portals and steam-power at the shaft. The air compressors consisted of four horizontal single-acting air cylinders with poppet valves and water injection. The compressors were designed by Mr. Thomas Deane the chief engineer of the tunnel. Palisades Tunnel. The Palisades tunnel was constructed to carry a double-track railway line through the ridge of rocks bordering the west bank of the Hudson River and known as the Palisades. It was located about opposite 116th St. in New York city. The material penetrated was a hard trap rock very full of seams in places, which caused large fragments to fall from the roof. The excavation was made by a single wide heading and bench, employing the center-cut method of blast- ing with eight center holes and 16 side holes for the 7 x 18 ft. heading. Ingersoll-Sergeant 2^ in. drills were used, four in each heading and six on each bench, and 30 ft. per 10 hours was considered good work for one drill. The power-plant was situated at the west portal of the tunnel, and the power was transmitted by electricity and com- pressed air to the middle shaft and east portal workings. The plant consisted of eight 100 H. P. boilers, furnishing steam to four Rand duplex 18 X 22 in. air compressors, and an engine running a 30 arc light dynamo. The compressed air was car- ried over the ridge by pipes varying from 10 ins. to 5 ins. in 126 TUNNELING diameter to the shaft and to the east portal, and was used for operating the hoisting engines as well as the drills at these workings. Inside the tunnel, specially designed derrick cars were employed to handle large stones, they being also operated by compressed air. This car ran on a center track, while the mucking cars ran on side tracks, and it was employed to lift the bodies of the cars from the trucks, place them close to the front, being worked where large stone could be rolled into them, and return them to the trucks for removal. In addition to handling the car bodies the derrick was used to lift heavy stones. The hauling was done at first by horse-power, and later by dummy locomotives. Croton Aqueduct Tunnel. In the construction of the Croton Aqueduct for the water supply of New York city, a tunnel 31 miles long was built, running from the Croton Dam to the Gate House at 135th St. in New York city. The section of the tunnel varies in form, but is generally either a circular or a horse-shoe section. In all cases the section was designed to have a capacity for the flow of water equal to a cylinder 14 ft. in diameter. To drive the tunnel, 40 shafts were employed. The material penetrated was of almost every character, from quicksand to granitic rock, but the bulk of the work was in rock of some character. The excavation in rock was conducted by the wide heading and bench method, employing the center- cut method of blasting. Four air drills, mounted on two double-arm columns, were employed in the heading. The drills for the bench work were mounted on tripods. Steam- power was used exclusively for operating the compressors, hoisting engines, ventilating fans and pumps ; but the size and kind of boilers used, as well as the kind and capacity of the machines which they operated, varied greatly, since a separate power-plant was employed for each shaft with a few exceptions. A description of the plant at one of the shafts will give an indication of the size and character of those at the other shafts, and for this purpose the plant at shaft 10 has been selected. MECHANICAL INSTALLATIONS FOR TUNNEL WORK 127 At shaft 10 steam was provided by two Ingersoll boilers of 80 H. P. each, and by a small upright boiler of 8 H. P. There were two 18 X 30 in. Ingersoll air compressors pumping into two 42 in. X 10 ft. and two 42 in. X 12 ft. Ingersoll receivers. In the excavation there were twelve 3 in. and six 3| in. Ingersoll drills, four drills mounted on two double-arm columns being used on each heading, and the remainder mounted on tripods being used on the bench. Two Dickson cages operated by one 12 x 12 in. Dickson reversible double hoisting engine provided transportation for material and supplies up and down the shaft. A Thomson-Houston ten-light dynamo operated by a Lidgerwood engine provided light. Drainage was effected by means of two No. 9 and one No. 6 Cameron pumps. At this particular shaft the air exhausted from the drills gave ample ventilation, especially when after each blast the smoke was cleared away by a jet of compressed air. In other workings, however, where this means of ventilation was not sufficient, Baker blowers were generally employed. Strickler Tunnel. -The Strickler tunnel for the water supply of Colorado Springs, Col., is 6,441 ft. long with a sec- tion of 4 ft. x 7 ft. It penetrates the ridge connecting Pike's Peak and the Big Horn Mountains, at an elevation of 11,540 ft. above sea level. The material penetrated is a coarse porphyritic granite and morainal debris, the portion through the latter material being lined. The mechanical installation consisted of a water-power electric plant operating air com- pressors. The water from Buxton Creek having a fall of 5,400 ft. was utilized to operate a 36 in. 220 H. P. Pelton water-wheel, which operated a 150 K. W. three-phase generator. From this generator a 3,500 volt current was transmitted to the east portal of the tunnel, where a step-down transformer reduced it to a 220 volt current to the motor. The transmis- sion line consisted of three No. 5 wires earned on cross-arm poles and provided with lightning arresters at intervals. The plant at the east portal of the tunnel consisted of a 75 H. P. 128 TUNNELING electric motor, driving a 75 H.P. air compressor, and of small motors to drive a Sturtevant blower for ventilation, to run the blacksmith shop, and to light the tunnel, shop, and yards. From the compressor air was piped into the tunnel at the east end, and also over the mountain to the west portal work- ings. Two drills were used at each end, and the air wa& also used for operating derricks and other machinery. For removing the spoil a trolley carrier system was employed. A longitudinal timber was fastened to the tunnel roof, directly in the apex of the roof arch. This timber carried by means- of hangers a steel bar trolley rail on which the carriages ran. Outside of the portal this rail formed a loop, so that the carriage oould pass around the loop and be taken back to- the working face. Each carriage carried a steel span of 1^ cu. ft. capacity, so suspended that by means of a tripping device it was automatically dumped when the proper point on the loop was reached. Niagara Falls Power , annel. The tail-race tunnel built to carry away the water discharged from the turbines of the Niagara Falls Power Co., has a horse-shoe section 19 x 21 ft. and a length of 6,700 ft. It was driven through rock from three shafts by the center-cut method of blasting. In sink- ing shaft No. very little water was encountered, but at shafts- Nos. 1 and 2 an inflow of 800 gallons and 600 gallons per minute, respectively, was encountered. The principal plant was located at shaft No. 2, and consisted of eight 100 H. P. boilers, three 18 x 30 in. Rand duplex air compressors, a Thomson-Houston electric-light plant, and a sawmill with a capacity of 20,000 ft. B. M. per day. The shafts were fitted with Otis automatic hoisting engines, with double cages at shafts Nos. 1 and 2, and a single cage at shaft No. 0. The drills used were 25 Rand drills and three Inge rsoll- Sergeant drills. The pumping plant at shaft No. 2 consisted of four No. 7 and ono Cameron pumps, and that at shaft No. 2 consisted of two N , -. 7 and two No. 9 Cameron pumps and MECHANICAL INSTALLATIONS FOR TUNNEL WORK 129 three Snow pumps. An auxiliary boiler plant consisting of two 60 H. P. boilers was located at shaft No. 1, and another, consisting of one 75 H. P. boiler, was located at shaft No* 0. Cascade Tunnel. The Cascade tunnel was built in 1886- 88 to carry the double tracks of the Northern Pacific Ry. through the Cascade Mountains in Washington. It is 9,850 ft. long with a cross-section 16i ft. wide and 22 ft. high, and is lined with masonry. The material penetrated was a basaltic rock, with a dip of the strata of about 5. The rock was excavated by a wide heading and one bench, using the center- cut system of blasting. A strutting consisting of five-segment timber arches carried on side posts, spaced from 2 ft. to 4 ft. apart, and having a roof lagging of 4 X 6 in. timbers packed above with cord- wood. The mechanical plant of the tunnel is of particular interest, because of the fact that all the machinery and supplies had to be hauled from 82 to 87 miles by teams, over a road cut through the forests covering the mountain slopes. This work required from Feb. 22 to July 15, 1886, to perform. In many places the grades were so steep that the wagons had to be hauled by block and tackle. The plant con- sisted of five engines, two water-wheels, five air compressors, eight 70 H. P. steam-boilers, four large exhaust fans, two com- plete electric arc-lighting plants, two fully equipped machine- shop outfits, 36 air drills, two locomotives, 60 dump care, and two sawmill outfits, with the necessary accessories for these vari- ous machines. This plant was divided about equally between the two ends of the tunnel. The cost of the plant and of the work of getting it into position was $125,000. G-raveholz Tunnel. The Graveholz tunnel on the Bergen Railway in Norway is notable as being the longest tunnel in northern Europe, and also as being built for a single-track narrow-gauge railway. This tunnel is 17,400 feet long, and is located at an elevation of 2,900 ft. above sea-level. Only about 3 % of the length of the tunnel is lined. The mechani- cal installation consists of a turbine plant operating the various 130 TUNNELING machines. There are two turbines of 100 H. P. and 120 H. P. taking water from a reservoir on the mountain slope, and furnishing 220 H. P., which is distributed about as follows : Boring-machines, 60 H. P. ; ventilation, 30 to 40 H. P. ; elec- tric locomotives, 15 H. P. ; machine shop, 15 H. P. ; electric- lighting dynamo, 25 H. P. ; electric drills, the surplus, or some 40 H. P. The boring-machines and electric drills will be operated by the smaller 100 H. P. turbine. Sonmtein Tunnel. - The Sonnstein tunnel in Germany is particularly interesting because of the exclusive use of Brandt rotary drills. The tunnel was driven through dolomite and hard limestone by means of a drift and two side galleries. The dimensions of the drift were 7i X 7^ ft. The power plant con- sisted of two steam pressure pumps, one accumulator, and four drills. The steam-boiler plant, in addition to operating the pumps, also supplied power for operating a rotary pump for drainage and a blower for ventilation. The hydraulic pressure required was 75 atmospheres in the dolomite, and from 85 to 100 atmospheres in the limestone. The drift was excavated with five 3^ in. holes, one being placed at the center and driven parallel to the axis of the tunnel, and four being placed at the corners of a rectangle corresponding to the sides of the drift, and driven at an angle diverging from the center hole. The average depths of the holes were 4.3 ft., and the efficiency of the drills was 1 in. per minute. One drill was employed at each front, and was operated by a machinist and two helpers, who worked eight-hour shifts, with a blast between shifts at first, and later twelve-hour shifts, with a blast between shifts. The 24 hours of the two shifts were divided as follows : boring the holes, 10.7 hours ; charging the holes, 1.1 hours ; removing the spoil, 11.7 hours; changing shifts, 0.5 hour. The average progress per day for each machine was 6.7 ft. The total cost of the plant was $17,450. St. Clair River Tunnel. The submarine double-track rail- way tunnel under the St. Clair River for the Grand Trunk Ry., MECHANICAL INSTALLATIONS FOR TUNNEL WORK 131 is 8,500 ft. long, and was driven through clay by means of a shield, as described in the succeeding chapter on the shield system of tunneling. The mechanical plant installed for pros- ecuting the work was very complete. To furnish steam to the air compressors, pumps, electric-light engines, hoisting-engines, etc., a steam-plant was provided on each side of the river, con- sisting of three 70 H. P. and four 80 H. P. Scotch portable boilers. The air-compressor plant at each end consisted of two 20 X 24 in. Ingersoll air compressors. To furnish light to the workings, two 100 candle-power Edison dynamos were in- stalled oft the American side, and two Ball dynamos of the same size were installed on the Canadian side. The dynamos on both sides were driven by Armington & Sims engines. These dynamos furnished light to the tunnel workings and to the machine-shops and power-plant at each end. Root blowers of 10,000 cu. ft. per minute capacity provided ventilation. The pumping plant consisted of one set of pumps installed for per- manent drainage, and another set installed for drainage during construction, and also to remain in place as apart of the permanent plant. The latter set consisted of two 500 gallon Worthington duplex pumps set first outside of each air lock, closing the ends of the river portion of the tunnel. For permanent drainage, a drainage shaft was sunk on the Canadian side of the river, and connected with a pump at the bottom of the open-cut approach. In this shaft were placed a vertical, direct acting, compound condensing pumping engine with two 19J in. high- pressure and two 33| in. low-pressure cylinders of 24 in. stroke, connected to double-acting pumps with a capacity of 3000 gallons per minute, and also two duplex pumps of 500 gallons capacity per minute. For permanent drainage on the American side, four Worthington pumps of 3,000 gallons' capacity were installed in a pump-house set back into the slope of the open- cut approach. For the permanent drainage of the tunnel proper two 400 gallon pumps were placed at the lowest point of the tunnel grade. Spoil coming from the tunnel proper was 132 TUNNELING hoisted to the top of the open cut by derricks operated by two 50 H. P. Lidgerwood hoisting-engines. The pressure pumping plant for supplying water to the hydraulic shield-jacks at each end of the tunnel consisted of duplex direct-acting engines with 12 in. steam cylinders and 1 in. water cylinders, supply- ing water at a pressure of 2000 Ibs. per sq. in. TUNNELS THROUGH SOFT GROUND 133 CHAPTER XIII. EXCAVATING TUNNELS THROUGH SOFT GROUND ; GENERAL DISCUSSION ; THE BELGIAN METHOD. GENERAL DISCUSSION. IT may be set down as a general truth that the excavation of tunnels through soft ground is the most difficult task which confronts the tunnel engineer. Under the general term of soft ground, however, a great variety of materials is included, be- ginning with stratified soft rock and the most stable sands and clays, and ending with laminated clay of the worst character. From this it is evident that certain kinds of soft-ground tunneling may be less difficult than the tunneling of rock, and that other kinds may present almost insurmountable dif- ficulties. Classing both the easy and the difficult materials together, however, the accuracy of the statement first made holds good in a general way. Whatever the opinion may be in regard to this point, however, there is no chance for dispute in the statement that the difficulty of tunneling the softer and more treacherous clays, peats, and sands is greater than that of tunneling firm soils and rock ; and if we describe the methods which are used successfully in tunneling very unstable materials, no difficulty need be experienced in modifying them to handle stable materials. Characteristics of Soft-Ground Tunneling. The principal char- acteristics which distinguish soft-ground tunneling are, first, that the material is excavated without the use of explosives, and second, that the excavation has to be strutted practically 134 TUNNELING as fast as it is completed. In treacherous soils the excavation also presents other characteristic phenomena: The material forming the walls of the excavation tends to cave and slide. This tendency may develop immediately upon excavation, or it may be of slower growth, due to weathering and other nat- ural causes. In either case the roof of the excavations tends to fall, the sides tend to cave inward and squeeze together, and the bottom tends to bulge or swell upward. In materials of very unstable character these movements exert enormous pres- sures upon the timbering or strutting, and in especially bad cases may destroy and crush the strutting completely. Out- side the tunnel the surface of the ground above sinks for a con- siderable distance on each side of the line of the tunnel. Methods of Soft-Ground Tunneling. There are a variety of methods of tunneling through soft ground. Some of these, like the quicksand method and the shield method, differ in char- acter entirely, while in others, like the Belgian, German, Eng- lish, Austrian, and Italian methods, the difference consists simply in the different order in which the drifts and headings are driven, in the difference in the number and size of these advance galleries, and in the different forms of strutting frame- work employed. In this book the shield method is considered individually ; but the description of the Belgian, German, Eng- lish, Austrian, Italian, and quicksand methods are grouped together in this and the three succeeding chapters to permit of easy comparison. THE BELGIAN METHOD OF TUNNELING THROUGH SOFT GROUND. The Belgian method of tunneling through soft ground was first employed in 1828 in excavating the Charleroy tunnel of the Brussels-Charleroy Canal in Belgium, and it takes its name from the country in which it originated. The distinctive char- acteristic of the method is the construction of the roof arch TUNNELS THROUGH SOFT GROUND 135 before the side walls and invert are built. The excavation, therefore, begins with the driving of a top center heading which is enlarged until the whole of the section above the springing lines of the arch is opened. Various modifications of the method have been developed, and some of the more important of these will be described farther on, but we shall begin its consideration here by describing first the original and usual mode of procedure. Excavation. Fig. 61 is the excavation diagram of the Bel- gian method of tunneling. The excavation is begun by open- ing the center top heading No. 1, which is carried ahead a greater or less distance, depending upon the nature of the soil, and is immediately strutted. This heading is then deepened FIGS. 61 and 62. Diagrams Showing Sequence of Excavations in the Belgian Method. by excavating part No. 2, to a depth corresponding to the springing lines of the roof arch. The next step is to remove the two side sections No. 3, by attacking them at the two fronts and at the sides with four gangs of excavators. The regularity and efficiency of the mode of procedure described consist in adopting such dimensions for these several parts of the section that each will be excavated at the same rate of speed. When the upper part of the section has been excavated as described, the roof arch is built, with its feet supported by the unexca- vated earth below. This portion of the section is excavated by taking out first the central trench No. 4 to the depth of the bottom of the tunnel, and then by removing the two side parts No. 5. As these side parts No. 5 have to support the arch, 136 TUNNELING they have to be excavated in such a way as not to endanger it. At intervals along the central trench No. 4, transverse or side trenches about 2 ft. wide are excavated on both sides, and struts are inserted to support the masonry previously supported by the earth which has been removed. The next step is to widen these side trenches, and insert struts until all of the material in parts No. 5 is taken out. When the material penetrated is firm enough to permit, the plan of excavation illustrated by the diagram, Fig. 62, is substi- tuted for the more typical one just described. The only differ- ence in the two methods consists in the plan of excavating the upper part of the profile, which in the second method consists in driving first the center top heading No. 1, and then in tak- ing out the remainder of the section above the springing lines of the arch in one operation, while in the first method it is done in two operations. The distance ahead of the masonry to which the various parts can be driven varies from 10 ft. to, in some cases, 100 ft., being very short in treacherous ground, and longer the more stable the material is. Strutting The longitudinal method of strutting, with the poling-boards running transversely of the tunnel, is always employed in the Belgian method of tunneling. In driving the first center top heading, pairs of vertical posts carrying a trans- verse cap-piece are erected at intervals. On these cap-pieces are carried two longitudinal bars, which in turn support the saddle planks. As fast as part No. 2, Fig. 61, is excavated, the vertical posts are replaced by the batter posts A and j5, Fig. 63. The excavation of parts No. 3 is begun at the top, the poling-boards a and b being inserted as the work pro- gresses. To support the outer ends of these poling-boards, the longitudinals X and Y are inserted and supported by the batter posts C and D. In exactly the same way the poling-boards c and d, the longitudinals Fand W, and the struts E and F, are placed in position ; and this procedure is repeated until the whole top part of the section is strutted, as shown by Fig. 63, TUNNELS THROUGH SOFT GROUND 137 the cross struts a:, #, z, etc., being inserted to hold the radial struts firmly in position. The feet of the various radial props rest on the sill M N. These fan-like timber structures are set up at intervals of from 3 ft. to 6 ft., depending *npon the quality of the soil penetrated. FIG. 63. Sketch Showing Radial Roof Strutting, Belgian Method. Centers. Either plank or trussed centers may be employed in laying the roof arch in the Belgian method, but the form of center commonly employed is a trussed center constructed as shown by Fig. 64. It may be said to consist of a king-post truss carried on top of a modified form of queen-post truss. The collar-beam and the tie-beam of the queen-post truss are spaced about 7 ft. apart, and the posts themselves are left far enough apart to allow the pas- sage of workmen and cars be- tween them. The tie beam of the king-post truss is clamped to the collar-beam of the queen- post truss by iron bands. On the rafters of the two trusses are fastened timbers, with their outer edges cut to the curve of the roof arch. These centers .are set up midway between the fan-like strutting frames previ- ously described. They are usually built of square timbers. The tie beams are usually 6x6 in., and the struts and posts 4 x 4 in. timbers. The reason for giving the larger sectional FIG. 64. Sketch Showing Roof Arch Center, Belgian Method. 138 TUNNELING dimensions to the tie beams, contrary to the usual practice in constructing centers, is that it has to serve as a sill for distrib- uting the pressure to the foundation of unexcavated soil which, supports the center. Sometimes a sub-sill is used to support the - center upon the soil ; and in any case wedges are employed to carry it, which can be removed for the purpose of striking the center. After the arch is completed, the centers may be removed immediately, or may be left in position until the masonry has thoroughly set. In either case the leading center over which the arch masonry terminates temporarily is left in position until the next section of the arch is built. Masonry. The masonry of the roof arch, which is the first part built, is of necessity begun at the springing lines, and the first course rests on short lengths of heavy planks. These planks, besides giving an even surface upon which to begin the masonry, are essential in furnishing a bearing to the struts inserted to support the arch while the earth beloAV them, part No. 5, Fig. 61, is being excavated. As the arch masonry progresses from the springing lines upward, the radial posts of the strutting are removed, and replaced by short struts rest- ing on the lagging of the centers, which support the crown bars or longitudinals until the masonry is in place, when they and the poling-boards are removed, and the space between the arch masonry and walls of the excavation is filled with stone or well-rammed earth. Considering now the side wall masonry, it will be re- membered that in excavating the part No. 5, Fig. 61, of the section, frequent side trenches were excavated, and struts inserted to take the weight of the masonry. These struts are inserted on a batter, with their feet near the center of the tunnel floor, so that the side wall masonry may be carried up behind them to a height as near as possible to the springing lines of the arch. When this is done the struts are removed, and the space remaining between the top of the partly fin- ished side wall and the arch is filled in. This leaves the arch TUNNELS THROUGH SOFT GROUND 139 supported by alternate lengths or pillars of unexcavated earth and completed side wall. The next step is to remove the remaining sections of earth between the sections of side wall, and fill in the space with masonry. Fig. 65 is a cross-section, showing the masonry completed for one-half and the inclined props in position for the other half; and Fig. ftfc is a longitudinal section showing the pillars of unexcavated earth be- tween the consecutive sets of in- clined struts and several other details of the lining, strutting, and excavating work. The invert masonry is built after the side walls are completed. This is regarded as a defect of tins method of tunneling, since the lateral pressures may squeeze the side walls together and dis- tort the arch l>efore the invert is in place to brace them apart. FIG. 65. Sketch Showing Method of Underpining Roof Arch with the Side Wall Masonry. To prevent as much as possible the distortion of the arch after the centers are removed, it is considered good practice to shore the maSOUry with hoii- _ ., . , zontal beams having their ends abutting against plank, as shown by Fig. 65. These hori- zontal beams should be placed at close intervals, and be supported at intermediate points by vertical posts, as shown FIG. 66. Longitudinal Section Showing Construction by the Belgian Method. 140 TUNNELING by the illustration. Since the roof arch rests for some time .supported directly by the unexcavated earth below, settle- ment is liable, particularly in working through soft ground. This fact may not be very important so long as the settle- ment is uniform, and is not enough to encroach 011 the space necessary for the safe passage of travel. To prevent the latter possibility the centers are placed from 9 ins. to 15 ins. higher than their true positions, depending upon the nature of the soil, so that considerable settlement is possible without any danger of the necessary cross-section being infringed upon. In conclusion it may be noted that the lining may be con- structed in a series of consecutive rings, or as a single cylin- drical mass. Hauling. Since in this method of tunneling the upper part of the section is excavated and lined before the excavation of the lower part is begun, the upper portion is always more ad- vanced than the lower. To carry away the earth excavated at the front, therefore, an elevation has to be surmounted ; and this is usually done by constructing an inclined plane rising from the floor of the tunnel to the floor of the heading, as shown by Fig. 66. This inclined plane has, of course, to be moved ahead as the work advances, and to permit of this movement with as little interruption of the other work as possible, two planes are employed. One is erected at the right-hand side of the section, and serves to carry the traffic while the left-hand side of the lower section is being removed some distance ahead and the other plane is being erected. The inclination given to these planes depends upon the size of the loads to be hauled, but they .should always have as slight a grade as practicable. Narrow- gauge tracks are laid on these planes and along the floor of the upper part of the section passing through the center opening mentioned before as being left in the centers and strutting. In excavating the top center heading there is, of course, an- other rise to its floor from the floor of the upper part of the .section. Where, as is usually the case in soft soils, this top TUNNELS THROUGH SOFT GROUND 141 heading is not driven very far in advance, the earth from the front is usually conveyed to the rear in wheelbarrows, and dumped into the cars standing on the tracks below. In % nnn soils, where the heading is driven too far in advance to make this method of conveyance inadequate, tracks are also laid on the floor of the heading, and an inclined plane is built connect- ing it with the tracks on the next level below. In place of these inclined planes, and also in place of those between the floor of the tunnel and the level above, some form of hoisting device is sometimes employed to lift the cars from one level to the other. There are some advantages to this method in point of economy, but the hoisting-machines are not easily worked in. the darkness, and accidents are likely to occur. In the advanced top heading and in the upper part of the section narrow-gauge tracks are necessarily employed, and these may be continued along the floor of the finished section, or the permanent broad-gauge railway tracks may be laid as fast as the full section is completed. In the former case the perma- nent tracks are not laid until the entire tunnel is practically completed ; and in the latter case, unless a third rail is laid, the loads have to be transshipped from the broad- to the narrow- gauge tracks or vice versa. It is the more general practice to use a third rail rather than to transship every load. Modifications. Considering the extent to which the Belgian method of tunneling has been employed, it is not surprising that many modifications of the standard mode of procedure have been developed. The modification which differs most from the standard form is, perhaps, that adopted in excavating the Roosebeck tunnel in Germany. This method preserves the principal characteristic of the Belgian method, which is the construction of the upper part of the section first ; but instead of building the side walls from the bottom upward, they are built in small sections from the top downward. The excavation begins by driving the center top heading No. 1, Fig. 67, whose floor is at the level of the springing lines of the roof arch, and 14 -2 TUNNELING then the two side parts No. 2 are excavated, opening up the entire upper portion of the section in which the roof arch is built, as in the regular Belgian method. The next step is to excavate part No. 3, shoring up the arch at frequent intervals. Between these sets of shoring the side walls are built, resting planks on the floor of part No. 3, and. then the sets of shores are removed and re- placed by masonry. Next part No. 4 is excavated, shored, and filled with masonry as was part No. 3. In exactly the same FIG. 67. Diagram Show- ing Sequence of Excava- way parts 5, 6, 7, and 8 are constructed tion in Modified Belgian Method. in the order numbered. To prevent the distortion of the arch during the side-wall is braced by horizontal struts, as described construction it above in Fig. 65. Advantages, The advantages of the Belgian method of tunneling may be summarized as follows: (1) The excavation progresses simultaneously at several points without the differ- ent gangs of excavators interfering with each other, thus secur- ing rapidity and efficiency of work; (2) the excavation is done by driving a number of drifts or parts of small section, which are immediately strutted, thus causing the minimum disturb- ance of the surrounding material ; (3) the roof of the tunnel, which is the part of the lining exposed to the greatest pressures, is built first. Disadvantages. : The disadvantages of the Belgian method of tunneling may be summarized as follows : (1) The roof arch which rests at first on compressible soil is liable to sink ; (2) before the invert is built there is danger of the arch and side walls being distorted or sliding under the lateral pressures; (3) the masonry of the side walls has to be underpinned to the arch masonry. Accidents and Repairs. One of the most frequent accidents in the Belgian method of tunneling is the sinking of the roof TUNNELS THROUGH SOFT GROUND arch owing to its unstable foundation on the unexcavated soil of the lower portion of the section. The amount of settlement may vary from a few inches in firm soil to over 2 ft. in Joose soils. To counteract the effect of this settlement it is the gene- ral practice to build the arch some inches higher than its nor- mal position. When the settlement is great enough to infringe seriously upon the tunnel section, repairs have to be made ; and the only way of accomplishing them is to demolish the arch and rebuild it from the side walls. It is usually considered best not to demolish the arch until the invert has been placed, so that no further disturbance is likely once the lining is completed anew. The rotation of the arch about its keystone, or the opening of the arch at the crown, by the squeezing inward of the haunches by the lateral pressures, is another characteristic accident. Fig. 68 shows the nature of the distortion produced ; the segments of the arch move toward each other by revolving on the intradosal edges of the keystone, which are broken away and crushed together with the operation, while the extradosal edges are opened. It is to prevent this occurrence that the horizontal struts shown in Fig. 65 are em- ployed. The manner of repairing this accident differs, depend- ing upon the extent of the injury 7 . When the intradosal edges of the keystone are but slightly crushed, the repairing is done as directed by Fig. 69. When the keystone is completely crushed, however, the indications are that the material of the keystone, usually brick, is not strong enough to resist the pressures coming upon it, and it is advisable to substitute a stronger material in the repairs, and a stone keystone is con- structed as shown by Fig. TO. The middle stone of this key- stone extends through the depth of the arch ring, and the two side stones only half-way through, their purpose being merely FIG. 68. Sketch Showing Failure of Roof Arch by Opening at Crown. 144 TUNNELuSG to resist the crushing forces which are greatest at the intrados* Sometimes, when the pressures are unsymmetrical, the arch ring breaks at the haunches as well as the crown, as shown by FIGS. 69 to 71. Sketches Showing Methods of Repairing Roof Arch Failures. Fig. 71, which also indicates the mode of repairing. This consists in demolishing the original arch, and rebuilding it with stone voussoirs inserted in place of the brick in which the rupture occurred. GERMAN METHOD 145 CHAPTER XIV. THE GERMAN METHOD OF EXCAVATING TUNNELS THROUGH SOFT GROUND; BALTIMORE BELT LINE TUNNEL. THE German method of tunneling was first used in 1803 in constructing the St. Quentin Canal. In 1837 the Konigs- dorf tunnel of the Cologne and Aix la Chapelle R.R. was excavated by the same method. The success of the method in these two difficult pieces of soft-ground tunneling led to its extensive adoption throughout Germany, and for this reason it gradually came to be designated as the German method. Briefly explained the method consists in excavating first an annular gallery in which the side walls and roof arch are built complete before taking out the center core and building the invert. Excavation. The excavation of tunnels by the German method is begun either by driving two bottom side drifts or by driving a center top heading. Fig. 73 shows the mode of FIGS. 72 and 73. Diagrams Showing Sequence of Excavation in German Method of Tunneling. procedure when bottom side drifts are used to start the work. The two side drifts Xo. 1 are made from 7 ft. to 8 ft. wide, and about one-third the total height of the full section ; the 146 TUNNELING width of each heading has to be sufficient for the construction of the masonry and strutting, and for the passage of narrow spoil cars alongside them. These drifts are increased in height to the springing line of the arch by taking out the two drifts No. 2. Next the top center heading No. 3 is driven, and finally the two haunch headings No. 4 are excavated. The center core No. 5 is utilized to support the strutting until the side walls and roof arch are completed, when it is broken down and removed. In case of very loose material, where the first side drifts cannot be carried as high as one-third the height of the section, it is the common practice to make them about one-fourth the height, and to take out the side portions of the annular gallery in three parts, as shown by Fig. 73. The top center heading plan of com- mencing the excavation is usually em- ployed in firm materials or when a vein of water is encountered in the upper part of the section. In the latter contingency a small bottom drift A., Fig. 74, is first FIG. 74. -Diagram show- (J r i ven to serve as a drain; but in any ing Sequence of Excava- tions in water Bearing case the excavation proper of the tunnel Material, German , . ~ i , i Method . consists in first driving the center top heading No. 1, and then by working both ways along the profile parts, Nos. 2, 3, 4, and 5 are removed. Part No. 6 is left to support the strutting until the side walls and roof arch are built, when it is also excavated. Strutting. - When the excavation is begun by bottom side drifts these drifts are 'strutted by erecting vertical posts close against the sides of the drift and placing a cap-piece trans- versely across the roof of the drift. The side posts are usually supported by sills placed across the bottom of the drift. These frameworks of posts, cap, and sill are erected at short intervals, and the roof, and, if necessary, the sides of the drift between them, are sustained by means of longitudinal poling- GERMAN METHOD 147 boards extending from one frame to the next. The cap-pieces of the strutting for the bottom drifts serve as sills for the exactly similar strutting of the heading next above. To sup- port the additional weight, and to allow the construction the side walls, the strutting of the bottom drifts is strengthened by inserting an intermediate post between the original side posts of each frame. These intermediate posts are not inserted at the center of the frames or bents, but close to the wall masonry line as shown by Fig. 75. This eccentric position of the post FIG. 75. Sketch Showing Work of Ex- cavating and Timbering Drifts and Headings. FIG. 76. Sketch Showing Method of Roof Strutting. avoids an} r interference with the hauling, and also allows the removal of the adjacent side post when the masonry is constructed. Two methods of strutting the soffit of the excavation are employed, one being a modification of the longitudinal system employed in the English method of tunneling described in a succeeding chapter, and the other a modification of the Belgian system previously described. Fig. 76 shows the method of m ploying the radial strutting of the Belgian system. At the beginning the center top heading is strutted with rectangular bents such as are employed for strutting the drifts. As this heading is enlarged by taking out the haunch sections, radial posts are inserted, as shown by Fig. 76, which also indicates 148 TUNNELING the method of strutting the side trenches when the excavation is carried downward from the center top heading instead of upward from bottom side drifts. Masonry. Whatever plan of excavation or strutting is employed, the construction of the masonry lining in the German method of tunneling begins at the foundations of the side walls and is carried upward to the roof arch. The invert, if one is required, is built after the center core of earth is removed. Centering. - - Tunnel centers are generally employed in the German method of tunneling, a common construction being shown by Fig. 77. It is essen- tially a queen-post truss, the tie beam of which rests on a transverse sill as shown by the illustration. The transverse sill is supported along its central portion by the unexcavated center core of earth, and at its ends either directly on the vertical posts or on longitudi- nal beams resting on these posts. The diagonal members of the queen-post truss form the bottom chords of small king-post trusses which are employed to build out the exterior member of the center to a closer approximation to the curve of the arch. Hauling. When the bottom side drift plan of excavation is employed, the spoil from the front of the drift is removed in narrow-gauge cars running on a track laid as close as practicable to the center core. These same cars are also employed to take the spoil from the drifts above, through holes left in the ceiling strutting of the bottom drifts. The spoil from the soffit sec- tions may be removed by the same car lines used in excavating the drifts, or a narrow-gauge track may be laid on the top of the center core for this special purpose. In the latter case the soffit tracks are usually connected by means of inclined planes with PlG. 77. Sketch Showing Roof Arch Centers and Arch Construction. GERMAN METHOD 149 the tracks on the bottoms of the side drifts. Generally, how- ever, the separate soffit car line is not used unless the material is of such a firm character that the headings and drifts can be carried a great distance ahead of the masonry work. With the center top heading plan of beginning the excavation, the car track has, of course, to be laid on the top of the center core. The center core itself is removed by means of car tracks along the floor of the completed tunnel. Advantages and Disadvantages. Like the Belgian method of tunneling, the German method has its advantages and dis- advantages. Since the excavation consists at first of a narrow annular gallery only, the equilibrium of the earth is not greatly disturbed, and the strutting does not need to be so heavy as in methods where the opening is much larger. The undisturbed center core also furnishes an excellent support for the strutting, and for the centers upon which the roof arches are built. Another important advantage of the method is that the con- struction of the masonry lining is begun logically at the bottom, and progresses upward, and a more homogeneous and stable construction is possible. The great disadvantage of the method is the small space in which the hauling has to be done. The spoil cars practically fill the narrow drifts in passing to and from the front, and interfere greatly with the work of the carpenters and masons. Another objection to the method is that the invert is the very last portion of the lining to be built. This may not be a serious objection in reasonably compact and stable materials, but in very loose soils there is always the danger of the side Avails being squeezed together before the invert masonry is in position to hold them apart. Altogether the difficulties are of a character which tend to increase the expense of the method, and this is the reason why to-day it is seldom used even in the country where it was first developed, and for some time extensively employed. For repairing accidents, such as the caving in of completed tunnels, the German method of tun- neling is frequently used, because of the ease with which the 150 TUNNELING timbering is accomplished. In such cases the cost of the method used cuts a small figure, so long as it is safe and expeditious. BALTIMORE BELT LINE TUNNEL. The Baltimore Belt Ry. Co. was organized in 1890 by officials of the Baltimore & Ohio, and Western Maryland rail- ways, and Baltimore Capitalists, to build 7 miles of double track railway, mostly within the city limits of Baltimore. This rail- way was partly open cut and embankment, and partly tunnel,, and its object was to afford the companies named facilities for reaching the center of the city with their passengers and freight. To carry out the work the Maryland Construction Co. was organized by the parties interested, and in September, 1890, this company let the contract for construction to Rayan & McDon- ald of Baltimore, Md. The chief difficulties of the work cen- tered in the construction of the Howard-street tunnel, 8,350 ft. long, running underneath the principal business section of the city. Material Penetrated. The soil penetrated by the tunnel was of almost all kinds and consistencies, but was chiefly sand of varying degrees of fineness penetrated by seams of loam, clay, and gravel. Some of the clay was so hard and tough that it could not be removed except by blasting. Rock was also found in a few places. For the most part, however, the work was through soft ground, furnishing more or less water, which necessitated unusual precautions to avoid the settling of the street, and consequent damage to the buildings along the line. A large quantity of water was encountered. Generally this water could be removed by drainage and pumps, and the earth be prevented from washing in by packing the space between the timbering with hay or other materials. At points where the inflow was greatest, and the earth was washed in despite the hay packing, the method was adopted of driving 6-in. per- GERMAN METHOD 151 forated pipes into the sides of the excavation, and forcing cement grout through them into the soil to solidify it These pipes penetrated the ground about 10 ft., and the method proved very efficient in preventing the inflow of water? Excavation. The excavation was carried out according to the German method of tunneling. Bottom side drifts were first driven, and then heightened to the springing line of the roof arch. Next a center top heading was driven, and the haunch sections taken out. The object of beginning the exca- vations by bottom side drifts, was to drain the soil of the upper part of the section. The center core was removed after the side walls and roof arch were completed, its removal being kept from 50 ft. to 75 ft. to the rear of the advanced heading. The dimensions of the side drifts proper were about 8x8 ft., but they were often carried down much below the floor level to secure a solid foundation bed for the side walls. Strntting. The side drifts were strutted by means of frames composed of two batter posts resting on boards, and having a cap-piece extending transversely across the roof of the drift. These frames were spaced about 4 ft. apart. The excavation was advanced in the usual way by driving poling-boards at the top and sides, with a slight outward and upward inclination, so that the next frame could be easily inserted with additional space enough between it and the sheeting to permit the next set of poling-boards to be inserted. These poling-boards were driven as close together as practicable so as to prevent as much as possible the inflow of water and earth. The center top heading was strutted in the same manner as were the side drifts. The arrangement of the strutting em- ployed in enlarging the center top heading is shown clearly by Fig. 78, which also shows the manner of strutting the side drifts and face of the excavation, and of building the masonry. Centers Both wood and iron centers were employed in building the roof arch. The timber centering was constructed 152 TUXXEL1XG of square timbers, as shown by Fig. 79. This construction of the iron centers is shown by Fig. 80. Each of the iron centers consisted of two 6 x 6 in. angles butted together, and bent into the form of an arch rib. Six of these ribs were set up 4 ft. apart. They were made of two half ribs butted together at the crown, and were held erect and the proper distance apart by PIG. 78. Sketch Showing Method of Excavating and Strutting Baltimore Belt Line Tunnel. spacing rods. The rearmost rib was held fast to the completed arch masonry, and in turn supported the forward ribs while the lagging was being placed. Masonry. The side walls of the lining were built first in the bottom side drifts, as shown by Fig. 78. They were gen- erally placed on a foundation of concrete, from 1 ft. to 2 ft. thick. As a rule the side walls were not built more than 20 ft. in advance of the arch, but occasionally this distance was increased to as much as 90 ft. The roof arch consisted ordina- , GERMAN METHOD - 153 rily of five rings of brick, but at some places in especially un- stable soil eight rings of brick were emplo}*ed. The arch was built in concentric sections about 18 ft. in length. All the timber of the strutting above the arch and outside of the side walls was left in place, and the voids were filled with rubble masonry laid in cement mortar. It required about 125 mason FIG. 79. Roof Arch Construction with Timber Centers, Baltimore Belt Line Tunnel. hours to build an 18-ft. arch section. Figs. 79 and 80 show various details of the masonry arch work. Owing to the very unstable character of the soil, consider- able difficulty was experienced in building the masonry invert. The process adopted was as follows; Two parallel 12 -i- 12 in. timbers were first placed transversely across the tunnel, abutting against longitudinal timbers or wedges resting against the side walls. Short sheet piles were then driven into the tunnel 154 TUNNELING bottom outside of these timbers, forming an inclosure similar to a cofferdam, from which the earth could be excavated with- out disturbing the surrounding ground. The earth being excavated, a layer of concrete 8 ins. thick was placed, and the brick masonry invert constructed on it. In less stable ground each of the above described cofferdams was subdivided by transverse timbers and sheet piling into three smaller coffer- dams. Here the masonry of the middle section was first con- structed, and then the side sections built. Where the ground PIG. 80. Roof Arch Construction with Iron Centers, Baltimore Belt Line Tunnel. was worst, still more care was necessary, and the bottom had to be covered with a sheeting of l -in. plank held down by struts abutting against the large transverse timbers. The invert masonry was constructed on this sheeting. Refuge niches 9 ft. high, 3 ft. wide, and 15 ins. deep were built in the side walls. Accidents. In this tunnel, owing to the quick striking of the centers, it was found that the masonry lining flattened at the crown and bulged at the sides. This was attributed to the insufficient time allowed for the mortar to set in the rubble GERMAN METHOD 155 filling. Earth packing was tried, but gave still worse results. Finally dry rubble rilling was adopted, with satisfactory results. There was necessarily some sinking of the surface. This, re- sulted partly from the necessity of changing and removing of the timbers, and from the compression and springing of the timbers under the great pressures. The crown of the arch also settled from 2 ins. to 6 ins., due to the compression of the mortar in the joints. The maximum sinking of the surface of the street over the tunnel was about 18 ins.; it usually ran from 1 to 12 ins. Some damage was done to the water and gas mains. This damage was not usually serious, but it of course necessitated immediate repairs, and in some instances it was found best to reconstruct the mains for some distance. At one point along the tunnel where very treacherous material was found, the surface settlement caused the collapse of an adjacent building, and necessitated its reconstruction. 156 TUNNELING CHAPTER XV. THE FULL SECTION METHOD CF TUNNELING: ENGLISH METHOD ; AUSTRIAN METHOD. ENGLISH METHOD. THE English method of tunneling through soft ground, as its name implies, originated in England, where, owing to the general prevalence of comparatively firm chalks, clays, shales, and sandstones, it has gained unusual popularity. The dis- tinctive characteristics of the method are the excavation of the full section of the tunnel at once, the use of longitudinal strut- ting, and the alternate execution of the masonry work and excavation. In America the method is generally designated as the longitudinal bar method, owing to the mode of strutting, which has gained particular favor in America, and is commonly employed there even when the mode of excavation is distinc- tively German or Belgian in other respects. Excavation. Although, as stated above, the distinctive characteristic of the English method is the excavation of the full section at once, the digging is usually started by driving a small heading or drift to locate and establish the axis of the tunnel, and to facilitate drainage in wet ground. These ad- vance galleries may be driven either in the upper or in the lower part of the section, as the local conditions and choice of the engineer dictate. Whether the advance gallery is located at the top or at the bottom of the section makes no difference in the mode of enlarging the profile. This work always begins at the upper part of the section. A center top heading is driven and strutted by erecting posts carrying longitudinal bars supporting transverse poling-boards. This heading is imme- THE FULL SECTION METHOD 157 tion in English Method of Tunneling. diately widened by digging away the earth at each side, and by strutting the opening by temporary posts resting on blocking, and carrying longitudinal bars supporting poling-boards. This process of widening is continued in this manner until thv full roof section, No. 1, Fig. 81, is opened, when a heavy transverse sill is laid, and permanent struts are erected from it to the longitudinal bars, the temporary posts and blocking being removed. The excavation of part No. 2 then begins by opening a center trench and widening it on each side, temporary posts being erected to support the sill above. As soon as part No. 2 is fully ex- cavated, a second transverse sill is placed Fm - si. -Diagram Show mg Sequence of Excava- below the first, and struts are , placed between them. The excavation of part No. 3 is carried out in exactly the same manner as was part No. 2. The lengths of the various sections, Nos. 1, 2, and 3, generally run from 12 ft. to 20 ft., depending upon the character of the soil. Strutting The strutting in the English method of tunnel- ing consists of a transverse framework set close to the face of the excavation, which supports one end of the longitudinal crown bars, the other ends of which rest on the completed lining. The transverse framework is composed of three hori- zontal sills arranged and supported as shown by Fig. 82. The bottom sill A is carried by vertical posts resting on blocking on the floor of the excavation. From the bottom sill vertical struts rise to support the middle sill B. The top sill, or miners' sill (7, is carried by vertical posts or struts rising from the middle sill B. The vertical struts are usually round timbers from 6 ins. to 8 ins. in diameter ; and the sills are square tim- bers of sufficient section to carry the vertical loads, and gener- ally made up of two posts scarf-jointed and butted to permit them to be more easily handled. In firm soils the struts be- 158 TUNNELING tween the sills are all set vertically, but those at the extreme sides of the roof section are inclined. In loose soils, however, where the sides of the excavation must be shored, the V- bracing shown by Fig. 82 is employed between one or more pairs of sills as the conditions necessitate. The manner of holding the transverse framework upright is explained quite clearly by Fig. 83 ; inclined props extending from the com- pleted masonry to the sills of the framework being employed. Two props are used to each sill. Sometimes, in addition to the FIGS. 82 and 83. Sketches Showing Construction of Strutting, English Method. props shown, another nearly horizontal prop extends from the crown of the arch masonry to the middle piece of the strutting. Referring to Fig. 83, it will be observed that the longitudinal crown bars are above the extrados of the roof arch. When, therefore, the lining masonry has been completed close up to the transverse framework, the latter is removed, leaving the crown bars resting on the arch masonry ; and excavation, which has been stopped while the masonry was being laid, is continued for another 12 ft. to 20 ft, and the transverse framework is erected at the face, and braced or propped against the completed lining as shown by Fig. 83. The next step is to place the THE FULL SECTION METHOD 159 crown bars, and this is done by pulling them ahead from their original position over the masonry of the completed section of the roof arch. It will be understood that the crown bars are not pulled ahead their full length at one operation, bvt are advanced by successive short movements as the excavation progresses, their outer ends being supported by temporary posts until the transverse framework is built at the face of the excavation. Centers Two standard forms of centers are employed in the English method of tunneling, as shown by Figs. 84 and 85. Both consist of an outer portion, constructed much like a typical plank center, which is strengthened against distortion by an interior truss framework. The elemental members of FIGS. 84 and 85. Sketches of Typical Timber Roof-Arch Centers, English Method. this truss framework take the form of a queen-post truss, as is shown more particularly by Fig. 84. In Fig. 85 the queen- post truss construction is less easily distinguished, owing to the cutting of the bottom tie-beam and other modifications, but it can still be observed. The possibility of cutting the tie-beam as shown in Fig. 85, without danger, is due to the fact that the lateral pressures on the haunches of the center counteract the tendency of the center to flatten under load, which is usually counteracted by the tie-beam alone. The object of cutting the tie-beam is to afford room for the props running from the completed masonry to the transverse framework of the strutting as shown by Fig. 83. Generally four or five centers are used for each length of arch built. They are set up so that the tie-beams rest on 160 TUNNELING double opposite wedges carried by a transverse beam below. This transverse beam in turn rests on another transverse beam which is supported by posts carried on blocking on the invert masonry. It is usually made with a butted joint at the middle to permit its removal, since it is so long that the masonry has to be built around its extreme ends. The lagging is of the usual form, and rests on the exterior edges of the curved upper member of the centers. Masonry. In the English method of tunneling, the masonry begins with the construction of the invert, and proceeds to the crown of the arch. The lining is built in lengths, or successive rings, corresponding to the length of excavation, which, as pre- viously stated, is from 12 ft. to 20 ft. Each ring or length of lining terminates close to the transverse strutting frame erected at the face of the excavation. Work is first begun on the invert at the point where the preceding ring of masonry ends, and is continued to the transverse strutting frame at the front of the excavation. As fast as the invert is completed, work is begun on the side walls. In very loose soils the longitudinal bars supporting the sides of the excavation are removed after the side walls are built ; but in firmer soils they may be taken out one by one just ahead of the masonry, or in very firm soils it may be possible to remove them entirely before beginning the side walls. In all cases it is necessary to fill the space between the masonry and the walls of the excavation with rip- rap or earth. To build the roof arch the centers are first erected as described above, and the crown bars are removed as previously described by putting them ahead after the arch ring is completed. As with the side walls, the vacant space be- tween the arch ring and the roof of the excavation must be filled in. Usually earth or small stones are used for filling ; but in very loose soils it is sometimes the practice not to remove the poling-boards, but to support them by short brick pillars resting on the arch ring and then to fill around these pillars. THE FULL SECTION METHOD 161 Hauling. To haul away the material and take in supplies, tracks are laid on the invert masonry. Generally the perma- nent tracks are laid as fast as the lining is completed. A short section of temporary track is used to extend this permanent track close to the work. Advantages and Disadvantages. The great advantage of the English method of tunneling is that the masonry lining is built in one piece from the foundations to the crown, making possible a strong, homogeneous construction. It also pos- sesses a decided advantage because of the simple methods of hauling which are possible : there being no differences of level to surmount, no hoisting of cars nor trans-shipments of loads are necessary. The chief disadvantage of the method is that the excavators and masons work alternately, thus making the progress of the work slower perhaps than in any other method of tunneling commonly employed under similar conditions. This disadvantage is overcome to a considerable extent when the tunnel is excavated by shafts, and the work at the different headings is so arranged that the masons or excavators when freed from duty at one heading may be transferred to another where excavation or lining is to .be done as the case may be. Another disadvantage of the English method arises from the excavation of the full section at once, which in unstable soils necessitates strong and careful strutting, and increases the danger of caving. The fact also that the arch ring has to carry the weight of the crown bars, and their loading at one end while the masonry is green, increases the chances of the arch being distorted. Conclusion. The English method of tunneling in its entirety is confined in actual practice pretty closely to the country from which it receives its name. A possible extension of its use more generally is considered by many as likely to follow the development of a successful excavating machine for soft material. The space afforded by the opening of the full sec- tion at once, especially adapts the method to the use of exca- 162 TUNNELING vators like, for example, the endless chain bucket excavator used on the Central London Ry., and illustrated in Fig. 12. The method also furnishes an excellent opportunity for electric hauling and lighting during construction. The English method of tunneling has been used in building the Hoosac, Musconetcong, Allegheny, Baltimore and Potomac, and other tunnels in America. The names of the European tunnels built by this method are too numerous to mention here. AUSTRIAN METHOD. The Austrian full-section method of tunneling through soft ground was first used in constructing the Oberau tunnel on the Leipsic and Dresden R.R., in Austria in 1837. It consists in excavating the full section and building up the lining masonry from the foundations as in the English, but with the important exception that the invert is built last instead of first in all cases except where the presence of very loose soil requires its con- struction first. A still more important difference in the two methods is that the excavation is carried out in smaller sections and is continuous in the Austrian method instead of alternating with the mason work as it does in the English method. FlGS. 86 and 87. Diagrams Showing Sequence of Excavation in Austrian Method of Tunneling. Excavation. The excavation in the Austrian method begins by driving the bottom center drift No. 1, Fig. 86, rising from the floor of the tunnel section nearly to the height of the THE FULL SECTION METHOD 163 springing lines of the roof arch. When this drift has been driven ahead a distance varying from 12 ft. to 20 ft. or some- times more, the excavation of the center top heading No. 2 is driven for the same distance. The next operation is to retiove part No. 3, thus forming a central passage the full depth of the tunnel section at the center. This trench is enlarged by removing parts Nos. 4, 5, 6, 7, and 8 in the order named until the full section is opened. A modification of this plan of excavation is shown by Fig. 87 which is used in firm soils. Strutting. Each part of the section is strutted as fast as it is excavated. The center bottom drift first excavated is strutted by laying a transverse sill across the floor, raising two side posts from it, and capping them with a transverse timber having its ends projecting beyond the side posts and halved as shown by Fig. 88. The top center heading No. 2, which is next excavated, is strutted by means of two side posts resting on blocking and carrying a transverse cap as also shown by Fig. 88. Sometimes the side posts in the heading strutting- frames are also carried on a transverse sill as are those of the bottom drift. This construction is usually adopted in loose soils. When the sill is employed, the middle part, No. 3, is strutted by inserting side posts between the bottom of the top sill and the cap of the frame in the drift below. When, how- ever, the posts of the top heading frame are carried on blocking, it is the practice to replace them with long posts rising from the cap of the bottom drift frame to the cap of the top heading frame. Further, when the intermediate sill is employed at the bottom level of the top heading it projects beyond the side posts and has its ends halved. After the completion of the center trench strutting the next task is to strut parts Nos. 4 and 5. This is done by continuing the upper sill by means of a timber having one end halved to join with the projecting end of the sill in position. This ex- tension timber is shown at a, Fig. 89. The next operation is to place the timber 5, having one end resting on the cap-piece 164 TUNNELING of the top heading frame and the other beveled and resting on the top of the sill a near the end. The timber b is laid tangent to the curve of the roof arch, and to support it against flexure the strut c is inserted as shown. To support the thrust of this strut the additional post d is inserted and the original bot- tom heading frame is rein- forced as shown. The next step is to insert the strut e, and when this and the previ- ous construction are dupli- cated on the opposite side of the tunnel section we have the strutting of the parts Nos. 1 to 5, inclusive, complete. Part No. 6 is then removed and strutted by extending the bottom drift cap-piece by a timber similar to timber a above, and then by inserting a side strut between the outer ends of these two timbers, as indicated by Fig. 90. As the final parts, Nos. 7 and 8, are removed, the inclined prop a, Fig. 90, is inserted as shown. When the soil FlGS. 88 to 90. Sketches Showing Construc- tion of Strutting, Austrian Method. THE FULL SECTION METHOD 165 is loose some of the members of the framework are doubled and additional bracing is introduced as shown by Fig. 90. The frames just described are placed at intervals of about 4 ft. along the excavation, and are braced apart by horizontal struts. Some of the longitudinal bearing beams, as at 5, Fig. 90, also extend through two or three frames, and help to tie them together. Finally, the longitudinal poling-boards extend- ing from one frame to the next along the walls of the excava- tion serve to connect them together. The short transverse beam e, Fig. 90, located just above the floor of the invert, serves to carry the planking upon which the train car tracks are laid. Besides the timber strutting peculiar to the Austrian method, the Rziha iron strutting described in a previous chapter is frequently used in tunneling by the Austrian process. Centers. The two forms of centers used in the English method of tunneling are also used in the Austrian method. One of the methods of support- ing these centers is shown by Fig. 91. The tie-beam of the tenter rests on longitudinal tim- bers carried by the strutting frames and intermediate props. In single-track tunnels it is the frequent practice also to carry the ends of the tie-beams in re- cesses left in the side wall ma- sonry, with intermediate props inserted to prevent flexure at the center. When the Rziha iron strutting is employed, it also serves for the centering upon which the arch masonry is built. Masonry. In the Austrian system of tunneling, the lining is built from the foundations of the side walls upward to the crown of the roof arch in lengths in consecutive rings equal to FIG. 91. Sketch Showing Manner of Constructing the Lining Masonry, Austrian Method. 166 TUNNELING the lengths of the consecutive openings of the full section, or from 12 ft. to 20 ft. long. Except in infrequent cases in very loose materials the invert is the last part of the masonry to be built, since to build it first requires the removal of the strutting which cannot easily or safely be accomplished until the side walls and roof arch are completed. As the side wall foundations are built, however, their interior faces are left inclined, as shown by Figs. 90 and 91, ready for the insertion of the invert, and are meanwhile kept from sliding inward by the insertion of blocking between them and the bottom of the strutting. Fig. 91 shows the nature of this blocking, and also the manner in which the side wall and roof arch masonry is carried upward. Finally when the roof arch is keyed and the centers are struck, the strutting is taken down and the invert is built. Advantages and Disadvantages. The principal advantages claimed for the Austrian method of tunneling are : (1) The excavation being conducted by driving a large number of con- secutive small galleries, which are immediately strutted, there is little disturbance of the surrounding material ; (2) the polygonal type of strutting adopted is easily erected and of great strength against symmetrical pressures ; (3) the masonry, being built from the foundations up, is a single homogeneous structure, and is thus better able to withstand dangerous pres- sures ; (4) the excavation is so conducted that the masons and excavators do not interfere, and both can work at the same time. The disadvantages which the method possesses are : (1) The strutting, while very strong under symmetrical pressures, either vertical or lateral, is distorted easily by unsymmetrical vertical or lateral pressures, and by pressure in the direction of the axis of the tunnel; (2) the construction of the invert last exposes the side walls to the danger of being squeezed together, causing a rotation of the arch of the nature discussed in de- scribing the Belgian method of tunneling. SPECIAL TKEACHEKOUS GilOUXD METHOD 167 CHAPTER XVI. SPECIAL TREACHEROUS GROUND METHOD; ITALIAN METHOD; QUICKSAND TUN- NELING; PILOT METHOD. ITALIAN METHOD. THE Italian method of tunneling was first employed in con- structing the Cristina tunnel on the Foggia & Benevento R.R. in Italy. This tunnel penetrated a laminated clay of the most treacherous character, and after various other soft-ground methods of tunneling had been tried and had failed, Mr. Procke, the engineer, devised and used successfully the method which is now known as the Italian or Cristina method. The Italian method is essentially a treacherous soil method. It consists in excavating the bottom half of the section by means of several successive drifts, and building the invert and side walls ; the space is then refilled and the upper half of the section is exca- vated, and the remainder of the side walls and the roof arch are built ; finally, the earth filling in the lower half of the section is re-excavated and the tunnel completed. The method is an expensive one, but it has proved remarkably successful in treacherous soils such as those of the Apennine Mountains, in which some of the most notable Italian tunnels are located. It is, moreover, a single-track tunnel method, since any soil which is so treacherous as to warrant its use is too treacherous to permit an opening to be excavated of sufficient size for a double-track railway, except by the use of shields. Excavation. The plan of excavation in the Italian method is shown by the diagram Fig. 92. Work is begun by driving 168 TUNNELING the center bottom heading No. 1, and this is widened by taking out parts No. 2. Finally part No. 3 is removed, and the lower half of the section is open. As soon as the invert and side wall masonry has been built in this excavation, parts No. 2 are filled in again with earth. The exca- vation of the center top heading No. 4 is then begun, and is enlarged by removing the earth of part No. 5. The faces of this last part are inclined so as to reduce their tendency to slide, and to permit of a greater number of radial struts to be placed. Next, parts No. 6 are excavated, and when this is done the entire section, except for the thin strip No. 7, has been opened. At the ends of part No. 7 nar- sunk to reach the tops of the side walls in the lower half of the section. The FIG. 92. Diagram Show- ing Sequence of Excava- tion in Italian Method of Tunneling. row trenches are already constructed masonry is then completed for the upper half of the section, and part No. 7 and the filling in parts No. 2 are removed. The various drifts and headings and ^ -^ the parts excavated to enlarge them N X are seldom excavated more than from / \ 6 ft. to 10 ft. ahead of the lining. Strutting. - - The bottom center drift, which is first driven, is strutted by means of frames consisting of side posts resting on floor blocks and car- rying a cap-piece. Poling-boards are placed around the walls, stretching from one frame to the next. As soon as the invert is sufficiently completed to permit it, the side posts of the strutting frames are replaced by short struts resting on the invert masonry as shown by Fig. 93. To permit the old side posts to be removed and the new shorter ones to be inserted, the cap-piece of the frame is temporarily supported FIG. 93. Sketch Showing Strut- ting for Lower Part of Section. SPECIAL TREACHEROUS GROUND METHOD 169 by inclined props arranged as shown by Fig. 97. When parts No. 2 are excavated the roof is strutted by inserting the trans- verse caps a, Fig. 93, the outer ends of which are carriedjby the system of struts >, c, J, and e. The longitudinal poling-boards supporting the ceiling and walls are held in place by the cap a and the side timber e. To stiffen the frames longitudinally of the tunnel, horizontal longitudinal struts are inserted between them. The excavation of the upper half of the tunnel section is strutted as in the Belgian method, with radial struts carrying longitudinal roof bars and transverse poling-boards. On ac- count of the enormous pressures developed by the treacherous soils in which only is the Italian method employed, the radial strutting frames and crown bars must be of great strength, FIGS. 94 and 95. Sketches Showing Construction of Centers, Italian Method. while the successive frames must be placed at frequent intervals, usually not more than 3 ft. After the masonry side walls have been built in the lower part of the excavation, longitudinal planks are laid against the side posts of the center bottom drift frames, to form an enclosure for the filling-in of parts No. 2. The object of this filling is principally to prevent the squeezing-in of the side walls. Centers. Owing to the great pressures to be resisted in the treacherous soils in which the Italian method is used, the con- struction of the centers has to be very strong and rigid. Figs. 94 and 95 show two common types of center construction used with this method. The construction shown in Fig. 94 is a strong one where only pressures normal to the axis of the tunnel have to be withstood, but it is likely to twist under 170 TUNNELING pressures parallel to the axis of the tunnel. In the construc- tion shown by Fig. 95, special provision is made to resist pressures normal to the plane of the center or twisting pres- sures, by the strength of the transverse bracing extending hori- zontally across the center. Masonry, The construction of the masonry lining begins with the invert, as indicated by Fig. 93, and is carried up to the roof of parts No. 2, as already indicated, and is then discon- tinued until the upper parts Nos. 4, 5, and 6 are excavated. The next step is to sink side trenches at the ends of part No. 7,. which reach to the top of the completed side walls. This operation leaves the way clear to finish the side walls and to construct the roof arch in the ordinary manner of such work in tunneling. Since this method of tunneling is used only in very soft ground which yields under load, the usual practice is to construct the in- vert and side walls on a continuous no. 96. -sketch showing invert foundation course of concrete as in- and Foundation Masonry, Italian dicated by Fig. 96. The lining is Method. .. . ., . . , usually built in successive rings, and the usual precautions are taken with respect to filling in the voids behind the lining. The thickness of the lining is based upon the figures for laminated clay of the third variety given in Table II. Hauling The system of hauling adopted with this method of tunneling is very simple, since the excavation of the various parts is driven only from 6 ft. to 10 ft. ahead, and the work pro- gresses slowly to allow for the construction of the heavy strutting required. To take away the material from the center bottom drift, narrow-gauge tracks carried by cross-beams between the side posts above the floor line are employed. This same narrow-gauge line is employed to take away a portion of parts No. 2, the remaining portion being left and used for the refill- ing after the bottom portion of the lining has been built, as SPECIAL TREACHEROUS GROUND METHOD 171 previously described. The upper half of the section being ex- cavated, as in the Belgian method, the system of hauling with inclined planes to the tunnel floor below, which is a character- istic of that method, may be employed. It is the more usual FIG. 97. Sketch Showing Longitudinal Section of a Tunnel under Construction, Italian Method. practice, however, since the excavation is carried so little a dis- tance ahead and progresses so slowly, to handle the spoil from the upper part of the section by wheelbarrows which dump it into the cars running on the tunnel floor below. Hand labor is also used to raise the construction ma- terials used in excavating the upper sec- tion. The tracks on the tunnel floor, besides extending to the front of the ad- vanced bottom center drift, have right and left switches to be employed in removing the refilling in parts No. 2, the spoil from the upper part of the section, and the material of part No. 7. Fig. 97 is a longi- tudinal section showing the plan of exca- vation and strutting adopted with the Italian method. Modifications. It often happens that the filling placed be- tween the side walls and the planking, which is practically the space comprised by parts No. 2, is not sufficient to resist the inward pressure of the walls, and they tip inward. In these cases a common expedient is to substitute for the earth filling 98. Sketch Showing Sequence of Excavation, Stazza Tunnel. 172 TUNNELING a temporary masonry arch sprung between the side walls with its feet near the bottom of the walls, and its crown, just below the level of their tops, as shown by Fig. 101. This construction was employed in the Stazza tunnel in Italy. In this tunnel the excavation was begun by driving the center drift, No. 1, Fig. 98, and immedi- ately strutting it as shown by Fig. 99. The other parts, Nos. 2 and 3, completing the lower portion of the section, were then taken out and strutted. While part No. 2 PIG. 99. sketch stowing was being excavated at the bottom, and Drif^sta f zza r T^nei Fir ' tne center part of the invert built, the longitudinal crown bars carrying the roof of the excavation were carried temporarily by the inclined props shown by Fig. 100. After completing the invert and the side walls to a height of 2 or 3 ft., a thick masonry arch was sprung between the side walls, as shown in transverse section by Fig. 101, and in longitudinal section by Fig. 100. This arch braced the side walls against tipping inward, and FIGS. 100 and 101. Sketches Showing Temporary Strutting Arch Construction, Stazza Tunnel. carried short struts to support the crown bars. The haunches of the arch were also filled in with rammed earth. The upper half of the section was excavated, strutted, and lined as in the standard Italian method previously described. When the lining was completed, the arch inserted between the side walls was broken down and removed. SPECIAL TREACHEROUS GROUND METHOD 17$ Advantages and Disadvantages. The great advantage claimed for the Italian method of tunneling is that it is built in two- separate parts, each of which is separately excavated, strutted, and lined, and thus can be employed successfully in very treacherous soils. Its chief disadvantage is its excessive cost, which limits its use to tunnels through treacherous soils where other methods of timbering cannot be used. QUICKSAND TUTOELING. When an underground stream of water passes with force through a bed of sand it produces the phenomenon known a& quicksand. This phenomenon is due to the fineness of the particles of sand and to the force of the water, and its activity is directly proportional to them. When sand is confined it furnishes a good foundation bed, since it is practically incom- pressible. To work successfully in quicksand, therefore, it is necessary to drain it and to confine the particles of sand so that they cannot flow away with the water. This observation suggests the mode of procedure adopted in excavating tunnels through quicksand, which is to drain the tunnel section by opening a gallery at its bottom to collect and carry away the water, and to prevent the movement or flowing of the sand by strutting the sides of the excavation with a tight planking. The sand having to be drained and confined as described, the ordinary methods of soft-ground tunneling must be employed, with the following modifications : (1) The first work to be performed is to open a bottom gallery to drain the tunnel. This gallery should be lined with boards laid close and braced sufficiently by interior frames to prevent distortion of the lining. The interstices or seams be- tween the lining boards snould be packed with straw so as to permit the percolation of water and } r et prevent the movement of the sand. (2) As fast as the excavation progresses its walls should 174 TUNNELING be strutted by planks laid close, and held in position by interior framework; the seams between the plank should be packed with straw. (8) The masonry Lining should be built in successive rings, and the work so arranged that the water seeping in at the sides and roof is collected and removed from the tunnel immediately. Excavation. The best and most commonly employed method of driving tunnels through quicksand is a modification of the Belgian method. At first sight it may appear a hazardous work to support the roof arch, as is the characteristic of this method, on the unexcavated soil below, when this soil is quicksand, but if the sand is well confined and drained the risk is really not very great. Next to the Belgian method the German method is perhaps the best for tunneling quicksand. In these compari- sons the shield system of tunneling is for the time being left out of consideration. This method will be described in suc- ceeding chapters. Whenever any of the systems of tunneling previously described are employed, the first task is always to open a drainage gallery at the bottom of the section. Assuming the Belgian method is to be the one adopted, the first work is to drive a center bottom drift, the floor of which is at the level of the extrados of the invert. This drift is im- mediately strutted by successive transverse frames made up of a sill, side posts, and a cap which support a close plank strut- ting or lining, with its joints packed with straw. Between the side posts of each cross-frame, at about the height of the intrados of the invert, a cross-beam is placed ; and on these cross- beams a plank flooring is laid, which divides the drift horizon- tally into two sections, as shown by Fig. 102; the lower section forming a covered drain for the seepage water, and the upper providing a passageway for workmen and cars. The bottom drift is driven as far ahead as practicable, in order to drain the sand for as great a distance in advance of the work as possible. After the construction of the bottom drainage drift the excava- tion proper is begun, as it ordinarily is in the Belgian method SPECIAL TREACHEROUS GROUND METHOD 175 FIG. 102. Sketch Showing Preliminary Drainage Gal- leries, Quicksand Method. by driving a top center heading, as shown by Fig. 102. This heading is deepened and widened after the manner usual to the Belgian method, until the top of the sec- tion is open down to the springing lines of the roof arch. To collect the seepage water from the center top heading it is provided with a center bottom drain con- structed like the drain in the bottom drift, as shown by Fig. 102. When the top heading is deepened to the level of the springing lines of the roof arch, its bottom drain is reconstructed at the new level, and serves to drain the full top section opened for the construction of the roof arch. This top drain is usually con- structed to empty into the drain in the bottom drift. Strutting. The method of strutting the bottom drift has already been described. For the remainder of the excavation the regular Belgian method of radial roof strutting-frames is employed, as shown by Fig. 103. Contrary to what might be expected, the number of radial struts required is not usually greater than would be used in many other soils besides quicksand. Single-track railway tun- nels have been constructed through quicksand in several instances where the number of radial props required on each side of the center did not exceed four or five. It is necessary, however, to place the poling-boards very close together, and to pack the joints between them to prevent the inflow of the fine sand. In strutting the lower part of the section it is also necessary to support the sides with tight planking. This is usually held in place by longitudinal FIG. 103. Sketch Showing Con- struction of Roof Strutting Quicksand Method. 176 TUNNELING FIG. 104. Sketch Showing Construc- tion of Masonry Lining, Quicksand Method. bars braced by short struts against the inclined props employed to carry the roof arch when the material on which they origi- nally rested is removed. This side strutting is shown at the right hand of Fig. 104. Masonry. As soon as the upper part of the section has been opened the roof arch is built with its feet resting on planks laid on the unex- cavated material below. This arch is builjb exactly as in the regular Belgian method previously de- scribed, using the same forms of centers and the same methods throughout, except that the poling- boards of the strutting are usually left remaining above the arch masonry. To prevent the possibility of water percolating through the arch masonry, many engineers also advise the plastering of the extrados of the arch with a layer of cement mortar. This plastering is designed to lead the water along the haunches of the arch and down behind the side walls. In constructing the masonry below the roof arch the invert is built first, contrary to the regular Belgian method, and the side walls are carried up on each side from the invert ma- sonry. Seepage holes are left in the invert masonry, and also in the side Avails just above the intrados of the invert. At the center of the invert a culvert or drain is constructed, as shown by Fig. 104, inside the invert masonry. This culvert is com- monly made with an elliptical section with its major axis hori- zontal, and having openings at frequent intervals at its top. The thickness of the lining masonry required in quicksand is shown by Table II. Removing the Seepage Water. After the tunnel is completed the water which seeps in through the weep-holes left in the ma- sonry passes out of the tunnel, following the direction of the SPECIAL, TREACHEROUS GROUND METHOD 177 descending grades. During construction, however, special means will have to be provided for removing the water from the excavation, their character depending upon the method of excavation and upon the grades of the tunnel bottom. When the excavation is carried on from the entrances only, unless the tunnel has a descending grade from the center toward each end, the tunnel floor in one heading will be below the level of the en- trance, or, in other words, the descending grade will be toward the point where work is going on, while at the opposite entrance the grade will be descending from the work. In the latter case the removal of the seepage water is easily accomplished by means of a drainage channel along the bottom of the excavation. In the former case the water which drains toward the front is collected in a sump, and if there is not too great a difference in level between this sump and the entrance, a siphon may be used to remove it. Where the siphon cannot be used, pumps are installed to remove the water. When the tunnel is excavated by shafts the condition of one high and one low front, as com- pared with the level at the shaft, is had at each shaft. Gene- rally, therefore, a sump is constructed at the bottom of the shaft ; the culvert from the high front drains directly to the shaft sump, while the water from the low-front sump is either siphoned or pumped to the shaft sump. From the shaft sump the water is forced up the shaft to the surface by pumps. THE PHOT METHOD. The pilot system of tunneling has been successfully em- ployed in constructing soft-ground sewer tunnels in America by the firm of Anderson & Barr, which controls the patents. The most important work on which the system has been em- ployed is the main relief sewer tunnel built in Brooklyn, N.Y., in 1892. This work comprised 800 ft of circular tunnel 15 ft. in diameter, 4400 ft 14 ft. in diameter, 3200 ft 12 ft. in diameter, and 1000 ft. 10 ft. in diameter, or 9400 ft. of tunnel 178 TUNNELING altogether. The method of construction by the pilot system is as follows : Shafts large enough for the proper conveyance of materials from and into the tunnel are sunk at such places on the line of work as are most convenient for the purpose. From these shafts a small tunnel, technically a pilot, about 6 ft. in diameter, composed of rolled boiler iron riveted to light angle irons on four sides, perforated for bolts, and bent to the required radius of the pilot, is built into the central part of the excavation on the axis of the tunnel. This pilot is generally kept about 30 ft. in advance of the completed excavation, as shown by Fig. 105. The material around the exterior of the pilot is then excavated, using the pilot as a support for braces which radiate from it and Bracing.'" v Arch Constriction. FIG. 105. Sketch Showing Pilot Method of Tunneling. secure in position the plates of the outside shell which holds the sand, gravel, or other material in place until the concentric rings of brick masonry are built. Ribs of T-iron bent to the radius of the interior of the brick work, and supported by the braces radiating from the pilot, are used as centering supports for the masonry. On these ribs narrow lagging-boards are laid as the construction of the arch proceeds, the braces holding the shell plates and the superincumbent mass being removed as the masonry progresses. The key bricks of the arches are placed in position on ingeniously contrived key-boards, about 12 ins. in width, which are fitted into rabbeted lagging-boards one after another as the key bricks are laid in place. After the masonry has been in place at least twenty-four hours, allowing the cement SPECIAL TREACHEROUS GROUND METHOD 179 mortar time to set, the braces, ribs, and lagging which support it are removed. In the meantime the excavation, bracing, pilot, and exterior shell have been carried forward, preparing the way for more masonry. The top plates of the shell are first placed in position, the material being excavated in advance and sup- ported by light poling-boards ; then the side-plates are butted to the top and the adjoining side-plates. In the pilot the plates are united continuously around the perimeter of the circle, while in the exterior shell the plates are used for about one- third of the perimeter on top, unless treacherous material is encountered, when the plates are continued down to the spring- ing lines of the arch. This iron lining is left in place. The bottom is excavated so as to conform to the exterior lines of the masonry. The excavation follows so closely to the outer lines of the normal section of the tunnel that very little loss occurs, even in bad material ; and there is no loss where suffi- cient bond exists in the material to hold it in place until the poling-boards are in position. In the Brooklyn sewer tunnel work, previously mentioned, the pilot was built of steel plates f in. thick, 12 ins. wide, and 37 ins. long, rolled to a radius of 3 ft. Steel angles 4 x 4 ins. were riveted along all four sides of each plate, and the plates were bolted together by f -in. machine-bolts. The plates weighed 136 Ibs. each, and six of them were required to make one com- plete ring 6 ft. in diameter. In bolting them together, iron shims were placed between the horizontal joints to form a footing for the wooden braces for the shell, which radiate from the pilot. The shell plates of the 15-ft. section of the tunnel were of No. 10 steel 12 ins. wide and 37 ins. long, with steel angles 2^ x 2^ x f ins., riveted around the edges the same as for the pilot, and put together with |-in. bolts. These plates weighed 61 Ibs. each, and eighteen of them were required to make one complete ring 15 ft. in diameter. The plates for the 12-ft. section were No. 12 steel 12 ins. wide with 2x 2xi-in. angles. Seventeen plates were required to make a complete ring. 180 TUNNELING CHAPTER XVII. OPEN-CUT TUNNELING METHODS; TUNNELS UNDER CITY STREETS; BOSTON SUBWAY AND NEW YORK RAPID TRANSIT. OPEN-CUT TUNNELING. WHEN a tunnel or rapid-transit subway has to be constructed at a small depth below the surface, the excavation is generally performed more economically by making an open cut than by subterranean tunneling proper. The necessary condition of small depth which makes open-cut tunneling desirable is most generally found in constructing rapid-transit subways or tun- nels under city streets. This fact introduces the chief difficul- ties encountered in such work, since the surface traffic makes it necessary to obstruct the streets as little as possible, and has led to the development of the several special methods commonly employed in performing it. These methods may be classed as follows : (1) The longitudinal trench method, using either a single wide trench or two narrow parallel trenches; (2) the transverse trench method. Single Longitudinal Trench. The simplest manner by which to construct open-cut tunnels is to open a single cut or trench the full width of the tunnel masonry. This trench is strutted by means of side sheetings of vertical planks, held in place by transverse braces extending across the trench and abutting against longitudinal timbers laid against the sheeting plank. The lining is built in this trench, and is then filled around and above with well-rammed earth, after which the surface of the ground is restored. An especial merit of the single longitudi- nal trench method of open-cut tunneling is that it permits the OPEN-CUT TUNNELING METHODS 181 L FIG. 106. Diagram Showing Se- quence of Construction in Open- Cut Tunnels. construction of the lining in a single piece from the bottom up, thus enabling better workmanship and stronger construction than when the separate parts are built at different times. The great objection to the method when B it is used for building subways under city streets is, that it occupies so much room that the street usually has to be closed to regular traffic. For this reason the single longi- tudinal trench method is seldom employed, except in those portions of city subways which pass under public squares or parks where room is plenty. Parallel Longitudinal Trenches. The parallel longitudinal trench method of open-cut tunneling consists in excavating two narrow parallel trenches for the side walls, leaving the center core to be removed after the side walls have been built. The diagram, Fig. 106, shows the sequence of opera- tions in this method. The two trenches No. 1 are first excavated a little wider than the side wall masonry, and strutted as shown by Fig. 107. At the bottoms of these trenches a foundation course of concrete is laid, as shown by Fig. 108, if the ground is soft ; or the masonry is started directly on the natural material, if it is rock. From the foundations the walls are carried up to the level of the springing lines of the roof arch, if an arch is Fio. 107. Sketch Showing Method of Timbering Open- Cut Tunnels, Double Parallel Trench Method. 182 TUNNELING used ; or to the level of its ceiling, if a flat roof is used. After the completion of the side walls, the portion of the excavation shown at No. 2, Fig. 106, is removed a sufficient depth to en- able the roof arch to be built. When the arch is completed, it is filled above with well-ram rned earth, and the surface is re- stored. The excavation of part No. 3 inclosed by the side walls and roof arch is carried on from the entrances and from shafts left at intervals along the line. A modification of the method just described was employed in constructing the Paris underground railways. It consists in excavating a single longitudinal trench along one side of the street, and building the side wall in it as previously described. When this side wall is completed to the roof, the right half of part No. 2, Fig. 106, is excavated to the line AB, and the right- hand half of the roof arch is built. The space above the arch is then refilled and the surface of the street restored, after which the left-hand trench is dug and the side wall and roof-arch masonry is built just as in the opposite half. Generally the work is prosecuted by opening up lengths of trench at considerable intervals along the street and alternately on the left- and right-hand sides. By this method one-half of the street width is everywhere open to traffic, the travel simply passing from one side of the street to the other to avoid the excavation. When the lining has been completed, the center core of earth inclosed by it is removed from the entrances and shafts, leaving the tunnel finished except for the invert and track construction, etc. Transverse Trenches. The transverse trench or " slice " method of open-cut tunneling has been employed in one work, the Boston Subway. This method is described in the specifica- tions for the work prepared by the chief engineer, Mr. H. A. Carson, M. Am. Soc. C. E., as follows : FIG. 108. Side -Wall Foundation Con- struction Open-Cut Tunnels. OPEN-CUT TUNNELING METHODS 183 "Trenches about 12 ft wide shall be excavated across the street to as great a distance and depth as is necessary for the construction of the subway. The top of this excavation shall be bridged during the night by strong beams and timbering, whose upper surface is flush with the surface of the street. These beams shall be used to support the railway tracks as well as the ordinary traffic. In each trench a small portion or slice of the subway shall be constructed. Each slice of the subway thus built is to be properly joined in due time to the contiguous slices. The contractor shall at all times have as many slice- trenches in process of excavation, in process of being filled with masonry, and in process of being back-filled with earth above the completed masonry, as is necessary for the even and steady progress of the work towards completion at the time named in the contract." In regard to the success of this method Mr. Carson, in his fourth annual report on the Boston Subway work, says : " The method was such that the street railway tracks were not disturbed at all, and the whole surface of the street, if de- sired, was left in daytime wholly free for the normal traffic." Tunnels on the Surface. It occasionally happens when filling-in is to take place in the future, or where landslides are liable to bury the tracks, that a railway tunnel has to be built on the surface of the ground. In such cases the construc- tion of the tunnel consists simply in building the lining of the section on the ground surface with just enough excavation to secure the proper grade and foundation. Generally the lining is finished on the outside with a waterproof coating, and is sometimes banked and partly covered with earth to protect the masonry from falling stones and similar shocks from other causes. A recent example of tunnel construction of this char- acter was described in " Engineering News " of Sept. 8, 1898. In constructing the Golden Circle Railroad, in the Cripple Creek mining district of Colorado, the line had to be carried across a valley used as a dumping-ground for the refuse of the surround- 184 TUNNELING ing mines. To protect the line from this refuse, the engineer constructed a tunnel lining consisting of successive steel ribs, filled between with masonry. Concluding Remarks. From the fact that the open-cut method of tunneling consists first in excavating a cut, and sec- ond in covering this cut to form an underground passageway, it has been named the l * cut-and-cover " method of tunneling. The cut-and-cover method of tunneling is almost never employed elsewhere than in cities, or where the surface of the ground has to be restored for the accommodation of traffic and business. When it is not necessary to restore the original surface, as is usually the case with tunnels built in the ordinary course of railway work, it would obviously be absurd to do so except in extraordinary cases. In a general way, therefore, it may be said that the cut-and-cover method of construction is confined to the building of tunnels under city streets ; and the discussion of this kind of tunnels follows logically the general description of the open-cut method of tunneling which has been given. TUNNELS UNDER CITY STREETS. The three most common purposes of tunnels under city streets are : to provide for the removal of railway tracks from the street surface, and separate the street railway traffic from the vehicular and pedestrian traffic; to provide for rapid transit railways from the business section to the outlying residence districts of the city ; and to provide conduits for sew- age or subways for water and gas mains, sewers, wires, etc. Within recent years the greatest works of tunneling under city streets have been designed and carried out to furnish improved transit facilities. Condition^ of Work. The construction of tunnels under city streets may be divided into two classes, which may be briefly defined as shallow tunnels and deep tunnels. Shallow tunnels, or those constructed at a small depth beneath the surface, are OPEN-CUT TUNNELING METHODS 185 usually built by one of the cut-and-cover methods ; deep tunnels, or those built at a great depth, beneath the surface are constructed by any of the various methods of tunneling described in this book, the choice of the method depending upon the character of the material penetrated, and the local conditions. In building tunnels under city streets the first duty of the engineer is to disturb as little as possible the various existing structures, and the activities for which these structures and the street are designed. The character of the difficulties encoun- tered in performing this duty will depend upon the depth at which the tunnel is driven. In constructing shallow tunnels by the cut-and-cover method care has to be taken first of all not to disturb the street traffic any more than is absolutely necessary. This condition precludes the single trench method of open cut tunneling in all places where the street traffic is at all dense, and compels the engineer to use the parallel trench method employed in Paris, as previously described, or else the transverse trench or slice method employed in the Boston Subway. Both of these methods have to be modified when the work is done on streets having underground trolley and cable roads, and in which are located gas and water pipes, conduits for wires, etc. Where underground trolley or cable railways are encountered, a common mode of procedure is to excavate parallel side trenches for the side walls, and turn the roof arch until it reaches the conduit carrying the cables or wires. The earth is then removed from beneath the conduit structure in small sections, and the arch completed as each section is opened. As fast as the arch is completed the conduit struc- ture is supported on it. Where pipes are encountered they may be supported by means of chains, suspending them from heavy cross-beams, or by means of strutting, or they may be removed and rebuilt at a new level. Generally the conditions require a different solution of this problem at different points. 186 TUNNELING Another serious difficulty of tunneling under city streets arises from the danger of disturbing the foundations of the adjacent buildings. This danger exists only where the depth of the tunnel excavation extends below the depth of the build- ing foundations, and where the material penetrated is soft ground. Where the tunnel penetrates rock there is no danger of disturbing the building foundations. To prevent trouble of this character requires simply that the excavation of the tunnel be so conducted that there is no inflow of the surround- ing material, which may, by causing a settlement of the neigh- boring material, allow the foundations resting on it to sink. The Baltimore Belt tunnel, described in a succeeding chap- ter, is an example of the method of work adopted in construct- ing a tunnel under city streets through very soft ground. This may be classed as a deep tunnel. Another method of deep tunneling under city streets is the shield method, ex- amples of which are given in a preceding chapter. Two notable examples of cut-and-cover methods of tunneling are the Boston Subway and the New York Rapid Transit Ry., a description of which follows. Boston Subway. The Boston Subway may be defined as the underground terminal system of the surface street railway system of the city, and as such it comprises various branches, loops, and stations. The subway begins at the Public Garden on Boylston St., near Charles St., and passes with double tracks under Boylston St. to its intersection with Tremont St., where it meets the other double-track branch, passing under Tremont St. and beginning at its intersection with Shawmut Ave. From their intersection at Tremont and Boylston streets the two double-track branches proceed under Tremont St. with four tracks to Scollay Square. At Scollay Square the subway divides again into two double-track branches, one passing under Hanover St., and the other under Washington St. At the intersection of Hanover and Washington streets the two double-track branches combine again into a four-track line, OPEN-CUT TUNNELING METHODS 1ST which runs under Washington St. to its terminus at Hay- market Square, where it comes to the surface by means of an incline. The subway, therefore, has three portals or entrances, located respectively at Boylston St., Shawmut Ave., and Hay- market Square. It also has five stations and two loops, the former being located at Boylston St., Park St., Scollay Square, Adams Square, and Haymarket Square, and the latter at Park St. and Adams Square. The total length of the subway is 10,810 ft. Material Penetrated. The material met with in construct- ing the subway is alluvial in character, the lower strata being generally composed of blue clay and sand, and the upper strata of more loose soil, such as loam, oyster shells, gravel, and peat. At many points the material was so stable that the walls of the excavation would stand vertical for some time after excava- ,tion. Surface water was encountered, but generally in small quantities, except near the Boylston St. portal, where it was so plentiful as to cause some trouble. Cross- Section. The subway being built for two tracks in some places and for four tracks in other places, it was neces- sary to vary the form and dimensions of the cross-section. The cross-sections actually adopted are of three types. Fig. 109 shows the section known as the wide arch type, in which the lining is solid masonry. The second type was known as the double- barrel section, and is shown by Fig. 110. The third type of section is shown by Fig. 111. The lining consists of steel columns carrying transverse roof girders ; the roof girders being filled between with arches, and the wall columns having concrete walls between them. The wide-arch type and the double-barrel type of sections were employed in some portions FIG. 109. Wide Arch Section, Boston Subway. 188 TUNNELING of the Tremont St. line, where the traffic was very dense, since it was possible to construct them without opening the street. Much of the wide arch line was constructed by the use of the roof shield, which is described in the succeeding chapter on the shield system of tunneling. Methods of Construction. Several different methods were employed in constructing the subway. Where ample space was available, the single wide trench method of cut-and-cover FIG. 110. Double Barrel Section, Boston Subway. construction was employed, the earth being removed as fast as excavated. In the streets, except where regular tunneling was resorted to, the parallel trench or transverse trench cut-and- -cover methods were employed. In the transverse trench method, trenches about 12 ft. wide were excavated across the street, their length being equal to the extreme transverse width of the tunnel lining, and their depth being equal to the depth of the tunnel floor. These trenches were begun during the night, and immediately roofed OPEN-CUT TUNNELING METHODS 189 over with a timber platform flush with the street surface. Under these platforms the excavation was completed and the lining built. As each trench or " slice " was completed, the street above it was restored and the platform reconstructed Cross Section of Side Wall. Tile ' W-proofingA HAYMARKET SQUARE Cross Section of Roof . FIG. 111. Four Track Rectangular Section, Boston Subway. over the succeeding trench or slice. During the construction of each slice the street traffic, including the street cars, was carried by the timber platform. In the parallel trench method, short parallel trenches were dug for the opposite side walls, and also for the intermediate Waterproofing FIG. 112. Section Showing Slice Method of Construction, Boston Subway. columns, and completely roofed over during the night. Under this roofing the masonry of the side walls and column founda- tions and the columns themselves were erected. When the side walls and columns had been erected, the surface of the street between them was removed, the roof beams laid, and a 190 TUNNELING platform covering erected, as shown by Fig. 112. This roofing work was also done at night. The subsequent work of build- ing the roof arches, removing the remainder of the earth, and constructing the invert, was carried on underneath the plat- form covering which carried the street traffic in the meantime. The successive repetition of the processes described con- structed the subway. Where the traffic was very dense on the street above, tunnel- ing was resorted to. For small portions of this work the ex- cavation was done in the ordinary way, using timber strutting, but much the greater portion of the tunnel work was performed by means of a roof shield. In the latter case, the side walls were first built in small bottom side drifts and were fitted with tracks on top to carry the roof shield. The construction and operation of this shield are described fully in the succeeding chapter on the shield system of tunneling. Masonry. The masonry of the inclined approaches to the subway consists simply of two parallel stone masonry retaining walls. In the wide-arch and double-barrel tunnel sections, the side walls are of concrete and the roof arches are of brick masonry. In the other parts of the subway the masonry consists of brick jack arches sprung between the roof beams and covered with concrete, of concrete walls embedding the side columns, and of the concrete invert and foundations for the columns. Figs. 109 to 112 inclusive show the general details of the masonry work for each of the three sections. The inside of the lining masonry is painted throughout with white paint. Stations. The design and construction of the stations for the Boston Subway were made the subjects of considerable thought. All the stations consist of two island platforms of artificial stone having stairways leading to the street above. The platforms are made 1 ft. higher than the rails. The station structure itself is built of steel columns and roof beams with brick roof arches, and concrete side walls. Its interior is lined with white enameled tiles. The intermediate columns are cased OPEN-CUT TUNNELING METHODS 191 with wood, and have circular wooden seats at their bottoms. Each stairway is covered by a light housing, consisting of a steel framework with a copper covering and an interior wood and tile finish. Ventilation. The subway is ventilated by means of ex- haust fans located in seven fan chambers, some of which con- tain two fans, and others only one fan. Each of the fans has a capacity of from 30,000 to 37,000 cu. ft. of air per minute, and is driven by electric motor, taking current from the trolley wires. This system of ventilation has worked satisfactorily. Disposal of Rainwater. The rainwater which enters the subway from the inclined entrances, together with that from leakage, is lifted from 12 ft. to 18 ft. by automatic electric pumps to the city sewers. The subway has pump- wells at the Public Garden, at Eliot St., Adams Square, and Haymarket Square. In each of these wells are two vertical submerged centrifugal pumps made entirely of composition metal. In each chamber above, are two electric motors operating the pumps. Each motor is started and stopped according to the height of water by means of a float and an automatic release starting box. The floats are so placed that only one pump is usually brought into use. The other, however, comes into service in case the first pump is out of order or the water enters more rapidly than one pump can dispose of it. In the latter case, both motors continue to run until the same low level has been reached. Very little dampness except from atmospheric condensation is to be found on tlie interior walls or roof of the subway, although numerous discolored patches, caused by dampness and dust, may be seen on some parts of the walls. Substantially all of the leakage comes through the small drains in the invert leading from hollows left in the side walls. Careful measure- ment was taken at the end of an unusually wet season to de- termine the actual amount of leakage, and the total amount for the entire subway was found to be about 81 gallons per minute. 192 TUNNELING Estimated Quantities. The estimated quantities of material used in constructing the subway were as follows : Excavation 369,450 cu. yds. Concrete 75,660 " " Brick 11,105 " " Steel 8,105 tons Granite 2,285 cu. yds. Piles 117,925 lin. ft. Ribbed tiles 12,440 sq. yds. Plaster 88,190 " Waterproofing (asphalt coating) . . . 117,980 " Artificial stone 6,790 " Enameled brick 2,210 " Enameled tiles 2,855 ' Cost of the Subway. The estimated cost of the subway made before the work was begun was approximately 14,000,000, and the cost of construction did not exceed 13,700,000. This includes ventilating and pump chambers, changes of water and gas pipes, sewers and other structures, administration, engineer- ing, interest on bonds, and all cost whatsoever. Dividing this number by the total length we obtain a cost per linear foot of 1342.30. New York Rapid Transit Railway The project of an under- ground rapid transit railway to run the entire length of Man- hattan Island, was originated some years previous to 1890. In 1894, however, a Rapid Transit Commission was appointed to prepare plans for such a road, and after a large amount of trouble and delay this commission awarded the contract for construction to Mr. John B. McDonald of New York City, on Jan. 15, 1900. Not enough work has been done to enable a description of the methods of construction, but the following is a brief account of the work to be done : Route. The road starts from a loop which encircles the triangular area occupied by the City Hall Park and the Post- Office. Within this loop the tunnel construction will be two- storied; but where the main line leaves the loop, all four tracks OPEN-CUT TUNNELING METHODS 193 will come to the same level, and continue side by side thereafter except at the points which will be noted as the description proceeds. Proceeding from the loop, the four-track line passes under Center and Elm Streets. It will continue under Lafay- ette Place, across Astor Place and private property between As tor Place and Ninth St. to Fourth Ave. The road will then pass under Fourth and Park avenues until 42d St. is reached. At this point the line turns west along 42d St., which it follows to Broadway. It turns northward again under Broad- way to the boulevard, crossing the Circle at 59th St. The road will then follow the boulevard until 97th St. is reached, where the four-track line is separated into two double-track lines. At a suitable point north of 96th St. the outside tracks will rise so as to permit the inside tracks, on reaching a point near 103d St., to curve to the right, passing under the north-bound track, and to continue thence across and under private property to 104th St. From there the two-track tunnel will go under 104th St and Central Park to 110th St., near Lenox Ave. ; thence under Lenox Ave. to a point near 142d St. ; thence across and under private property and the intervening streets to the Harlem River. The road will pass under the Harlem River and across and under private property to 149th St., which street it will follow to Third Ave., and will then pass under Westchester Ave., where at a convenient point the tracks will emerge from the tunnel, and be carried on a viaduct along and over Westchester Ave., Southern Boulevard, and Boston Road to Bronx Park. This portion of the line, from 96th St. to Bronx Park, will be known as the East Side Line. From the northern side of 96th St. the outside tracks will rise, and after crossing over the inside tracks they will be brought together on a location under the center line of the street and proceed along under the boulevard to a point between 122d and 123d streets. At this point the tracks will com- mence to emerge from the tunnel, and be carried on a viaduct along and over the boulevard at a point between 134th and 194 TUNNELING 135th streets, where they will again pass into the tunnel under and along the boulevard and Eleventh Ave. to a point about 1,350 ft. north of the center line of 190th St. There the tracks will again emerge from the tunnel, and be carried on a viaduct across and over private property to El wood St., and over and along Elwood St. to Kingsbridge St. to Kingsbridge Ave., private property, the Harlem Ship Canal and Spuyten Duyvil Creek, private property, Riverdale Ave., or 230th St. to a ter- minus near Bailey Ave. That portion of the line from 96th St. to the above mentioned terminus at Bailey Ave. will be known as the West Side Line. The total length of "the rapid transit road, including the parts above and below the surface ground of the streets, as well as both the East and West Side Lines, will be about 20^ miles. Material Penetrated. The soil through which the road will be excavated, according to numerous borings taken along the line, will be a varied one. The lower portion of the road, or the part including the loop around the Post-Office up to nearly Fourth St., will be undoubtedly excavated through loose soil, but from Fourth St. to the ends it will be excavated in rock. The loose soil forming the southern part of Manhattan Island is chiefly composed of clay, sand, and old rubbish a soil very easy to excavate. There is no fear of any damage to the build- ings along the line since, with the exception of the loop around the Post-Office, no high buildings are met ; and at the loop the underground road passes far above the plane of the foundations of the high buildings fronting Park Row. Water will be met at some points, but not in such quantities as to be a serious inconvenience, except, perhaps, in crossing Canal St., where the meeting of a large body of water is expected. From Fourth St. to the ends of both the east and west side lines, the soil will be chiefly composed.of rock of gneissoid and mica-schistose char- acter, these rocks prevailing nearly throughout the whole of Manhattan Island. The rock, as a rule, will not be compact, but will have seams and fissures, and at many points it will be OPEN-CUT TUNNELING METHODS 195 found disintegrated and alternated with strata of loose soils, and even pockets of quicksand will be met with along the line of the road. Cross-Sections. The section of the underground road will be of three different types, the rectangular, the barrel- vault, and the circular. The rectangular section, Fig. 113, will be used for the greater part of the road, of which a portion will be for four tracks and a portion for two tracks. The dimensions adopted for the four tracks are 50 x 13 ft., and for the double tracks 25 x 13 ft. The barrel-vault section, composed of a Water Proofing Minimum Tfiidnea lobe 8: tntckntss Increased in Beta unx/ftd. FIG. 113. Double Track Section, New York Rapid Transit Railway. polycentric arch, having the flattest curve at the crown, whose dimensions are 16 ft. high and 24 ft. wide, has been adopted for the portions of the road to be tunneled. The circular sec- tion of 15-ft. diameter will be used under the Harlem River, and being for single track, two parallel tunnels will be built side by side. The main line from the post-office loop to about 102d St., consists of four tracks built side by side in one conduit, except for that portion under the present Fourth Ave. tunnel where two parallel double-track tunnels will be em- ployed. The West Side Line will consist of double tracks 196 TUNNELING laid in one conduit, except across Manhattan St. and beyond 190th St., where it will be carried on an elevated structure. The East Side Line will consist of a double-track tunnel driven from 102d St., and the boulevard under Central Park to 110th St. and Lenox Ave., and two parallel circular tun- nels excavated under the Harlem River, the other portions of the road being double-track, subway and elevated structure. The subway, both for four and two tracks, may be built by open excavation, cut-and-cover methods. For the main line the Slice method, so successfully em- ployed in the Boston Subway, will be adopted as the most convenient in a case where the width of the excavation is great and the traffic enormous, as is the case especially below 43d St. and along the boulevard. For the double-track sub- way, the method of the side trench will perhaps be adopted on account of it being the least expensive ; and since the streets where such a trench will be opened are very wide, with only a light traffic. Lining. The lining of the subway is of concrete, carried by a framework of steel. The floor consists of a foundation layer of concrete at least eight inches thick on good founda- tion, but thicker, according to conditions, where the founda- tion is bad. On top of this is placed another layer of concrete, with a layer of waterproofing between the two. In this top layer are set the stone pedestals for the steel columns, and the members making up the tracks. In the four-track subway, the steel framework consists of transverse bents of columns, and I-beams spaced about five feet apart along the tunnel. The three interior columns of each bent are built up bulb angle and plate columns of H-section. The wall columns are I-beams, as are also the roof beams ; between the I-beams, wall columns, and roof beams there is a concrete filling. So that the roof of the subway will be made up of concrete arches resting on the flanges of the I-beams of the roof. The concrete to be used is of one part Portland OPEN-CUT TUNNELING METHODS 197 cement, two parts sand, and four parts broken stones. The double-track subway will be built in the same way, except that only one column is placed between the tracks for the" support of the roof. All the concrete masonry of the roof, foundations, and side walls, must contain a layer of waterproofing, so as to keep perfectly dry the underground road, and prevent the perco- lation of water. This waterproofing must be made up as follows : On the lowest stratum of concrete, whose surface is made as smooth as possible, a layer of hot asphalt is spread. On this asphalt are immediately laid sheets or rolls of felt ; another layer of hot asphalt is then spread over the felt, and tTtf - Jftf- FIG. 114. Park Avenue Deep Tunnel Construction, New York Rapid Transit Railway. then another layer of felt laid, and so on, until no less than two, and no more than six, layers of felt are laid, with the felt between layers of asphalt. On top of the upper surface of asphalt the remainder of the concrete is put in place so as to reach the required thickness of the concrete wall. Tunnels. At three points the Standard Subway will be replaced by tunnel lines. The location of the three tunnels will be between 33d and 42d St. on Park Ave. ; under Central Park, northeast of 104th St., and under the Har- lem River. The Park Ave. construction (Fig. 114) will consist of two parallel double-track tunnels, located on each side of the street, and about 10 ft. below the present tunnel. The soil being composed of good rock, the tunnels will be 198 TUNNELING driven by a wide heading, and one bench, since no strutting will be required, and the masonry lining, even of the roof, may be left far behind the front of the excavation. The masonry lining will consist of concrete walls and brick arches. The tunnel under Central Park being driven through a similar rock, the same method of excavation and the same manner of lining will be used. The tunnel under the Harlem River is to be driven through soft ground ; and it will be constructed as a submarine tunnel, according to the shield and compressed air combined process. FIG. 115. Harlem River Tunnel, New York Rapid Transit Railway. The tunnels will be lined with iron made up of segments, with radial and circumferential flanges. Concrete will be placed inside and flush with the flanges. The tracks, both in the subway and tunnels, are an inti- mate part of the concrete flooring. The rail rests on a con- tinuous bearing of wooden blocks, laid with the grain running transversely with respect to the line of the rail, and held in place by two channel iron guard rails. The guard rails are bolted to metal cross-ties embedded in the concrete. OPEN-CUT TUNNELING METHODS 199 Viaduct. A considerable portion of the double track branch lines north of 103d St. will be viaduct, or elevated structure. The viaduct construction on the West Side Line will extend, in- cluding approaches, from 122d St. to very near 135th St. Of this distance, 2,018 ft. 8 ins. will be viaduct proper, consisting of plate girder spans carried by trestle bents at the ends, and by trestle towers for the central portion. The columns of the bents and towers are to be built up bulb-angle and plate columns of H-section of the same form as those used in the bents inside the subway. The approaches proper will be built of masonry. The elevated line proper consists of plate girder spans, supported on plate girder plate cross girders carried by columns set at the curb lines. Stations. Many stations will be built along the line. These will be located on each side of the street. The entrances at the stations will consist of iron framework, with glass roofs covering the descending stairways. The passage- ways leading down will be walled with white enameled bricks and wainscoted with slabs of marble. The stations for the local trains will be located on each side of the road close to the walls, since the outside tracks are reserved for the local trains, while the middle ones will be reserved for the expresses. The few stations for the express trains will be located between the middle and outside tracks. Stations will be provided with all the conveniences required, having toilet rooms, news stands, benches, etc., and will be lighted day and night by numerous arc lamps. G-eneral. The contractor is compelled to complete the work in four and one-half years, but he has promised to have it in full running order within three years. There is no diffi- culty in doing this, since the great extension of the road and the great width of the avenues under which it runs allow work all along the line at the same time. The work, briefly summar- ized, comprises the following items : 200 TUNNELING Length of all sections, ft 109,570 Total excavation of earth, cu. yds 1,700,228 Earth to be filled back, " . . 773,093 Rock excavated, " 921,128 Rock tunneled, " 368,606 Steel used in structure, tons 65,044 Cast iron used, "...<. 7,901 Concrete, cu. yds 489,122 Brick, " .............. 18,519 Waterproofing, sq. yds 775,795 Vault lights, " .',';.'. . . 6,640 Local stations, number ......' 43 Express stations, " 5 Station elevators, " 10 Track total, lin. ft 305,380 " underground, lin. ft 245,514 " elevated, " 59,766 In addition to the construction of the railway itself, it will be necessary to construct or reconstruct certain sewers, and to adjust, readjust, and maintain street railway lines, water pipes, subways, and other surface and subsurface structures, and to relay street pavements. The total cost of the work, according to the contract signed by Mr. McDonald, will be 135,000,000. Dividing this amount by the total length of the road, which is 109,570 lineal feet, we have the unit cost a lineal foot $315, or a little less than unit of cost of the Boston subway, which was $342 per lineal foot. The road belongs to the city. The contractor acts as an agent for the city in carrying out the work, and he is the leaser of the road for fifty years. The work is paid for as soon as the various parts of the road are completed, and the money is obtained from an issue of city bonds. During the fifty years' lease the contractor will pay the interest plus 1 % of the face value of the bonds. This 1 % goes to the sinking-fund, which within the fifty years at compound interest forms the total sum required for the redemption of bonds. SUBMARINE TUNNELING 201 CHAPTER XVIII. SUBMARINE TUNNELING: GENERAL DISCUS- SION. THE SEVERN TUNNEL. GENERAL DISCUSSION. SUBMARINE tunnels, or % tunnels excavated under the beds of rivers, lakes, etc., have been constructed in large numbers during the last quarter of a century, and the projects for such tunnels, which have not yet been carried to completion, are still more numerous. Among the more notable completed works of this character may be noted the tunnel under the River Severn and those under the River Thames in England, the one under the River Seine in France, that under the St. Clair River for railway, that under the East River for gas mains, that under Dorchester Bay, Boston, for sewage, and those under Lakes Michigan and Erie for the water supply of Chicago and Cleveland in America. Among the partly com- pleted submarine tunnels which have been abandoned the most notable example is, perhaps, the Hudson River tunnel. For the details of the various projected submarine tunnels of note, which include tunnels under the English and Irish Channels, under the Straits of Gibraltar, under the sound between Copenhagen in Denmark and Malino in Sweden, under the Messina Straits between Italy and Sicily, and under the Straits of Northumberland between New Brunswick and Prince Edward Island, the reader is referred to the periodical litera- ture of the last few years. Previous to attempting the driving of a submarine tunnel it is necessary to ascertain the character of the material it will 202 TUNNELING penetrate. This fact is generally determined by making dia- mond-drill borings along the line, and the object of ascertaining it is to determine the method of excavation to be adopted. If the material is permeable and the tunnel must pass at a small depth below the river bed, a method will have to be adopted .which provides for handling the difficulty of inflowing water. If, on the other hand, the tunnel passes through impermeable material at a great depth, there will be no unusual trouble from water, and almost any of the ordinary methods of tun- neling such materials may be employed. Shallow submarine tunnels through permeable material are usually driven by the shield method or by the compressed jiir method, or by a method which is a combination of the first and second. It is not an uncommon experience for a submarine tunnel to start out in firm soil and unexpectedly to find that this material becomes soft and treacherous as the wwk proceeds, or that it is intersected by strata of soft material. The method of dealing with this condition will vary with the circumstances, but generally if any considerable amount of soft material has to be penetrated, or if the inflow of water is very large, the firm- ground system of work is changed to one of the methods employed for excavating soft-ground submarine tunnels. The Milwaukee water supply tunnel and the East River gas tunnel, described elsewhere, are notable examples of submarine tunnels began in firm material which unexpectedly developed a treacher- ous character after the work had proceeded some distance. Occasionally the task of building a submarine tunnel in the river bed arises. In such cases the tunnel is usually built by means of cofferdams in shallow water, and by means of caissons in deep water. Submarine tunnels under rivers are usually built with a de- scending grade from each end which terminates in a level middle position, the longitudinal profile of the tunnel corresponding to the transverse profile of the river bottom. Where, however, such tunnels pass under the water with one end submerged, and SUBMARINE TUNNELING 203 the other end rising to land like the water supply tunnels of Chicago, Milwaukee, and Cleveland, the longitudinal profile is commonly level, or else descends from the shore to a level position reaching out under the water. The drainage of submarine tunnels during construction is one of the most serious problems with which the engineer has to deal in such works. This arises from the fact that, since the entrances of the tunnel are higher than the other parts, all of the seepage water remains in the tunnel unless pumped out, and from the possibility of encountering faults or permeable strata, which reach to the stream bed and give access to water in greater or less quantities. Generally, therefore, the excavation is conducted in such a manner that the inflowing water is led directly to sumps. To drain these sumps pumping stations are necessary at the shore shafts, and they should have ample capacity to handle the ordinary amount of seepage, and enough surplus capacity to meet probable increases in the inflow. For extraordinary emergencies this plant may have to be greatly enlarged, but it is not usual to provide for these at the outset unless their likelihood is obvious from the start. The character and size of the pumping plants used in constructing a number of well-known tunnels are described in Chapter XII. In this book submarine tunnels will be classified as follows: (1) Tunnels in rock or very compact soils, which are driven by any of the ordinary methods of tunneling similar materials 011 land; (2) tunnels in loose soils, which may be driven, (a) by compressed air, (5) by shields, or (c) by shields and compressed air combined; (3) tunnels on the river bed, which are con- structed, (a) by cofferdams, or (&) by caissons ; (4) tunnels partly in firm soil and partly in treacherous soils, which are driven partly by one of the firm-soil methods, and partly by one of the soft-soil methods. To illustrate tunnels of the first class, the River Severn tunnel in England has been selected ; to illustrate those of the second class, the several tunnels discussed in the chapter on the shield system of tunneling and the Mil- 204 TUNNELING waukee tunnel is sufficient ; to illustrate those of the third class, the Yan Buren Street tunnel in Chicago is selected ; and to illustrate those of the fourth class, the East River gas tunnel and the Milwaukee water supply tunnels are excellent examples. THE SEVERN TUNNEL. The Severn tunnel, which carries the Great Western Hail- way, of England, beneath the estuary 01 a large river, is 4 miles 642 yards long. It penetrates strata of conglomerate, limestone, carboniferous beds, marl, gravel, and sand, at a minimum depth of 44f ft. below the deepest portion of the estuary bed. The following description of the work is abstracted from a paper by Mr. L. F. Yernon-Harcourt. * The first work was the sinking of a shaft, 15 ft. in diameter, lined with brickwork, on the Monmouthshire bank of the Severn, 200 ft. deep. After the Monmouthshire shaft had been sunk, a heading 7 ft. square was driven under the river, rising with a gradient of 1 in 500 from the shaft on the Monmouthshire shore, so as to drain the lowest part of the tunnel. Near to the first, a second shaft was sunk and tubbed with iron, in which the pumps were placed for removing the water from the tunnel works, connection being made by a cross-heading with the heading from the first shaft. There was also a shaft on the Gloucestershire shore ; and two shafts inland from the first on the Monmouthshire side, to expedite the construction of the tunnel. Headings were then driven in both directions along the line of the tunnel, from the four shafts ; and the drainage head- ing from the old shaft was continued, in the line of the tunnel, under the deep channel of the estuary, and up the ascending gradient towards the Gloucestershire shore, till, in October, 1879, it had reached to within about 130 yards of the end of the descending heading from the Gloucestershire shaft. During this period, though the work had progressed slowly, no large * Proceedings Inst. C.E., vol. cxxi. SUBMARINE TUNNELING 205 quantity of water had been met with in any of the headings, in spite of their already extending under almost the whole width of the estuary. On October 18, 1889, however, a great spring was tapped by the heading which was being driven landwards from the old shaft, about 40 ft. above the level of the drainage heading ; and the water poured out from this land spring in such quantity that, flowing along the heading, falling down the old shaft, and thus finding its way into the drainage heading and the long heading of the tunnel under the estuary in con- nection with it, it flooded the whole of the workings in com- munication with the old shaft, which it also tilled within twenty- four hours from the piercing of the spring. To obtain increased security against any influx of water from the deep channel of the estuary into the tunnel, the proposed level portion of the tunnel, rather more than a furlong long under this part, was lowered 15 ft. by increas- ing the descending gradient on the Monmouthshire side from 1 in 100 to 1 in 90, and lowering the proposed rail level on. the Gloucestershire side 15 ft. throughout the ascent, so as not to increase the gradient of 1 in 100 against the load. A new shaft, 18 ft. in diameter, was sunk slightly nearer the estuary 011 the Monmouthshire shore than the old one ; two shafts also were sunk on the land side of the great spring for pumping purposes; and additional pumping machinery was erected. The flow from the spring into the old shaft was arrested by a shield of oak fixed across the heading; and at last, after numerous failures and breakdowns of the pumps, the headings were cleared of water, after a diver, supplied with a knapsack of compressed oxygen, had closed a door in the long heading under the estuary ; and the works were resumed nearly fourteen months after the flooding occurred. The great spring was then shut off from the workings by a wall across the heading leading to the old shaft ; and, owing to the lower- ing of the level of the tunnel, a new drainage heading had to be driven from the bottom of the new shaft at a lower level, 206 TUNNELING which was made 5 ft. in diameter, and lined with brickwork, whilst the old drainage heading was enlarged to 9 ft. in diam- eter, and lined with brickwork, so as to aid in the permanent ventilation of the tunnel. The lowering of the level, moreover, converted the bottom tunnel headings into top headings, so that along more than a mile of the tunnel the semicircular arch, 26 ft. in diameter, was built first, and then, after lowering the headings, the invert was laid and the side walls were built up. Bottom headings were driven along the remainder of the tunnel, and the work was expedited by means of break-ups. Ventila- tion was effected in the works by a fan 18 inches in diameter and 7 ft. wide, fixed at the top of the new deep shaft ; the rock was bored by drills worked by compressed air ; the blasting was eventually effected exclusively by tonite, owing to its being freer from deleterious fumes than any other explosive ; and the workings were lighted by Swan and Brush electric lamps. The tunnel is lined throughout with vitrified brickwork, between 2] ft. to 3 ft. thick, set in cement, and has an invert 1| ft. to 3 ft. in thickness ; the lining was commenced towards the end of 1880, the headings under the river were joined in Septem- ber, 1881, and the last length of the tunnel, across the line of the great spring, was completed in April, 1885. Water came in from the river through a hole in a pool of the estuary, close to the Gloucestershire shore, in April, 1881, during the lining of a portion of the tunnel, but fortunately before the headings were joined. This influx was stopped by allowing the water to rise in the tunnel to tide-level, to prevent the enlargement of the hole, which was then filled up at- low water with clay, weighted on the top with clay in bags. The great spring broke out again in October, 1883, and flooded the works a second time ; but within four weeks the water had been pumped out and the spring again imprisoned. During this period an exceptionally high tide, raised still higher by a southwesterly gale, inundated the low-lying land on the Mon- mouthshire side of the estuary, and, flowing down one of the SUBMARINE TUNNELING 207 inland shafts, flooded a section of the tunnel, but the pumps removed this water within a week. In order to construct the portion of tunnel traversing the line of the great spring, the water was diverted into a side heading below the level of the tunnel, leading to the old shaft, whence it was pumped, and the fissure below the tunnel was filled with concrete, over which the invert was built. An attempt to imprison the spring, on the completion of this length of tunnel, having resulted in imposing an excessive pres- sure on the brickwork, leading to fractures and leakage, a shaft, 29 ft. in diameter, was sunk at the side of the tunnel at this point in 1886, and pumps were erected powerful enough to deal with the entire flow of the spring. The tunnel was opened for traffic in December, 1886, and gives access to a double line of railway, connecting the lines converging to Bristol with the South Wales railway and the western lines. The pumping power provided at the shaft con- nected with the great spring, and at four other shafts, is capable of raising 66,000,000 gallons of water per day, the maximum amount pumped from the tunnel being 30,000,000 gallons a day. The ventilation of the tunnel is effected by fans placed in the two main shafts on each bank of the estuary, and the fan in the Monmouthshire shaft is 40 ft. in diameter, and 12 ft. wide. The tunnel gives passage to a large traffic, numerous through-trains between the north and southwest of England making use of it. 208 TUNNELING CHAPTER XIX. SUBMARINE TUNNELING (Continued) ; THE EAST RIVER GAS TUNNEL. VAN BUREN ST. TUNNEL, CHICAGO. THE East River gas tunnel is a notable example of a tunnel begun in firm soil which unexpectedly developed treacherous strata. It is also remarkable from the fact that the shield which was employed to overcome the trouble was driven from rock into soft material and from the soft material into rock again with the utmost success. The following description of the work is abstracted from a paper by Mr. Walton I. Aims, the engineer in charge of the work, published in the Journal of the Association of Engineering Societies for May, 1895, and in Engineering Neivs of July 11, 1895. The accompanying cuts are reproduced from the last-named publication. During 1891 and 1892 the East River Gas Co., of Long Island City, a corporation with works situated on the Long Island shore of the East River, obtained from the New York State Legislature a new charter, and such necessary legislation as to permit the extension of their mains across the East River into the city of New York. The feasibility of constructing a tunnel under the river through which the gas mains might be laid was discussed, and after some preliminary surveys and examinations a route was decided upon from the works of the company at Ravenswood, Long Island City, to between 70th and 71st Sts., New York, passing under Blackwell's Island and the east and west chan- nels of the East River. On about this line of location some eight or ten pipe soundings were made in the two river chan- SUBMARINE TUNNELING 209 nels, all of which indicated a rock bottom ; and the results of these, together with surface indications, where at both the Long Island and New York shores, as well as on Blackwell's Island, bedrock lay exposed, led all to conclude that nothing but rock was to be encountered. On these investigations a contract was entered into on June 25, 1891, for the construction of a sup- posedly rock tunnel, which the contractor guaranteed to com- plete by April, 1893. Work was begun at the Ravenswood or Long Island side on June 28 by sinking a shaft 9 ft. square about 200 ft. back from the river to a depth of about 148 ft. below the surface ; while at New York, on July 7, a shaft of the same dimensions was sunk to a depth from the surface of 139 ft. In both these shafts rock was entered after about 8 ft. of soil ; but while the rock at New York was quite dry, at Ravenswood it proved seamy and very wet. The tunnel-roof grade had been established at 109 ft. below mean high water at the New York shaft, with a grade for drain- age of ^ / towards Ravenswood. This gave a minimum cover of 41 ft. at the deepest point in the west or New York channel on the East River, where there is 70 ft. of water at mean high tide. The east or Long Island channel is comparatively shallow, the deepest point being only 35 ft. below mean high water level. The one thing feared was that fissures yielding large volumes of water might extend to the tunnel roof and largely augment the cost of pumping. The size of the tunnel section was to be 8 ft. 6 ins. in height by 10 ft. 6 ins. in width, this giving suffi- cient room for the laying of two 3-ft. gas mains and one 4-ft. main. In the shafts, on both sides of the river, the headings were now turned. At Ravenswood the work was delayed by meet- ing considerable quantities of silty water, but at New York the tunnel was practically dry until towards the end of December, 1892, when, at a distance of 338 ft. from the shaft, a fissure was struck yielding about a 3-in. stream of salt water. The 210 TUNNELING rock to within 20 ft. of this point had been the regular hard New York gneiss, with a dip towards Long Island of 10 from the vertical, and a strike north and south at right angles to the direction of the tunnel. Here it gradually began to soften, becoming more and more micaceous until when about 20 ft. beyond the water-bearing fissure the rock suddenly terminated, running into a vein of soft material with the same dip and strike as that of the rock. This new material proved to be a vein, principally of decom- posed feldspar, gray in color, crumbling easily, and with no perceptible grit. It still preserved a rock structure, and was perfectly dry when undisturbed. But its exposed surfaces were quickly acted upon by water, which it would absorb and then wash away quite rapidly. The water-bearing fissure and this soft vein were connected ; more water was also met at the junction of the rock and the soft material, and later experience proved that in passing through these soft veins water was always to be found next to the rock a sort of water-course on both sides of the soft vein. Had it not been for encounter- ing this water, the tunnel might have been carried through the soft material without employing compressed air, though the prudence of attempting this might be questioned, for nothing more insures the safety of both the men and the work than compressed air in sub-aqueous tunneling. The finding of this soft material, so unexpected, was quite a set-back to all concerned. However, it was decided to drive a small timbered drift about 4 ft. wide by 6 ft. high to investi- gate the ground ahead, and find how much of this material was to be penetrated before solid rock was again met. This drift was started and driven in for about 6 ft. Meanwhile a most destructive action was going on between the water and the soft material. The water running along the face of the rock had washed out a cavity overhead in the soft ground. The walls of this cavity were gradually breaking away, and the clay-like substance falling down would close the outlet of the water into SUBMARINE TUNNELING 211 the tunnel. The water would then accumulate in this pocket, softening up fresh material on the sides until it had gained a sufficient head to burst through the dam which confined it, when it would come rushing into the tunnel, carrying with it large quantities of the softened material. These rushes were accompanied by a loud bubbling sound that quite mystified the men, which was, of course, the sound of the air displacing the water in the cavity. As soon as the pocket had emptied itself, for a time the trouble was over, until with the falling of more material the outlet was again closed and the operation was repeated. These rushes of water, with the accompanying sound of the bubbling air, soon became more and more alarming to the men. The cavity was constantly increasing in size, and extending up toward the river-bed. Each recurrence would now send the men running for the shaft, by no means certain that the river had not at last made a connection wiflh the tunnel. All work in the small drift was abandoned, and on Dec. 31 a bulkhead was hurriedly constructed at the face to prevent the threatened flooding of the shaft. Up to this time over 25 yards of material had been washed into the tunnel, all of which had come from along the rock face. With the river-bed only 45 ft. above the tunnel-roof, there is every reason to believe that this .bulkhead was put in none too soon, and a connection with the river narrowly averted. The bulkhead was well packed with hay to prevent, as much as possible, further washing of the material, and a discussion was now entered into as to the method of future procedure. The contractors were in favor of abandoning the heading and returning to the shaft, to sink to a lower level and start anew in hopes of meeting more favorable conditions at a greater depth. There had been a somewhat similar experience on the Croton Aqueduct, where that tunnel passes under the Harlem River. Soft material had been en- countered on the first established level, which proved so trouble- some that after two or three unsuccessful attempts had been 212 TUNNELING made to pass through it, it was finally decided to abandon the heading and return to the shaft, sinking some 150 ft. deeper. On this new level nothing but rock was encountered. In the East River tunnel, however, the soft material was clearly a decomposed vein, and to what depth this decomposition might extend was unknown ; so that as there were no well-founded reasons, in this case, for expecting any better conditions at a lower level, it was decided to first attempt to drive the present heading, in compressed air, leaving the sinking as a later ex- pedient should the proposed means fail. An arrangement was made with the contractors by which the company was to share the expense of the work in soft ground. It was at this time that the writer became connected with, the work, having charge of installing and conducting the com- pressed air operations for the company. To form the com- pressed air-working chamber, a solid brick wall or bulkhead 8 ft. thick was built across the tunnel into gains -in the rock about 40 ft. back from the heading, and containing a cylindrical steel air-lock 6 ft. in diameter and 10 ft. long. In the engine room, the 18 x 24-in. Ingersoll piston-inlet compressor, used heretofore for running the rock-drills, was supplemented by a small Rand compressor, and both arranged to supply, independently, compressed air to the working cham- ber below. Incandescent electric lighting was introduced into the tunnel, which is almost a necessity in compressed air opera- tions, as common illuminants produce an enormous quantity of smoke when burning in compressed air. A telephone was also taken into the working chamber, by which instant communica- tion could be had with the engine room in case any sudden increase of air pressure should be desired. On Feb. 25, 1893, operations were commenced, in the heading, under 35 Ibs. of air pressure. The previous work here had greatly increased the difficulties, and it was not long before the air pressure had to be raised to 42 Ibs. to control the water. The excavation was advanced under a cylindrical SUBMARINE TUNNELING 213 steel roof, built up of plates 3 ft long and 1 ft. wide, of $-in. sheet steel, to the four sides of which were riveted angle bars 2i X 2| x in. These plates were bolted together in a heading about 6 ft. high. In the erection of this roof, poling-boards were used for each plate, and a bulkhead carried down with each ring as erected. When the heading had been advanced about 20 ft. from the rock, a 12 x 12 in. yellow-pine mudsill was introduced along the bottom of the heading, and on this the roof was covered by means of radial timber bracing. The excavation was now carried down on both sides of this mudsill, to a distance of about 10 ft. from the rock, the steel roof being extended well down on the sides. A circular section was thus excavated, in which brickwork was laid, four courses thick, and with an internal diameter of 10ft. Between March 4 and 6 a great deal of trouble was experienced. Air pressure was several times to 48 Ibs., and the work progressed very slowly on account of the many inrushes of water and softened mate- rial. It was not until April. 8 that the last section of brickwork in the soft material was completed and rock again entered, after passing through 29 ft. of this decomposed material. Of the material met in driving through this vein, at first 9 ft. of the gray decomposed feldspar was penetrated, a vein of 4 ins. of hard quartz was then met, and this was followed by 6 ft. of pure white decomposed feldspar, smooth and soft as plaster. The remaining 14 ft. was made up of layers of feldspar and chlorite. This chlorite, deep green in color, flaky and grease-like to the touch when wet, proved to be very troublesome material, as it was easily converted into a fluid state by the water, which was again encountered next to the rock. At the Long Island shaft, the work up to this time had pro- gressed to about 250 ft. from the shaft. The material so far encountered on this side was a hard, seamy gneiss, bearing con- siderable quantities of salty water, containing iron, lime, and magnesia. Soft ground was now met at this end, in a seam about 4 ft. wide, of chlorite. As this material was perfectly 214 TUNNELING dry and not thoroughly disintegrated, the tunnel was timbered through this seam without difficulty. Several similar veins were thus met and passed through, until at a point 285 ft. from the shaft, where after drilling for about 2 ft. through rock a soft green, almost liquid chlorite vein was struck, which began flowing in through the drill holes with great force. These holes were plugged ; but as it was necessary to know what was ahead, and as with 100 ft. of cover between the tunnel roof and the river bottom it was thought that the condition of affairs could not be very serious, it was decided to continue driving ahead without air pressure, and with a timbered heading. To see what the material would do, several hand-holes were put into the rock- face with the object of blasting out a hole about 2 ft. square through the remaining 2 ft. of rock, to the chlorite. Before blasting, however, the precaution was taken to build a bulk- head, some 40 ft. back from the face. On firing the holes an inrush of many yards of material took place, which was finally checked by some rock fragments closing the opening through the rock. After several desperate attempts on the part of the contractors to control this material and make progress, the work was finally abandoned in the latter part of March, and as a 4-in. stream of water was now flowing from the heading, pump- ing was discontinued, and the shaft and tunnel allowed to flood. At the New York end work was still being carried on in compressed air. The rock encountered at the other side of the soft seam closely resembled the decomposed material which had been penetrated before, and consisted of alternate layers of feldspar and chlorite, with an occasional vein of quartz. It was quite soft, though requiring drilling and blasting, and eventually it had to be lined. After the heading had been driven about 69 ft. into this rock the company decided, in spite of the uncertainty as to the material ahead, to remove the air pressure, and to call upon the contractors to resume their contract. Upon removing the air pressure, however, the brick- work through the soft seam proved so unsatisfactory in exclud- SUBMARINE TUNNELING 215 flrt ing the water that air pressure was again put on, and it was decided to line the brickwork with a circular cast-iron lining (Fig. 116). Although this brickwork was only 10 ft. in in- side diameter, a lining was designed 10 ft. 2 ins. in the clear, as it was now desired to make the tunnel bore as large as possible. To put in this lining, some of the brickwork had to be cut out, which was then removed in sections, enough for one ring of plates at a time. The lining consisted of rings of plates or segments, mo****n ru.iTt"! 1 ft. 4 ins. wide, with internal flanges 4 ins. deep, from the back of the plate. The metal in both the back of the plate and the flanges was 1^ ins. thick. All the joint- faces of the segments were planed, and 1-in. bolts used for fasten- ing them together. A complete tunnel ring was composed of nine segments and a small inverted key, about 8 ins. wide. Difficulties between the company and the contractors, which had been brewing for some time, now culminated and the courts were appealed to, to settle their differences. This caused a cessation of work for a short time until the com- pany were empowered to take possession and resume the work of construction for themselves. The work of putting the cast- iron lining into the brickwork was necessarily a very slow operation. The lining was extended well into the rock on both sides of the soft vein, and a wall built at both ends, be- Ooss Section. Long: Section. FIG. 116. Sections of Cast Iron Lining, East River Gas Tunnel. 216 TUNNELING tween the rock and the iron lining, to confine the Portland cement grout, which was now introduced back of the plates. To effect this grouting 1^-in. holes had been drilled and tapped through the back of several plates in each ring. Through these holes the grout was pumped by means of a Cameron pump ; and after the space between the brickwork and the lining had been thoroughly grouted, the work was found, on taking off the air pressure from the heading, to be perfectly water-tight. It was not until towards the end of July that the work of lining the brickwork was completed and driving ahead in the rock was resumed. Then, when an advance of only 10 ft. had been made, a second soft seam was encountered about 80 ft. beyond the first one, and a test pipe was driven to a horizontal depth of 70 ft., without encountering anything solid. To avoid further delay, the driving of the test-pipe was discontinued at this depth, and preparations made for advancing the heading. For this test- pipe 1^-in. common wrought-iron pipe was used, which was driven in by a small machine-drill, and washed out at each lengthening of the pipe with a l|-in. wash-pipe. From these washings the differ- ent materials penetrated were sampled, with the following tabulated results : 3 ft. gray decomposed feldspar and chlorite. 11 ft. soft black mud, containing lumps of carbonized wood like charcoal. 19 ft. hard black mud and sand, with nodules of pyrites. 22 ft. gray decomposed feldspar. 4 ft. decomposed feldspar and chlorite. 11 ft. gray decomposed feldspar. Water was again found next to the rock, but was consider- ably held in check by the compressed air. As from the results of the test-pipe there were no special difficulties to apprehend from the indicated material, it was decided to drive ahead, under the open heading method, as this involved no delays in waiting for special machinery. The light steel cylindrical roof was again used in advancing the excavation, but for the perma- SUBMARINE TUNNELING 217 nent lining the cast-iron rings were to be introduced instead of brickwork, as heretofore. A start was made on Aug. 7 to drive the heading into the soft material, but two days later, after the work had been advanced 6 ft. into the soft vein, orders were received to suspend all work on account of the great financial depression of the time. This was unfortunate ; and could it have been anticipated a few days the heading into the soft material would have been left unopened. As it was now, from being first disturbed and then abandoned, the water was first allowed to soften up the black mud in the heading, and, in spite of the bulkhead, a considerable quantity of the material was washed into the tunnel. This stay of proceedings was utilized by making a horizon- tal test boring in the heading on the Long Island side. At this shaft no work had been done since the departure of the contractors, beyond the building of a brick bulkhead and air- lock in the tunnel. Compressed air had then been put on, which considerably reduced the amount of water flowing into the tunnel from the heading. The action of the compressed air had been somewhat peculiar ; for notwithstanding the great depth of the tunnel below the river bed, at 10 Ibs. pressure the air began to escape through the heading, and with a pressure of 35 Ibs. per sq. in. small bubbles of escaping air could be seen rising to the surface for over 300 ft. up and down the river. This seemed to indicate that the ground above the tunnel had been honeycombed up to the river bottom by the previous washing-in of such quantities of the soft green chlorite. As it was known that there were detached lumps of rocks in this soft vein, 2-in. heavy pipe was used for the test boring, with drive-well couplings, and a circular, hollow steel bit for the cutting end. This pipe was driven in the same way as the one on the New York side, and after passing through chlorite and various kinds of sofkrock fragments, solid rock was again met at 32 ft. Into this rock a hole was drilled to a depth of 54 ft., using a small bit on the end of a 1-in. pipe and 218 TUNNELING drilling through the test-pipe. The rock beyond the soft seam was a soft white limestone. With the prospect of resuming work the question now arose as to the best method of proceeding; and, as a great deal depended upon the success of driving through the present headings, it was strongly recommended that the safest and surest method, that of shield tunneling, be adopted in both headings, although necessarily entailing a large expenditure in plant, and delay in time for installation. This plan met with *&}* longitudinal Section. End Viewof Head. FIG. 117. Section and Elevation of Shield, East River Gas Tunnel. the company's approval, and a shield and hydraulic plant were designed. As the nature of the material to be penetrated be- yond the test-pipes was unknown, this shield was so made that in passing from rock to soft material, or back again to rock, it could be erected or taken apart again with a minimum of time and labor, so that it might almost be called a portable shield (Fig. 117). As in both the tunnel headings there was but one air-lock, and as it was inadvisable to remove the air pressure from the headings, the different parts of the shield had to be of such size as could be passed through the air-lock doors. This SUBMARINE TUNNELING 219 was accomplished by dividing the shield transversely, separatr ing the tail-end section, or that which overlaps the tunnel, from the cutting-edge section containing the working chambers. These two sections were, of course, circular, 11 ft. f in. out- side diameter. The tail end section was 3 ft. 6 ins. long, and the cutting-edge section 3 ft. 8 ins. long. Both of these sections were again divided, longitudinally, into four quadrants. The outside shell, in both tail-end and cutting-edge sections, was made up of one ^ in. and one | in. steel plates riveted together ; and at the four quadrant joints, there were |-in. butt- straps 12 ins. wide running the whole length of the shield and uniting the quadrants and the two sections. The middle diaphragm, separating the cutting-edge and tail-end sections, was made of two plates, one riveted to each of the two sections, and these two plates bolted together with the butt-straps united the sections. The cutting-edge section contained two platforms, one vertical and one hori- zontal, of the same length as the section. To erect this shield the only rivet- ing necessary was at the four butt- strap joints in the tail-end section, where it was necessary ta preserve a flush surface on both sides of the outer shell. In the cutting-edge part countersunk bolts were used through the butt-straps. About 380 f-in. bolts and 160 rivets were used ta erect the shield. Two doors closing each of the four working chambers were hung on the vertical platform, and were pro- vided with fastenings so that the whole face could be easily closed. To drive the shield 12 5-in. hydraulic jacks were used, designed for a working pressure of 5,000 Ibs. per sq. in., or FIG. 118. Elevation and Section of Hydraulic Jack, East River Gas Tunnel. 220 TUNNELING 700 tons on the whole shield (Fig. 118). These jacks were controlled by two block-valves, one placed on each side of the shield. Each of these block-valves consisted of six inde- pendent valves all in one compact casting, each of which had a pressure and exhaust stem. Half-inch XX pipe was used for connecting each jack with its valve, and 1-in. hydraulic pipe was used for the pressure main, which was connected with the shield block-valves by three swivel-joint connections. To fur- nish the pressure, a very compact little pump, designed by Watson & Stillnian, of New York, was used without an accu- mulator, the pressure being very nicely governed by a steam- regulating valve. On Sept. 22 work was resumed on the New York side, with a small force of men working days only, to excavate in the rock an enlarged chamber about 15 ft. back from the face, in which to erect the shield. This chamber was made circular, about 15 ft. in diameter and 10 ft. long. Back from this, the rock was taken out in a circular form of about 11 ft. diameter, for some 14 ft., or enough for about 10 rings of the cast-iron segments which were here erected in the rock, the spaces be tween being thoroughly grouted with Portland cement. These rings were thus made solid in the rock to withstand the thrust of the shield-jacks upon the lining. The blasting necessary in this work was made as light as possible ; but it was not without its effect upon the soft material in the heading, a considerable quantity of the black mud being washed through the bulkhead, while the braces showed signs of a heavy strain from the squeezing of the material. The shield arrived at the works on Nov. 10, and the work of erection was immediately begun. The sections were lowered down the shaft and taken through the air-lock to the shield-chamber. On Nov. 17 the shield was all assembled, and riveting the tail-end sections was commenced. For heating the rivets in the air-chamber a forge was used, with a hood to which was connected at the top a 2-in. pipe with a valve which extended through the air-lock bulkhead. By SUBMARINE TUNNELING 221 means of this pipe all the obnoxious gases from the furnace were removed from the air-chamber. After the riveting was finished, the shield was brought to its right position for line and grade, the hydraulic jacks and valves put in place, and the necessary connections made. On Nov. 24 word was received that the work on the New York side was to be pushed wiih all possible speed, and a force was at once organized of three gangs, working in eightrhour shifts. More rings were built on the ten rings already anchored in the rock, until the tunnel lining was brought within the tail-end of the shield. The shield was now advanced until it was necessary to disturb the bulkhead, the remaining bench ahead of the shield being blasted out as the shield progressed. The most difficult part of the work was now reached, for at the point where the shield entered the soft, black mud on top there still remained about 12 ft. of hard rock in the bottom, as the dip of this vein was over 40 toward Long Island. Blasting had therefore ta be continued in the bottom pockets of the shield after the top had entered the much-softened material. As soon as the bulk- head was passed it was with great difficulty that the bottom pockets could be kept clear of the black slush from overhead. The material had become so softened along the rock face that it was almost impossible to confine it, and several rushes of inflowing material occurred, until finally an open connection with the river was established, and the tunnel was visited by crabs and mussels, together with boulders, old boots and shoes, brick, and tinware, direct from the river bottom. Notwith- standing these adverse circumstances the work was still pro- gressing, although in 45 Ibs. of compressed air, which was now escaping through the heading, and causing a very violent ebullition on the river surface. This upward current of air held in check the downward current of water, so that no efforts were made to prevent its escape. On Dec. 13 the shield finally cleared the rock and was now fully entered into the soft, black mud. The main difficulty was now surmounted, the work 222 TUNNELING progressed more rapidly, and the shield soon reached undis- turbed material, which was found quite dry and hard. It was still the same black mud, with occasional lumps like chare oal, and numerous nodules like pyrites, which glistened like silver in the black, peat-like mud. Mattocks were used by the men in the working chambers, who would clean out these four com- partments to within a foot of the cutting edge. As soon as this was done hydraulic pressure was put upon the jacks, some- times to the amount of 5,000 Ibs. per sq. in., and the shield forced ahead 16 or 18 ins., enough for another ring of plates, the working chambers again being filled with the displaced material. On Dec. 24 the last of the black mud was passed through, and lying next to it, at an angle of 40 towards Long Island, white decomposed feldspar was found, containing fragments of decomposed quartz charged with sulphureted hydrogen. An important departure was now made in the method of erecting the cast-iron lining rings by breaking joints with the segments. In all the iron-lined tunnels it has been the estab- O lished custom to erect the rings with continuous horizontal joints. For some reason it was thought inadvisable to attempt breaking joints with the segments. The writer's experience in the Hudson tunnel had shown him the importance of obtain- ing, in soft, squeezing ground, a perfectly rigid tunnel-ring. In a material exerting hydrostatic pressure the tunnel lining is subjected to a resultant strain, tending to flatten the ring, or decrease its vertical diameter. Any yielding to this strain results both in increasing the deforming pressure and in de- creasing the power of the ring to resist the strain. In a lining erected with continuous joints the rigidity of the ring is dependent upon the bolting in the horizontal joints. At the Hudson River tunnel a ring of plates was bolted together lying flat on the ground, the plates all brought to a true circle, and the two 1^-in. bolts in each joint well tightened. Upon raising this ring with a derrick, so that it stood erect, the ring- SUBMARINE TUNNELING 223 was flattened 3 ins. by its own weight. At the East River tunnel a similar experiment was made ; two rings of plates were bolted together, breaking joints, one ring being revolved two holes. These two rings were then raised upright, but no flattening could be detected. By means of a turnbuckle a measured strain was now brought upon the rings along the vertical diameter. At 16,000 Ibs. the vertical diameter was shortened i-in., the flanges of the plates cracking where the turnbuckle was attached. In these two instances there was, of course, a great difference in the size of the rings, those in the Hudson tunnel being 18 ft inside diameter, while those in the gas tunnel were only 10 ft. 2 ins. inside diameter. Aside from the rigidity gained, breaking joints has proved much the better in other ways. With continuous joints, two things are apt to occur: (1) The joint^face where two rings meet may become slightly warped ; that is, all points on this face of the ring will no longer lie in the same plane. This may be caused by carelessness in allowing dirt to get into the joints between the rings. When this once occurs the warping increases with every additional ring till true joints can no longer be made. (2) The rings may be erected so as to depart gradually from a true circular form. This latter case is im- possible where the joints are broken, and, in the former in : stance, by breaking joints, the error is divided and distributed around the ring until it disappears. On Jan. 16, 1894, the end of the soft seam was reached with the shield, and rock was again entered after having passed through 98 ft. of soft ground. This rock resembled slightly the rock on BlackwelTs Island. It was in a much shattered condition, with many loose heads and small, soft veins. As this material required support in the heading and a permanent lining, and as, in its present condition, there was no assurance that it might not again pass into soft material shield tunnel- ing was still continued. Small machine-drills were set up in the four working-chambers of the shield upon arms bolted to 224 TUNNELING the vertical platform, and the rock was drilled and blasted just ahead of the shield. The progress of 4 ft. per day was made in this material, of which there was about 65 ft. The rock then became much more solid, with a roof that was self-sustain- ing, and arrangements were made for removing the shield. On Feb. 18 the work of removing the shield was begun, and two days later everything was ready for the regular rock-tunnel work in the heading, the shield having been taken apart and removed in that time. At about the time that shield tunneling was being discon- tinued at New York, it was being installed at Long Island. An entire duplicate plant had been ordered for this side ; for, although it had been originally intended to use one shield for both headings, it was later deemed advisable to provide a shield for each heading, so that there might be no delay, should soft ground be met in both headings at the same time. In passing through the soft seam at Ravenswood with the shield, no especial difficulties were met. - The material proved to be a mass of soft-rock fragments, boulders and cinder-like stones im- bedded in soft green chlorite. About a month was consumed in passing through this seam, removing the shield, and prolong- ing the cast-iron lining well into the rock on both sides of the vein. With both tunnel headings now in rock, remarkably rapid progress was made ; and as progress now had become of great importance to the company, a liberal bonus, arranged on a sliding scale, was given the foremen for work done over stated amounts. Up to the time of the headings meeting, an average progress of 69 ft. per week was made, while in rock, on both the New York and Long Island sides. The record week of the work was the one ending June 27, when at Ravenswood 95 ft. was driven, while on the New York side, the heading was advanced 101 ft., making a total for the week of 196 ft. of tunnel driven. Soon after the rock tunneling had been re- sumed on the New York side, this heading reached Blackwell's Island, and the troubles on this side were over. But at Ravens- SUBMARINE TUNNELING 225 wood, with the heading in white limestone, there was every reason to expect further soft seams where the rock should change to the granite gneiss of Blackwell's Island. These expectations were not disappointed ; for after passing through 350 ft. of the limestone, and when within 200 ft of Blackwell's Island, a soft seam was met, and air pressure had to be once more used in the heading. As this seam was but 14 ft. in width, and presented no especial difficulties, the tunnel was carried through it without using the shield, the cast-iron seg- ments being erected under a timber roof. Gneiss was encoun- tered on the other side of this soft vein, which brought with it the assurance that the last of the soft ground had been passed. On May 16 serious loss and delay were caused by a fire which destroyed the New York works. The fire started in an adjoin- ing picnic ground, containing many light frame structures, which caused so fierce a conflagration that it was impossible to save our works. This caused a delay of three weeks in the time of the tunnel's completion. On July 11, 1894, the re- maining 15 ft. of rock between the headings was blasted away, thus opening the pioneer tunnel under the East River, two years from the time when ground was first broken. Some weeks were spent in clearing up and shutting out the water in the wet places. A 3-ft. gas main was now laid through to New York, and on Oct. 15 gas was delivered into the city, accomplishing the purpose of the tunnel. VAN BUREN STREET TUNNEL, CHICAGO. The Van Buren Street tunnel in Chicago belongs to that class of submarine tunnels which has been designated as tunnels on the river bed, by which it is meant that the top of the tunnel is flush with, or extends slightly above, the bed of the stream. Two methods are available for constructing these tunnels ; viz., the cofferdam method and the caisson method. The cofferdam method has been actually employed in several 226 TUNNELING instances ; but the caisson method, although proposed for sev- eral projected works, has never actually been employed. The Van Buren Street tunnel, built to carry a double-track street railway under the Chicago River, was completed in 1894 by the cofferdam method. The special features of the tunnel * are : (1) the unusually large dimensions of the cross-section of 30ft. X 15ft. 9 ins.; (2) its construction inside of coffer- dams of great length and wdith; (3) the construction under some very high buildings calling for great care and very strong temporary and permanent supports. The special feature of the work for our present purpose was the construction of the tunnel across the river. To accom- plish this a cofferdam was built out from the west shore of the river to its middle, and the tunnel constructed within it like the building of any other structure within a cofferdam. Trans- verse and longitudinal sections of this cofferdam are shown by Fig. 119. As will be seen, it was a simple double-wall coffer- dam, with a clear width between the walls of 58 ft., and braced transversely as shown. Inside of this a single-wall cofferdam of piles was constructed, with a clear width just sufficient to allow the construction of the masonry within it. When the tunnel end reached the channel end of the cofferdam, a crib-wall was built over the end of the completed tunnel, as shown by the drawings. This crib wall was intended to form the end wall of another cofferdam, which was built out from the east shore, and within which the remaining half of the tunnel was built as the first half had been. The drawings show the char- acter of the tunnel masonry and of the centering upon which it was built. In this connection it will be interesting to mention briefly the most pretentious proposition for tunnel construction by means of caissons. Some years ago, Prof. Winkler proposed to construct a tunnel under the River Danube to connect the various portions of the Vienna, Austria, underground railway, * " Eng. News," April 12, 1892. SUBMARINE TUNNELING 227 228 TUNNELING and to use caissons in the construction. Prof. Winkler pro- posed to build caissons from 30 ft. to 45 ft. long, with a width depending upon the lateral dimensions adopted for the tunnel masonry. The caisson was to be made of metal plates and angle iron with riveted connections on all sides except those running vertically transverse to the tunnel axis, whose connec- tions were to be bolted. The roof of the caisson was to be made of T-irons resting upon templates placed on the edge of the longitudinal sides of the caisson, and strutted in the middle by the crown of an iron arch having its springers upon brackets inserted on the vertical angle irons forming the frame of the caisson. Between the T-irons of the roof small brick vaults were to be built, and a very thick stratum of concrete laid on their extrados so as to obtain a level surface. In the middle of the roof an opening was to be left ; this was for the shaft having the air-locks to allow the passage of men, mate- rials, and compressed air. Across the river two parallel rows of piles were to be driven into the river bed, to fix the place where the caisson was to be sunk. Then the first caisson near the shore was to be lowered in the ordinary way, and a second caisson was to be immediately sunk very close to the first one. When both cais- sons had reached the plane of the tunnel floor, the sides which were in contact were to be unbolted and removed, and the small space between made water-tight by filling them with yarn and tar. The chambers of the two caissons were to be opened into a single large one communicating above by means of two shafts. At the same time that the masonry was being built in the two first caissons, from the inverted arch up, a third cais- son was to be sunk ; and when by excavation it had reached the plane of the projected tunnel floor, the partitions were to be removed so that the three caissons were in communica- tion, forming a large single caisson. To limit the compressed air to the working-place, walls were to be built across the tun- nel near the advanced part completely lined. The first wall SUBMARINE TUNNELING 229 was to be built after four caissons were sunk. Then the outer partition of the first caisson was to be removed, and the ma- sonry of the submarine tunnel connected with the portion of the tunnel built on land. In a similar manner all the caissons were to be sunk ; and when the last one was placed, and the ma- sonry lining constructed, and connected with the portion of the tunnel built on the other shore of the river, the partition walls were to be battered down, and the submarine tunnel com- pletely constructed and open to traffic. 230 TUNNELING CHAPTER XX. SUBMARINE TUNNELING (Continued). THE MILWAUKEE WATER-WORKS TUNNEL. THE new water supply intake tunnel for the city of Mil- waukee, Wis., is one of the most difficult examples of tunnel construction which American engineering practice has afforded. The difficulties were in a large measure unexpected when the work was decided upon and put under way. The tunnel began and ended in a hard, impervious clay, practically a rock, and all the preliminary investigations led to the conclusion that the same favorable material would be encountered for its entire length. With such material a brick-lined tunnel 7^ ft. in diameter presented no unusual problems ; but after about 1,640 ft. had been excavated from the shore end the tunnel ran out of the hard clay, and for the next 600 ft. or more a variety of water-bearing material was encountered, which tried the skill and patience of the engineer to their utmost. Other difficulties were indeed met with, but these were of minor importance in comparison with that of safely and successfully penetrating the water-bearing drift. The work of sinking the shore shafts and excavating the first 1,600 ft. of tunnel did not prove especially difficult. A hard, compact, and rock-like clay, bearing very little moisture, was encountered all along, and was blasted and removed in the ordinary manner. The only mishap which occurred with this portion of the work was the destruction of the contractor's boiler plant by fire on Jan. 12, 1891, which allowed the tunnel to fill with water, and delayed work about a month. By Oct. 21, 1891, 1,640 ft. had been driven, averaging about 6 ft. SUBMARINE TUNNELING 231 per day, all in the hard clay. No timbering had been necessary, and except for the first 100 ft. of the tunnel there was very little seepage. On the afternoon of Get 21 water was observed coming out from one of the diill holes in the heading, 'but no attention was paid to it. Shortly after a blast was fired, and was immediately followed by a rush of water from the heading. An unsuccessful attempt was made to check the flow, and the pumps were started ; but they were unable to keep the water down, and after seven hours' hard work the tunnel was aban- doned. By the next morning the tunnel and shaft were full of water. Several attempts were made to empty the tunnel ; but the limited pumping capacity was not equal to the task, and it was finally decided to install larger pumps. The pumping had, how- ever, shown that about 1,000 gallons of water a minute was coming through the leak. With the increased pumping plant the tunnel was finally laid dry Feb. 13, 1892. Upon examina- tion the head of the drift was found to be in the same undis- turbed condition in which it was left when the water broke in three months before. A brick bulkhead was built into the end of the brickwork of the tunnel, and provided with a timber door for passage, and two 10-in. pipes for the outlet of the water. With these open- ings closed, the flow was checked sufficiently to allow the pla- cing of pumps at the bottom of the shore shaft. Meanwhile the pressure of the water against the bulkhead caused dangerous leakage, and so after the pumps were in position the 10-in. pipes were opened, relieving the pressure and allowing the water its normal rate of flow. Trouble with the pumps now arose, and after various stoppages and breaks the discharge pipe finally fell, disabling the whole plant. It became necessary to close the 10-in. pipes in the bulkhead and draw up the pumps. This allowed the tunnel to again fill with water. After thoroughly overhauling the pumping machinery, the contractor again laid the tunnel dry on March 19; and after 232 TUNNELING the pumps had been permanently placed so as to take care of the water, an examination of the work was made. It was found that the water was coming from the north, and with the hope of avoiding the difficulties of the old heading, it was decided to make a detour of the south. On April 16 work was begun at a point about 90 ft. back from the face, and deflecting the line about 38 toward the south. About 38 ft. from the angle of junction a brick bulkhead with two 8-in. openings was built ;jr ~v f V V%: , FlG. 120. Sketch showing underground stream. Milwaukee Water- Works Tunnel. into the new bore. The work progressed successfully for about 75 ft., when water was again encountered ; and upon pushing forward the heading, gravel and sand came in such quantities that it was found impracticable to continue the work further. On June 1 the bulkhead was permanently closed, and the work in this direction was abandoned. A further and closer examination was now made of the heading first abandoned. Upon breaking through the rock-like clay it was found that the water came from an underground SUBMARINE TUNNELING 233 stream flowing from the north through a well defined channel in red clay. This channel was about 13 ft. above the grade of the tunnel ; and above it in every direction visible was a bed of hard, dry, red clay, while immediately in front of the face of the work was a bank of coarse gravel. Fig. 120 is a sketch of the channel and stream where they entered the work. In this last drawing the photograph has been followed exactly, no particu- lar being exaggerated in the slightest. The water from this stream was clear and pure; and a chemical analysis showed that it was not lake water, but must come from some separate source. While the engineer did not consider the difficulty of pro- ceding along the old Line insurmountable, it was decided to be less difficult on the whole to go back from 150 ft. to 175 ft and deflect the line to the north and upward, so as to pass over the underground entrance. Instead of allowing the water to flow at its normal rate and take care of it by pumping, the contrac- tors desired to reduce the pumping, and to this end they con- structed a bulkhead just west of the deflection toward the south with a view of shutting off the water. The water, how- ever, accumulated with a pressure of some 50 Ibs. per sq. in., and penetrated the filling around the brick lining of the tunnel, preventing the cutting through of the lining for the new line. A second bulkhead was then built about 20 ft. west of the first, but with not much better results, for upon closing it the water was found to leak through the brickwork for a long distance west. Finally on Aug. 2, 1892, the contractors lifted their pumps and allowed the tunnel to fill again with water. No further work was done on the tunnel by the contractors, although they continued work on the lake shaft for some months. Difficulties had, however, arisen here, which will be described further on ; and finally a disagreement arose between the contractors and the city over the delay in prosecuting the tunnel work and over one or two other questions, which 234 TUNNELING resulted in the City Council suspending their contract and ordering the Board of Public Works to go ahead with the work. The first step to be taken by the engineer was to purchase adequate pumping machinery and empty the tunnel. This was effected Jan. 17, 1894 ; and as soon as practicable thereafter the two bulkheads were removed and the tunnel cleaned, tram-car tracks laid, and everything prepared for work. It was now determined to go ahead on the original line of the tunnel if possible, and the bulkhead here was removed and work begun. Meanwhile, a safety bulkhead had been built to replace the first one torn away. This was provided with a door and drain- age pipes. Work was begun on the original heading, but had proceeded only a little way when the water broke in, driving out the workmen. This was removed three or four times, when the flow suddenly increased to 3,000 gallons per minute. An examination of the lake bottom above the break showed that it had settled down, indicating that the new break connected with the lake bottom, and making further work along the original line out of the question. The question now arose what it was best to do. It was impracticable to use a shield, as the material ahead of the break required blasting, and the pressure from above was enormous. On account of its expense and difficulty of application the freezing process did not seem advisable, and the plenum process was likewise out of the question on account of the great pressure which would be required at this depth. The detour to the south which had been made by the contractor had been unsuccessful, and had left the ground in a treacherous condi- tion. To depress the tunnel was not advisable, for it was not by any means certain that the bed of gravel could be avoided in that way ; and, moreover, it would be necessary to ascend again further on, and thus leave a trap which would effectually cut off escape to those at work on the face if water again broke into the tunnel. SUBMARINE TUNNELING 235 It was finally decided that the old plan of deflecting the line toward the north and upward so as to pass over the under- ground stream should be tried. A hole was therefore cut through the tunnel lining 1,433 ft. from the shore, and work was begun on a detour of 20 toward the north and an upward grade of 10 %. Fair progress was made on this new line, gradually ascending into solid rock, until May 10, when the test borings, which were constantly made in every direction from the face, showed that sand was being approached. A brick bulkhead was therefore built into the masonry as a safe- guard, should it happen that water was encountered in large quantities. As the borings seemed to indicate that the top surface of the rock underlying the sand was nearly level, the lower half of the tunnel was first excavated, leaving about 18 ins. of the rock to serve as a roof (Sketch a, Fig. 121), and the brick invert was built for a distance of 52 ft. The rock roof was then carefully broken through for short distances at a time, and short sheeting driven ahead into the sand, which proved to be a very fine q/iicksand flowing through the smallest openings. Extreme care had to be taken in this work, but little by little the brickwork was pushed ahead until at a distance of 90 ft. from the point where the sand was first net, and 208 ft. from the old tunnel, the sand stopped and the heading entered a hard clay. All this work had been done on an ascending grade, and the ascent was continued about 40 ft. farther in the clay. By this time a sufficient elevation was gained to pass over the under- ground stream, and the tunnel line was changed to head toward the lake shaft, and the grade reduced to a level. The under- ground stream was passed without trouble and the tunnel continued for a distance of 54 ft. without difficulty. On July 10 the clay in the heading suddenly softened, and before the miners could secure it by bracing, the water rushed in, followed by gravel, filling up solidly some 34 ft. of the tunnel before it was stopped by a timber bulkhead hastily built. 236 TUNNELING Upon examining the lake bottom a cavity over 60 ft. deep and 10 ft. in diameter was found directly over the end of the tunnel, which had been caused by the gravel breaking into the tunnel. Having now reached an elevation where it was possible to use compressed air, it was determined to put in double air-locks and use the plenum process. The locks were built, and some r. n- / * Bench Face, ' $ Packed with Clay. ,* I ? Longitudinal Section Showing Method of \' H \ /- Construction in Rock Covered with Quicksand. ^^ Sketch "a". Section A-B-C-D. Sketch "c". Cross Section Showing Manner of Longitudinal Section at Tunnel . Constructing Lining around Boulder. Sketch tt b". Sketch "d." FlG. 121. Sketch Showing Methods of Lining, Milwaukee Water- Works Tunnel. 670 cu. yds. of clay were dumped into the hole in the lake bottom. On Aug. 4 the air-locks were tried with 26 Ibs. air pressure ; but, upon a temporary release of the pressure, the water passed around the locks and back of the tunnel lining for some distance, and even forced through the lining, carrying considerable clay and fine sand with it. Upon sounding the SUBMARINE TUNNELING. 237 lake bottom it was found that the cavity had again increased to a depth of 65 ft, whereupon an additional 600 cu. yds. of clay were dumped into it. On account of the water leaking through the brickwork, the only dry place to cut through the brickwork and build in an air-lock was just ahead of the brick bulkhead. This lock wa& completed Aug. 27, and to avoid encountering the danger of the direct connection with the lake at the end of the drift, it was decided to make another detour to the north. On Aug. 28 r therefore, the brick on the north side of the tunnel 12 ft. back from the end of the brickwork was cut through under 25 Ibs. air pressure, and work proceeded in good, hard clay. The original air-lock was cut out and a new lock built into this clay about 34 ft. from the last detour, to be used in case of further difficulties. After building the tunnel for about 80 ft. from the detour, the soundings again indicated the approach to gravel and water, and on Oct. 14 the water broke through from the bottom in such volume and with such force that the men ran out, closing every air-lock and the valves of every drain in their haste to escape, until the brick bulkhead was reached. It was with great difficulty that the portion of the tunnel up to the last air-lock was recovered and cleaned out. It was now recognized that a pressure of from 38 to 40 Ibs. of air would be needed to hold this water, and accordingly an- other compressor was added to the plant. With a pressure of 36 Ibs. the water was driven out and the work again started. At this time also an additional 350 cu. yds. of clay were dumped into the hole in the lake bottom. Altogether, 1,620 cu. yds. of clay had been put into this hole. Loose gravel and boulders, some of immense size, were now encountered, and the work became exceedingly difficult on account of the great escape of air. The interstices between the gravel and boulders were not filled with silt or sand, but con- tained water. Moreover, this material extended upward to the lake bottom, as was shown by the escape of air at the surface of 288 TUNNELING the lake. For an area of several hundred square feet the surface of the water resembled a pot of boiling water. At times the air would escape very rapidly, and again only a few bubbles would show. It need hardly be said that the work in this gravel was very slow. It was impossible to blast or to tear out the large boulders whole, as so much surface would be exposed that an inrush of water would take place despite the air pressure. The method of procedure was to excavate a heading and build the brick roof arch first, and then to take out the bench and build the in- vert. Fig. 121 gives a number of sketches showing how the work was done. A short piece of heading was taken out, the top and face of the bench being meanwhile plastered with clay (Sketches b and c, Fig. 121) to reduce the escape of air, and then the roof arch was built and supported on side sills resting on the bench. Bit by bit the roof arch was pushed forward until some little distance had been completed, then the heading was plastered with clay and the bench taken out little by little and the invert built. All the gravel except the small area upon which work was actually in progress was kept thoroughly plastered with clay ; and as the air escaped through the com- pleted brick work very rapidly, water was allowed to cover a portion of the invert (see Sketch c, Fig. 121), so as to reduce the area of escape. When a large boulder was reached, which lay partly within and partly without the tunnel section, the lining was built out and around it, as shown in Sketch d, Fig. 121. The boulder was then broken and taken out. All through this gravel bed the cross-section of the lining is made irregular by the con- struction of these pockets in the lining to get around boulders. Sometimes they were on one side and sometimes on the other, or on both, or at the top or bottom. In fact, there was no regularity. Despite the hazard and danger of this work, con- tinual progress was made, though sometimes it was only 4 ft. of completed tunnel per week, working night and day ; and, if SUBMARINE TUKXEL1XG 239 some cases of caisson disease be excepted, the only mishap oc- curring was a fire which got into the timber packing behind the lining and caused some trouble. From the gravel the tunnel ran into clay and quicksand, and then into hard, dry clay similar to that encountered near the shore. Some difficulty was had with the quicksand, but it was successfully overcome : and when the hard clay was struck, the trouble, as far as the work from the shore shaft was concerned, was virtually over. Meanwhile, a different set of afflictions had come upon the engineer and contractors in sinking the lake shaft and driving the heading toward shore. This shaft was intended to be built by sinking a cast-iron cylinder 10 ft. in diameter, made up of sections bolted together. Work was begun July 5, 1892, and the sinking was accomplished first by weighting the cylinder, and afterwards by pumping out the sand and water within it until the pressure from the outside broke through under the cutting edge and forced the sand into the cylinder, allowing it to sink a little. From 10 to 30 cu. yds. of sand were carried into the cylinder each time, and finally it was feared that if the process continued, the crib, which had been previously erected, would be undermined. On Sept. 6, therefore, the contractors were ordered to discontinue this method of work. No change was made, however, until Oct. 1, when the cylinder had reached a depth of 68 ft., and by this time there was quite a large cavity underneath the crib. This was refilled, and the cylinder pumped out, and excavation begun inside of it. On Oct. 11 a 2^-ft. deep ring of brick work was laid underneath the cutting edge ; but in trying to put in another ring beneath the first, two days later, the sand and water broke through the bottom, driving the men out, and filling the cylinder to a depth of 16 ft. with sand. The pumps w^ere started, but the water could not be lowered to a greater depth than 60 ft. At the request of the contractors, the city engineer had a boring made at the center of the shaft to. determine the character of the material to be further penetrated. This 240 TUNNELING boring showed that sand mixed with loam and gravel would be found for a depth of 26 ft., then would come 15 ft. of red clay, and finally a layer of hard clay like that penetrated by th& shore end of the tunnel. About the middle of December the contractors made another attempt to pump the shaft, but find- ing that the water came in at the rate of 25 gallons a minute,, abandoned the attempt. In the latter part of February prepa- rations were made to put an air-lock in the shaft and use compressed air. Hardly had the work been begun by this system, when, on April 20, 1893, a terrific easterly storm swept the top of the crib bare of the buildings and machinery, and drowned all but one of the 15 men at work there. This disaster delayed the work for some time, but in June the contractors erected a new building and new machinery, and resumed work. Very little progress was made ; and the air es- caped so rapidly that it loosened the sand surrounding the shaft and reduced the friction to such an extent that on July 28 the entire cylinder lifted bodily about 6 ft., and sand rushed in, filling the lower part of the cylinder to within 45 ft. of the lake surface. No further work was done by the contractors, although they submitted a proposition to sink a steel cylinder inside the cast-iron cylinder and extending from 5 ft. above datum to 100 ft. below datum for $300 per ft. This proposi- tion was refused by the city; and since work on the tunnel proper has been abandoned by the contractors some time before, as had already been described, the city suspended their contract on Oct. 19. On Oct. 30 a contract was made with Mr. Thos. Murphy, of Milwaukee, Wis., to sink a steel cylinder inside the old iron cylinder. The water was first pumped out of the old cylinder, and a timber bulkhead built at the bottom. On this the steel cylinder was built, and then the bulkhead was removed. Air pressure was put on, and the excavation proceeded successfully until the bottom layer of clay was met with, when all chances for trouble ceased. SUBMARINE TUNNELING 241 The cylinder, as it was completed, penetrated 9 ft. into the hard clay, and was underpinned with brickwork for a depth of 29 ft. or more, to a point 4 ft. below the grade line of the tunnel. At the lower end, the section of the shaft was changed from a circle to a square. Later the steel cylinder was lined with brick. On March 28, 1894, an agreement was made with Mr. Thos. Murphy to construct the tunnel from the lake shaft toward the shore. Except that considerable water was en- countered, which, owing to inadequate pumping machinery, filled the tunnel and shaft at two different times, and had to be removed, no very great difficulty was had with this part of the work. On July 28, 1895. the headings from the lake and shore shafts met. Meanwhile the cast-iron pipe intake, the intake crib, etc., had been completed, and practically all that remained to be done was to clean the tunnel and lift the pumping machinery at the shore shaft. During the cleaning, the air pressure had been kept up on account of the leakage through the brick lining, and, indeed, the pressure was kept up until the last possible moment, and everything made ready for removing the air locks, bulkheads, pumps, etc., in the least possible time. The pumps were the last to come out. 242 TUNNELING CHAPTER XXL SUBMARINE TUNNELING (Continued). THE SHIELD SYSTEM. Historical Introduction. The invention of the shield system of tunneling through soft ground is generally accredited to Sir Isambard Brunei, a Frenchman born in 1769, who emigrated to the United States in 1793, where he remained six years, and then went to England, in which country his epoch-making in- vention in tunneling was developed and successfully employed in building the first Thames tunnel, and where he died in 1849, a few years after the completion of this great work. Sir Isambard is said to have obtained the idea of employing a shield to tunnel soft ground from observing the work of ship-worms. He no- ticed that this little animal had a head provided with a boring apparatus with which it dug its way into the wood, and that its body threw off a secretion which lined the hole behind it and rendered it impervious to water. Toduplicate this operation by mechanical means on a large enough scale to make it ap- plicable to the construction of tunnels was the plan which occurred to the engineer ; and how closely he ,f Allowed his ani- mate model may be seen by examining tht> drawings of his first shield, for which he secured a patent in 1818. Briefly described, this device consisted of an iron cylinder having at its front end an auger-like cutter, whose revolution was in- tended to shove away the material ahead and thus advance the cylinder. As the cylinder advanced the perimeter of the hole behind was to be lined with a spiral sheet-iron plating, which was to be strengthened with an interior lining of masonry. It will be seen that the mechanical resemblance of this device to SUBMARINE TUNNELING 243 the ship-worm, on which it is alleged to have been modeled, was remarkably close. In the same patent in which Sir Isambard secured protection for his mechanical ship-worm he claimed equal rights of inven- tion for another shield, which is of far greater importance in being the prototype of the shield actually employed by him in constructing the first Thames tunnel. This alternative inven- tion, if it may be so termed, consisted of a group of separate cells which could be advanced one or more at a time or all together. The sides of these cells were to be provided with friction rollers to enable them to slide easily upon each other ; and it was also specified that the preferable motive power for advancing the cells was hydraulic jacks. To summarize briefly, therefore, the two inventions of Brunei comprehended the pro- tecting cylinder or shield, the closure of the face of the exca- vation, the cellular division, the hydraulic-jack propelling power, and cylindrical iron lining, which are the essential characteris- tics of the modern shield system of tunneling. The next step required was the actual proof of the practicability of Brunei's inventions, and this soon came. Those who have read the history of the first Thames tunnel will recall the early unsuccessful attempts at construction which had discouraged English engineers. Five years after Brunei's patent was secured a company was formed to undertake the task again, the plan being to use the shield system, under the personal direct* *^ of its inventor as chief engineer. For this work Brunei selected the cellular shield mentioned as an alter- native construction in his original patent. He also chose to make this shield rectangular in form. This choice is commonly accounted for by the fact that the strata to be penetrated by the tunnel were practically horizontal, and that it was assumed by the engineer that a rectangular shield would for spme reason best resist the pressures which would be developed. Whatever the reason may have been for the choice, the fact remains that a rectangular shield was adopted. The tunnel as designed con- 244 TUNNELING sisted of two parallel horseshoe tunnels, 18ft. 9 ins. wide and 16 ft. 4 ins. high and 1200 ft. long, separated from each other by a wall 4 ft. thick, pierced by 64 arched openings of 4 ft. span, the whole being surrounded with massive brickwork built to a rectangular section measuring over all 38 ft. wide and 22ft. high. The first shield designed by Brunei for the work proved in- adequate to resist the pressures, and it was replaced by another somewhat larger shield of substantially the same design, but of improved construction. This last shield was 22 ft. 3 ins. high and 37 ft. 6 ins. wide. It was divided vertically into twelve separate cast-iron frames placed close side by side, and each frame was divided horizontally into three cells -capable of sepa- rate movement, but connected by a peculiar articulated con- struction, which is indicated in a general \fay by Fig. 122. To close or cover the face of the excavation, poling-boards held in place by numerous small screw-jacks were employed. Each cell or each frame could be advanced independently of the others, the power for this operation being obtained by means of screw-jacks abutting against the completed masonry lining. Briefly described, the mode of procedure was to remove the poling-boards in front of the top cell of one frame, and excavate the material ahead for about 6 ins. This being done, the top cell was advanced 6 ins. by means of the screw-jacks, and the poling-boards were replaced. The middle cell of the frame was then advanced 6 ins. by repeating the same process, and finally the operation was duplicated for the bottom cell. With the advance of the bottom cell one frame had been pushed ahead 6 ins., and by a succession of such operations the other eleven frames were advanced a distance of 6 ins., one after the other, until the whole shield occupied a position 6 ins. in advance of that at which work was begun. The next step was to fill the 6-in. space behind the shield with a ring of brickwork. The illustration, Fig. 122, is the section parallel to the ver- tical plane of the tunnel through, the center of one of the SUBMARINE TUNNELING 245 frames, and it shows quite clearly the complicated details of the shield construction. Two features which are to be particu- larly noted are the suspended staging and centering for con- Fio. 122. - Longitudinal Section of Brunei's Shield, First Thames Tunnel. structing the roof arch, and the top plate of the shield extending back and overlapping the roof masonry so as to close completely the roof of the excavation and prevent it falling. Notwithstand- ing its complicated construction and unwieldy weight of 120 246 TUNNELING tons, this shield worked successfully, and during several months the construction proceeded at the rate of 2 ft. every 24 hours. There were two irruptions of water and inud from the river during the work, but the apertures were effectually stopped by heaving bags of clay into the holes in the river bed, and cover- ing them over with tarpauling, with a layer of gravel over all. The tunnel was completed in 1843, at a cost of about $5600 per lineal yard, and 20 years from the time work was first commenced, including all delays. The next tunnel to be built by the shield system was the tunnel under London Tower constructed by Barlow and Greathead and begun in 1869. In 1863 Mr. Peter W. Barlow secured a patent in England for a system of tunnel con- struction comprising the use of a circular shield and a cylindri- cal cast-iron lining. The shield, as shown by Fig. 123, was simply an iron or steel plate cylinder. The cylinder plates were thinned down in front to form a cutting edge, and they extended far enough back at the rear to enable the advance ring of the cast-iron lining to be set up within the cylinder. In simplicity of form this shield was much superior to Brunei's ; but it seems very doubtful, since it had no diametrical bracing of any sort, whether it would ever have withstood the com- bined pressure of the screw-jacks and of the surrounding earth in actual operation without serious distortion, and, probably, total collapse. It should also be noted that Barlow's shield made no provision for protecting the face of the excavation, although the inventor did state that if the soil made it neces- sary such a protection could be used. The patent provided for the injection of liquid cement behind the cast-iron lining to fill the annular space left by the advancing tail-plates of the FIG. 123. First Shield Invented by Barlow. SUBMARINE TUNNELING 247 shield. Although Barlow made vigorous efforts to get his shield used, it was not until 1868 that an opportunity pre- sented itself. In the meantime the inventor had been studying how to improve his original device, and in 1868 he secured addi- tional patents covering these improvements. Briefly described, they consisted in partly closing the shield with a diaphragm, as shown by Fig. 124. The uninclosed portion of the shield is here shown at the center, but the patent specified that it might also be located below the center in the bottom part of the shield. The idea of the construction was that in case of an irruption of water the upper portion of the shield could be kept open by air pressure, and work prosecuted in this open space until the shield had been driven ahead sufficiently to close the aperture, when the normal condition of affairs would be resumed. This was obviously an improve- ment of real merit. The partial diaphragm also served to stiffen the shield somewhat against collapse, but the thin plate cutting-edges and most of the other structural weaknesses were left unaltered. To summarize briefly the improvements due to Balow's work, we have : the construction of the shield in a single piece ; the use of compressed air and a partial diaphragm for keeping the upper part of the shield open in case of irruptions of water ; and the injection of liquid cement to fill the voids behind the lining. Turning now to the London Tower tunnel work, it may first be noted that Barlow found some difficulty in finding a contractor who was willing to undertake the job, so little confidence had engineers generally in his shield system. One man, however, Mr. J. H. Greathead, perceived that Barlow's Longitudinal Section.. Cross Section . FIG. 124. Second Shield Invented by Barlow. 248 TUNNELING device presented merit, although its design and construction were defective, and he finally undertook the work and carried it to a brilliant success. The tunnel was 1,350 ft. long and 7 ft. in diameter, and penetrated compact clay. Work was begun on the first shore shaft on Feb. 12, 1869, and the tunnel was completed the following Christmas, or in something short of eleven months, at a cost of c 14,5 00. The shield used was Barlow's idea put into practical shape by Greathead. It consisted of an iron cylinder, or, more properly, a frustum of a cone whose circumferential sides were very slightly inclined to the axis, the idea being that the friction would be less if the front end of the shield were slightly larger than the rear end. The shell of the cone was made of J in. plates. The thinned plate cutting-edge of Barlow's shield was replaced by Greathead with a circular ring of cast iron. Greathead also altered the construction of the diaphragm by arranging the angle stiffeners so that they ran horizontally and vertically, and by fastening the diaphragm plates to an interior cast-iron ring connected to the shell plates. This was a decided structural improvement, but it was accom- panied with another modification which was quite as decided a retrogression from Barlow's design. Greathead made the diaphragm opening rectangular and to extend very nearly from the top to the bottom of the shield, thus abandoning the element of safety provided by Barlow in case of an irruption of water. Fortunately the material penetrated by the shield for the Tower tunnel was so compact that no trouble was had from water ; but the dangerous character of the construction was some years afterwards disastrously proven in driving the Yarra River tunnel at Melbourne, Australia. To drive his shield Greathead employed six 2^ in. screw-jacks capable of developing a total force of 60 tons. The tails of the jack bore against the completed lining, which consisted of cast-iron rings 18 ins. wide and in. thick, each ring being made up of a crown piece and three segments. The different segments SUBMARINE TUNNELING 249 and rings were provided with double (exterior and interior) flanges, by means of which they were bolted together. The soil behind the lining was filled with liquid cement injected through small holes by means of a hand pump. The remarkable suc- cess of the London Tower tunnel encouraged Barlow to form in 1871 a company to tunnel the Thames between South- FlG - 125 Shield Suggested by Greathead for the Proposed North and South Woolwich Subway. wark and the City, and Greathead, in 1876, to project a tunnel under the same water- way known as the North and South Woolwich Subway. Bar- low's concession was abrogated by Parliament in 1873, without any work having been done. Greathead pro- gressed far enough with his enterprise to construct a shield and a large amount of the iron lining when the contractors abandoned the work. From the brief descrip- tion of his shield given by Greathead to the Lon-^ don Society of Civil En- gineers, it contained sev- eral important differences from the shield built by him for the London Tower tunnel, as is shown by Fig. 125. The changes which deserve particular notice are the great extension of the shield behind the diaphragm, the curved form of the diaphragm, PIG. 126 Beach's Shield Used on Broadway Pneumatic Railway Tunnel. 250 TUNNELING and the use of hydraulic jacks. Greathead had also designed for this work a special crane to be used in erecting the cast-iron segments of the lining. While these works had been progressing in England, Mr. Beach, an American, received a patent in the United States for a tunnel shield of the construction shown by Fig. 126, which was first tried practically in constructing a short length of tunnel under Broadway for the nearly forgotten Broadway FIG. 127. Shield for City and South London Railway. Pneumatic Underground Ry. This shield, as is indicated by the illustration, consisted of a cylinder of wood with an iron- cutting-edge and an iron tail-ring. Extending transversely across the shield at the front end were a number of horizontal iron plates or shelves with cutting-edges, as shown clearly by the drawing. The shield was moved ahead by means of a number of hydraulic jacks supplied with power by a hand pump attached to the shield. By means of suitable valves all or any lesser number of these jacks could be operated, and by S? 0< d* SUBMARINE TUXXELIXG 251 aJA-MsVVW^V; ft! t T i iSaJKT.t "i - tit: V nkrJ?i'61/i/ ^HolKfbr^bohs^ JJj2^_L.J- e.o__o..o_''_e.. ...,_ SECTION SHOWING HALF OF VV\LL F. ~ -j> ^SECTION SHOWING HALF OF WALL E, 252 TUNNELING thus regulating the action of the motive power the direction of the shield could be altered at will. Work was abandoned on the Broadway tunnel in 1870. In 1871-2 Beach's shield was used in building a short circular tunnel 8 ft. in diameter in Cincinnati, and a little later it was introduced into the Cleve- land water-works tunnel 8 ft. in diameter. In this latter work, which was through a very treacherous soil, the shield gave a great deal of trouble, and was finally so flattened by the Longitudinal Section, Cross Section. Fm. 129. Shield for BlackAvall Tunnel. pressures that it was abandoned. The obviously defective fea- tures of this shield were its want of vertical bracing and the lack of any means of closing the front in soft soil. With the foregoing brief review of the early development of the shield system of tunneling, we have arrived at a point where the methods of modern practice can be studied intelligently. In the pages which follow we shall first illustrate fully the construction of a number of shields of typical and special construction, and follow these illustrations with a general dis- cussion of present practice in the various details of shield construction. SUBMARINE TUNNELING 253 Transverse Section. Longitudinal Section. FIG. 130. Elliptical Shield for Clichy Sewer Tunnel, Paris. Mr. Raynald Legouez, in his excellent book upon the shield system of tunneling, considers that tunnel shields may be di- vided into three classes structurally, according to the character 254 TUNNELING of the material which they are designed to penetrate. In the first class he places shields designed to work in a stiff and com- paratively stable soil, like the well-known London clay ; in the second class are placed those constructed to work in soft clays r nd silts ; and in the third class those intended for soils of an Longitudinal Section. Cross Section. FIG. 131. Semi Elliptical Shield for Clichy Sewer Tunnel. unstable granular nature. This classification will, in a general way, be kept by the writer. As a representative shield of the first class, the one designed for the City and South London Ry. is illustrated in Fig. 127. The shields for the London Tower tunnel, the Waterloo & City Ry., the Glasgow District Subway, the Siphons of Cliehy and Concorde in Paris, and the SUBMARINE TUNNELING 255 Glasgow Port tunnel, are of the same general design and con- struction. To represent shields of the second class, the St. Clair River and Black wall shields are shown in Figs. 128 and 129. The shields for the Mersey River, the Hudson River, and the East River tunnels also belong to this class. To represent shields of the third class, the elliptical and semi- > ,(f , ;..__.. *V . Details of Casting Supporting Ends of Jack*. Details of Casting under Ends of Girders. Longitudinal Section C-Q. FIG. 132. Roof Shield for Boston Subway. elliptical shields of the Clichy tunnel work in Paris are shown by Figs. 130 and 131. The semi-circular shield of the Boston Subway is illustrated by Fig. 132. SHIELD CONSTRUCTION. General Form. Tunnel shields are usually cylindrical or semi-cylindrical in cross-section. The cylinder may be circular, elliptical, or oval in section. Far the greater number of shields used in the past have been circular cylinders ; but in one part of the sewer tunnel of Clichy, in Paris, an elliptical shield 256 TUNNELING with its major axis horizontal, was used, and the German en- gineer, Herr Mackensen, has designed an oval shield, with its major axis vertical. A semi-elliptical shield was employed on the Clichy tunnel, and semi-circular shields were used on the Baltimore Belt Line tunnel and the Boston Subway in Amer- ica. Generally, also, tunnel shields are right cylinders ; that is, the front and rear edges are in vertical planes perpendicular to the axis of the cylinder. Occasionally, however, they are oblique cylinders ; that is, the front or rear edges, or both, are in planes oblique to the axis of the cylinder. One of these visor-shaped shields was employed on the Clichy tunnel. The Shell, It is absolutely necessary that the exterior sur- face of the shell should be smooth, and for this reason the exterior rivet heads must be countersunk. It is generally admitted, also, that the shell should be perfectly cylindrical, and not conical. The conical form has some advantage in reducing the frictional resistance to the advance of the shield ; but this is generally considered to be more than counterbalanced by the danger of subsidence of the earth, caused by the exces- sive void which it leaves behind the iron tunnel lining. For the same reason the shell plate, which overlaps the forward ring of the lining, should be as thin as practicable, but its thickness should not be reduced so that it will deflect under the earth pressures from above. Generally the shell is made of at least two thicknesses of plating, the plates being arranged so as to break joints, and, thus, to avoid the use of cover joints, to inter- rupt the smooth surface which is so essential, particularly on the exterior. The thickness of the shell required will vary with the diameter of the shield, and the character and strength of the diametrical bracing. Mr. Raynald Legouez suggests as a rule for determining the thickness of the shell, that, to a minimum thickness of 2 mm., should be added 1 mm. for every meter of diameter over 4 meters. Referring to the illustrations, Figs. 128 to 132 inclusive, it will be noted that the St. Clair tunnel shield, 21^ ft. in diameter, had a shell of 1-in. steel SUBMARINE TUNNELING 257 plates with cover-plate joints and interior angle stiffeners ; the shell of the East River tunnel shield, 11 ft. in diameter, was made up of one -in. and one f-in. plate; the Blackwall tunnel shield, 27 ft. 9 ins. in diameter, had a shell consisting of four thicknesses of f-in. plates ; and the Clichy tunnel shield, with a diameter of 2.06 meters, had a shell 2 millimeters thick. Front-End Construction. By the front end is meant that portion of the shield between the cutting-edge and the vertical diaphragm. The length of this portion of the shield was formerly made quite small, and where the material penetrated is very soft, a short front-end construction yet has many advo- cates ; but the general tendency now is to extend the cutting- edge far enough ahead of the diaphragm to form a fair-sized working chamber. Excavation is far more easy and rapid when the face can be attacked directly from in front of the diaphragm than where the work has to be done form behind through the apertures in the diaphragm. So long as the roof of the excava- tion is supported from falling, experience has shown that it is easily possible to extend the excavation safely some distance ahead of the diaphragm. In reasonably stable material, tike compact clay, the front face will usually stand alone for the short time necessary to excavate the section and advance the shield one stage. In softer material the face can usually be sustained for the same short period by means of compressed air ; or the face of the excavation, instead of being made vertical, can be allowed to assume its natural slope. In the latter case a visor-shaped front-end construction, such as was used on some portions of the Clichy tunnel, is particularly advantageous. The following figures show the lengths of the front ends of a number of representative tunnel shields. City and South London . 1 ft. Mersey River 3 ft. St. Clair River .... 11.25 " East River 31 " Hudson River .... 53 " Blackwall 6.5 " Two general types of construction are employed for the cutting-edge. The first type consists of a cast-iron or cast-steel 258 TUNNELING ring, beveled to form a chisel-like cutting-edge, and bolted to the ends of the forward shell plates. This construction was first employed in the shield for the London Tower tunnel, and has since been used on the City and South London, Waterloo and City, and the Clichy tunnels. The second construction consists in bracing the forward shell plates by means of right triangular brackets, whose perpendicular sides are riveted respectively to the shell plates and the diaphragm, and whose inclined sides slant backward and downward from the front edge, and carry a conical ring of plating. The shields for the St. Clair River, East River, and Blackwall tunnels show forms of this type of cutting-edge construction. A modification of the second type of construction, which consists in omitting the conical plating, was employed on some of the shields for the Clichy tunnel. This modification is generally considered to be allowable only in materials which have little stability, and which crumble down before the advance of the cutting-edge. Where the material is of a sticky or compact nature, into which the shield in advancing must actually cut, the beveled plating is necessary to insure a clean cutting action without wedging or jamming of the material. Cellular Division. It is necessary in shields of large diam- eter to brace the shell horizontally and vertically against distortion. This bracing also serves to form stagings for the workmen, and to divide the shield into cells. The following table shows the arrangement of the vertical and transverse bracing in several representative tunnel shields. NAME OF TUNNEL. DlAMETEB. HORI- ZONTAL. PLATES, DlST. APAKT. VERT. BRACES. Hudson River Ft. 19 19.4 21 24 27 11 In. 11 6 10* 8 1 No. 2 2 2 2 2 None Ft. 6.54 6.54 6.98 7.12 6.0 No. 2 None 3 None 3 1 Clichy St. Clair River Waterloo (Station) .... Blackwall East River STJBMAKINE TUNNELING 259 Referring first to the horizontal divisions, it may be noted that they serve different purposes in different instances. In the Clichy tunnel shield the horizontal divisions formed simply working platforms , in the Waterloo tunnel shield they were designed to abut closely against the working face by means of special jacks, and so to divide it into three separate divisions ; in the St. Glair tunnel they served as working platforms, and also had cutting-edges for penetrating the material ahead; and in the Blackwall tunnel shield they served as working platforms, and had cutting-edges as in the St. Clair tunnel shield, and in addition the middle division was so devised that the two lower chambers of the shield could be kept under a higher pressure of air than the two upper chambers. Passing now to the vertical divisions, they serve to brace the shell of the shield against ver- tical pressures, and also to divide the horizontal chambers into cells; but unlike the horizontal plates they are not provided with cutting-edges. The St. Clair, Hudson River, and Black- wall tunnel shields illustrate the use of the vertical bracirg for the double purpose of vertical bracing and of dividing the hori- zontal chambers into cells. The Waterloo tunnel shield is an example of vertical bracing employed solely as bracing. The vertical division of the East River tunnel shield was employed in order to allow the shield to be dissembled in quadrants. The Diaphragm. The purpose of the shield diaphragm is to close the rear end of the shield and the tunnel behind from an inrush of water and earth from the face of the excavation. It also serves the secondary purpose of stiffening the shell diamet- rically. Structurally the diaphragm separates the front-end con- struction previously described from the rear-end construction, which will be described farther on ; and it is usually composed of iron or steel plating reinforced by beams or girders, and pierced with one or several openings by which access is had to the working face. In stable material, where caving or an inrush of water and earth is not likely, the diaphragm is 260 TUNNELING omitted. The shield of the Waterloo tunnel is an example of this construction. In more treacherous materials, however, not only is a diaphragm necessary, but it is also necessary to diminish the size of the openings through it, and to provide means for closing them entirely. Sometimes only one or two openings are left near the bottom of the diaphragm, as in the St. Clair and Mersey tunnel shields ; and sometimes a number of smaller openings are provided, as in the East River and Hudson River tunnel shields. In highly treacherous materials subject to sudden and violent irruptions of earth from the excavation face, it some- times is the case that openings, however small, closed in the ordinary manner, are impracticable, and special construction .has to be adopted to deal with the difficulty. The shields for the Mersey and for the Blackwall tunnels are examples of such special devices. In the Mersey tunnel a second diaphragm was built behind the first, extending from the bottom of the shield upward to about half its total height. The aperture in the first diaphragm being near the bottom, the space between the second and first diaphragms f ormed a trap to hold the inflowing material. The Blackwall tunnel shield, as previously indicated, had its front end divided into cells. Ordinarily the face of the excava- tion in front of each cell was left open, but where material was encountered which irrupted into these cells a special means of closing the face was necessary. This consisted of three poling- boards or shutters of iron held one above the other against the face of the excavation. These shutters were supported by means of strong threaded rods passing through nuts fastened to the vertical frames, which permitted each shutter to be ad- vanced against or withdrawn from the face of the excavation independently of the others. Various other constructions have been devised to retain the face of the excavation in highly treacherous soils, but few of them have been subjected to conclusive tests, and they do not therefore justify considera- tion. SUBMARINE TUNNELING 261 Rear-End Construction. By the rear end of the shield is meant that portion at the rear of the diaphragm. It may be divided into two parts, called respectively the body and the tail of the shield. The chief purpose of the body of the shield is to furnish a place for the location of the jacks, pumps, motors, etc., employed in manipulating the shield. It also serves a purpose in distributing the weight of the shield over a large area. To facilitate the passage of the shield around curves, or in changing from one grade to another, it is desirable to make the body of the shield as short as possible. In the Mersey, Clichy, and Waterloo tunnel shields, and, in fact, in most others which have been employed, the shell plates of the body have been reinforced by a heavy cast-iron ring, within and to which are attached the jacks and other apparatus. The latest opinion, however, seems to point to the use of brackets and beams for strengthening the shell for the purpose named, rather than to this heavy cast-iron construction. In the Hudson River, St. Clair River, and East River tunnel shields, with their long and strongly braced front-end construction to carry the jacks, the body of the shield, so to speak, is omitted, and the -rear-end construction consists simply of the tail plat- ing. In the Black wall shield, the body of the shield shell provides the space necessary for the double diaphragms and the cells which they inclose. In a general way, it may be said that the present tendency of engineers is to favor as short and as light a body construction as can be secured. The tail of the shield serves to support the earth while the lining is being erected; and for this reason it overlaps the forward ring of the lining, as shown clearly by most of the shields illustrated. To fulfill this purpose, the tail-plates should be perfectly smooth inside and outside, so as to slide easily between the outside of the lining plates and the earth, and should also be as thin as practicable, in order not to leave a large void behind the lining to be filled in. In soils which are fairly stable, the tail construction is often visor-shaped : 262 TUNNELING that is, the tail-plates overlap the lining only for, say, the roof from the springing lines up, as in one of the shields for the Clichy tunnel. In unstable materials, the tail-plating ex- tends entirely around the shield and excavation. The length of the tail-plating is usually sufficient to overlap two rings of the lining, but in one of the Clichy tunnel shields it will be noticed that it extended over three rings of lining. This seemingly considerable space for thin steel plates is made possible by the fact that the extreme rear end of the tail always rests upon the last completed ring of lining. In closing these remarks concerning the rear-end con- struction, the accompanying table, prepared by Mr. Raynald Legouez, will be of interest, as a general summary of principal dimensions of most of the important tunnel shields which have been built. The figures in this table have been converted from metric to English measure, and some slight variation. from the exact dimensions necessarily exists. The different columns of the table show the diameter, total length, and the length of each of the three principal parts into which tunnel shields are ordinarily divided in construction as previously described : NAME OF SHIKLD. LENGTH IN FEET. DIAMETER. TAIL. BODY. FRONT. TOTAL. Concorde Siphon .... 6 75 2.51 2.55 1.16 6.67 Clichy Siphon 8.39 2.51 2.55 1.16 6.16 Mersey 9.97 5.61 2.98 2.98 11.58 East River 10.99 3.51 0.32 3.67 7.51 City and South London 10.99 2.65 2.82 1 01 641) Glasgow District . . . 12.07 2.65 2.82 1.01 6.49 Waterloo and City . . . 12.99 2.75 2.98 1.24 6.T8 Glasgow Harbor .... 17.25 2.75 2.98 1.08 8.49 Hudson River 19.91 4.82 2.1)8 5.67 10.49 St Clair River 21 52 4 00 2 98 11 25 1") 25 Clichy Tunnel 237 198 4 00 2 98 6 88 17 2 Clichy Tunnel 23 8-19 4 7 44 11 90 4 46 23 6") Blackwall . .... 27 t 6 98 5 90 6 59 19 48 Waterloo Station .... 24.86 3.34 5.51 1.14 10.00 SUBMARINE TUNNELING 263 Jacks The motive power usually employed in driving modern tunnel shields is hydraulic jacks. In some of the earlier shields screw-jacks were used, but these soon gave way to the more powerful hydraulic device. The manner of attaching the hydraulic jacks to the shield is always to fasten the cylinder castings at regular intervals around the inside of the shell, with the piston rods extending backward to a bearing against the forward edge of the lining. In the older forms of shield, having an interior cast>iron reinforcing ring construc- tion, the jack cylinder castings were always attached to this cast-iron ring; but in many of the later shields constructed without this cast-iron reinforcing ring, the cylinder castings are attached to the shell by means of bracket and gusset con- nections. The number and size of the jacks employed, and the distance apart at which they are spaced, depend upon the size of the shield and the character of the material in which it is designed to work. In stiff and comparatively stable clays, the skin friction of the shield is comparatively small, and an ag- gregate jack-power of from 4 to 5 tons per square yard of the exterior friction surface of the shield has usually been found ample. The cylinders are spaced about 5| ft. apart, and have a working diameter of from 5 to 6 ins., with a water pressure of about 1,000 Ibs. per sq. in. In soft, sticky material, giving a high skin friction, the aggregate jack- power required per square yard of exterior shell surface rises to from 18 to 24 tons; the jacks are spaced about 3 ft. apart; and the working cylinder diameter and water pressure are, re- spectively, about 6 or 7 ins., and from 4,000 Ibs. to 6,000 Ibs. per sq. in. With these high pressures, power pumps are necessary to give the required water pressure ; but where the pressure required does not exceed 1,000 Ibs. per sq. in., hand pumps may be, and usually are, employed. The number of jacks required depends upon the diameter of the shield, and, of course, upon the distance apart which they are placed. In the City and South London tunnel shield six jacks were used, and 264 TUNNELING in the Blackwall shield 24 were used. The construction of the jacks for tunnel shields presents no features out of the usual lines of such devices used elsewhere. The jacks used on the East River tunnel shield are shown by Fig. 118, and those for the St. Clair River tunnel by Fig. 133. Part Transverse Section. Longitudinal Section. FIG. 133. Cast Iron Lining, St. Clair River Tunnel. Two general methods are employed for transmitting the thrust of the piston rods against the tunnel lining. The object sought in each is to distribute the thrust in such a manner that there is no danger of bending the thin front flange of the forward lining ring. In English practice the plan SUBMARINE TUNNELING 265 usually adopted, is to attach a shoe or bearing casting to the end of the piston rod, which will distribute the pressure over a considerable area. An example of this construction is the shield for the City and South London tunnel. In the East River and St. Clair River tunnels, built in America, the tail of the piston rod is so constructed that the thrust is carried directly to the shell of the lining. LINING. Either iron or masonry may be used for lining shield-driven tunnels but present practice is almost universally in favor of iron lining. As usually built, iron lining consists of a series of successive cast iron rings, the abutting edges of which are pro- vided with flanges. These flanges are connected by means of butts, the joints being packed with thin strips of wood, oakum, cement, or some other material to make them water-tight. Each lining ring is made up of four or more segments, which are provided with flanges for bolted connections similar to those fastening the successive rings. Generally the crown seg- ment is made considerably shorter than those forming the sides and bottom of the ring. The erection of the iron segments forming the successive rings of the lining may be done by hand in tunnels of small diameter where the weights to be handled are comparatively light, but in tunnels of large size special cranes attached to the shield or carried by the finished lining are employed. The construction of the iron lining for the East River tunnel is illustrated in Chapter XIX., and that for the St. Clair River tunnel is shown by Fig. 133. 266 TUNNELING CHAPTER XXII. ACCIDENTS AND REPAIRS IN TUNNELS DURING AND AFTER CONSTRUCTION. IN the excavation of tunnels it often happens that the dis- turbance of the equilibrium of the surrounding material by the excavation develops forces of such intensity that the timbering or lining is crushed and the tunnel destroyed. To provide against accidents of this kind in a theoretically perfect manner would require the engineer to have an accurate knowledge of the character, direction and intensity of the forces developed, and this is practically impossible, since all of these factors differ with the nature and structure of the material penetrated. The best that can be done, therefore, is to determine the general character and structure of the material penetrated, as fully as practicable, by means of borings and geological surveys, and then to employ timbering and masonry of such dimensions and character as have withstood successfully the pressures devel- oped in previous tunnels excavated through similar material. If, despite these precautions, accidents occur, the engineer is compelled to devise methods of checking and repairing them, and it is the purpose of this chapter to point out briefly the most common kinds of accidents, their causes, and the usual methods of repairing them. Accidents During Construction. Accidents may happen both during or after construction, but it is during construction, when the equilibrium of the surrounding material is first disturbed, and when the only support of the pressures developed is the timber strutting that they most commonly occur. ACCIDENTS AND REPA1KS IN TUNNELS 267 Causes of Collapse. Collapse in tunnels may be caused : (1) by the weight of the earth overhead, which is left unsupported by the excavation ; (2) by defective or insufficient strutting ; and (3) by defective or weak masonry. (1) The danger of collapse of the roof of the excavation is influenced by several conditions. One of these is the method of excavation adopted. It is obvious that the larger the volume of the supporting earth is, which is removed, the greater will be the tendency of the roof to fall, and the more intense will be the pressures which the strutting will be called upon to support. Thus the English and Austrian methods of tunneling, where the full section is excavated before any of the lining is placed, and where, as the consequence, the strutting has to sustain all of the pressures, piesent more likelihood of the roof caving in than any of the other common methods. The character and structure of the material penetrated also influence the danger of a collapse. A loose soil with little cohesion is of course more likely to cave than one which is more stable. Rock where strata are horizontal, or which is seamy and fissured, is more likely to break down under the roof pressures than one with vertical strata and of homogeneous structure. Soft sod containing boulders whose weight develops local stresses in the roof timbering is likely to be more danger- ous than one which is more homogeneous. A factor which greatly increases the danger of collapse, especially in soft soils, is the presence of water. This element often changes a soil which is comparatively stable, when dry, into one which is highly unstable and treacherous. The liability of the material to disintegration by atmospheric influences and various other conditions, which will occur to the reader, may influence its stability to a dangerous extent, and result in collapse. (2) Collapse is often the result of using defective or insuf- ficient strutting. Of course, in one sense, any strutting which fails under the pressures developed, however enormous they may be, can be said to be insufficient, but as used here the term 268 TUNNELING means a strutting with an insufficient factor of safety to meet probable increases or variations in pressure. Insufficient strut- ting may be due to the use of too light timbers, to the spacing of the roof timbers too far apart, to the yielding of the founda- tions, to insufficient bearing surface at the joints, etc. Collapse is often caused by the premature removal of the strutting dur- ing the construction of the masonry. The masons, to secure more free space in which to work, are very likely, unless watched, to remove too many of the timbers and seriously weaken the strutting. (3) The third cause of collapse is badly built masonry. Poor masonry may be due to the use of defective stone or brick, to the thinness of the lining, to poor mortar, to weak centers which allow the arch to become distorted during construction, to poor bonding of the stone or bricks, to the premature removal of the centers, to driving some of the roof timbers inside it, etc. Prevention of Collapse, Tunnels very seldom collapse with- out giving some previous warning of the possible failure, and also of the manner in which the failure is likely to occur. From these indications the engineer is often able t.> foresee the nature of the danger and take steps to check it. The danger may occur either during excavation or after the lining is built. During excavation the danger of collapse is indicated before- hand by the partial crushing or deflection of the strutting tim- bers. If the timbers are too light or the bearing surfaces are too small, crushing takes place where the pressures are the greatest, and the timbers bend, burst, or crack in places, and the joints open in other places. The remedy in such cases is to in- sert additional timbers to strengthen the weak points, or it may be necessary to construct a double strutting throughout. When the distance spanned by the roof timbers is too great, failure is generally indicated by the excessive deflection of these timbers, and this may often be remedied by inserting intermediate struts or props. In some respects the best remedy ACCIDENTS AND REPAIRS IN TUNNELS 269 under any of these conditions is to construct the masonry as soon as possible. When collapse is likely to occur after the masonry is com- pleted, its probability is generally indicated by the cracking and distortion of the lining. A study of the cause is quite likely to show that it is the percolation of water through the material surrounding the lining which causes cavities behind the lining in some places, and an increase of the pressures in other places. When it is certain that this water comes from the surface streams above, these streams may often be diverted or have their beds lined with concrete to prevent further perco- lation. When percolating water is not the cause of the trouble, a usually efficient remedy is to sink a shaft over the weak point, and refill it with material of more stable character. These, and the remedies previously suggested, are designed to prevent failure without resorting to reconstruction. When they or similar means prove insufficient, reconstruction or repairs have to be resorted to. Repairing Failures. Tunnels may collapse in several ways : (1) The front and sides of the excavation may cave in; (2) the floor or bottom may bulge or sink ; (3) the roof may fall in ; (4) the material above the entrances may slide and fill them up. (1) One of the most common accidents is the caving of the front and sides of the excavation. This may often be prevented by taking care that the face of the excavation -follows the natu- ral slope of the material instead of being more or less nearly vertical. When, however, caving does occur it may usually be repaired by removing the fallen material, strongly shoring the cavity, and filling in behind with stone, timber, or fascines. (2) The bulging or rising of the bottom of the tunnel may usually be considered as a consequence of the squeezing together of the side walls. It usually occurs in very loose soils, and is chiefly important from the fact that the reconstruction of the walls is made necessary. The sinking of the tunnel bot- 270 TUNNELING torn is a more serious occurrence. It seldom happens unless there is a cavity beneath the floor, due either to natural causes or to the fact that mining operations have gone on in the hill or mountain penetrated by the tunnel. When the bottom of the tunnel sinks, three cases may be considered : (a) when the sinking is limited to the middle of the tunnel floor ; (>) when only a portion of the foundation masonry is affected ; and, ( Cross Section. Longitudinal Section. FIG. 144. Kelining Timber-Lined Tunnel, Great Northern Ky. 284 TUNNELING and has a grade of 20 dynamo of 20 arc light capacity, one arc light being placed on each side of the tunnel at all working-places. Each lamp carried a coil of wire 20 or 30 ft. long to allow it to be shifted from place to place without delay. Mullan Tunnel. This tunnel is 3,850 ft. long, and crosses the main range of the Rocky Mountains, about 20 miles west of Helena, Mont. The tunnel is on a tangent throughout, falling toward the east. The summit of the grade, west of the tun- nel, is 5,548 ft. above sea level, and the mountain above the line of the tunnel rises to an elevation of 5,855 ft. Owing to the treacherous nature of the material through which the tunnel passed, it had been a constant menace to traffic ever since its con- struction in 1883, and numer- ous delays to trains had been caused by the falls of rock and fires in the timber lin- Permanent Work. FIG. 145. Relining Timber Lined Tunnel, Great Northern Ry. ing. For these reasons it was finally decided to build a per- manent masonry lining, and work on this was begun in July, 1892. The original timbering consisted of sets spaced 4 ft. apart c. to