MODEM KOAD CONSTRUCTION
A PRACTICAL TREATISE ON THE ENGINEERING PROBLEMS OF
ROAD BUILDING, WITH CAREFULLY COMPILED SPECI-
FICATIONS FOR MODERN HIGHWAYS, AND
CITY STREETS AND BOULEVARDS
AUSTIN T. BYRNE
CIVIL ENGINEER
AUTHOR OF
'HIGHWAY CONSTRUCTION," "MATERIALS AND WORKMANSHIP'
ILLUSTRATED
AMERICAN TECHNICAL SOCIETY
CHICAGO
1917
COPYRIGHT, 1917, BY
AMERICAN TECHNICAL SOCIETY
COPYRIGHTED IN GREAT BRITAIN
ALL RIGHTS RESERVED
INTRODUCTION
THE science of good road building is an old one as evidenced
by the many highways in Europe which have withstood the
wear of travel for centuries. Most of these famous roads were
cut from solid rock or built of crushed stone of such a character
as to be unaffected by weather conditions. Modern road building,
however, has been largely influenced within the past fifteen years
by the enormous increase in the amount of travel due to the auto-
mobile. This has not only been the means of developing new road
surface to meet the more severe requirements of this type of vehicle
but it has developed a country-wide interest in good roads, thus
making it possible for the enthusiastic travelers to take long tours
without meeting the formerly ever-present bugaboo of bad roads,
besides making the ordinary town-to-town travel more satisfactory.
<I It is with the idea of giving a clear conception of the engineering
problems involved in road building, that is, laying out of the road
by the best and easiest route, the questions of grade, contour,
and drainage, and the construction of culverts and bridges, that
this treatise has been written. The author has had long experience
in the field of highway construction and has treated the different
types of roads in a very complete and practical manner. Natural
soil, gravel, broken stone, bituminous macadam and concrete
roads are all carefully treated, not only as to material, but as to the
best methods of laying them. The city pavements are also given
due consideration, accompanied by typical specifications for the
new surfaces developed for boulevards.
<I Altogether, the article covers the entire field of road building,
both city and country, and should appeal either to the highway
engineer or to the untrained reader who has merely a passing
interest in the subject.
365550
CONTENTS
COUNTRY ROADS AND BOULEVARDS
PAGE
Resistance to movement of vehicles 1
Resistance to traction 1
Tractive power and gradients ." 7
Axle friction 11
Resistance of air 12
Location of roads 12
Reconnoissance 13
Preliminary survey 15
Topography 15
Map 20
Memoir 22
Bridge sites 22
Final selection of route 22
Preliminary road construction methods. . 35
Width and transverse contour 35
Drainage 38
Types of drainage 38
Nature of soils 39
Location of drains 39
Proper fall for drains 40
Materials used for drains 40
Sizes of drams 42
Silt basins 42
Protection of drain ends from weather 42
Drain outlets. 43
Side ditches 43
Treatment of springs found in cuttings 44
Drainage for hillside roads 45
Inner and outer road gutters 45
Culverts 46
Earthenware pipe culverts 49
Iron pipe culverts. . . 51
Box culverts 52
Arch culverts 53
Short span bridges used as culverts 53
Earthwork 55
Balancing cuts and fills 55
Side slopes 55
Shrinkage of earthwork 57
Prosecution of earthwork 58
Methods of forming embankments 58
CONTENTS
Preliminary road construction methods (Continued) PAGE
Tools for construction work 60
Natural-soil roads 74
Earth roads 74
Sand roads 77
Sand-clay roads 77
Application of oil to sand and gravel soils 78
Roads with special coverings 79
Foundations 79
Materials 79
Thickness 79
Types of foundation to be used '. 81
Wearing surfaces 82
Maintenance and improvement of roads 109
Repair and maintenance of broken-stone roads 109
Systems of maintenance 1 10
Improvement of existing roads 110
Traffic census Ill
CITY STREETS AND HIGHWAYS
Foundations ' 121
Stone-block pavements , 123
Materials 124
Cobblestone pavement 125
Belgian-block pavement 125
Granite-block pavement 126
Blocks .' ,.... 127
Manner of laying blocks 127
Foundation 129
Cushion coat 129
Laying blocks 130
Ramming 130
Fillings for joints 130
Stone pavement on steep grades 132
Brick pavements 133
Qualifications of brick 133
Tests for paving brick 136
Brick-pavement qualifications 137
Foundation 137
Sand cushion 137
Manner of laying 138
Joint fillings 139
Tools used by hand in the construction of block pavements 145
Concrete-mixing machine 145
Gravel heaters. . 146
CONTENTS
PAGE
Wood-block pavements 147
Creosoting 147
Laying the blocks 149
Qualifications of wood pavements 152
Asphalt pavements 153
Sheet-asphalt pavement 153
Laying the pavement. . 155
Foundation 157
Qualifications of asphalt pavements 157
Failure of asphalt pavement 159
Rock asphalt pavement 161
Asphalt blocks ! 161
Tools employed in construction of asphalt pavements. 162
Miscellaneous pavements '164
Burnt clay 164
Straw 164
Oyster-shell 164
Chert 164
Slag 164
Clinker 165
Petrolithic 165
Kleinpflaster 165
Iron 165
Trackways 165
National pavement 166
Filbered asphalt pavement 166
Miscellaneous street work 166
Curbstones and gutters 170
Curbstones 170
Combination curb and gutter 171
Street cleaning 172
Cleaning methods 172
Removal of snow 175
Street sprinkling 176
Selecting the pavement 176
Qualifications 176
Interests affected 177
Problem involved in selection 177
Economic benefit 181
Relative economies. \ 181
Gross cost of pavements 184
Comparative rank of pavements 185
Specifications 185
Contracts . . ... 187
HIGHWAY CONSTRUCTION
PART I
COUNTRY ROADS AND BOULEVARDS
RESISTANCE TO MOVEMENT OF VEHICLES
The object of a road is to provide a way for the transportation
of persons and goods from one place to another with the least
expenditure of power and expense. The facility with which this
traffic or transportation may be conducted over any given road
depends upon the resistance offered to the movement of vehicles.
This resistance is composed of: (1) resistance offered by the road-
way, which consists of (a) "friction" between the surface of the
road and the wheel tires, (b) resistance offered to the rolling of
the wheels occasioned by the want of uniformity in the road surface
or lack of strength to resist the penetrating efforts of loaded wheels,
(c) resistance due to gravity called "grade resistance"; (2) resistance
offered by vehicles, termed "axle friction"; and (3) resistance of
the air. The magnitude of each of the components has a wide
range, varying with the kind and condition of the road and its
surface, the form and condition of the vehicle, the load, and the
speed.
Resistance to Traction. The combination of road resistances
is designated by the general term "resistance to traction", the
magnitude of which is measured by the number of pounds of effort
per ton of the load required to overcome it; this is ascertained by
a form of spring-balance variously called "dynograph", "tracto-
graph", etc., one end of which is attached to the vehicles and the
other end to the draft animals.
The road which offers the least resistance to traffic should
combine a surface on which the friction of the wheels is reduced
to the least possible amount, while possessing sufficient roughness
to afford good foothold for the draft animals and good adhesion
for motor vehicles; and should be so located as to give the most
direct route with the least gradients.
HIGHWAY CONSTRUCTION
TABLE I
Resistance to Traction on Different Road Surfaces
ROAD SURFACE
V
TRACTION RESISTANCE
Pounds per Ton
In Terms of Load
Earth road ordinary condition
Gravel
Sand
Macadam
Plank road
Steel wheelway
50 to 200
50 to 100
100 to 200
30 to 100
30 to 50
15 to 40
4 T tO A
4*0- tO 2*0
^0 tO T"O
17 tO. ^5
iV to^
ik tO rif
Friction. The resistance of friction arises from the rubbing
of the wheel tires against the surface of the road; its amount can
be determined only by experiment for each kind of road surface.
From many experiments the following deductions are drawn:
(1) The resistance to traction is directly proportional to the pressure.
(2) On solid unyielding surfaces, the resistance is independent of the
width of the tire; but on compressible surfaces it decreases as the width of the
tire increases. There is no material advantage gained, however, in making
a tire more than 4 inches wide, for the reason that it is impossible to distribute
the load evenly over the road owing to the irregularities and curvatures of its
surface.
(3) On uniformly smooth surfaces, the resistance is independent of the
speed.
(4) On rough irregular surfaces, which give rise to constant concussion,
the resistance increases with the speed.
Table I shows the relative resistance to traction of various
surfaces. These coefficients refer to the power required to keep the
load in motion. It requires from two to six or eight times as much
force to start a load as it does to keep it in motion at two or three
miles per hour. The extra force required to start a load is due in
part to the fact that during the stop the wheel may settle into the
road surface; in part to the fact that the axle friction at starting
is greater than after motion has begun; and in part to the fact
that energy is consumed in accelerating the load.
Resistance to Rolling. Resistance to rolling is caused partly
by the wheel penetrating or sinking below the surface of the road,
forming a depression or rut, as shown in Fig. 1, thus compelling
the wheel to be continually rolling up a short incline. The measure
of this resistance is the horizontal force necessary at the axle to
HIGHWAY CONSTRUCTION
roll it up the incline; and is equal to the product of the load multi-
plied by one-third of the semi-chord of the submerged arc of the
wheel.
Resistance to rolling is also caused by the wheel striking or
colliding with loose or projecting stones, which suddenly checks
Road Surface
fJl Rest 3
Natural Soil/ ^Rood Covering
Fig. 1. Exaggerated Section of Road under Pressure of Loaded Vehicle
the motive power; the momentum thus destroyed varies with the
height of the stone or obstacle and is often considerable.
In both cases the power required to overcome the resistance
is affected largely by the diameter of the wheel, as the larger the
wheel the less force is required to lift it over the obstruction or
to roll it up the inclination due
to the indentation of the sur-
face.
Illustrative Example. The
power required to draw a wheel
over ^a stone or any obstacle,
such as S in Fig. 2, may be thus
calculated :
Let P represent the power
sought, or that which would
just balance the weight on the
point of the stone, and the slightest increase of which would draw
it over. This power acts in the direction CP with the leverage of
BC or DE. The force of gravity W resists in the direction
CB with the leverage BD. The equation of equilibrium will be
PXCB = WXBD, whence
R 5
Fig. 2. Diagram for Calculating Power
Required to Draw Wheel over
Resisting Object
BD =
CB CA
AB
4 HIGHWAY CONSTRUCTION
Let the radius of the wheel equal CD = 26 inches, and the height
of the obstacle equal AB = 4: inches. Let the weight 11' = 500
pounds, of which 200 pounds may be the weight of the wheel and
300 pounds the load on the axle. The formula then becomes
p gnn V676-484 rm 13.85
P = 500 - =500 =314.7 Ib.
2o 4 22
The pressure at the point D is compounded of the weight and the
power, and equals
CD 26
H/ li =500x l= 5911b -
Therefore this pressure acts with this great effect to destroy the road
in its collision with the stone; in addition there is to be considered
the effect of the blow given by the wheel in descending from it.
For minute accuracy the non-horizontal direction of % the draft and
the thickness of the axle should be taken into account. The power
required is lessened by proper springs to vehicles, by enlarged wheels,
and by making the line of draft ascending.
Illustrative Example. The mechanical advantage of the wheel
in surmounting an obstacle may be computed from the principle
of the lever. Let the wheel,
Fig. 3, touch the horizontal line
of traction in the point A and
meet a protuberance ED. Sup-
pose the line of draft CP to be
parallel to AB. Join CD and
draw the perpendiculars DE and
DF. We may suppose the power
Fig. 3. Force Diagram for Wheel Drawn to be applied at E and the Weight
over Obstacle ,, . ,1 (1
at F, and the action is then the
same as the bent lever EDF turning round the fulcrum at D. Hence
P : W :: FD : DE. But FD : DE :: tan FCD : 1 ; and tan FCD =
tan 2 DA B; therefore P = Wtsm2DAB. Now it is obvious that the
angle DAB increases as the radius of the circle diminishes; there-
fore, the weight W being constant, the power required to overcome
an obstacle of given height is diminished when the diameter is
increased. Large wheels are, therefore, the best adapted for sur-
mounting inequalities of the road.
HIGHWAY CONSTRUCTION
TABLE II
Resistance Due to Gravity on Different Inclinations
Grade 1 inch
20
30
40
50
60
' 70
80
90
100
200
300
400
Rise in feet per mile
264
176
132
105
88
75
66
58
52
26
17
13
Resistance in pounds
per ton
100
66$
50
40
33f
28|
25
221
20
10
6f
5
There are, however, circumstances which provide limits to
the height of the wheels of vehicles. If the radius AC exceeds
the height of that part of the horse to which the traces are attached,
the line of traction CP will be inclined to the horse, and part of the
power will be exerted in pressing the wheel against the ground.
The best average size of wheels is considered to be about 6 feet in
diameter. Wheels of large diameter do less damage to a road than
small ones, and cause less draft for the horses. With the same load,
a two-wheeled cart does far more damage than one with four wheels,
and this because of their sudden and irregular twisting motion in
the trackway.
Grade Resistance. Grade resistance is due to the action of
gravity, and is the same on good and bad roads. On level roads
its effect is immaterial, as it acts in a direction perpendicular to
the plane of the horizon and neither accelerates nor retards motion.
On inclined roads it offers considerable resistance, proportional to
the steepness of the incline. The resistance due to gravity on any
incline in pounds per ton is equal to
2000
rate of grade
Table II shows the resistance due to gravity on different grades.
The additional resistance caused by inclines may be investigated
in the following illustrated example.
Illustrative Example. Suppose the whole weight to be borne
on one pair of wheels, and that the tractive force is applied in a
direction parallel to the surface of the road.
Let AB, Fig. 4, represent a portion of the inclined road, C
being a vehicle just sustained in its position by a force acting in the
direction CD. It is evident that the vehicle is kept in its position
by three forces : namely, by its own weight W acting in the vertical
direction CF; by the force F applied in the direction CD parallel
6
HIGHWAY CONSTRUCTION
Fig. 4. Forces Acting on Vehicle When
Drawn up Inclined. Road
to the surface of the road; and by the pressure P which the vehicle
exerts against the surface of the road acting in the direction CE
perpendicular to the same. * To determine the relative magnitude
of these three forces, draw the horizontal line AG and the vertical
line BG; then, since the two lines CF and BG are parallel and are
both cut by the line AB, they must make the two angles CFE and
ABG equal; also the two angles CEF and AGE are equal; there-
fore, the remaining angles FCE and BAG are. equal, and the two
triangles CFE and ABG are similar. And as the three sides of
the former are proportional to
the three forces by which the
vehicle is sustained, so also
are the three sides of the lat-
ter; namely, the length of the
road AB is proportional to
W, or the weight of the
vehicle; the vertical rise BG
is proportional to F, or the
force required to sustain the
vehicle on the incline; and the horizontal distance AG in which
the rise occurs is proportional to P, or the force with which the
vehicle presses upon the surface of the road. Therefore,
W : AB : : F : GB
and
W : AB : : P : AG
If to AG such a value be assigned that the vertical rise of the
road is exactly one foot, then
W W
and
in which A is the angle BAG.
To find the force requisite to sustain a vehicle upon an inclined road
(the effects of friction being neglected), divide the weight of the vehicle
and its load by the inclined length of the road, the vertical rise of
which is one foot, and the quotient is the force required.
HIGHWAY CONSTRUCTION
TABLE III
Tractive Power of Horses at Different Velocities
MILES PER HOUR
TRACTIVE FORCE
db.)
MILES PER HOUR
TRACTIVE FORCE
(lb.)
I
333.33
21
111.11
250
2
100
11
200
2|
90.91
H
166.66
3
83.33
if
142.86
3
71.43
2
125
4
62.50
To find the pressure of a vehicle against the surface of an inclined
road, multiply the weight of the loaded vehicle by the horizontal
length of the road, and divide the product by the inclined length
of the same; the quotient is the pressure required. The force
with which a vehicle presses upon an inclined road is always less
than its actual weight; the difference is so small that, unless the
inclination is very steep, it may be taken equal to the weight of the
loaded vehicle.
To find the resistance to traction in passing up or down an incline,
ascertain the resistance on a level road having the same surface as
the incline, to which add, if the vehicle ascends, or subtract, if
it descends, the force requisite to sustain it on the incline; the sum
or difference, as the case may be, will express the resistance.
Tractive Power and Gradients. Although transportation by
mechanically propelled vehicles will continue to increase, it is not
probable that for many years it will become more important than
traffic drawn by animals; and as mechanically propelled vehicles
can ascend any grade feasible for animals, it is only necessary to
discuss the effect of grades on horse-drawn traffic.
Tractive Power of Horses. The necessity for easy grades is
dependent upon the power of the horse to overcome the resistance
to motion, which is composed of four forces, viz, friction, collision,
gravity, and resistance of air. All estimates on the tractive power
of horses must to a certain extent be vague, owing to the different
strengths and speeds of animals of the same kind, as well as to the
extent of their training to any particular kind of work.
The draft or pull which a good average horse, weighing 1,200
pounds, can exert on a level, smooth road at a speed of 2 J miles per
HIGHWAY CONSTRUCTION
TABLE IV
Variation of Tractive Power with Time
HOURS PER DAY
TRACTIVE FORCE
db.)
HOURS PER DAY
TRACTIVE FORCE
db.)
10
100
7
146$
9
111*
6
166|
8
125
5
200
NOTE: The tractive power of teams may be found by multiplying the
above values by the following constants:
1 horse =1
2 horses 0.95X2 = 1.90
3 horses 0.85X3=2.55
4 horses 0.80X4 = 3.20
hour is 100 pounds; which is equivalent to 22,000 foot-pounds per
minute, or 13,200,000 foot-pounds per day of 10 hours. The tractive
power diminishes as the speed increases and, perhaps, within
certain limits, say from f mile to 4 miles per hour, nearly in inverse
proportion to it. Thus the average tractive force of a horse, on a
level, and actually pulling for 10 hours, may be assumed approx-
imately as shown in Table III.
The work done by a horse is greatest when the velocity with
which he moves is one-eighth of the greatest velocity with which
he can move when unloaded; and the force thus exerted is 0.45
of the utmost force that he can exert at a dead pull. The tractive
power of a horse may be increased in about the same proportion
as the time is diminished, so that when working from 5 to 10 hours
on a level, it will be about as shown in Table IV.
Loss of Tractive Power on Inclines. In ascending inclines a
horse's power diminishes rapidly; a large portion of his strength is
expended in overcoming the resistance of gravity due to his own
weight and that of the load. Table V shows that as the steepness
of the grade increases, the efficiency of both the horse and the road
surface diminishes; that the more of the horse's energy which is
expended in overcoming gravity, the less remains to overcome the
surface resistance.
Table VI shows the gross load which an average horse, weighing
1,200 pounds, can draw on different kinds of road surfaces, on a
level and on grades rising 5 and 10 feet per 100 feet.
HIGHWAY CONSTRUCTION
TABLE V
Effects of Grades upon the Load a Horse Can Draw on Different
Pavements
GRADE
EARTH
BROKEN STONE
STONE BLOCKS
ASPHALT
Level
1.00
1.00
1.00
1.00
1 100
.80
.66
.72
.41
2 100
.66
.50
.55
.25
3 100
.55
.40
.44
.18
4 100
.47
.33
.36
.13
5 100
.41
.29
.30
.10
10 100
.26
.16
.14
.04
15 100
.10
.05
.07
20 100
.04
.03
The decrease in the load which a horse can draw upon an incline
is not due alone to gravity; it varies with the amount of foothold
afforded by the road surface. The tangent of the angle of inclination
should not be greater than the coefficient of tractional resistance.
Therefore, it is evident that the smoother the road surface, the easier
should be the grade; the smoother the surface the less the foothold,
and consequently the less the possible load.
The loss of tractive power on inclines is greater than any inves-
tigation will show; for, besides the increase of draft caused by
gravity, the power of the horse is much diminished by fatigue upon
a long ascent, and even in greater ratio than man, owing to its
anatomical formation and great weight. Though a horse on a
level is as strong as five men, on a grade of 15 per cent, it is less
strong than three; for three men carrying each 100 pounds will
ascend such a grade faster and with less fatigue than a horse with
300 pounds.
A horse can exert for a short time twice the average tractive
pull which he can exert continuously throughout the day's work;
hence, so long as the resistance on the incline is not more than double
the resistance on the level, the horse will be able to take up the full
load which he is capable of drawing.
Steep grades are thus seen to be objectionable, and particularly
so when a single one occurs on an otherwise comparatively level
road, in which case the load carried over the less inclined portions
must be reduced to what can be hauled up the steeper portion.
The bad effects of steep grades are especially felt in winter,
10
HIGHWAY CONSTRUCTION
TABLE VI
Gross Loads for Horse on Different Pavements on Different Grades
DESCRIPTION OF SURFACE
LEVEL
GRADE
(5 per cent)
GRADE
(10 per cent)
Asphalt
13,216
Broken stone (best condition)
6,700
1^840
1^060
Broken stone (slightly muddy)
4,700
1,500
1,000
Broken stone (ruts and mud)
3,000
1,390
890
Broken stone (very bad condition)
1,840
1,040
740
Earth (best condition)
3,600
1,500
930
Earth (average condition)
1,400
900
660
Earth (moist but not muddy)
1,100
780
600
Stone-block pavement (dry and clean)
8,300
1,920
1,090
Stone-block pavement (muddy)
6,250
1,800
1,040
Sand (wet)
1,500
675
390
Sand (dry)
1,087
445
217
when ice covers the roads, for the slippery condition of the surface
causes danger in descending, as well as increased labor in ascending;
during heavy rains the water also runs down the road and gulleys
it out, destroying its surface, thus causing a constant expense for
repairs. The inclined portions are subject to greater wear from
horses ascending, thus requiring thicker covering than the more
level portions, and hence increasing the cost of construction.
It will rarely be possible, except in a flat or comparatively level
country, to combine easy grades with the best and most direct
route. These two requirements will often conflict. In such a case,
increase the length of the road. The proportion of this increase
will depend upon the friction of the covering which is adopted.
But no general rule can be given to meet all cases as respects the
length which may thus be added, for the comparative time occupied
in making the journey forms an important element in any case
which arises for settlement. Disregarding time, the horizontal
length of a road may be increased to avoid a 5 per cent grade, seventy
times the height.
Table VII shows, for most practical purposes, the force required
to draw loaded vehicles over inclined roads. In the fifth column
the length given is the length which would require the same motive
power to be expended in drawing the load over it, as would be
necessary to draw over a mile of the inclined road. The loads
given in the sixth column are the maximum loads which average
HIGHWAY CONSTRUCTION
11
TABLE VII
Data for Loaded Vehicles over Inclined Roads
Rate of Grade
(ft. per 100
ft.)
Pressure on
Plane
(Ib. per ton)
Tendency
down Plane
(Ib. per ton)
Power
Required to
Haul 1 Ton
up Plane
(Ib.)
Equivalent
Length of
Level Road
(mi.)
Maximum
Load Horse
Can Haul
(Ib.)
0.00
2240
.00
45.00
1.000
6270
0.25
2240
5.60
50.60
1.121
5376
0.50
2240
11.20.
56.20
1.242
4973
0.75
2240
16.80
61.80
1.373
4490
1.00
2240
22.40
67.40
1.500
4145
1.25
*2240
28.00
73.00
1.622
3830
1.50
2240
33.60
78.60
1.746
3584
1.75
2240
39.20
84.20
1.871
3290
2.00
2240
45.00
90.00
2.000
3114
2.25
2240
50.40
95.40
2.120
2935
2 . 50.
2240
56.00
101.00
2.244
2725
2.75
2240
61.33
106.33
2.363
2620
3.00
2239
67.20
112.20
2.484
2486
4.00
2238
89.20
134.20
2.982
2083
5.00
2237
112.00
157 . 00
3.444
1800
6.00
2233
134.40
179 . 40
3.986
1568
7.00
2232
156.80
201 . 80
4.844
1367
8.00
2232
179.20
224 . 20
4.982
1235
9.00
2231
201.60
246.60
4.840
1125
10.00
2229
224.00
269.00
5.977
1030
* Near enough for practice; actually 2239.888
Pressure on plane == weight Xnat cos of angle of plane
horses weighing 1,200 pounds can draw over such inclines, the friction
of the surface being taken at -V of the load drawn.
Axle Friction. The resistance of the hub to turning on the
axle is the same as that of a journal revolving in its bearing, and has
nothing to do with the condition of the road surface. The coefficient
of journal friction varies with the material of the journal and its
bearing, and with the lubricant. It is nearly independent of the
velocity, and seems to vary about inversely as the square root of the
pressure. For light carriages when loaded, the coefficient of friction
is about 0.020 of the weight on the axle; for the ordinary thimble-
skein wagon when loaded, it is about 0.012. These coefficients are
for good lubrication; if the lubrication is deficient, the axle friction
is 2 to 6 times as much as above.
The tractive power required to overcome the above axle friction
for carriages of the usual proportions is about 3 to 3f pounds per
ton of the weight on the axle; and for truck wagons, which have
medium sized wheels and axles, it is about 3J to 4J pounds per ton.
12
HIGHWAY CONSTRUCTION
TABLE VIII
Wind Pressure for Various Vehicles
DESCRIPTION
VELOCITY OF WIND
(mi. per hour)
WIND PRESSURE
(Ib. per sq. ft.)
Pleasant breeze
15
1.107
Brisk gale
20
25
1.968
3.075
High wind
30
35
4.428
6.027
Very high wind
40
45
7.782
9.963
Storm
50
12.300
Effect of Springs on Vehicles. Experiments have shown that
springs mounted in vehicles materially decrease the resistance to
traction and diminish the effects caused in the vertical plane by
irregularities of the surface ; but they do not diminish the horizontal
component which is the one that causes the greatest wear of the
road, especially at speeds beyond a walking pace. The vehicles
with springs were found not to cause more wear with the horses
going at a trot than vehicles without springs when the horses were
walking, all other conditions being similar. Vehicles with springs
improperly fixed cause considerable concussion which, in turn,
destroys the road covering.
Resistance of Air. The resistance arising from the force of
the wind will vary with the velocity of the wind, with the velocity
of the vehicle, with the area of the surface acted upon, and also
with the angle of incidence of direction of the wind with the plane
of the surface. Table VIII gives the force per square foot for
various velocities.
LOCATION OF ROADS
The considerations governing the location of roads are
dependent upon the commercial condition of the country to be trav-
ersed. In old and long-inhabited sections, the controlling element
will be the character of the traffic to be accommodated. In such
a section, the route is generally predetermined and, therefore, there
is less liberty of choice and selection than in a new and sparsely
settled district, where the object is to establish the easiest, shortest,
HIGHWAY CONSTRUCTION 13
and most economical line of intercommunication according to
the physical character of the ground.
Whichever of these two cases may have to be dealt with, the
same principle governs the engineer, namely, so to lay out the road
as to effect the conveyance of the traffic with the least expenditure
of motive power consistent with economy of construction and main-
tenance.
Economy of motive power is promoted by easy grades, by the
avoidance of all unnecessary ascents and descents, and by a direct
line; but directness must be sacrificed to secure easy grades and to
avoid expensive construction.
RECONNOISSANCE
Object of Reconnoissance. The selection of the best route
demands much care and consideration on the part of the engineer.
To obtain the requisite data upon which to form his judgment,
he must make a personal reconnoissance of the district. This
requires that the proposed route be either ridden or walked over
and a careful examination made of the principal physical contours
and natural features of the district. The amount of care demanded
and the difficulties attending the operations will altogether depend
upon the character of the country. The immediate object of the
reconnoissance is to select one or more trial lines, from which the
final route may be ultimately determined. When there are no
maps of the section traversed, or when those which can be procured
are indefinite or inaccurate, the work of reconnoitering will be much
increased.
Points to Consider. In making a reconnoissance there are several
points which, if carefully attended to, will very considerably lessen
the labor and time otherwise required. Lines which would run along
the immediate bank of a large stream must of necessity intersect
all the tributaries confluent on that bank, thereby demanding a
corresponding number of bridges. Those, again, which are situated
along the slopes of hills are more liable in rainy weather to suffer
from the washing away of the earthwork and the sliding of the
embankments; the others which are placed in valleys or on elevated
plateaux, when the line crosses the ridges dividing the principal
watercourses, will have steep ascents and descents.
14 HIGHWAY CONSTRUCTION
In making an examination of a tract of country, the first point
to attract notice is the unevenness or undulation of its surface,
which appears to be entirely without system, order, or arrangement;
but upon closer examination it will be perceived that one general
principle of configuration obtains even in the most irregular countries.
The country is intersected in various directions by main watercourses
or rivers, which increase in size as they approach the point of their
discharge. Towards these main rivers lesser rivers approach on
both sides, running right and left through the country, and into
these, again, enter still smaller streams and brooks. The streams
thus divide the hills into branches or spurs having approximately
the same direction as themselves, and the ground falls in every
direction from the main chain of hills towards the watercourses,
forming ridges more or less elevated.
The main ridge is cut down at the heads of the streams into
depressions called gaps or passes; the more elevated points are
called peaks. The water which has fallen upon these peaks is the
origin of the streams which have hollowed out the valleys. Further-
more, the ground falls in every direction towards the natural water-
courses, forming ridges more or less elevated running between them
and separating from each other the districts drained by the streams.
The natural watercourses mark not only the lowest lines, but
the lines of the greatest longitudinal slope in the valleys through
which they flow. The direction and position of the principal streams
give also the direction and approximate position of the high ground
or ridges which lie between them. The positions of the tributaries
to the larger stream generally indicate the points of greatest depres-
sion in the summits of the ridges and, therefore, the points at which
lateral communication across the high ground separating con-
tiguous valleys can be most readily made.
Instruments Used. The instruments employed in reconnoiter-
ing are: the compass, which is used to ascertain direction; the
aneroid barometer, to fix the approximate elevation of summits,
etc.; and the hand level, to ascertain the elevation of neighboring
points. If a vehicle can be 1 used, an odometer may be added, but
distances can usually be guessed or ascertained by time estimates
closely enough for preliminary purposes. The best maps obtainable
and traveling companions who possess a local knowledge of the
HIGHWAY CONSTRUCTION 15
country, together with the above outfit, are all that will be necessary
for the first inspection.
PRELIMINARY SURVEY
The routes selected through the reconnoissance are examined
in detail by a survey called a "preliminary survey" from the results
of which the exact location can be determined.
Features to Be Considered. In making the preliminary survey,
the topographical features are noted to the right and left of the
transit line for a convenient distance. The data required for drawing
the topography are obtained by levels taken with a leveling instru-
ment or with a transit provided with stadia wires, on lines per-
pendicular to the transit line of the survey. The location of build-
ings, fences, streams, roads, railroads, and other objects, is determined
by measurements made with a tape on lines perpendicular to the
survey line; or, when the distance to the object required is con-
siderable, the location is found by angles measured from two stations
on the transit line and the distance is measured by stadia. The
following information is also noted: the importance, magnitude,
and direction of the streams crossed; the character of the material
to be excavated or available for embankments; the position of
quarries; the mode of access thereto, and the kind of stone; the posi-
tion of unloading points on railroads; and any other information
that might affect a selection.
Topography. Levels. Levels should be taken along the course
of each line, usually at every 100 feet, or at closer intervals,
depending upon the nature of the country. In taking the levels, the
heights of all existing roads, railroads, rivers, or canals should be
noted. "Bench marks" should be established at least every half
mile, that is, marks made on any fixed object, such as a gate post,
side of a house, or, in the absence of these, a cut made on a large
tree. The height and exact description of each bench mark should
be recorded in the level book.
Cross Levels. Wherever considered necessary, levels at right
angles to the center line should be taken. These will be found
useful in showing what effect a deviation to the right or left of the
surveyed line would have. Cross levels should be taken at the
intersection of all roads and railroads to show to what extent, if
16
HIGHWAY CONSTRUCTION
Fig. 5. Contour Map Used in Road Surveys
HIGHWAY CONSTRUCTION 17
%
any, these levels will have to be altered to suit the levels of the
proposed road.
Contours. The levels of the transit and cross lines are worked
into a map that shows the irregularities of the ground with reference
to its elevations and depressions. Various methods are employed
for delineating these upon paper. For the purpose of the engineer
the method of contours, Fig. 5, is the most serviceable, since by
it the true shape of the hills and valleys can be shown.
Contours are lines drawn through the points of equal altitude;
that is, every point of the ground over which a contour line passes
is at a certain height above a known fixed plane called the "datum".
Fig. 6. Diagram Showing Method of Approximating Elevation of Successive
Contours of Inclined Road
Mean sea level is the datum plane universally employed; when
this cannot be conveniently used, an arbitrary plane is adopted
which is below the lowest point in the territory under survey.
The difference of elevation between adjacent contour lines
is called the "contour interval"; this may be one, five, ten, or more,
feet. Whatever the difference adopted, it must be constant for
all contours on the same map. Contours are designated by their
height, expressed in feet, above the datum plane. The elevation
of each contour is shown in figures at points close enough together
to allow the eye to run from one to the other with ease. It is best
to break the contour and write the numbers between the ends. If
18
HIGHWAY CONSTRUCTION
Method of Showing Contours of Banks
. of Streams
written alongside, the numbers should be placed on the higher
side of the contour.
The theory of contours is given in order that no error will be
made by supposing the
slope of the ground from a
point in one contour to a
point in the next, to be a
straight line. The less the
contour interval, the less
error will be made. If in
Fig. 6 the curved line AB
represents the actual surface
of the ground, and points
1,3,5, the elevation of suc-
cessive contours, the broken
line 1,3,5 will represent the
assumed ground surface, and its departure from the line AB is the
error introduced. If now the points 2, 4, 6 are also determined,
or the contour intervals be reduced one-half, the assumed slope
is 1, 2, S, 4, 5, 6, which
differs less from the line
AB than the line 1, 3, 5,
and hence introduces less
error. With points deter-
mined at very short inter-
vals, the error is practically
eliminated.
A knowledge of the
shape of the ground is
obtained from the distance
of the contours from one
another. The steeper the
slopes, the closer will the
contours be. If in a hill
the upper contours, near the
summit, are closer together than those near the bottom, the inter-
vening ground is concave; if the lower contours are closer than the
higher ones, the intervening ground is convex.
Fig. 8.
Method of Showing Contour of Small
Stream or Dry River Bed
HIGHWAY CONSTRUCTION
19
Every contour must close upon itself in a loop or else must
extend unbroken from one point on the margin of the map to some
other point on the margin. An exception is made in the case of
large streams, the contour on each bank being carried up-stream
until it cuts the water surface, when it is dropped, as shown in Fig. 7.
In a small stream or dry bed, the contour crosses at the point where
the elevation of the bed is that of the contour, as shown in Fig. 8.
too
Fig. 9. Typical Profile as Obtained from Contour Map
Profile. A profile is a longitudinal section of the route. The
profile in any given direction is easily made from the contour map
in the manner shown in Fig. 9. Assuming that a profile is required
along the line AB, the contours show that the ground rises from
A to B y and also that a small isolated elevation occurs at C. The
short distance between the contours near B indicates that the
rise is steep. To obtain the profile, draw parallel lines at a distance
apart equal to the vertical interval between the contours on any
convenient exaggerated scale. Number these lines to correspond
20
HIGHWAY CONSTRUCTION
pooy
OO'SS-
-OI'S9-
09'frS-
OS'tff-
poojj
00 '09
with the numbers on the contours.
From each point on the line AB,
where it intersects a contour, draw
vertical lines to intersect the corre-
sponding horizontal line. Connect
the several points thus found, remem-
bering the distinction between convex
and concave surfaces. The profile
thus obtained gives the relative heights
of different points in the line AB,
but it does not give the true gradient.
The true gradient cannot be repre-
sented accurately unless the vertical
intervals are drawn on the same scale
as the horizontal scale of the map.
If this is done, the elevations will
generally be so minute that the
profile will not give a sufficiently
striking representation of the surface
features. It is, therefore, necessary
to exaggerate the vertical scale in a
certain fixed proportion. A conven-
ient scale is 400 feet horizontal and
40 feet vertical. A typical prelimi-
nary profile, with all the information
which it is supposed to give, is shown
in Fig. 10.
Map. The map, Fig. 11, should
show the lengths and direction of
the different portions of the line,
the topography, rivers, water-courses,
roads, railroads, and other matters
of interest, such as town and county
lines, dividing lines between property,
timbered and cultivated lands, etc.
Any convenient scale may be adopted;
400 feet to an inch will be found the
most useful.
HIGHWAY CONSTRUCTION
21
00=l Inch.
Fig. 11. Typical Map Showing Layout in the Region of the Proposed Road
22 HIGHWAY CONSTRUCTION
Memoir. The descriptive memoir should give with minute-
ness all information, such as the nature of the soil, character of the
several excavations whether earth or rock, and such particular
features as cannot be clearly shown upon the map or profile. Special
information should be given regarding the rivers crossed, as to their
width, depth at highest known flood, velocity of current, character
of banks and bottom, and their angle of skew with the line of road.
Bridge Sites. The question of choosing the site of bridges is
an important one. If the selection is not restricted to a particular
point, the river should be examined for a considerable distance
above and below what would be the most convenient point for
crossing; and if a better site is found, the line of the road must be
made subordinate to it. If several practicable crossings exist, they
must be carefully compared in order to select the one most
advantageous. The following are controlling conditions: (1) Good
character of river bed, affording a firm foundation. If rock is
present near the surface of the river bed, the foundation wdll be
easy of execution, and stability and economy will be insured. (2)
Stability of river banks, thus securing a permanent concentration
of the waters in the same bed. (3) Axis of bridge at right angles
to direction of current. (4) Bends in rivers, not being suitable
localities, to be avoided if possible. A straight reach above the
bridge should be secured if possible.
FINAL SELECTION OF ROUTE
Elements Entering into Choice. In making the final selection,
the following principles should be observed as far as practicable:
(1) To follow that route which affords the easiest grades.
The easiest grade for a given road will depend on the kind of covering
adopted for its surface.
(2) To connect the places by the shortest and most direct
route commensurate with easy grades.
(3) To avoid all unnecessary ascents and descents. When a
road is encumbered with useless ascents, the wasteful expenditure
of power is considerable.
(4) To give the center line such a position, with reference to
the natural surface of the ground, that the cost of construction
shall be reduced to the smallest possible amount.
HIGHWAY CONSTRUCTION 23
(5) To cross all obstacles, where structures are necessary, as
nearly as possible at right angles. The cost of skew structures
increases nearly as the square of the secant of the obliquity.
(6) To cross ridges through the lowest pass which occurs.
(7) To cross either under or over railroads; for grade crossings
mean danger to every user of the highway.
Treatment of Typical Cases
Connecting Two Towns in Same or Adjacent Valleys. In
laying out the line of a road, there are three cases which may have
to be treated, and each of these is exemplified in the contour map,
Fig. 5. First, the two places to be connected, as the towns A and B
on the plan, may both be situated in the same valley, and upon the
same side of it; that is, they are not separated from each other by
the main stream which drains the valley. This is the simplest
case. Second, although both in the same valley, the two places
may be on opposite sides of the valley, as at A and C, being sep-
arated by the main river. Third, the two places may be situated
in different valleys, separated by an intervening ridge of ground
more or less elevated, as at A and D. In laying out an extensive
line of road, it frequently happens that all these cases have to be
dealt with. The most perfect road is that of which the course
is perfectly straight and the surface practically level; and, all other
things being the same, the best road is that which answers nearest
to this description.
Case 1. Now, in the first case, that of the two towns situated
on the same side of the main valley, there are two methods which
may be pursued in forming a communication between them. A
road following the direct line between them, shown by the thick
dotted line AB, may be made, or a line may be adopted which
will gradually and equally incline from one town to another, sup-
posing them to be at different levels; or, if they are on the same level,
the line should keep at that level throughout its entire course,
following all the sinuosities and curves which the irregular formation
of the country may render necessary for the fulfillment of these
conditions. According to the first method, a level or uniformly
inclined road might be made from one to the other; this line
would cross all the valleys and streams which run down to the
24 HIGHWAY CONSTRUCTION
main river, thus necessitating deep cuttings, heavy embankments,
and numerous bridges; or these expensive works might be avoided
by following the sinuosities of the valley. When the sides of the
main valley are pierced by numerous ravines with projecting spurs
and ridges intervening, instead of following the sinuosities, it will
be found better to make a nearly straight line cutting through
the projecting points in such a way that the material excavated
should be just sufficient to fill the hollows.
Of all these, the best is the straight uniformly inclined or level
road, although at the same time it is the most expensive. If the
importance of the traffic passing between the places is not sufficient
to warrant so great an outlay, it will become a matter of consider-
ation whether the course of the road should be kept straight, its
surface being made to undulate with the natural face of the country;
or whether, a level or equally inclined line being adopted, the course
of the road should be made to deviate from the direct line and follow
the winding course which such a condition is supposed to necessitate.
Case 2. In the second case, that of two places situated on
opposite sides of the same valley, there is, in like manner, the choice
of a perfectly straight line to connect them, winch would probably
require a big embankment if the road were kept level; or steep
inclines if it followed the surface of the country; or by winding
the road, it might be carried across the valley at a higher point,
where, if the level road be taken, the embankment would not be
so high, or, if kept on the surface, the inclination would be reduced.
Case 8. In the third case, there is, in like manner, the alter-
native of carrying the road across the intervening ridge in a perfectly
straight line, or of deviating it to the right and left, and crossing
the ridge at a point where the elevation is less. The proper deter-
mination of the question which of these courses is the best under
certain circumstances involves a consideration of the comparative
advantages and disadvantages of inclines and curves. What
additional increase in the length of the road would be equivalent
to a given inclined plane upon it; or conversely, what inclination
might be given to a road as an equivalent to a given decrease in
its length? To satisfy this question, the comparative force required
to draw different vehicles with given loads must be known, both
upon level and variously inclined roads.
HIGHWAY CONSTRUCTION 25
The route which will give the most general satisfaction consists
in following the valleys as much as possible and rising afterward
by gentle grades. This course traverses the cultivated lands,
regions studded with farmhouses and factories. The value of such
a line is much more considerable than that of a route by the ridges.
The watercourses which flow down to the main valley are, it is
true, crossed where they are the largest and require works of large
dimensions, but also they are fewer in number.
Treatment of Intermediate Towns. Suppose that it is desired
to construct a road between two distant towns, A and B, Fig. 12,
and let us for the present neglect altogether the consideration of
the physical features of the intervening country, assuming that it
is equally favorable whichever line we select. Now at first sight
it would appear that, under such circumstances, a perfectly straight
line drawn from one town to
the other would be the best xj\
that could be chosen. On
more careful examination,
however, of the locality, we
may find that there is a
third town (7, situated some- fi
, . Fig. 12. Diagram Showing Method of Deter-
Wliat On One Side Or the mining Line of Road between Successive
Towns
straight line which we have
drawn from A to B; and although our primary object is to connect
only the two latter, it would nevertheless be of considerable service
if all three towns were put into mutual connection with each other.
This may be effected in three different ways, any one of which
might, under the circumstances, be the best. In the first place,
we might, as originally suggested, form a straight road from A
to B, and in a similar manner two other straight roads from A to C,
and from B to C, and this would be the most perfect way of effecting
the object in view, the distance between any of the two towns being
reduced to the least distance possible. It would, however, be
attended with considerable expense, and it would be necessary to
construct a much greater length of road than according to the second
plan, which would be to form, as before, a straight road from A
to B, and from C to construct a road which should join the former
at a point D, so as to be perpendicular to it The traffic between
\
26 HIGHWAY CONSTRUCTION
A or B and C would proceed to the point D and then turn off to C.
With this arrangement, while the length of the roads would be
very materially decreased, only a slight increase would be occasioned
in the distance between C and the other two towns. The third
method would be to form only the two roads AC and CB, in which
case the distance between A and B would be somewhat increased,
while that between A and C or B and C would be diminished, and
the total length of road to be constructed also would be lessened.
As a general rule it may be taken that the last of these methods
is the best and most convenient for the public; that is to say, that
if the physical character of the country does not determine the
course of the road, it generally will be found best not to adopt a
perfectly straight line, but to vary the line so as to pass through all
the principal towns near its general course.
Treatment of Mountain Roads. The location of roads in
mountainous countries presents greater difficulties than in an ordi-
nary undulating country; the same latitude in adopting undulating
grades and choice of position is not permissible, for the maximum
must be kept before the eye perpetually. A mountain road has
to be constructed on the maximum grade or at grades closely approx-
imating it, and but one fixed point can be obtained before com-
mencing the survey, and that is the lowest pass in the mountain
range ; from this point the survey must be commenced. The reason
for this is that the lower slopes of the mountain are flatter than
those at their summit; they cover a larger area; and they merge
into the valley in diverse undulations. Consequently, a road at the
foot of a mountain may be carried at will in the desired direction
by more than one route, while at the top of a mountain range any
deviation from the lowest pass involves increased length of line.
The engineer having less command of the ground, owing to the
reduced area he has to deal with and the greater abruptness of the
slopes, is liable to be frustrated in his attempt to get his line carried
in the desired direction.
It is a common practice to run a mountain survey up hill, but
this should be avoided. Whenever an acute-angled zigzag is met
with on a mountain road near the summit, the inference to be drawn
is that the line, being carried up hill, on reaching the summit was
too low and the zigzag was necessary to reach the desired pass.
HIGHWAY CONSTRUCTION 27
The only remedy in such a case is a resurvey beginning at the summit
and running down hill. This method requires a reversal of that
usually adopted. The grade line is first staked out and its horizontal
location surveyed afterwards. The most appropriate instrument for
this work is a transit with a vertical circle on which the telescope
may be set to the angle of the maximum grade.
Loss of Height. "Loss of height" is to be carefully avoided
in a mountain road. By loss of height is meant an intermediate
rise in a descending grade. If a descending grade is interrupted by
the introduction of an unnecessary ascent, the length of the road
will be increased, over that due to the continuous grade, by the
length of the portion of the road intervening between the summit
of the rise and the point in the road on a level with that rise a
length which is double that due on the gradient to the height of
the rise. For example, if a road descending a mountain rises, at
some intermediate point, to cross over a ridge or spur, and the height
ascended amounts to 110 feet before the descent is continued, such
a road would be just one mile longer than if the descent had been
uninterrupted; for 110 feet is the rise due to a half-mile length at
a slope of 1 : 24.
Water on Mountain Roads. - Water is needed by the workmen
and during the construction of the road. It is also very necessary
for the traffic, especially during hot weather; and if the road exceeds
5 miles in length, provision should be made to have the water either
close to or within easy reach of the road. With a little ingenuity
the water from springs above the road, if such exist, can be led
down to drinking fountains for men, and to troughs for animals.
In a tropical country it would be a matter for serious con-
sideration if the best line for a mountain road 10 miles in length
or upwards, but without water, should, not be abandoned in favor
of a worse line with a water supply available.
Halting Places. On long lines of mountain roads, halting
places should be provided at frequent intervals.
Alignment of Roads. No rule can be laid down for the align-
ment of a road it will depend upon both the character of its traffic
and the "lay of the land". To promote economy of transportation,
it should be straight; but, if straightness is obtained at the expense
of easy grades that might have been obtained by deflections and
28
HIGHWAY CONSTRUCTION
increase in length, it will prove very expensive to the community
that uses it.
The curving road around a hill often may be no longer than
the straight one over it, for the latter is straight only with reference
to the horizontal plane, while it is curved as to the vertical plane;
the former is curved as to the horizontal plane, but straight as to
the vertical plane. Both lines curve, and we call the one passing
over the hill straight only because its vertical curvature is less
apparent to our eyes. Excessive crookedness of alignment is to
be avoided, for any unnecessary length causes a constant threefold
waste : first, of the interest of the capital expended in making that
unnecessary portion; second, of the ever recurring expense of repair-
ing it; and third, of the time and labor employed in traveling over it.
Fig. 13. Diagram Showing Method of Laying Out Curve in Road
Location and Construction of Curves. Curves, on a road used
exclusively by horse-drawn traffic, should have a center radius of
not less than 50 feet. On roads used by both horse-drawn and
motor- vehicle traffic, the greatest possible radius should be employed
and not less than 150 feet at the inner margin. Curves should
not be placed at the foot of a steep ascent; and, when they occur
on an ascent, the grade at that point should be decreased in order
to compensate for the additional resistance of the curve.
Curves may be either circular or parabolic in form. The
latter will be found exceedingly useful for joining tangents of unequal
length and for following contours; when the curvature is least
at its beginning and ending, the deviations from a straight line
HIGHWAY CONSTRUCTION 29
are less strongly marked than by a circular arc. The connection
between a circular curve and its tangents should be made by a
parabolic arc.
The width of the wheelway on curves should be greater than
on tangents; the position in which the additional width will be of
the greatest service to the traffic is at the entry arcs, as shown at
A and A, Fig. 13, and not at the center B of the curve, which is
the point commonly widened. The minimum radius of the outer
curve to provide the increased width may be determined by the
formula
in which R is radius of inner curve; W is width of road on tangents;
w is width of vehicles; and I is maximum length of vehicles, including
teams. If the traffic requires it, a further widening may be obtained
by flattening the inner curve as indicated by abc. The radius of
the reversing curves should be not less than 15 feet.
The outer half of the wheelway on curves used by fast motor-
vehicle traffic should be raised, as shown in Fig. 14, the amount
I ~i.'". 14. Diagram Showing Adjustment of Profile on Curves for Rapidly
Moving Motor Vehicles
of elevation being 4 inches for a curve of 150-foot radius and
decreasing to 2 inches for a curve of 300-foot radius.
The approach to curves should afford an unobstructed view
for at least 300 feet, and obstructions which prevent the entire
length of the curve from being seen by approaching vehicles should
be removed.
Zigzags. The method of surmounting a height by a series of
zigzags or by a series of reaches with practicable curves at the
turns, is objectionable for the following reasons :
(1) An acute-angled zigzag obliges the traffic to reverse its
direction without affording it convenient room for the purpose.
The consequence is that with slow traffic a single train of vehicles
is brought to a stand, while if two trains of vehicles, traveling in
opposite directions, meet at the zigzag, a block ensues.
30
HIGHWAY CONSTRUCTION
00'99OL 'Off
00-99 OI'SJL
-00 'PJL 21 'LL
0'L+00'9L00'9
-02'2I+ -^O0'0l 02'SII^
2 3 &
(2) With zigzags little
progress is made toward the
ultimate destination of the
road; height is surmounted,
but horizontal distance is in-
creased without compensation.
(3) Zigzags are dangerous.
Incase of a runaway down hill,
the zigzag must prove fatal.
(4) If the drainage can-
not be carried clear of the
road at the end of each reach,
it must be carried under the
road in one reach, only to
appear again at the next, when
a second bridge, culvert, or
drain will be required, and so
on at the other reaches. If the
drainage can be carried clear
at the termination of each
reach, the lengths between the
curves will be very short, en-
tailing numerous zigzag curves,
which are expensive to con-
struct and maintain.
Details after Choosing Route
Final Location. With the
route finally determined upon,
it must be located. This
consists in tracing the line,
placing a stake at every 100
feet on the straight portions
and at every 50 or 25 feet on
the curves. At the tangent
point of curves, and at points
of compound and reverse
curves, a larger and more per-
HIGHWAY CONSTRUCTION 31
manent stake should be placed. Lest those stakes should be dis-
turbed in the process of construction, their exact distance from several
points outside of the ground to be occupied by the road should be
carefully measured and recorded in the notebook, so that they may
be replaced. The stakes above referred to show the position of
the center line of the road, and form the base line from which all
operations of construction are carried on. Levels are taken at each
stake, and cross levels are taken at every change of longitudinal slope.
Construction Profile. The construction or working profile is
made from the levels obtained on location. It should be drawn
to a horizontal scale of 400 feet to the inch and a vertical scale
of 20 feet to the inch. Fig. 15 represents a portion of such a profile.
The figures in column A represent the elevation of the ground at
every 100 feet, or where a stake has been driven, above datum.
The figures in column B are the elevations of the grade above datum.
The figures in column C indicate the depth of cut or height of fill;
they are obtained by taking the difference between the level of
the road and the level of the surface of the ground. The straight
line at the top represents the grade of the road; the upper surface
of the road when finished would be somewhat higher than this, while
the given line represents, what is termed the sub-grade or formation
level. All the dimensions refer to the formation level, to which the
surface of the ground is to be formed to receive the road covering.
At all changes in the rate of inclination of the grade line a
heavier vertical line should be drawn.
Gradient. The grade of a line is in its longitudinal slope, and
is designated by the proportion between its length and the difference
of height of its two extremes. The ratio of these two qualities
gives it its name; if the road ascends or falls one foot in every twenty
feet of its length, it is said to have a grade of 1 : 20, or a 5 per cent
grade. Grades are of two kinds: maximum and minimum. The
maximum grade is the steepest which is to be permitted and which
on no account is to be exceeded; the minimum grade is the least
allowable for good drainage. Table IX gives different methods
of designating grades.
Determination of Gradients. The maximum grade is fixed by
two considerations: the one relating to the power expended in
ascending, and the other to the acceleration in descending, the
32
HIGHWAY CONSTRUCTION
TABLE IX
Methods of Designating Grades
AMERICAN METHOD
(ft. per 100 ft.)
ENGLISH METHOD
FEET PER MILE
ANGLE WITH HORIZON
i
1 400
13.2
8' 36"
-
1 200
26.4
17 11
1 150
39.6
22 55
1
1 100
52.8
34 23
U
1 80
66
42 58
i|
1 66|
79.2
51 28
if
1 57J
92.4
1 51
2
1 50
105.6
186
2|
1 44|
118.8
1 17 39
2
1 40
132
1 25 57
2|
1 36|
145.2
1 34 22
3
1 33
158.4
1 43 08
3|
1 30|
171.6
1 51 42
3|
1 28^
184.8
2 16
3f
1 26f
198
2 8 51
4
1 25
211.2
1 17 26
4j
1 23|
224.4
2 26 10
4
1 22|
237.6
2 34 36
4|
1 21
250.8
2 43 35
5
1 20
284
2 51 44
6
1 13f
316.8
3 26 12
7
1 14f
369.6
4 15
8
1 12i
422.4
4 34 26
9
i Hi
475.2
5 8 31
10
1 10
. 528
5 42 37
incline. There is a certain inclination, depending upon the degree
of perfection given to the surface of the road, which cannot be
exceeded without a direct loss of tractive power. This inclination
is that, on which, in descending at a uniform speed, the traces
slacken, or which causes the vehicles to press on the horses; the
limiting inclination within which this effect does not take place is
the angle of repose.
Angle of Repose. The angle of repose for any given road surface
can be ascertained easily from the tractive force required upon a
level with the same character of surface. Thus if the force necessary
on a level to overcome the resistance of the load is ^V of its weight,
then the same fraction expresses the angle of repose for that surface.
On all inclines less steep than the angle of repose, a certain
amount of tractive force is necessary in the descent as well as in
the ascent, and the mean of the two drawing forces, ascending and
descending, is equal to the force along the level of the road. Thus,
on such inclines, as much mechanical force is gained in the descent
HIGHWAY CONSTRUCTION 33
as is lost in the ascent. From this it might be inferred that when a
vehicle passes alternately each way along the road, no real loss is
occasioned by the inclination of the road; which, however, is not
the fact with animal power, for w r hile the up and down slopes in
the ascending journey will demand, respectively, a greater and a
less number of horses than that required on a level road, no actual
compensation for this fluctuation can be made in the descending
journey. On inclines which are more steep than the angle of repose,
the load presses on the horses during their descent, so as to impede
their action, and their power is expended in checking the descent
of the load; or if this effect be prevented by the use of any form of
drag or brake, then the power expended on such a drag or brake
corresponds to an equal quantity of mechanical power expended
in the ascent, for which no equivalent is obtained in the descent.
Grade Problems. Maximum Grade. The maximum grade for
a given road will depend: (1) upon the class of traffic that will
use it, whether fast and light, slow and heavy, or mixed, consisting
of both light and heavy; (2) upon the character of the pavement
adopted; and (3) upon the question of cost of construction. Econ-
omy of motive power and low cost of construction are antagonistic
to each other, and the engineer will have to weigh the two in the
balance.
For fast and light traffic the grades should not exceed 2 per
cent; for mixed traffic 3 per cent may be adopted; while for slow
traffic combined with economy 5 per cent should not be exceeded.
This grade is practicable but not convenient.
Minimum Grade. From the previous considerations it would
appear that an absolutely level road was the one to be sought for,
but this is not so; there is a minimum, or a least allowable grade,
of which the road must not fall short, as well as a maximum one
which it must not exceed. If the road were perfectly level in its
longitudinal direction, its surface could not be kept free from water
without giving it so great a rise in its middle as would expose vehi-
cles to the danger of overturning. The minimum grade commonly
used is 1 per cent.
Undulating Grades. From the fact that the power required
to move a load at a given velocity on a level road is decreased on
a descending grade to the same extent it is increased in ascending
34 HIGHWAY CONSTRUCTION
the same grade, it must not be inferred that the animal force
expended in passing alternately each way over a rising and falling
road will gain as much in descending the several inclines as it will
lose in ascending them. Such is not the case. The animal force
must be sufficient, either in power or number, to draw the load over
the level portions and up the steepest inclines of the road, and in
practice no reduction in the number of horses can be made to corre-
spond with the decreased power required in descending the inclines.
The popular theory that a gently undulating road is less fatiguing
to horses than one which is perfectly level is erroneous. The asser-
tion that the alternations of ascent, descent, and levels, call into
play different muscles, allowing some to rest while others are exerted,
and thus relieving each in turn, is demonstrably false, and con-
tradicted by the anatomical structure of the horse. Since this
doctrine is a mere popular error, it should be rejected utterly, not
only because it is false in itself, but still more because it encourages
the building of undulating roads, and this increases the labor and
cost of transportation upon them.
Level Stretches. On long ascents it is generally recommended
that level or nearly level stretches be introduced at frequent intervals
in order to rest the animals. These are objectionable when they
cause loss of height, and animals will be more rested by halting
and unharnessing for half an hour than by traveling over a level
portion. The only case which justifies the introduction of levels
into an ascending road is where such levels will advance the road
towards its objective point; where this is the case there will be no
loss of either length or height, and it will simply be exchanging a
level road below for a level road above.
Establishing the Grade. When the profile of a proposed route
has been made, a grade line is drawn upon it (usually in red) in
such a manner as to follow its general slope, but to average its
irregular elevations and depressions. If the ratio between the whole
distance and the height of the line is less than the maximum grade
intended to be used, this line will be satisfactory; but if it be found
steeper, the cut or the length of the line will have to be increased.
The latter is generally preferable.
The apex or meeting point of all curves should be rounded off
by a vertical curve, as shown in Fig. 16, thus slightly changing
HIGHWAY CONSTRUCTION
35
the grade at and near the point of intersection. A vertical curve
rarely need extend more than 200 feet each way from that point.
Let AB and BC be two grades in profile intersecting at station
B, and let A and C be the adjacent stations. It is required to join
the grades by a vertical curve extending from A to C. Imagine a
chord drawn from A to C. The elevation of the middle point of
the chord will be a mean of the elevations of the grade at A and (7,
and one-half of the difference between this and the elevation of
the grade at B will be the middle ordinate of the curve. Hence
we have
TIT 1 /grade A+ grade C , D \
M = l^ ^- graded 1
in which M equals the correction in elevation for the point B. The
correction for any other point is proportional to the square of its
I | J __L j
Fig. 16. Typical Road Section Showing Rounding Off of Meeting Points of Curves
distance from A to C. Thus, assuming the distance between
successive ordinates, Fig. 16, as 50 feet, the correction ^4+25 is
T&M; at ^4+50 it is \M; at A +7 '5 it is &M; and the same for
corresponding points on the other side of B. The corrections
in this case shown are subtractive, since M is negative. They
are additional when M is positive, and the curve concave upward.
PRELIMINARY ROAD CONSTRUCTION METHODS
WIDTH AND TRANSVERSE CONTOUR
Width of Road. A road should be wide enough to accommodate
the traffic for which it is intended, and should comprise a wheelway
for vehicles and a space on each side for pedestrians.
The wheelway of country highways need be no wider than is
absolutely necessary to accommodate the traffic using it; in many
36 HIGHWAY CONSTRUCTION
places a track wide enough for a single team is all that is necessary.
But the breadth of the land appropriated for highway purposes
should be sufficient to provide for all future increase of traffic.
The wheelways of roads in rural sections should be double; that is,
one portion paved (preferably the center), and the other left with
the natural soil. The latter, if kept in repair, will be preferred by
teamsters for at least one-half the year.
The minimum width of the paved portion, if intended to carry
two lines of travel, is fixed by the width required to allow two
vehicles to pass each other safely. This width is 1 6 feet. If intended
for a single line of travel, 8 feet is sufficient, but suitable turnouts
must be provided at frequent intervals. The most economical
width for any roadway is some multiple of eight. Wide roads
are the best; they expose a larger surface to the drying action of
the sun and wind, and require less supervision than narrow ones.
Their first cost is greater than that of narrow ones, and nearly
in the ratio of the increased width.
The cost of maintaining a mile of road depends more upon
the extent of the traffic than upon the extent of its surface, and
unless -extremes be taken, the same quantity of material will be
necessary for the repair of roads, either wide or narrow, which are
subjected to the same amount of traffic. The cost of spreading
materials over the wide road will be somewhat greater, but the
cost of the materials will be the same. On narrow roads the traffic
being confined to one track, will wear more severely than if spread
over a wider surface.
The width of land appropriated for road purposes varies in
the United States from 49 J feet to 66 feet; in England and France
from 26 to 66 feet. And the width or space macadamized is also
subject to variation; in the United States the average width is
16 feet; in France it varies between 16 and 22 feet; in Belgium
8J feet seems to be the regular width, while in Austria, from 14J
to 26i feet.
Transverse Contour. The centers of roadways in most cases
should be higher than the sides, the object being to facilitate the
flow of the rain water to the gutters. Where a good surface is
maintained a very moderate amount of rise is sufficient for this
purpose, but the rise should bear a certain proportion to the width
HIGHWAY CONSTRUCTION
37
TABLE X
Proportionate Rise of Center to Width of Carriageway for
Different Road Materials
KIND OF SURFACE
PROPORTIONS OF RISE AT
CENTER TO WIDTH OF
CARRIAGEWAY
Earth
Gravel
Broken stone
1:40
1:50
1:60
of the carriageway. Earth roads require the most and asphalt
the least. The most suitable proportions for the different paving
materials is shown in Table X.
Form of Contour. All authorities agree that the form should
be convex, but they differ in the amount and form of the convexity.
Circular arcs, two straight lines joined by a circular arc, and ellipses,
all have their advocates. For country roads a curve of suitable
Fig. 17. Typical Section of Road, Showing Contour
convexity may be obtained as follows: At J of the width from
center to side, make the rise } of the total rise, and at J of the width
make the rise f of the total, Fig. 17.
Excessive height and convexity of cross section contract the
width of the wheelway by concentrating the traffic at the center,
that being the only part where a vehicle can run upright. The force
required to haul vehicles over such cross sections is increased because
an undue proportion of the load is thrown upon two wheels instead
of being distributed equally over the four. The continual tread
of horses' feet in one track soon forms a depression which holds
water, and the surface is not so dry as with a flat section which
allows the traffic to distribute itself over the whole width. Sides
formed of straight lines are also objectionable. They wear hollow,
retain water, and, by raising the center, defeat the object sought.
The required convexity should be obtained by rounding the forma-
tion surface, and not by diminishing the thickness of the covering
at the sides.
38 HIGHWAY CONSTRUCTION
Although on hillside and mountain roads it is generally recom-
mended that the surface should consist of a single slope inclining
inwards, there is no reason for or advantage gained by this method.
The form -best adapted to these roads is the same as for a road under
ordinary conditions.
With a roadway raised in the center and the rain water draining
off to gutters on each side, the drainage will be more effectual and
speedy than if the drainage of the outer half of the road has to pass
over the inner half. The inner half of such a road is usually sub-
jected to more traffic than the outer half. If formed of a straight
incline, this side will be worn hollow and retain water. The inclined
flat section never can be properly repaired to withstand the traffic.
Consequently it never can be kept in good order, no matter how
constantly it may be mended. It is always below par and when
heavy rain falls it is seriously damaged.
DRAINAGE
Types of Drainage. In the construction of roads, drainage is of
the first importance. The ability of earth to sustain a load depends
in a large measure upon the amount of moisture retained by it.
Most earths form a good firm foundation so long as they are kept
dry, but when wet they lose their sustaining power, becoming soft
and incoherent.
The drainage of roadways is of two kinds, viz, subsurface and
surface. . The first provides for the removal of the underground
water found in the body of the road; the second provides for the
speedy removal of all water falling on the surface of the road.
Experience has shown that a thorough removal of the underground
water is of the utmost importance and is essential to 'the life of the
road. A road covering placed on a wet undrained bottom will be
destroyed by both water and frost, and will always be troublesome
and expensive to maintain; perfect subsoil drainage is a necessity
and will, be found economical in the end even if in securing it
considerable expense is required.
Subsoil Drainage
The methods employed for securing the subsoil drainage must
be varied according to the character of natural soil, each kind of
soil requiring different treatment.
HIGHWAY CONSTRUCTION
39
Nature of Soils. The natural soil may be divided into the
following classes: siliceous, argillaceous, and calcareous; rock,
swamps, and morasses. The siliceous and calcareous soils, the
sandy loams and rock, present no great difficulty in securing a
dry and solid foundation. Ordinarily they are not retentive of
water and therefore require no underdrains; ditches on each side
of the road will generally be found sufficient. The argillaceous
soils and softer marls require more care; they retain water and are
difficult to compact, except at the surface; and they are very unstable
under the action of water and frost.
Location of Drains. The removal of water from the subsoil
is effected by drains so placed as to intercept the underground
circulation of the water. Regarding the best location for the drains
to accomplish this, three cases in general will present themselves:
Marginal Drains. Where the subsoil is continually wet and
without a well-defined flow of water from either side. Under
Fig. 18. Typical Road Section Showing Marginal Drains
this condition marginal drains, as shown in Fig. 18, will be found
satisfactory.
Side Drains. Where there is a regular flow from one side to
the other, as on a hillside road, a single drain placed on the side
Fig. 19. Typical Road Section Showing Side Drains
from which the water comes, as in Fig. 19, will be sufficient usually.
Center and Cross Drains. Where the subsoil is so retentive
of water as to require a system of drains under the roadbed, these
40
HIGHWAY CONSTRUCTION
drains may be constructed in a variety of ways. The simplest
method is to place a drain under the center of the roadway, as in
Fig. 20, connecting it at intervals by cross drains with drains placed
at the sides which discharge into the natural watercourses. Where
Fig. 20. Typical Road Section Showing Center Drain
the ground is level or has but a slight inclination, the cross drains
may be placed at right angles to the axis of the road. Where there
is a steep grade it is better to lay the cross drains in the form of
an inverted V with the point in the center of the roadway and
directed uphill.
The distance apart of the cross drains depends upon the ease
with which the subsoil yields its water. In porous soils the drains
will prove efficient at distances of from 30 to 40 feet; in retentive
clay the spacing may range from 10 to 20 feet.
Proper Fall for Drains. The fall to be given the drains
depends upon the size of the drain and the amount of water to be
carried off. It is not advisable to employ a fall greater than 1 foot
in 100 feet. Too great a fall will produce a swift current that is
Fig. 21. "Blind" Stone Drain
Fig. 22. "Throat" Stone Drain
liable to undermine the drain as well as to choke it by foreign
matter, which a less rapid stream could not have transported.
Materials Used for Drains. The materials employed for
drains are: stone, vitrified clay pipe, porous tile, and concrete.
HIGHWAY CONSTRUCTION 41
Stone Drains. The stone drain is constructed in two forms,
shown in Figs. 21 and 22. The first form, called a "blind" drain,
consists of a trench excavated to the required depth and filled
with cobblestones or rounded pebbles. To prevent the soil from
washing in and choking it, the larger stones are covered first with
a layer of small gravel, and then with a layer of coarse gravel, by
which means the water is filtered before passing into the porous
mass beneath. Angular stones are not suitable for this type of
drain. The second form of stone drain, an open channel called a
"throat", is formed in the bottom of the trench with rough slabs
of stone, and the trench is filled in the same manner as for a
blind drain.
Vitrified Pipe Drains. Vitrified pipe drains are constructed
by placing the pipe in the bottom of the trench, filling the hubs
with oakum and back-filling the trench with gravel, broken stone,
or a mixture of these.
Porous Tile Drains. Porous tile, Fig. 23, form very satisfactory
drains. They carry off the water with great ease, rarely if ever
get choked, and require only a slight
inclination to keep the water moving
through them. The tile have plain
ends which are placed in contact in
the trench and wrapped with tar
paper or burlap. They are sur-
rounded and covered with gravel or
. . Fig. 23. Porous Tile Drain
broken stone not exceeding 1 inch in
size for a depth ranging from 6 to 12 inches, and the remaining
depth of the trench is filled with large gravel or broken stone.
Concrete Tile Drains. Tile made of concrete have been in
satisfactory service for several decades. They are generally made in
lengths of one foot with plain ends and. are laid in the same manner
as the porous tile. They can be made in portable machines in the
vicinity where they are to be used an advantage that tends
toward low first cost.
For the manufacture of concrete tile the best quality of
hydraulic (Portland) cement, clean sand, and fine gravel or broken
stone should be used in the proportion of one part cement, two
parts sand, and four parts stone; the stone should contain no par-
42
HIGHWAY CONSTRUCTION
tides exceeding f inch in size. Sufficient clean water should be
used to produce a "wet mix", which should be poured into the
molds before setting begins and rammed lightly. After setting is
completed, the tile should be cured for about 90 days.
Sizes of Drains. The size of the drain to be adopted for a
given situation depends upon the amount
of water to be carried and the fall that can
be given the drain. These two factors being
given, there are several formulas that can
be used to determine the required size. But,
in the subsoil drainage of a road, the amount
of water to be moved can be guessed at
only; therefore, experience as to what a
drain has accomplished in a given locality
is a better guide than the result given by
any formula. Experience shows that the
least practicable size is 4 inches. The
amount of water to be moved is generally
assumed to vary between J inch and 1 inch per acre, per 24 hours,
on the area to be drained.
Silt Basins. Silt basins should be constructed at all junctions
and wherever else they may be considered necessary; they may be
made from a single 6-inch pipe, Fig. 24, or constructed of brick
masonry.
Protection of Drain Ends from Weather. As tile drains are
more liable to injury from frost than those of either brick or stone,
Fig. 24. Typical Construction
for Silt Basin
Fig. 25. Proper Method of Covering Drain Outlet
their ends at the side ditches should not be exposed directly to
the weather in very cold climates, but may terminate in blind
HIGHWAY CONSTRUCTION 43
drains, or a few lengths of vitrified clay pipe reaching under the
road a distance of about 3 or 4 feet from the inner slope of the ditch.
Drain Outlets. Drain outlets may be formed by building a
dwarf wall of brick or stone, whichever is the cheapest or most
convenient in the locality. The outlet, Fig. 25, should be covered
with an iron grating to prevent vermin entering the drain pipes
and building nests, thus choking the waterway.
Side Ditches. Side ditches are provided to carry away the
subsoil water from the base of the road, and the rain water which
falls upon its surface; to do this speedily they must have capacity
and inclination proportionate to the amount of water reaching
them. The width of the bed should not be less than 18 inches;
the depth will vary with circumstances, but should be such that
the water surface shall not reach the subgrade, but remain at least
12 inches below the crown of the road. The sides should slope
at least 1J to 1.
The longitudinal inclination of the ditch follows the configura-
tion of the general topography, that is, the lines of natural drainage.
When the latter has to be aided artificially, grades of from 1 in
500 to 1 in 800 will usually answer.
In absorbent soil less fall is sufficient, and in certain cases
level ditches are permissible. The slopes of the ditches must be
protected where the grade is considerable. This can be accomplished
by sod revetments, riprapping, or paving.
Surface Drainage
The drainage of the roadway surface depends upon the preser-
vation of the cross section, with regular and uninterrupted fall
to the sides and without hollows or ruts in which the water can lie,
and also upon the longitudinal fall of the road. If this is not suffi-
cient the road becomes flooded during heavy rainstorms and melting
snow, and is considerably damaged.
Side Ditches and Gutters. The removal of surface water
from country roads may be effected by the side ditches, into which,
when there are no sidewalks, the water flows directly. When there
are sidewalks, gutters are formed between the roadway and foot-
path, and the water is conducted from these gutters into the side
ditches by tile piping laid under the walks at intervals of about
44 HIGHWAY CONSTRUCTION
50 feet. The entrance to these pipes should be protected against
washing by a rough stone paving. In the case of covered ditches
under the footpath, the water must be led into them by first passing
through catch basins. These are small masonry vaults covered
with iron gratings to prevent the ingress of stones, leaves, etc.
Connection from the catch basin is made by a tile pipe about 6
inches in diameter. The mouth of this pipe is placed a few feet
above the bottom of the catch basin, and the space below it acts
as a depository for the silt carried by the water, and is cleaned
out periodically. The catch basins may be placed from 200 to
300 feet apart. They should be made of dimensions sufficient
to convey the amount of water which is liable to flow into them
during heavy and continuous rains.
If on inclines the velocity of the water is greater than the nature
of the soil will withstand, the gutters should be roughly paved.
In all cases, the slope adjoining the footpath should be covered
with sod. A velocity of 30 feet a minute will not disturb clay
with sand and stone; 40 feet per minute will move coarse sand;
60 feet a minute will move gravel; 120 feet a minute should move
round pebbles 1 inch in diameter, and 180 feet a minute will move
angular stones If inches in diameter.
The scour in the gutters on inclines may be prevented by
small weirs of stones or wood stick fascines constructed by the
roadmen at a nominal cost. At junctions and crossroads the
gutters and side ditches require careful arrangement so that the
water from one road may not be thrown upon another; cross drains
and culverts will be required at such places.
Treatment of Springs Found in Cuttings. In cuttings, springs
are frequently encountered and become a source of constant danger
to the stability of the slopes. In such cases the slope should be
excavated at the point where the water appears, until, if possible,
the source is reached. When the source has been reached, an outlet
is provided by constructing a drain and connecting it with the
drain at the roadside. Sometimes it may be impossible to trace
the water to a single source, the whole face of the cutting being
saturated for some distance. In such cases the treatment may
be difficult and expensive, but a series of drains may be run up the
slope to such height as will tap all the water appearing.
HIGHWAY CONSTRUCTION 45
In cuttings, the ditch at the toe of the slope is liable to be
filled with silt carried down the slope by rain; and where this might
occur, covered drains should be constructed.
Drainage for Hillside Roads. On hillside or mountain roads
catch- water ditches should be cut on the mountain side above the
road, to cut off and convey the drainage of the ground above them
to the neighboring ravines. The size of these ditches will be
determined by the amount of rainfall, extent of drainage from the
mountain which they intercept, and by the distances of the ravine
watercourses on each side.
Inner and Outer Road Gutters. The inner road gutter should
be of dimensions ample to carry off the water reaching it; when
in soil, it should be roughly paved with stone. When paving is
not absolutely necessary, but is desirable to arrest the scouring
action of running water during heavy rains, stone weirs may be
erected across the gutter at convenient intervals. The outer gutter
need not be more than 12 inches wide and 9 inches deep. The
gutter is formed by a depression in the surface of the road close
to the parapet or revetted earthen protection mound. The drainage
which falls into this gutter is led off through the parapet, or other
roadside protection, at frequent intervals. The guard stones on
the outside of the road are placed in and across the gutter, just
below the drainage holes, so as to turn the current of the drainage
into these holes or channels. On straight reaches, with parapet
protection, drainage holes with guard stones should be placed every
20 feet apart. Where earthen mounds are used, and it may not be
convenient to have the drainage holes or channels every 20 feet,
the guardstones are to be placed in advance of the gutter to allow
the drainage to pass behind them. This drainage is either to be
run off at the cross drainage of the road, or to be turned off as before
by a guard stone set across the gutter.
At re-entering turns, where the outer side of the road requires
particular protection, guard stones should be placed every 4 feet.
As all re-entering turns should be protected by parapets, the drainage
holes through them may be placed as close together as desired.
Where the road is in embankment the surface water must be
prevented from running down the slopes by providing ample gutters
suitably connected to the natural watercourses.
46 HIGHWAY CONSTRUCTION
Water Breaks. Water breaks to turn the surface drainage
into the side ditches, should not be constructed on improved roads.
They increase the grade and are an impediment to convenient
and easy travel. W T here it is necessary that water should cross
the road, a culvert should be built.
CULVERTS
Functions of Culverts. Culverts are necessary for carrying
the cross streams under a road, and also for conveying the surface
water collected in the side ditches from the upper side to that side
on which the natural watercourses lie.
Especial care is required to provide an ample way for the water
to be passed. If the culvert is too small, it is liable to cause a
washout, entailing interruption of traffic and cost of repairs, and
possibly may cause accidents that will require payment of large
sums for damages. On the other hand, if the culvert is made
unnecessarily large, the cost of construction is needlessly increased.
Factors Considered in Design of Culverts. The area of water-
way required depends upon a number of important factors, which
will be discussed briefly.
Rate of Rainfall. It is the maximum rate of rainfall during
the severest storms which is required in this connection. This
varies greatly in different sections of the country.
The maximum rainfall as shown by statistics is about one inch
per hour (except during heavy storms); equal to 3,630 cubic feet
per acre. Owing to various causes, not more than 50 to 75 per
cent of this amount will reach the culvert within the same hour.
Inches of rainfall X 3,630 = cubic feet per acre
Inches of rainf all X 2,323,200 = cubic feet per square mile
Kind and Condition of Soil. The amount of water to be drained
off will depend upon the permeability of the surface of the ground,
which will vary greatly with the kind of soil, the degree of saturation,
the condition of the cultivation, the amount of vegetation, etc.
Character and Inclination of Surface. The rapidity with which the
water will reach the watercourse depends upon whether the surface is
rough or smooth, steep or flat, barren or covered with vegetation, etc.
Condition and Inclination of Stream Bed. The rapidity with
which the water will reach the culvert depends upon whether there
HIGHWAY CONSTRUCTION 47
is a well-defined and unobstructed channel or whether the water finds
its way in a broad, thin sheet. If the watercourse is unobstructed and
has a considerable inclination, the water may arrive at the culvert
nearly as rapidly as it falls; but if the channel is obstructed, the
water may be much longer in passing the culvert than in falling.
Shape of Area to be Drained and Position of Stream Branches.
The area of waterway depends upon the amount of the area to
be drained ; but in many cases the shape of this area and the position
of the branches of the stream are of more importance than the
amount of the territory. For example, if the area is long and
narrow, the water from the lower portion may pass through the
culvert before that from the upper end arrives; or, on the other
hand, if the upper end of the area is steeper than the lower, the water
from the former may arrive simultaneously with that from the
latter. Again, if the lower part of the area is supplied better with
branches than the upper portion, the water from the former will
be carried past the culvert before the arrival of that from the latter;
or, on the other hand, if the upper part is supplied better with branch
watercourses than is the low r er, the water from the whole area
may arrive at the culvert at nearly the same time. In large areas
the shape of the area and the position of the watercourses are very
important considerations.
Mouth of Culvert and Inclination of Bed. The efficiency of a
culvert may be increased very materially by arranging the upper
end so that the water may enter into it without being retarded.
The discharging capacity of a culvert can be increased greatly by
increasing the inclination of its bed, provided the channel below will
allow the water to flow away freely after having passed the culvert.
Provision for Discharge of Water Under Head. The discharging
capacity of a culvert can be increased greatly by allowing the water
to dam up above it. A culvert will discharge twice as much under
a head of four feet as under a head of one foot. This can be done
safely only with a well-constructed culvert.
The determination of the values of the different factors entering
into the problem is almost wholly a matter of judgment. An
estimate for any one of the above factors is liable to be in error
from 100 to 200 per cent, or even more, and of course any result
deduced from such data must be very uncertain. Fortunately,
48 HIGHWAY CONSTRUCTION
mathematical exactness is not required by the problem nor warranted
by the data. The question is not one of 10 or 20 per cent of increase;
for if a 2-foot pipe is insufficient, a 3-foot pipe probably will be
the next size, an increase of 225 per cent; and if a 6-foot arch culvert
is too small, an 8-foot will be used, an increase of 180 per cent.
The real question is whether a 2-foot pipe or an 8-foot arch culvert
is needed.
Valuable data on the proper size of any particular culvert may
be obtained as follows: (1) by observing the existing openings on
the same stream; (2) by measuring, preferably at time of high
water, a cross section of the stream at some narrow place; and (3)
by determining the height of high water as indicated by drift and
debris, and from the evidence of the inhabitants of the neighborhood.
On mountain roads, or roads subjected to heavy rainfall,
culverts of ample dimensions should be provided wherever required,
and it will be more economical to construct them of masonry. In
localities where boulders and debris are likely to be washed down
during wet weather, it will be a good precaution to construct catch
pools at the entrance of all culverts and cross drains for the reception
of such matter. In hard soil or rock these catch pools will be
simple well-like excavations, with their bottoms two or three feet
below the entrance sill or floor of the culvert or drain. Where
the soil is soft they should be lined with stone laid dry; if very soft,
with masonry. The size of the catch pools will depend upon the
width of the drainage works. They should be wide enough to
prevent the drains from being injured by falling rocks and stones
of a not inordinate size.
The use of catch pools obviates the necessity of building culverts
and drains at an angle to the axis of the road. Oblique structures
are objectionable, as being longer than if set at right angles and by
reason of the acute- and obtuse-angled terminations to their piers,
abutments, and coverings.
Types of Culverts
General Classification. Three types of culverts are employed,
namely: pipe, box, and arch. The pipe culvert is employed for
small streams, in sizes from 12 to 24 inches. Box culverts are
employed in sizes from 24 inches up to 8 feet. Arch culverts are
HIGHWAY CONSTRUCTION 49
used for spans 8 feet and over. In the construction of culverts
a variety of materials are used. Pipe culverts are constructed
of earthenware or vitrified clay, cast iron, corrugated steel, brick,
or concrete; box and arch culverts are built of stone, brick, or con-
crete. Short span concrete bridges are also often employed as
culverts. The type of culvert and the material to be used are
determined in some cases by the cost; in others by the load to be
supported, as where the depth of fill over the culvert is considerable,
or where a large area of waterway is required.
Earthenware Pipe Culverts. Construction. In laying the pipe
the bottom of the trench should be rounded out to fit the lower
half of the body of the pipe, with proper depressions for the sockets.
If the ground is soft or sandy, the earth should be rammed carefully,
but solidly, in and around the lower part of the pipe. The top
surface of the pipe, as a rule, never should be less than 18 inches
below the surface of the roadway, but there are many cases where
pipes have stood, for several years, under heavy loads, with only
8 to 12 inches of earth over them. No danger from frost need be
apprehended, provided the culverts are so constructed that the
water is carried away from the level end. Ordinary soft drain
tiles are not affected in the least by the expansion of frost in the
earth around them.
The freezing of water in the pipe, particularly if more than half
full, is liable to burst it; consequently the pipe should have a suffi-
cient fall to drain itself, and the outside should be so low that there
is no danger of backwaters reaching the pipe. If properly drained,
there is no danger from frost.
Jointing. In many cases, perhaps in most, the joints are
not calked. If this is not done, there is danger of the water being
forced out of the joints and of washing away the soil from around
the pipe. Even if the danger is not very imminent, the joints
of the larger pipes, at least, should be calked with hydraulic cement,
since the cost is very small compared with the insurance against
damage thereby secured. Sometimes the joints are calked with
clay. Every culvert should be built so it can discharge water
under a head without damage to itself.
Use of Bulkheads. Although often omitted, the end sections
should be protected with a masonry, Fig. 26, or timber bulkhead.
50
HIGHWAY CONSTRUCTION
The foundation of the bulkhead should be deep enough not to be
disturbed by frost. In constructing the end wall, it is well to
increase the fall near the outlet to allow for a possible settlement
Fig. 26. Typical Design for Masonry Bulkhead
of the interior sections. When stone and brick abutments are too
expensive, a fair substitute can be made by setting posts in the
ground and spiking plank to them. When planks are used, it
is best to set them with considerable inclination towards the road-
bed to prevent their being crowded outward by the pressure of the
embankment. The upper end of the culvert should be so protected
1
I
Fig. 27. Section Showing Typical Layout for Double Pipe Culvert
that the water will not readily find its way along the outside of the
pipes, in case the mouth of the culvert should become submerged.
When the capacity of one pipe is not sufficient, two or more
may be laid side by side as shown in Fig. 27. Although the two
HIGHWAY CONSTRUCTION
51
small pipes do not have as much discharging capacity as a single
large one of equal cross section, yet there is an advantage in laying
two small ones side by side, since the water need not rise so high
to utilize the full capacity of the two pipes as would be necessary
to discharge itself through a single one of large size.
Iron Pipe Culverts. During recent years iron pipe, Fig. 28,
has been used for culverts on many prominent railroads, and may
be used on roads in sections where other materials are unavailable.
In constructing a culvert with cast-iron pipe the points requiring
particular attention are: (1) tamping the soil tightly around the
pipe to prevent the water from forming a channel along the outside;
Fig. 28. Section Showing Construction of Iron Pipe Culvert
and (2) protecting the ends by suitable head walls and, when neces-
sary, laying riprap at the lower end. The amount of masonry
required for the end walls depends upon the relative width of the
embankment and the number of sections of pipe used. For example,
if the embankment is, say, 40 feet wide at the base, the culvert
may consist of three 12-foot lengths of pipe and a light end wall
near the toe of the bank; but if the embankment is, say, 32 feet
wide, the culvert may consist of two 12-foot lengths of pipe and a
comparatively heavy end wall well back from the toe of the bank.
The smaller sizes of pipe usually come in 12-foot lengths, but some-
times a few 6-foot lengths are included for use in adjusting the
length of the culvert to the width of the bank. The larger sizes
are generally 6 feet long.
52 HIGHWAY CONSTRUCTION
Box Culverts. Box culverts, Fig. 29, consist of two side walls
with a flat deck. When stone is used, they are generally built of
dry rubble masonry. The walls should be well founded at about
42 inches below the bed of the stream. The thickness of the walls
varies according to the height. The wings are formed by extending
the walls out straight and stepping them down. The deck may be
made of stone slabs or reinforced concrete; with the latter it is
possible to use wider spans than with stone slabs. Where the force
of the stream is sufficient to scour the bed, it will be necessary to
End View and Section
Lonqiludinal Section
Fig. 29. Typical Design of Concrete Box Culvert
pave it with stone or concrete. When reinforced concrete is used
instead of stone, the side walls are made from 4 to 8 inches thick,
depending upon the height. Where it is not necessary to pave
the stream bed, the walls are carried down about 2 feet below the
bed, and founded upon a footing 9 to 12 inches thick and sufficiently
wide to secure ample area of the soil to support the load. Where
scouring of the bed is liable to occur, a concrete bottom is con-
structed throughout the entire width and length of the culvert,
and the side walls are founded on it; if necessary, a cut-off wall
HIGHWAY CONSTRUCTION
53
is constructed across each end to a depth of about 2 feet below
the bottom.
Arch Culverts. The arch form of culvert is more costly than
the other forms, but it is often preferred on account of its appearance,
Fig. 30. When masonry and plain concrete are used, very heavy
abutments are required in order that no movement can take place
under a live load, to cause bending moments in the arch. In design-
ing reinforced-concrete arches, bending is provided for by consider-
Section and End Vie
Side Elevation.
Fig. 30. Design for Arch Culvert
ing the arch as a curved beam, with a consequent reduction in the
weight of the abutments.
Short Span Bridges Used as Culverts. Three types of rein-
forced-concrete bridges are employed for short spans: (a) the flat
slab; (6) the T-beam; (c) the steel I-beam incased in concrete,
Fig. 31. The length of span over which reinforced slabs may be
built with safety depends upon the load to be carried; under normal
conditions the maximum span is 12 feet. The thickness of the slab
for a span 2 feet should be not less than 6 inches and should increase
with increase of span. The slabs are reinforced with steel bars,
expanded metal, or other forms of reinforcing metal; the cross-
54
HIGHWAY CONSTRUCTION
sectional area of the reinforcing steel required is about 1 per cent
of that of the slab.*
The T-beam type is practicable for spans from 12 to 30 feet.
The I-beam type may be used for all spans up to 30 feet. In this
type, the I-beam is designed to transmit the load to the abutments,
while the reinforced-concrete floor transmits the load to the I-beams.
This type of construction is noted for its safety and ability to with-
stand severe and unfavorable conditions, such as the settlement
of the abutments, which may cause rupture of the concrete. The
wr
5eclion of Flal Slab CL
Reinforcement^
Section Through Top For T Beam Culvert
(Reinforcement ,^. rr ^. Trr ..^ 7T .. r ^ : ^ r -,..^,. r _ Steel I-
Section Through Top For I Beam
Fig. 31. Sections of Typical Short Span Concrete Bridges Used as Culverts
I-beam may or may not be incased in the concrete; the object sought
in so doing is to protect it from rust. This may be accomplished
also by painting, but as this needs to be repeated frequently and
as there is a possibility that it will not be done, it is better to incase
the beams in the concrete during construction, and so insure their
permanent protection. This type also admits of arch construction
between the beams for the floor system, thus decreasing the depth
required for the floor; this feature may be of value in locations where
the area of the waterway or the "head room" is a controlling factor.
"The theory of design of concrete bridges and culverts is discussed in Masonry and Rein-
forced Concrete, Part III.
HIGHWAY CONSTRUCTION 55
EARTHWORK
The term "earthwork" is applied to all the operations per-
formed in the making of excavations and embankments. In its
widest sense it comprehends work in rock as well as in the looser
materials of the earth's crust.
Balancing Cuts and Fills. In the construction of new roads,
the formation of the roadbed consists in bringing the surface of the
ground to the adopted grade. This grade should be established so as
to reduce the earthwork to the least possible amount, both to render
the cost of construction low, and to avoid unnecessarily marring
the appearance of the country in the vicinity of the road. The
most desirable position of the grade line is usually that which makes
the amounts of cutting and filling equal to each other, for any sur-
plus embankment over cutting must be made up by borrowing,
% and surplus cutting must be wasted; both of these operations
involving additional cost for labor and land.
Side Slopes. Inclination. The proper inclination for the
side slopes of cutting and embankments depends upon the nature
of the soil, the action of the atmosphere, and the action of internal
moisture upon it. For economy the inclination should be as steep
as the nature of the soil will permit.
The usual slopes in cuttings are:
Solid rock to 1
Earth and gravel 1 to 1
Clay 3 or 6 to 1
Fine sand 2 or 3 to 1
The slopes of embankment are usually made 1J to 1.
Form of Slopes. The natural, strongest, and ultimate form
of earth slopes is a concave curve, in which the flattest portion
is at the bottom. This form is very rarely given to the slopes in
constructing them; in fact, the reverse is often the case, the slopes
being made convex, thus saving excavation by the contractor
and inviting slips.
In cuttings exceeding 10 feet in depth the forming of concave
slopes will aid materially in preventing slips, and in any case they
will reduce the amount of material which eventually will have to
be removed when cleaning up. Straight or convex slopes will
continue to slip until the natural form is attained.
56 HIGHWAY CONSTRUCTION
A revetment or retaining wall at the base of a slope will save
excavation.
In excavations of considerable depth, and particularly in soils
liable to slips, the slope may be formed in terraces, the horizontal
offsets or benches being made a few feet in width with a ditch on
Fig. 32. Section Showing Correct Slopes of Embankments
the inner side to receive the surface water from the portion of the
side slope above them. These benches catch and retain earth
Fig. 33. Section Showing Correct Slopes of Excavations
that may fall from the slopes above them. The correct forms for
the slopes of embankment and excavation are shown in Figs. 32
and 33.
Covering of Slopes. It is not usual to employ any artificial
means to protect the surface of the side slopes from the action of
the weather; but it is a precaution which in the end will save much
labor and expense in keeping the roadways in good order. The
simplest means which can be used for this purpose consist in cover-
ing the slopes with good sods, or else with a layer of vegetable
mold about four inches thick, carefully laid and sown with grass
seed. These means are amply sufficient to protect the side slopes
from injury when they are not exposed to any other cause of
deterioration than the wash of the rain and the action of frost
on the ordinary moisture retained by the soil.
A covering of brushwood or a thatch of straw may also be used
with good effect; but from their perishable nature they will require
frequent renewal and repairs.
Where stone is abundant a small wall of stone laid dry may be
constructed at the foot of the slopes to prevent any wash from them
being carried into the ditches.
HIGHWAY CONSTRUCTION 57
Shrinkage of Earthwork. All materials when excavated in-
crease in bulk, but after being deposited in banks subside or shrink
(rock excepted) until they occupy less space than in the pit from
which excavated.
Rock, on the other hand, increases in volume by being broken
up, and does not settle again into less than its original bulk. The
increase may be taken at 50 per cent.
The shrinkage in the different materials is about as follows:
Gravel 8 per cent
Gravel and sand 9 per cent
Clay and clay earths 10 per cent
Loam and light sandy earths 12 per cent
Loose vegetable soil 15 per cent
Puddled clay 25 per cent
Thus an excavation of loam measuring 1000 cubic yards will
form only about 880 cubic yards of embankment, or an embankment
of 1000 cubic yards will require about 1120 cubic yards, measured
in excavation, to make it. A rock excavation measuring 1000 yards
will make from 1500 to 1700 cubic yards of embankment, depending
upon the size of the fragments.
The lineal settlement of earth embankments will be about in
the ratio given above; therefore either the contractor should be
instructed, in setting his poles to guide him as to the height of grade
on an earth embankment, to add the required percentage to the
fill marked on the stakes, or the percentage may be included in
the fill marked on the stakes. In rock embankments this is not
necessary.
Classification of Earthwork. Excavation is usually classified
as earth, hardpan, loose rock, or solid rock. For each of these
classes a specific price is usually agreed upon, and an extra allowance
is sometimes made when the haul, or distance to which the excavated
material is moved, exceeds a given amount.
The characteristics which determine the classes to which a
given material belongs are usually described with clearness in the
specifications, as :
Earth, to include loam, clay, sand, and loose gravel.
Hardpan, to include cemented gravel, slate, cobbles, and
boulders containing less than 1 cubic foot, and all other material
of an earthy nature, however compact it may be.
58 HIGHWAY CONSTRUCTION
Loose rock t to include shale, decomposed rock, boulders, and
detached masses of rock containing not less than 3 cubic feet, and
all other material of a rock nature which may be loosened with a
pick, although blasting may be resorted to in order to expedite the
work.
Solid rock, to include all rock found in place in ledges and
masses, or boulders measuring more than 3 cubic feet, and which
can only be removed by blasting.
Prosecution of Earthwork. No general rule can be laid down
for the exact method of carrying on an excavation and disposing
of the excavated material. The operation in each case can be
determined only by the requirements of the contract, character
of the material, magnitude of the work, length of haul, etc.
Methods of Forming Embankments. General Case. Where
embankments are to be formed less than 2 feet in height, all stumps,
weeds, etc., should be removed from the space to be occupied by
the embankment. For embankments exceeding 2 feet in height
stumps need only be close cut. Weeds and brush, however, ought
to be removed and if the surface is covered with grass sod, it is
advisable to plow a furrow at the toe of the slope. Where a cut
passes into a fill all the vegetable matter should be removed from
the surface before placing the fill. The site of the bank should
be examined carefully and all deposits of soft, compressible matter
removed. When a bank is to be made over a swamp or marsh,
the site should be drained thoroughly, and if possible the fill should
be started on hard bottom.
Perfect stability is the object aimed at, and all precautions
necessary to this end should be taken. Embankments should be
built in successive layers: banks 2 feet and under in layers from 6
inches to 1 foot; heavier banks in layers 2 and 3 feet thick. The
horses and vehicles conveying the materials should be required to
pass over the bank for the purpose of consolidating it, and care
should be taken to have the layers dip towards the center. Embank-
ments which have been first built up in the center, and after-
wards widened by dumping the earth over the sides, should never
be allowed.
Embankments on Hillsides. When the axis of the road is
laid out on the side slope of a hill, and the road is formed partly
HIGHWAY CONSTRUCTION 59
by excavating and partly by embanking, the usual and most simple
method is to extend out the embankment gradually along the whole
line of the excavation. This method is insecure; the excavated
material if simply deposited on the natural slope is liable to slip,
and no pains should be spared to give it a secure hold, particularly
at the toe of the slope. The natural surface of the slope should be
cut into steps, as shown in Fig. 34. The dotted line AB represents
the natural surface of the ground, CEB the excavation, and ADC
the embankment, resting on steps which have been cut between A
and C. The best position for these steps is perpendicular to the
axis of greatest pressure. If AD is inclined at the angle of repose
of the material, the steps near A should be inclined in the oppo-
site direction to AD, and at an angle of nearly 90 degrees thereto,
Fig. 34. Section of Embankment Showing Reinforcing by Means of Steps
while the steps near C may be level. If stone is abundant in the
locality, the toe of the slope may be further secured by a dry wall
of stone.
On hillsides of great inclination the above method of construc-
tion will not be sufficiently secure; retaining walls of stone must
be substituted for the side slopes of both the excavations and embank-
ments. These walls may be made of stone laid dry, when stone
can be procured in blocks of sufficient size to render this kind of
construction of sufficient stability to resist the pressure of the
earth. When the stones laid dry do not offer this security, they
must be laid in mortar. The wall which forms the slope of the
excavation should be carried up as high as the natural surface
of the ground. Unless the material is such that the slope may be
safely formed into steps or benches, as shown in Fig. 34, the wall
that sustains the embankment should be built up to the surface
60
HIGHWAY CONSTRUCTION
of the roadway, and a parapet wall or fence raised upon it, to protect
pedestrians against accident, Fig. 35.
For the formula for calculating the dimensions of retaining
walls see Instruction Paper on Masonry and Reinforced Concrete,
Part III.
Treatment of Roadways on Rock Slopes. On rock slopes, when
the inclination of the natural surface is not greater than 1 on the
vertical to 2 on the base, the road may be constructed partly in
excavation and partly in embankment in the usual manner, or
by cutting the face of the slope into horizontal steps with vertical
faces, and building up the embankment in the form of a solid stone
wall in horizontal courses, laid either dry or in mortar. Care is
required in proportioning the steps, as all attempts to lessen the
Fig. 35. Reinforcing Roadway by Parapet Wall or Fence
quantity of excavation, by increasing the number and diminishing
the width of the steps, require additional precautions against settle-
ment in the built-up portion of the roadway.
When the rock slope has a greater inclination than 1:2 the
whole of the roadway should be in excavation.
In some localities roads have been constructed along the face
of nearly perpendicular cliffs, on timber frameworks consisting of
horizontal beams firmly fixed at one end by being let into holes
drilled in the rock, the other end being supported by an inclined
strut resting against the rock in a shoulder cut to receive it. There
are also examples of similar platforms suspended instead of being
supported.
Tools for Construction Work
Picks. Picks are made in various styles, according to the class
of material in which they are to be used. Fig. 36 shows the form
HIGHWAY CONSTRUCTION
61'
usually employed in street work. Fig. 37 shows the form generally
used for clay or gravel excavation.
Fig. 36. Grading Pick
Courtesy of Acme Road Machinery Company, Frankfort, New York
The eye of the pick is formed generally of wrought iron, while
the points are of steel. The weight of picks ranges from 4 to 9
pounds.
Fig. 37. Clay Pick
Courtesy of Acme Road Machinery Company, Frankfort, New York
Grubbing Tools. In handling brush, stumps, etc., such tools
as the bush hooks, Fig. 38, the bush mattock, Fig. 39, and the axe
mattock, Fig. 40, are com-
monly used. These are
cutting as well as grading
tools.
Shovels. Shovels, Fig.
41, are made in tw T o forms,
square and round pointed,
usually of pressed steel.
Plows. Plows are em-
, , ... Fig. 38. Bush Hooks
ployed extensively in grad-
Fig. 39. Bush Mattock
Fig. 40. Axe Mattock
ing, special forms being manufactured for the purpose. They are
known as "grading plows", "road plows", "township plows", etc.
62
HIGHWAY CONSTRUCTION
They vary in form according to the kind of work they are intended
for, viz, loosening earth, gravel, hardpan, and some of the softer rocks.
These plows are of great strength ;
selected white oak, rock elm, wrought
steel, and iron generally being used in
their construction. The cost of oper-
ating plows ranges from 2 to 5 cents
per cubic yard, depending upon the
compactness of the soil. The quan-
tity of material loosened will vary
from 2 to 5 cubic yards per hour.
Grading Plow. Fig. 42 shows the
form usually adopted for loosening
earth. This plow does not turn the
soil, but cuts a furrow about 10 inches
wide and of a depth adjustable up to
11 inches.
In light soil the plows are oper-
ated by 2 or 4 horses; in heavy soil
as many as 8 are employed. Grad-
ing plows vary in weight from 100
to 325 pounds.
Fig. 41.
Round Pointed and Square
Shovels
Courtesy of Acme Road Machinery
Company, Frankfort, New York
Fig. 42. Typical Road Plow
Courtesy of Western Wheeled Scraper Company, Aurora, Illinois
Hardpan Plow. Fig. 43 illustrates a plow specially designed
for tearing up macadam, gravel, or similar material. The point is
a straight bar of cast steel drawn down to a point, and can be
repaired easily r
HIGHWAY CONSTRUCTION
63
Scrapers. Scrapers are used generally to move the material
loosened by plowing; they are made of either iron or steel, and in a
Fig. 43. Typical Hardpan or Rooter Plow
Courtesy Western Wheeled Scraper Company, Aurora, Illinois
variety of forms, and are known by various names, as "drag", "buck",
"pole", and "wheeled". The drag scrapers are employed usually
on short hauls, the wheeled
ones on long hauls.
Drag Scrapers. Drag
scrapers, Fig. 44, are made
in three sizes. The smallest,
for one horse, has a capacity
of 3 cubic feet; the others,
for two horses, have a
capacity of 5 to 7J cubic
feet. The smallest weighs
about 90 pounds, and the
Fig. 44. Drag Scraper
Courtesy Western Wheeled Scraper Company,.
Aurora, Illinois
Fig. 45. Buck Scraper
Courtesy Western Wheeled Scraper Company,
Aurora, Illinois
larger ones from 94 to 102 pounds.
Buck Scrapers. Buck scrapers,
Fig. 45, are made in two sizes
two-rhorse, carrying 7J cubic feet;
four-horse, ,12 cubic feet.
Pole Scrapers. Pole scrapers
are designed for use in making and
leveling earth roads and for cutting
and cleaning ditches; they are well
adapted also for moving earth short
distances at a minimum cost.
64 HIGHWAY CONSTRUCTION
Wheeled Scrapers. Wheeled scrapers, Fig. 46, consist of a metal
box, usually steel, mounted on wheels, and furnished with levers for
raising, lowering, and dumping. They are operated in the same
Fig. 46. Typical Wheeled Scraper
Courtesy Western Wheeled Scraper Company, Aurora, Illinois
manner as drag scrapers, except that all the movements are made
by means of the levers, and without stopping the team. By their
use the excessive resistance to traction of the drag scraper is avoided.
Fig. 47. Contractor's Barrow with Pressed-Steel Tray
Courtesy of Acme Road Machinery Company, Frankfort, New York
Various sizes are made, ranging in capacity from 10 to 17 cubic feet.
In weight they range from 350 to 700 pounds.
Wheelbarrows. Wheelbarrows sometimes are constructed of
wood and are employed most commonly for earthwork. Their
HIGHWAY CONSTRUCTION 65
capacities range from 2 to 2| cubic feet. Weight is about 50 pounds.
The barrow, Fig. 47, has pressed-steel tray, oak frame, and
steel wheels, and will be found more durable in the maintenance
department than the all-wood barrow. Capacity is from 3J to 5
cubic feet, dependent on size of tray.
The barrow, Fig. 48, is constructed with tubular-iron frames
and steel tray, and is adaptable to the heaviest work, such as mov-
Fig. 48. All Steel and Iron Concrete Barrow
Courtesy of Acme Road Machinery Company, Frankfort, New York
ing heavy broken stone, etc., or it may be employed with advantage
in the cleaning department. Capacity from 3 to 4 cubic feet.
Weight from 70 to 82 pounds.
The maximum distance to which earth can be moved econom-
ically in barrows is about 200 feet. The wheeling should be per-
Fig. 49. Typical Dump Carts for Hauling Earth, Etc.
Courtesy of Western Wheeled Scraper Company, Aurora, Illinois
formed upon planks, whose steepest inclination should not exceed 1
in 12. The force required to move a barrow on a plank is about -%%
part of the weight; on hard dry earth, about y^ part of the weight.
The time occupied in loading a barrow will vary with the
character of the material and the proportion of wheelers to shovel-
ers. Approximately, a shoveler takes about as long to fill a barrow
66
HIGHWAY CONSTRUCTION
with earth as a wheeler takes to wheel a full barrow a distance of
about 100 or 120 feet on a horizontal plank and return with the
empty barrow.
Fig. 50. Rear View of Typical Dump Wagon Showing Bottom Open
Courtesy of Western Wheeled Scraper Company, Aurora, Illinois
Carts. The cart usually employed for hauling earth, etc., is
shown in Fig. 49. The average capacity is 22 cubic feet, and the
average weight is 800 pounds. These carts are furnished usually
Fig. 51. Twenty-Yard Dump Car
Courtesy ojf Western Wheeled Scraper Company, Aurora, Illinois
with broad tires, and the body is balanced so that the load is evenly
divided about the axle.
The time required to load a cart varies with the material.
One shoveler will require about as follows: clay, 7 minutes; loam,
6 minutes; sand, 5 minutes.
HIGHWAY CONSTRUCTION 67
Dump Wagons. The use of dump wagons, Fig. 50, for moving
excavated earth, etc., and for transporting materials such as sand,
gravel, etc., materially shortens the time required for unloading the
ordinary form of contractor's wagon; having no reach or pole con-
necting the rear axle with the center bearing of the front axle, they
may be cramped short and the load deposited just where required.
They are operated by the driver, and the capacity ranges from 35
to 45 cubic feet.
Fig. 52. Typical Grader
Courtesy of Acme Road Machinery Company, Frankfort, New York
Dump Cars. Dump cars, Fig. 51, are made to dump in several
different ways, viz, single or double side, single or double end, and
rotary or universal dumpers.
Dump cars may be operated singly or in trains, as the magni-
tude of the work may demand. They may be moved by horses
or small locomotives. They are made in various sizes, depending
upon the gage of the track on which they are run. A common
gage is 20 inches, but it varies from that up to the standard railroad
gage of 56J inches.
Mechanical Graders. Mechanical graders are used extensively
in the making and maintaining of earth roads. They excavate
and move earth more expeditiously and economically than can be
HIGHWAY CONSTRUCTION 69
done by hand; they are called by various names, such as "road
machines", "graders", "road hones", etc.
Simple Graders. Briefly described, graders consist of a large
blade, Fig. 52, made entirely of steel, iron, or wood shod with steel,
which is so arranged by a mechanism attached to the frame from
which it is suspended that it can be adjusted and fixed in any
direction by the operator. In their action they combine the work
of excavating and transporting the earth. They have been
employed chiefly in the forming and maintenance of earth roads,
but also may be used advantageously in preparing the subgrade
surface of roads for the reception of broken stone or other
improved covering.
Elevating Graders. Some graders combine the function of
elevating the material, of excavating it from side ditches, and
of loading it automatically into carts or wagons. Briefly described,
the machine, Fig. 53, consists of a plow which loosens and raises
the earth, depositing it upon a transverse carrying belt, which con-
veys it from excavation to embankment. Carrier frames of two
or three different lengths are provided with the machine, the distance
of the end of the elevator from the plow varying from 15 to 30 feet.
The carrier belt is of heavy 3-ply rubber 3 feet wide.
The plow and carrier are supported by a strong trussed frame-
work resting on heavy steel axles and broad wheels. The large
rear wheels are ratcheted upon the axle, and connected with strong
gearing which propels the carrying belt at right angles to the direc-
tion in which the machine is moving.
The wheels and trusses are low and broad, occupying a space
8 feet wide and 14 feet long, exclusive of the side carrier. This
enables it to work on hillsides where any wheeled implements can
be used. Notwithstanding its large size it is so flexible that it may
be turned around on a 16-foot embankment. Pilot wheels and
levers enable the operator to raise or lower the plow or carrier at
pleasure.
For motive power, 12 horses 8 driven in front, 4 abreast, and
4 in the rear on a push cart are usually employed.
When the teams are started, the operator lowers the plow and
throws the belting into gear, and as the plow raises and turns the
earth to the side the belt receives and delivers it at the distance for
70
HIGHWAY CONSTRUCTION
which the carrier is adjusted, forming either excavation or embank-
ment, as the case may be.
When it becomes necessary to deliver the excavated earth
beyond the capacity of the machine, the earth is loaded upon wagons,
Fig. 54. Two Views of Elevating Graders Loading Earth into Dump Wagons
Courtesy of Western Wheeled Scraper Company, Aurora, Illinois
then conveyed to any distance. By adjusting the height of the
carrier, the wagons are driven under it, Fig. 54, and loaded with
H to 1J yards of earth in from 20 to 30 seconds. When one wagon
turns out with its load, another drives under the carrier, and the
HIGHWAY CONSTRUCTION 71
machine thus loads 600 to 800 wagons per day. It is claimed that
with six teams and three men it is capable of excavating and placing
in embankment from 1,000 to 1,500 cubic yards of earth in 10 hours,
or of loading from 600 to 800 wagons in the same time, and that the
cost of this handling is from 1J to 2J cents per cubic yard.
Points to be Considered in Selecting a Road Machine. In the
selection of a road machine the following points should be carefully
considered : thoroughness and simplicity of its mechanical construc-
tion; material and workmanship used in its construction; safety
to the operator; ease of operation; lightness of draft; and adapta-
bility to general road work, ditching, etc.
Care of Road Machines. The road machine when not in use
should be stored in a dry house and thoroughly cleaned, its blade
brushed clean from all accumulations of mud, wiped thoroughly
dry, and well covered with grease or crude oil. The axles, journals,
and wearing parts should be kept well oiled when in use, and an
extra blade should be kept on hand to avoid stopping the machine
while the dulled one is being sharpened.
Surface Graders. The surface grader is used for removing
earth previously loosened by a plow. It is operated by one horse.
Fig. 55. Simple Road Leveler
The load may be retained and carried a considerable distance, or
it may be spread gradually as the operator desires. It is also
employed to level off and trim the surface following the scrapers.
The blade is of steel, \ inch thick, 15 inches wide, and 30 inches
long. The beam and other parts are of oak and iron. Weight
about 60 pounds.
Road Leveler. The road leveler, Fig. 55, is used for trimming
and smoothing the surface of earth roads. It is largely employed in
the spring when the frost leaves the ground.
72
HIGHWAY CONSTRUCTION
The blade is of steel, J-inch thick by 4 inches by 72 inches, and
is provided with a seat for the driver. It is operated by a team of
horses. Weight about 150 pounds.
Ditching Tools. The tools employed for digging the ditches
and shaping the bottom to fit the drain tiles are shown in Fig. 56.
They are convenient to use, and expedite the work by avoiding
unnecessary excavation.
The tools are used as follows: Nos. 3, 4, and 5 are used for
digging the ditches; Nos. 6 and 7 for cleaning and rounding the
Fig. 56. Typical Tools Used for Digging Ditches
bottom of the ditch for round tile; No. 2 is used for shoveling out
loose earth and leveling the bottom of the ditch; No. 1 is used for
the same purpose when the ditch is intended for "sole" tile.
Sprinkling Wagons. A convenient form of sprinkling wagon
for suburban streets and country roads is shown in Fig. 57. The
tank is of 12 gage steel and its capacity is 380 to 600 gallons.
HIGHWAY CONSTRUCTION
73
Road Rollers. Horse-Drawn Rollers. There are a number of
types of horse-drawn rollers on the market, consisting essentially
Fig. 57. Steel 'lank Sprinkling Wagon
Courtesy of Acme Road Machinery Company, Frankfort, New York
of a hollow cast-iron cylinder 4 to 5 feet long, 5 to 6 feet in
diameter, and weighing from 3 to 6 tons. Some forms are
Fig. 58. Ten-Ton Steam-Driven Road Roller
Courtesy of Charles Longenecker & Company, New York City
provided with boxes in which stone or iron may be placed to
increase the weight, and some have closed ends and may be filled
74 HIGHWAY CONSTRUCTION
with water or sand. The use today of small gasoline road rollers
makes this type less prevalent than formerly.
Power-Propelled Rollers. The rollers employed for compacting
the natural soil and all forms of broken-stone pavements usually
are of the three-wheel type, operated by steam or gasoline, Fig. 58.
They generally are arranged to move at two speeds, low and high;
the low speed is from 2 to 3 miles per hour and the high speed from
4 to 5 miles. The low speed is employed for compacting the natural
soil and the foundation; the high speed is employed for finishing
the surface. The driving wheels are furnished with lock pins or
differential gears to permit them to accommodate themselves auto-
matically to the difference in speeds when operating on sharp curves.
They vary in weight from 10 to 20 tons.
Scarifiers. The implement used for breaking up a broken-
stone road preparatory to applying a new surface is called a "scari-
ng. 59. Scarifier, for Quick and Economical Repair of Macadam Roads
Courtesy of Charles Longenecker & Company, New York City
fier", Fig. 59. It usually consists of a cast-iron block, weighing
about 3 tons, mounted on 2 or 4 wheels; the block is fitted with a
series of spikes or picks, arranged either in one line, or in two lines
forming a V; means are provided for adjusting the depth to which
the picks penetrate, the maximum depth being about 6 inches.
The scarifier is operated by being attached to the rear of a steam
roller or traction engine which hauls it over the road.
NATURAL=SOIL ROADS
Earth Roads. The term "earth road" is applied to roads
where the surface consists of the native soil; this class of road is the
HIGHWAY CONSTRUCTION 75
most common and cheapest in first cost. At certain seasons of the
year earth roads, when properly cared for, are second to none, but
during the spring and wet seasons they" are very deficient in the
important requisite of hardness, and are almost impassable.
For the construction of new earth roads, all the principles pre-
viously discussed relating to alignment, grades, drainage, width, etc.,
should be followed carefully. The crown or transverse contour
should be greater than in stone roads; 12 inches at the center in 25
feet will be sufficient.
Drainage is especially important, because the material of the
road is more susceptible to the action of water, and more easily
destroyed by it than are the materials used in the construction of the
better class of roads. When water is allowed to stand upon the
road, the earth is softened, the wagon wheels penetrate it, and the
horses' feet mix and knead it until it becomes impassable mud. The
action of frost is also apt to be disastrous upon the more permeable
surface of the earth road, having the effect of swelling and heaving
the roadway and throwing its surface out of shape. It may in fact
be said that the whole problem of the improvement and mainte-
nance of ordinary country roads is one of drainage.
In the preparation of the wheelway all stumps, brush, vegetable
matter, rocks, and boulders should be removed from the surface and
the resulting holes filled in with clean earth. The roadbed, having
been brought to the required grade and crown, should be thoroughly
rolled; all inequalities appearing during the rolling should be filled
up and re-rolled.
Care of Earth Roads. If the surface of the roadway is prop-
erly formed and kept smooth, the water will be shed into the side
ditches and do comparatively little harm; but if it remains upon the
surface, it will be absorbed and convert the road into mud. All
ruts and depressions should be filled up as soon as they appear.
Repairs should be attended to particularly in the spring. At that
season the judicious use of a road machine and rollers will make a
smooth road. In summer when the surface gets rough it can be
improved by running a harrow over it; if the surface is a little muddy
this treatment will hasten the drying.
During the fall the surface should be repaired, with special
reference to putting it in shape to withstand the ravages of winter.
76 HIGHWAY CONSTRUCTION
Saucer-like depressions and ruts should be filled up with clean earth
similar to that of the roadbed and tamped into place.
The side ditches should be examined in the fall to see that they
are free from dead weeds and grass, and late in winter they should
be examined again to see that they are not clogged. The mouths
of culverts should be cleaned of rubbish and the outlet of tile drains
opened. Attention to the side ditches will prevent overflow and
washing of the roadway, and also will prevent the formation of
ponds at the roadside and the consequent saturation of the roadbed.
Holes and ruts should not be filled with stone, bricks, gravel,
or other material harder than the earth of the roadway as the hard
material will not wear uniformly with the rest of the road, but
produce bumps and ridges, and usually result in making two holes,
each larger than the original one. It is bad practice to cut a gutter
Fig. 60 fcsteel Road Drag
Courtesy of Western Wheeled Scraper Company, Aurora, Illinois
from a hole to drain it to the side of the road. Filling is the proper
course, whether the hole is dry or contains mud.
The maintaining of smooth surfaces on all classes of earth roads
will be assisted and cheapened greatly by the frequent use of a
roller (either steam or horse) and any one of the various forms of
road grading and scraping machines. In repairing an earth road
the plow should not be used. It breaks up the surface which has
been compacted by time and travel.
In the maintenance of earth roads the road drag, Fig. 60, or
some similar device, is indispensable. The drag should be light
and should be hauled along the road at an angle of about 45 degrees,
so that only a small amount of earth is pushed to the center of the
HIGHWAY CONSTRUCTION 77
road. The driver should ride on the drag and not drive faster
than a walk. Dragging should begin on the side of the road, or
wheel track, and return on the opposite side. Unless the road is
in good condition, it should be dragged after every heavy rain.
In the maintenance of clay roads neither sods nor turf should
be used to fill holes or ruts; for, though at first deceptively tough,
they soon decay and form the softest mud. Neither should the ruts
be filled with field stones; they will not wear uniformly with the
rest of the road, but will produce hard ridges.
Trees and close hedges should not be allowed within 200 feet
of a clay road. It requires all the sun and wind possible to keep its
surface in a dry and hard condition.
Sand Roads. The aim in the improvement of sand roads is to
have the wheelway as narrow and well defined as possible, so as to
have all vehicles run in the same track. An abundant growth
of vegetation should be encouraged on each side of the wheelway,
for by this means the shearing of the sand is, in a great measure,
avoided. Ditching beyond a slight depth to carry away the rain
water is not desirable, for it tends to hasten the drying of the sands,
which is to be avoided. Where possible the roads should be over-
hung with trees, the leaves and twigs of which, catching on the
wheelway, will serve still further to diminish the effect of the wheels
in moving the sands about. If clay can be obtained, a coating 6
inches thick will be found a most efficient and economical improve-
ment. A coating of 4 inches of loose straw will, after a few days'
travel, grind into the sand and become as hard and firm as a dry
clay road.
Sand=Clay Roads. A sand-clay road is formed by mixing
clay and sand in such proportions that the clay will just fill the
voids in the sand, and produce a mixture that is neither sticky nor
friable, but coheres in a comparatively dry plastic mass when com-
pacted with pressure. If an insufficient amount of clay is used,
the mixture will not bind; if an excess of clay is used, the road will
be sticky and muddy after a fall of rain.
The grains of sand furnish the hard material to resist the
abrasion of the traffic; the clay provides the cementing or binding
medium to hold the sand together. All clays are not equally satis-
factory as binders, owing to the diversity of their origin. A common
78 HIGHWAY CONSTRUCTION
test for clay suitable for road purposes is to apply a wet finger
to a piece of clay; if the clay adheres to the finger, it may be assumed
reasonably that it will adhere to the sand.
The natural sand soils and the naturel clay soils are improved
by the application of the sand-clay mixture, the method of applying
it being varied according to which kind of soil is to be treated.
Applying Sand-Clay Mixture to Clay Soil. In the treatment
of a clay soil, the soil is plowed to a depth of 6 to 8 inches; then
pulverized by harrowing, and, if necessary, by rolling with a light
roller and again harrowing. After the clay is thoroughly pulverized,
the sand is spread over the surface in a layer from 6 to 8 inches
thick, and the sand and clay are thoroughly mixed by continued
harrowing. After the dry mixing has been accomplished satis-
factorily, the surface is moistened slightly by sprinkling with water,
then compacted by rolling, after which a road machine or grader
is used to give the required crown; and then the roller is again applied
until the surface becomes smooth and hard.
Applying Sand-Clay Mixture to Sand Soil. In the treatment
of a sand soil, the clay is spread over the surface in a layer, ranging
from 4 to 8 inches thick; then mixed with the sand by harrowing.
After that it is sprinkled heavily with water and again worked
with the harrow; then it is shaped and rolled in the same manner
as stated above for a clay soil.
The sand-clay roads require considerable attention, after
completion, to eliminate weak or defective spots by applying sand
or clay, as may be required.
Application of Oil to Sand and Gravel Soils. Sand and gravel
soils are improved by the application of crude petroleum or asphal-
tic oils. The oil abates dust; forms a non-absorbent surface which
turns off rain water and decreases the amount of mud; and furnishes
a dark-colored road surface which is more pleasing to the eye than
the ordinary light color.
The roadbed is prepared to receive the oil by grading, shaping,
and rolling. The oil is applied to the prepared surface by sprinkling
from tank wagons; the oil coat is covered with a thin layer of sand,
after which the roller is applied again. If during the rolling the
surface becomes sticky, or dry and dusty, dry sand or more oil is
added as required.
HIGHWAY CONSTRUCTION 79
ROADS WITH SPECIAL COVERINGS
Elements of a Road Covering. The wheelways of roads and
streets are prepared for traffic by placing upon the natural soil a
covering of some suitable material which will furnish a comparatively
smooth surface on which the resistance to traction will be reduced
to the least possible amount, and over which all classes of vehicles
may pass with safety and expedition at all seasons of the year.
The covering usually consists of two parts: a foundation, and a
wearing surface.
The functions of the foundation are as follows: (1) to protect
the soil from disturbance and the injurious effects of surface water;
(2) to transmit to and distribute over a sufficiently large area of
the soil the weight of the loads imposed upon the wearing coat;
(3) to support unyieldingly the wearing surface and the loads coming
upon it.
The efficiency of the wearing surface depends entirely upon
the quality of the foundation. If the foundation be weak, the
wearing surface will be disrupted speedily, no matter how well
constructed.
FOUNDATIONS
Materials. The foundation, when once constructed, should
not require to be disturbed nor reconstructed. The materials
employed in its construction may be the cheapest available, such
as local rock, gravel, sand, furnace slag, etc., the important point
in the design being to provide sufficient thickness, so that when
consolidated it will maintain its form under the heaviest traffic
liable to come upon it. If the foundation and the covering yield
under the load, an upheaval is caused that disrupts the bond and
hastens the destruction of the road.
Thickness. The thickness of the foundation depends upon
the supporting power of the natural soil and the weight of the loads
coming upon the wearing surface. The supporting or bearing
power of the soil can be ascertained by direct test, and the weight
of the loads by a survey of the traffic plus a provision for future
increase.
Recent tests indicate that non-porous soils from which the
subsoil water is removed by drainage will support in their worst
80 HIGHWAY CONSTRUCTION
condition a load of about 4 pounds per square inch; and that if
the thickness of the foundation be adjusted to the traffic on this
basis it will be safe at all seasons of the year.
Methods of Calculating Thickness of Covering. There are two
theories as to the manner in which pressure of a loaded wheel is
transmitted from the surface of the covering to the natural soil:
(1) that the pressure on the soil varies inversely as the cube of the
thickness of the foundation and the wearing surface; (2) that the
pressure is transmitted downwards in the form of a truncated cone,
the lines of which diverge at an angle varying from 30 to 50 degrees
from the vertical, according to the solidity of the covering. If
the surface of the road is uneven or obstructed by loose stones,
the lines of pressure are more concentrated when the wheels pass
over such obstacles.
The latter theory is the one most frequently applied. The
calculation is performed as follows: Let P be distributed pressure
on the soil, per square inch; A, length of arc of wheel tire in contact
with surface in square inches; W, width of tire in inches; L, load
carried by wheel in pounds; F, depth of wearing surface and foun-
dation in inches; C, area of contact equal to AxW; and B, area
of base at surface of natural soil. The area of the base is
B = (2F+A) &F+W)
The distributed pressure is
p= _ L_ = L
(2F+A) (2F+W) B
Assuming that the load is 1,000 pounds per inch of tire width; the
tire, 3 inches; length of contact 3 inches; total thickness of the
wearing coat and the foundation 12 inches; the pressure on the
soil is
1000X3 _ 3000 = = llolb j
"(2X12+3) (2X12+3)" 27X27" 729 ~
According to this theory the thickness of the covering varies
from 4 to 16 inches, the smallest thickness being placed upon gravel
or sand and the greatest upon clay.
Preparation of Foundation. The preparation of the foundation
involves two distinct operations: (1) preparation of the natural
soil; and (2) placing an artificial foundation upon the Drepared
natural soil.
HIGHWAY CONSTRUCTION 81
The essentials necessary to the preparation of the natural
soil are: (1) the entire removal of perishable vegetable and yielding
matter; (2) the drainage of the soil where necessary; (3) the improv-
ing of the bearing power of the soil where required; and (4) compact-
ing the soil.
All soils are improved by rolling, and weak spots, which other-
wise would pass unnoticed, are discovered. However, care must
be taken that the weight of the roller employed is not too great for
the bearing power of the soil ; if it exceed this the surface of the soil
will be formed into a series of undulations that will cause the wear-
ing coat to fail; the same condition may be produced by excessive
rolling with a comparatively light roller. Each soil requires different
treatment.
Soils of a siliceous and calcareous nature may be improved by
drainage and the addition of a layer of clay 2 to 6 inches thick,
mixed with the soil and compacted by rolling. The argillaceous
and allied soils, owing to their retentive nature, are very unstable
under the action of water and frost, and in their natural condition
afford a defective foundation. They are improved by thorough
drainage and the admixture of sand well rolled, together with the
placing upon the surface of the compacted soil a layer 2 to 6 inches
thick of sand, slag, cinders, or other material of a similar nature,
and then compacting it by sprinkling with water and rolling.
Types of Foundation to Be Used. The essential requisite in the
construction of the artificial foundation is that it be a dense mass,
and the type of foundation to be employed varies with the char-
acter of the wearing surface. For the various types of broken-
stone surfaces, the foundation may be composed of blocks of stone
(ledge rock or fieldstones), roughly shaped to a rectangular form,
ranging in width and depth from 6 to 8 inches and in length from
6 to 16 inches. They are set by hand on the soil bed with the length
at right angles to the axis of the roadway, so arranged that they
break joints. The edges that project above the subgrade level are
broken off with hand hammers, and the spaces between them are
filled with chips of stone well packed and wedged in. The blocks
are brought then to a firm bearing by rolling with a steam roller,
after which the wearing surface is laid. The foundation also may
be composed of broken stone, gravel, or furnace slag so graded that
82 HIGHWAY CONSTRUCTION
the voids will be reduced to the smallest possible amount. The
voids may be filled with stone dust; a mixture of sand and clay; a
mortar and grout composed of hydraulic cement and sand; bitu-
minous cement; or hydraulic-cement concrete, mixed and placed
upon the soil bed.
WEARING SURFACES
Functions of Wearing Surface. The office of the wearing sur-
face is to protect the foundation from the wear of the traffic and the
effects of surface water, and to support the weight of the traffic and
transmit it to the foundation. To render efficient service to the
traffic, it must furnish a comparatively smooth unyielding surface
that affords good foothold for draft animals and good adhesion for
motor vehicles, and on which the resistance to traction will be a
minimum. To fill its office satisfactorily the material of which it is
composed must possess strength to resist crushing and abrasion,
and its fabric must be practically impervious. To render economical
service, it must possess the power of resisting the action of the
destroying agencies for a reasonable length of time before it becomes
unfit for use. For this purpose it must possess the resisting quali-
ties previously stated, and it must also possess a certain thickness;
this thickness will depend upon the character of the material
employed and its rate of wear under the given traffic and atmospheric
conditions. Economy is not promoted by using a thick wearing
surface, as under heavy traffic it will be so worn in a few years as to
be unserviceable, and under light traffic it will be decomposed before
it is worn out. In either case it must be removed and the portion
so removed is waste; therefore, only such thickness as will give
efficient service during a few years should be adopted.
Thickness. The measure for the economical thickness of any
type of wearing surface is that the annual interest charge on the
first cost plus the annual depreciation shall be a minimum. To
apply this measure it is necessary to know the amount of traffic and
the loss of thickness due to wear.
Classification of Wearing Surfaces. The wearing surfaces
most commonly employed for roads and streets are composed of:
(1) gravel, broken stone, furnace slag, and similar granular materials
bound with colloidal cement formed by the action of water on the
plastic elements of rock and clay; (2) broken stone, gravel, and sand
HIGHWAY CONSTRUCTION 83
bound with: (a) bituminous cement; (b) hydraulic cement; (3) stone
blocks; (4) brick; (5) wood blocks.
In type (1), a certain amount of moisture is essential to success-
ful binding. When this is lacking, as in the summer season, the
binding material becomes dry and brittle, and the fragments at the
surface are displaced by the action of the traffic; an excess of mois-
ture destroys the binding power; and the surface is quickly broken
up by the traffic.
Wearing surfaces of type (2a) are usually limited in life not
merely by the wear of traffic, but by the fact that all bitumens. slowly
alter in chemical composition when exposed to atmospheric action,
and in time become brittle. Type (2b) is subject to cracking under
expansion and contracting, due to changes of temperature, and is
liable to w r ear unevenly owing to irregularity in mixing and the segre-
gation of the ingredients while the concrete is being put in place.
When a defective spot begins to wear, it extends very rapidly under
the abrasive action of the traffic.
The materials of types (3) and (4) seldom rot or disintegrate
and, when the pavement is well constructed, are eminently enduring
and generally render satisfactory service. Since the use of creosote
and other preservatives has increased the service life of wood blocks,
type (5), by lessening their tendency to decay, they have come into
extensive use for street paving.
Gravel Roads
Gravel. Gravel consists of smooth and somewhat rounded
stones, varying in size from small grains to pebbles 4 or more inches
in diameter. It is found mixed with sand, on the banks and in the
beds of rivers; and in deposits on the land, mixed with clay and other
mineral substances, such as limestone and oxide of iron, from which
it derives a distinctive name. Gravel of the latter class is called
cementatious and when suitably prepared cements together, forming
a very satisfactory roadway for light traffic, producing but little
dust in dry weather and costing little to maintain.
Preparation of Gravel. Gravel is best prepared for use by
screening into three grades: grade (I), containing the stones retained
by a IJ-inch mesh screen and passing a 2J-inch mesh; grade (2),
containing the stones retained by a i-inch mesh and passing a 1 J-inch
84 HIGHWAY CONSTRUCTION
mesh; grade (3), containing all the material passing the J-inch
mesh. The voids in grade (1) are determined, and enough of grade
(3) added slightly more than to fill them; the two are intimately
and evenly mixed and the mixture is used for the first or lower
course. The voids in grade (2) are determined and a sufficient
quantity of grade (3) added to fill them ; the two are mixed and used
for the top course. The mixture should be combined very evenly
so that the fine material is mixed uniformly with the coarse ; and in
spreading the mixture, care should be taken to avoid separating it
or allowing the fine material to settle to the bottom.
If the gravel is deficient in binding material, the latter may be
added in the form of clay, loam, limestone screenings, shale, or marl,
the amount added ranging from 10 to 15 per cent. An excess (20
per cent) of clay causes the gravel to pack quickly and to present a
good appearance under the rolling; but in dry weather the road will
ravel, become defective and dusty, and in wet weather it will be
muddy. Clean smooth gravel will not consolidate without a binder
and, unless this is of very good quality, a road made with it will
prove unsatisfactory.
Laying the Gravel. On the natural-soil bed properly graded
and compacted, the prepared gravel is spread uniformly to the
depth desired usually 6 inches. Then it is compacted by rolling
with a steam roller, after which it is moistened by sprinkling with
water, and the rolling is repeated. The sprinkling and rolling are
repeated as often as may be required, until the stones cease to rise
or creep in front of the roller. The second course then is spread to
a depth of about 4 inches, rolled, sprinkled, and again rolled in the
same manner and to the same extent as the first course. After this,
a thin coat of the fine screenings is spread over the surface and the
traffic is admitted.
If, during the rolling, the first course appears to be deficient in
binding material, more may be added by spreading a thin layer of
the fine material over the surface of the course, sprinkling and
rolling, as above described.
If, during the rolling of the top course, any stones larger than
1| inches appear, they must be removed.
Gravel shrinks in rolling about 20 per cent of its loose depth;
therefore, to obtain a thickness of 8 inches when compacted, the
HIGHWAY CONSTRUCTION 85
loose material should have a depth of about 10 inches. The thick-
ness of the gravel coating varies according to the nature of the
roadbed, a thicker layer being necessary on impermeable soil than
on a well-drained soil.
The pebbles in a gravel road are imbedded in a paste and can be
displaced easily. It is for this reason, among others, that such
roads are subject to internal destruction.
The binding power of clay depends in a large measure upon
the state of the weather. During rainy periods a gravel road
becomes soft and muddy, while in very dry weather the clay will
contract and crack, thus releasing the pebbles, and causing a loose
surface. The most favorable conditions are obtained in moderately
damp or dry weather, during which a gravel road offers several
advantages for light traffic, the character of the drainage, etc.,
largely determining durability, cost, maintenance, etc.
Repair. Gravel roads constructed as above described will need
only small repairs for some years, but daily attention is required in
making these. A garden rake should be kept at hand to draw any
loose gravel into the wheel tracks, and for filling any depressions
that may occur.
In making repairs, it is best to apply a small quantity of gravel
at a time, unless it is a spot which actually has cut through. Two
inches of gravel at once is more profitable than a larger amount.
Where a thick coating is applied at once it does not all pack, and if,
after the surface is solid, a cut be made, loose gravel will be found;
this holds water and makes the road heave and become spouty
under the action of frost. It will cost no more to apply 6 inches of
gravel at three different times than to do it at once.
At every J mile a few cubic yards of gravel should be stored to
be used in filling depressions and ruts as fast as they appear, and
there should be at least one laborer to every 5 miles of road.
Broken=Stone Roads
Methods of Construction. Broken-stone roads are formed in
several different ways. For example, the road may be formed by
placing one or two layers of stone broken into small fragments
upon: (1) the natural soil; (2) a foundation composed of large stone
set by hand upon the natural soil ; or (3) a foundation layer of cement
86
HIGHWAY CONSTRUCTION
HIGHWAY CONSTRUCTION 87
concrete. The layers of broken stone are compacted by rolling
with a heavy roller and the interstices, or spaces between the stones
are filled with a binder composed either of stone dust; stone
dust and clay; a grout of hydraulic or Portland cement; or a bitu-
minous cement derived from either coal tar or asphalt, and used
alone or mixed with sand. The broken stone forming the lower
surface layer often is coated with a bituminous cement before placing
it upon the foundation. This applies particularly when the upper
layer is of bituminous cement. The broken stone also may be
mixed with Portland cement and sand, forming a concrete, which
is placed either upon the prepared natural soil or upon a concrete
or broken-stone foundation.
The several methods for constructing broken-stone roads are
distinguished by either a specific name or the name of the introducer.
Thus, the types known as Telford and Macadam are named
from Thomas Telford and John L. McAdam, Scottish engineers,
who introduced them in England during the early part of the 19th
Century, as an improvement of the method employed in the 18th
Century by M. Tresaguet on the roads of France. Telford used a
base of large stones, Fig. 61, upon which the small stone was placed.
McAdam omitted the base contending that it was useless and
injurious. Both constructors insisted on thorough drainage of the
subsoil, but neither used a binder and rolling was unknown. The
stones were left to be compacted by the traffic. The introduction of
stone-crushing machinery and rollers as well as the practice (con-
demned by McAdam, but advocated by Mr. Edgeworth, an Irish
landowner in his treatise on Road Building published in 1817) of filling
the voids with a binder has caused material departures from the
methods of the pioneers whose names are still but improperly applied.
The cement grouting was introduced in England by Sir John
Macneil. The coating of the stone with coal tar was first prac-
ticed in England about 1840, and was called "tar-macadam". In
recent times, to distinguish the several varieties of bituminous con-
struction, several specific terms have been coined, as "bitulithic",
"tarmac", "warrenite", "bituminous macadam", "asphalt macadam",
etc. Since the use of bituminous binding has become extensive, the
term "water-bound macadam" has come into use, to distinguish the
earlier macadam type from the types recently introduced.
88 HIGHWAY CONSTRUCTION
Quality of Stones. The materials used for broken-stone pave-
ments of necessity must vary very much according to the locality.
Owing to the cost of haulage, local stone generally must be used,
especially if the traffic be only moderate. If, however, the traffic is
heavy, it sometimes will be found better and more economical to
obtain a superior material, even at a higher cost, than the local
stone; and in cases where the traffic is very great, the best material
that can be obtained is the most economical.
There are a number of qualities required in a stone to render
efficient service. Hardness and toughness, to resist the effects of
abrasion and impact. These two properties, while closely related,
are not always coincident; some rocks, although extremely hard, yet
are so brittle that they crush easily under impact. In others the
cohesion between the component particles is so weak that they are
worn quickly by abrasion. Durability, or power to resist the disin-
tegrating influences of the weather and humus acids. The quality
of durability depends chiefly upon the chemical stability of the
minerals present. Physical defects and abrasion generally cause
the destruction of the stone long before it is injured by chemical
changes. Capability of binding into a compact mass. This quality
is essential to stone used for water-bound macadam. The binding
or cementing property is possessed to a greater or less extent by all
rocks when in a state of disintegration. It is caused by the action
of water upon the chemical constituents of the stone contained in
the detritus material worn off produced by crushing the stone,
and by the friction of the fragments on each other while being com-
pacted; its strength varies with the different species of rock, but it
exists in some measure with them all, being greatest with limestone
and least with gneiss.
The essential condition of the stone to produce this binding
effect is that it be sound. No decayed stone retains the property of
binding, though in some few cases, where the material contains iron
oxides, it may, by the cementing property of the oxide, undergo a
certain amount of binding.
A stone of good binding nature frequently will wear much
better than one without, although it is not so hard. A limestone
road well made and of good cross section will be more impervious
than any other, owing to this cause, and will not disintegrate so
HIGHWAY CONSTRUCTION 89
soon in dry weather, owing partly to this and partly to the well-
known quality which all limestone has of absorbing moisture from
the atmosphere. Mere hardness without toughness is not of much
use, as a stone may be very hard but so brittle as to be crushed to
powder under a heavy load, while a stone not so hard but having a
greater degree of toughness will be uninjured.
A stone for a road surface should be as little absorptive of
moisture as possible in order that it may not suffer injury from the
action of frost. Many limestones are objectionable on this account.
The stone used should be uniform in quality, otherwise it will
wear unevenly, and depressions will appear where the softer material
has been used. As the under parts of the road covering are not
subject to the wear of traffic, and have only the weight of loads to
sustain, it is not necessary that the stone of the lower layer be so
hard or so tough as the stone for the surface, hence it is frequently
possible by using an inferior stone for that portion of the work, to
reduce greatly the cost of construction.
Testing the Rock. In order to ascertain the probable resist-
ance of the different rocks to the destructive action of the traffic and
weather, tests are made in the laboratory to determine the resistance
to impact and abrasion, absorptive capacity, hardness, toughness,
and specific gravity.
Abrasion. The test for abrasion is conducted in the Deval type
of machine. It consists of two or more cast-iron cylinders mounted
on a shaft so that the axis of each cylinder is inclined an angle of 30
degrees from the axis of rotation. The cylinders' are charged with
11 pounds of the rock broken into fragments, ranging from 1J to 2|
inches. The cylinders are then rotated at a uniform speed of 2,000
revolutions per hour for five hours, or until the automatic recorder
shows 10,000 revolutions; the charge then is removed and placed
on a sieve having meshes of $ inch. The material retained on the
sieve is washed, dried, and weighed. The difference in weight
between the weight of the charge and the residue larger than & inch
shows the loss by abrasion.
Impact and Toughness. The test for impact and toughness is
made in a machine, consisting of an anvil, plunger, and hammer,
mounted in vertical guides. The test piece is placed on the anvil;
the hammer weighing 4.40 pounds is raised and allowed to fall a
90 HIGHWAY CONSTRUCTION
distance of one centimeter for the first blow and an increased fall of
one centimeter for each succeeding blow, until the test piece fails.
The number of blows required to destroy it is used to represent the
toughness; 13 blows is considered to indicate low resistance, 13 to
19 medium, and above 19 high.
Hardness. The test for hardness is made on a Dory machine,
which consists of a steel disk mounted so as to be revolved. The
test pieces are cylinders cut from the rock by a core drill, and the
ends ground level. Two pieces are used for a test; each is weighed,
then placed in the guides of the machine with its face resting upon
the grinding disk. The machine is revolved until 1000 revolutions
have been made, and during the operation, quartz sand is fed onto
the disk. The test piece is removed and weighed, and the hardness
is determined from the formula
W
Hardness = 20
o
in which W is loss in grams per 1000 revolutions. Rocks having a
hardness less than 14 are considered soft; from 14 to 17 medium;
and over 17 hard.
Water Absorption. The capacity of the stone to absorb water
is determined by using a thoroughly dry sample of stone weighing
about 12 grams. The sample is weighed in air, then immersed in
water where it is weighed immediately; after 96 hours' immersion it
is weighed again in the water. The absorptive capacity then is
calculated by the formula
/> _ T>
Lb. water absorbed = - X62.37 per cu. ft. of rock
in which A is the weight in air; B is the weight in water immediately
after immersion; C is the weight in water after immersion for 96
hours; and 62.37 is the normal weight in pounds of a cubic foot of
water.
The durability of a stone used for roads is affected to a certain
extent by its capability of absorbing water. In cold climates a low
absorptive capacity is essential to resist the disintegrating effects
of alternate freezing and thawing.
Specific Gravity. The specific gravity is determined either by
HIGHWAY CONSTRUCTION 91
weighing in a specific gravity balance or by weighing in air and
water, and applying the formula
Specific gravity = j^rr^r
in which W is the weight in air, and W\ is the weight in water.
Specific gravity and porosity are closely related. The specific
gravity varies with the density or compactness of the aggregation of
the mineral grains forming the stone. The closer the grains the more
compact the stone, and the less will be the amount of interstitial
space and hence the less the porosity.
From the specific gravity the weight per ton or per cubic yard
may be determined. A knowledge of the weight is useful in decid-
ing between two otherwise good stones; the heavier will be the more
expensive, due to increased cost of transportation. On a water-
bound macadam road it is an advantage to have a detritus with a
high specific gravity, as it will not be moved so easily by rain and
wind as one of low specific gravity.
Cementing Quality. The cementing quality of the stone dust is
determined by placing 500 grams of the rock, broken to pass a J-inch
mesh screen, in a ball mill, together with 90 cubic centimeters of
water and 2 steel balls weighing 20 pounds. The mill and its charge
are revolved for 2J hours at a rate of 2000 revolutions per hour.
The operation produces a stiff dough, of which 25 grams are placed
in a metal die 25 millimeters in diameter, and subjected to a pressure
of 132 kilograms per square centimeter, producing a cylindrical test
piece. The test piece is dried in the air for 20 hours, after which it
is heated in a hot-air oven for 4 hours at a temperature of 200
Fahrenheit and then cooled in a desiccator for 20 minutes. When
cool it is tested in the impact testing machine in the same manner
as the test for toughness, using a hammer weighing 1 kilogram and
a fixed height of fall of 1 centimeter. Blows are struck until the
test piece fails. The average of the number of blows on 5 test
pieces is taken as the result of the test. A result of 10 is considered
to indicate a low cementing quality; 10 to 25 is considered fair; 26
to 75 good; 76 to 100 very good; over 100 excellent.
Species of Stone. The rocks most extensively used for broken-
stone roads are trap, granite, limestone, sandstone, boulders, or field-
stone.
92 HIGHWAY CONSTRUCTION
Trap rock is hard and tough and has good wearing and binding
qualities. Granite is brittle and its cementing value is low. Lime-
stone is deficient in hardness and toughness but possesses good
binding qualities and for light traffic roads is eminently suitable.
Sandstones are rocks made up of grains of sand cemented together
by siliceous, ferruginous, calcareous, or argillaceous material; they
are usually deficient in binding quality and resistance to abrasion.
With a bituminous binder good results are obtained. Fieldstones
are a mixture of the hardest parts of the granites, sandstones, lime-
stones, etc., distributed by glacial action and which have resisted
the disintegrating effects of the weather. Owing to their variable
character and unequal hardness they wear irregularly and make a
very rough road.
Shape and Size of Stone. The shape and size of the fragments
of stone affect the enduring qualities of the road. The nearer the
fragments approach the cubical form with irregular jagged sides,
the more satisfactory will be the results.
The size of the stone must be such as will not fracture or crush
under the action of the roller during compaction nor become loosened
under traffic. For the harder rocks the size varies from 1J to 2|
inches and for the softer rocks from 2J to 4 inches. The sizes do
not refer to the actual dimensions of the stone, but to the size of the
hole in the screen; thus IJ-inch stone is that which is retained by a
1^-inch opening and passes through a 2- or 2J-inch opening.
Thickness of the Broken Stone. The thickness of the broken
stone is determined for the given conditions by the formula pre-
viously stated, and ranges from 4 to 16 inches. Where the thickness
exceeds 6 inches, the excess may be composed of sand, gravel, field-
stone, ledge rock, or broken stone, as previously stated in the dis-
cussion on foundations; the choice depending on availability and
cost. For use in the base all are equally effective.
Spreading the Stone. The method employed for laying the
covering varies with the thickness. When the finished thickness is
4 inches all the stone to be used is laid in one course. When the
finished thickness exceeds 4 inches the stone is laid in two or more
courses; the top or wearing course, being composed of the best and
most expensive stone, is made the least thickness compatible with
good construction and maintenance. To provide for the shrinkage
HIGHWAY CONSTRUCTION
93
of the stone under the roller, the depth of the courses of loose stone
should exceed the finished depth by from 25 to 30 per cent.
The stone is hauled upon the roadbed in vehicles of various
types provided with broad-tired wheels. In some types of vehicle
it is spread in layers as the vehicle is drawn along the roadbed ; with
others it is dumped in heaps and spread by hand with forks and
brought to an even surface by raking, Fig. 62.
Compacting the Broken Stone. The stone is compacted by
rolling with heavy rollers drawn by horses or propelled by steam or
other power, Fig. 63. The steam roller is more effective than horse-
Fig. 62. View Showing Spreading of Lower Course of Macadam Road
Courtesy of United States Department of Agriculture
drawn rollers. The usual weights of steam rollers are 5, 10, and 15
tons; the 10-ton being the one generally used, although the weight
of the roller should be selected in accordance with the bearing power
of the natural soil. A roller having excessive weight may cause
injury to the roadbed, by rolling it into undulations that will permit
water to collect and consequently cause damage. A roadbed which
will stand a heavy roller in dry weather may be injured by it during
wet weather. For a weak roadbed it is well to use two rollers, one of
light weight to form a crust and a narrow, heavy roller to compact it.
94
HIGHWAY CONSTRUCTION
The roller should commence at one edge or border of the road-
way, and move along that edge until within about 25 feet of one
end of the spread stone; it then should cross over to the other edge
and proceed along this edge to the beginning, crossing over and
overlapping the strips previously rolled until the center of the road
is reached. The rolling is continued in this manner until the stones
cease to creep in front or sink under the roller. If, during the first
passages of the roller, low spots appear, they should be filled to
grade with stone of the same size as is in the course being rolled.
Fig. 63. Compacting Broken Stone by Steam Roller
Courtesy of United States Department of Agriculture
After about two passages of the roller, the binder, consisting of
the screenings from the stone being used for the course, is spread in
a thin layer over the surface of the partly compacted stone and
sprinkled with water, which washes it into the voids in the stone;
the rolling then is continued, Fig. 64. The operation of applying
the binder, sprinkling, and rolling is repeated until a wave of water
and screenings rises in front of the roller. Each course is treated
and rolled in the same manner. If the screenings from the rock
that is being used are not suitable for binding, screenings from other
rock, clay, sand, or loam are substituted.
HIGHWAY CONSTRUCTION
95
An excess of binder and water will shorten the time required to
consolidate the stone and produce the appearance of a good piece of
work, but under traffic it will wear unevenly and go to pieces quickly.
Suppression of Dust on Macadam and Telford Roads. Since
the introduction of mechanically , propelled vehicles, broken-stone
roads constructed according to the principles of Telford and McAdam,
have proven inadequate to the demands of the changed traffic.
The adhesion between the particles of stone is insufficient to
react against the propulsive force exerted by the driving wheels,
Fig. 64. Rolling and Sprinkling Second Course of Macadam Road to Complete
Binding Process
Courtesy of United States Department of Agriculture
hence the stones are loosened, and although the rubber tires with
which the motor vehicles are equipped produce little dust by attri-
tion or wearing away, the vehicle moving at high speed creates a
partial vacuum. The current of air which then rushes in to re-estab-
lish the equilibrium picks up the small particles of stone displaced
and loosened by the thrust of the driving wheels and distributes
them in the form of dust, which is very disagreeable to other users
of the road and residents along it. The large stones that are loosened
96 HIGHWAY CONSTRUCTION
are thrown about and ground upon one another and thus increase
the amount of fine material ready to be scattered as dust.
The frequent repetition of these actions causes the pavement
to pit and. disintegrate. The destructive effect is intensified, the
greater the speed, and where the irregularities of the surfaces are
such as to cause the wheels to leave it, there is produced a bounding
motion that is continued for some distance and is particularly dis-
astrous. Shearing of the road fabric is very severe on steep grades
and curves due to the slipping of the driving wheels when the pro-
pulsive force is greater than the adhesion between the tire and the
road surface. The damage arising from this is more extensive
during wet weather and is intensified when the wheels are equipped
with bars, chains, studs, and other anti-skidding devices.
The formation bf dust and mud cannot be prevented absolutely,
because all materials, by attrition and the disintegrating action of
the elements, yield dust when dry and mud when wet. If the sur-
face of a water-bound macadam road could be maintained in a
moist condition, there would be no dust, but moistening with water
even in cities, towns, and villages is expensive, and in rural districts
the cost is prohibitive and the practice would be impossible, owing
to the absence of water available for the purpose. Hence in dealing
with existing road surfaces a remedy has been sought in more fre-
quent cleansing and in the use of some substitute for water which
would be cheap, effective, lasting, and easily applied. To meet this
demand several "dust-laying" compositions have been placed on
the market, and experiments have been made with some of these,
but it has been demonstrated clearly that, with but few exceptions,
they have a very temporary effect, and their application must be
frequent and thorough.
Under the head of exceptions, that is, of the more or less
permanent methods, are included the following: (1) the cementing
of the surface stone by a bituminous cement or binder. When
the binder is applied by the penetration method, the surface is
described by the general term " bituminous-macadam"; and when it is
desired to indicate the kind of binder, the descriptive names, "asphalt-
macadam", "tar-macadam", etc., are used. When the binder is
applied by the mixing method, the construction is called "bitumi-
nous-concrete", or specifically designated by the trade or patented
HIGHWAY CONSTRUCTION 97
name as, "bitulithic", "warrenite", "amiesite", "filbertine", "rock-
asphalt", etc.; (2) binding the stone with hydraulic cement, the
surface so formed being called "concrete-macadam", or "concrete
pavement". These will be discussed later under their respective
headings.
Turning to the details of the various temporary methods, we
find the following :
(1) Fresh Water. This is the simplest remedy, but not always
the most practicable nor the cheapest.
(2) Sea Water. This is a simple remedy but available only on
the seacoast. The salts contained in sea water are highly anti-
septic and deliquescent; a light sprinkling will suppress the dust for
several hours. Its use, however, is objected to for the reason that
it injures the varnish and running gear of vehicles, corrodes cast-
iron street fittings, and when the road surface on which it has been
used has dried the dust then produced, containing salt, injures food
and other goods exposed to it. Moreover, after a few weeks' use
the dust is converted into a pasty mud that adheres to the wheels
and causes the surface of the road to be "picked up".
(3) Deliquescent Salts. The chief advantage of these salts is
that their effect is more lasting than that of water. The salt used
most extensively is calcium chloride obtained as a by-product in the
manufacture of soda by the ammonia process. The salt may be
applied either in solution or in the dry form. It takes up water
rapidly and proves very efficient where the atmospheric moisture is
sufficient to feed the salt. Glutrin, the commercial name for the
waste sulphite liquor obtained in the manufacture of paper from
wood pulp by the sulphite process, reduces the formation of dust,
but the treatment must be repeated frequently. Waste molasses
or "black strap" from sugar refineries mixed with milk of lime
possesses good dust-suppressing qualities.
(4) Coal- Tar Coating. Refined coal tar applied either hot or
cold in the form of a spray minimizes the production of dust, renders
the surface waterproof, and reduces wear. The success attending
its use depends upon the quality of the tar, the state of the weather,
which must be clear and dry, the condition of the road surface,
which must be dry and free from dust and dirt, and, in the case of
hot application, that the tar is not overheated.
98 HIGHWAY CONSTRUCTION
(5) Solutions of Coal Tar and Petroleum. Several patented
preparations of coal tar are on the market. The principle of all is
practically the same, namely, the solution of the tar or oil in water
by a volatile agent, which on evaporation leaves a more or less
insoluble coating on the road surface. The more favorably known
of these preparations are "tarvia" and "westrumite".
(6) Crude Petroleum and Residuum Oil. Crude petroleum
containing a large percentage of asphalt gives the best results.
Petroleum having paraffin and naphtha as a base refuses to bind,
and produces a greasy slime. The residuum oils obtained in the
distillation of petroleum having asphaltum for a base have yielded
good results in many cases.
Two methods are followed in applying the oil: (a) The sur-
face of the road to be oiled is prepared by removing the dust with
hand or power brooms. The oil, in the cold method, is applied by
specially designed sprinkling wagons, at the rate of from one-third
to one-half gallon per square yard. After being applied the oil is
covered with sand or stone screenings and may or may not be
rolled. The oil is applied once or twice a year according to whether
the traffic is light or heavy. The surface of the road must be dry
when the oil is applied.
(b) The oil is sprinkled over the surface and mixed with the
dust. If the oil is merely sprinkled, the mixture of dust and oil
made by the action of the traffic will become very sticky and will be
removed in spots by adhering to the wheels. For the purpose of
facilitating the handling and of securing a deeper penetration
than is possible with cold oil, the oil is heated to a temperature of
about 140 Fahrenheit and applied in the same manner as the
cold oil.
(7) Oil Tar and Creosote. Oil tar is the residual liquid from
the manufacture of carbureted water gas and oil gas. The tar used
for road purposes is obtained by distilling the original tarry liquid to
remove the light oil, naphthalene, and creosote. Various grades of
tar are produced according to the temperature at which the dis-
tillation is stopped. The higher the temperature of distillation, the
harder and more brittle the tar.
The oil tar either alone or mixed with creosote is applied in the
same manner as coal tar.
HIGHWAY CONSTRUCTION 99
Bituminous=Macadam
Features of Bituminous=Macadam. A bituminous-macadam
wearing surface differs from the previously described water-bound
broken-stone surface only in the kind of binder and the quality
of the stone. The bituminous binder is prepared from asphalt,
asphaltic oils, refined water-gas tars, refined coal tars, and com-
binations of refined tars and asphalts.
The bituminous binders adhere to comparatively porous and
relatively soft stone, such as limestone, better than to the hard
stones, such as trap and granite. Consequently, the stone used
with the bituminous binders may be inferior in hardness and binding
quality to that required for water-bound macadam.
Methods of Construction. The essentials necessary to the
successful construction of a bituminous covering are: (1) the exclu-
sion of both subsoil and surface water from the foundation; (2) a
solid unyielding foundation; (3) a stone of suitable quality and
size; (4) that the stone shall be entirely free from dust, otherwise
the dust will interpose a thin film between the stone and the bitu-
minous binder and prevent the latter from adhering to the stone;
(5) if the stone is to be used hot, that it shall not be overheated;
and if is to be used cold, that it shall be dry, for if wet or damp,
the bituminous material will not adhere to it; (6) that the bituminous
cement shall be of suitable quality; free from water, for which the
stone has a greater affinity than for bitumen, and would thus pre-
vent adhesion; free from ammoniacal liquor, which is apt to saponify
some of the oily constitutents and thus render them capable of
combining with water and therefore apt to be washed out; free
from an excess of light oils and naphtha, which act as diluents
and volatilize on the surface of the road, forming a skin that is
not durable; free from an excess of free carbon, because it has no
binding value and is liable to be converted into dust and mud.
Two general methods with various modifications in the minor
details are employed for applying the bituminous binder to form
the wearing surface, viz, the penetration method, and the mixing
method.
Penetration Method. In this method, the stone is spread and
packed slightly by rolling. The bituminous binder is then applied
by one of the following ways: by hand from pouring pots; by a
100
HIGHWAY CONSTRUCTION
nozzle leading from a tank cart; or by a mechanical distributor using
air pressure to discharge the material through nozzles that spread
it in a finely divided stream or spray, Fig. 65. The binder is heated,
usually by steam from the roller, but when hand pots are used, it
is heated in kettles over fires. The quantity applied is about If
gallons per square yard. After the binder is distributed, it is covered
with a light coating of stone
dust, sand, or gravel, and the
rolling is continued. In some
cases, after the rolling is com-
pleted, another application of
the binder is made at the rate
of about one-half gallon per
square yard; this is called a
"paint coat" and is covered
with a light sprinkling of stone
screenings.
Mixing Method. In this
method the stone to be used
for the wearing surface, vary-
ing in size from J to 1J
inches, is cleaned and dried,
then mixed with a sufficient
quantity of the binder to coat
all the stones thoroughly.
The mixing is performed by
manual labor on a mixing
board, Fig. 66, or by raking
the stones through a bath of
liquid binder, or by passing
through a mechanical mixing
machine, Fig. 67. The coated stones are spread upon the foundation
in a layer having a thickness of about 3 inches and are covered
with a light coating of stone screenings free from dust; then are
compacted by rolling, Fig. 68. Wherever the binder flushes to
the surface it is covered with screenings and rolled. When the
rolling is completed, the surplus screenings are swept from the
surface. The cleaned surface then is covered with a coat of the
Fig. 65. Spreading Bituminous Binder by Pressure
Nozzle, Penetration Method
Courtesy of Barrett Manufacturing Company,
New York City
HIGHWAY CONSTRUCTION 101
Fig. 66. Hand Method of Mixing Stone and Binder
Courtesy of Barrett Manufacturing Company, New York City
Fig. 67. Machine Method of Mixing Stone and Binder
Courtesy of Barrett Manufacturing Company, New York City
Fig. 68. Rolling Bituminous Macadam Road Surface
Courtesy of Barrett Manufacturing Company, New 'York City
102
HIGHWAY CONSTRUCTION
binder called a "seal coat", for the purpose of insuring the water-
proofing and complete filling of the voids, Fig. 69. For this coat,
about one-half gallon of binder is used per square yard of surface.
Screenings again are spread and may or may not be rolled.
Advantages and Disadvantages of the Penetration Method. The
advantage of the penetration method is the ease and rapidity with
which it can be carried out, and the low cost for equipment and labor.
The disadvantages of the penetration method are: (1) the
difficulty of obtaining an absolutely uniform distribution of the binder,
thus producing "lean" and "fat" spots that will prove defective under
traffic; (2) it is wasteful, in that it is necessary to use more binder
than actually is required to coat the stones and bind them together;
7T\
Fig. 69. Spraying Seal Coat by Auto Truck, One-Half Gallon to the Yard
Courtesy of Barrett Manufacturing Company, New York City
(3) it is difficult and sometimes impossible to use a binder of suffi-
cient original consistency to produce a satisfactory bond, owing to
the bitumen setting too rapidly when applied to cold stone.
Advantages and Disadvantages of the Mixing Method. The
advantages of the mixing method are : (1) the producing of a uniform
fabric in which the cement is distributed uniformly and cements
each individual stone; (2) that construction can be carried on in
colder weather than is permissible with the penetration method.
If hot stone is used, a bitumen can be employed of such original
consistency as is required to sustain the traffic satisfactorily.
The disadvantage of the mixing method is the greater cost,
HIGHWAY CONSTRUCTION 103
due (1) to the increased labor, and (2) to the more elaborate equip-
ment and apparatus required.
Bitulithic. Bitulithic is composed of stone, ranging in size from
2 inches to ^iU of an inch, and dust, which are dried, heated,
and mixed in predetermined proportions, so as to reduce the voids
to about 10 per cent, and cemented by a hot bituminous cement
manufactured from either coal tar, asphalt, or a combination of
both. The cement is added in sufficient quantity not only to coat
every particle and to fill all of the remaining voids but with enough
surplus to result in a rubbery and slightly flexible condition of the
mixture after compression.
The mixture is spread, while hot, to such depth as will give
a thickness of 2 inches after compressing with a 10-ton roller.
After rolling, a composition coating called a "flush coat" is spread
over the surface; this being covered while sticky with hot stone
chips which are rolled until cool. The purpose of the stone chips
is to form a gritty surface to prevent slipping.
Amiesite. Amiesite is a patented preparation of crushed stone
or gravel, coated w r ith an asphaltic cement. It is laid in two courses
and a surface finish. The first course, -composed of stone ranging
from \ inch to \\ inches, is spread to a depth of 3 inches, blocks
or strips of wood being used to insure uniformity of depth, then
rolled once. The second course is composed of stone \ inch and
less, spread 1 inch deep, then rolled. The surface finish consists
of screenings or sand, used in sufficient quantity to fill the voids.
Rock Asphalt. The rock asphalt most used in the United
States is a sandstone containing from 7 to 10 per cent of asphalt.
It is prepared for use by pulverizing and is used either hot or cold.
It is spread upon the surface of the stone to a depth of about \\
inches and rolled with a steam roller; the rolling is repeated daily for
several days, or until the asphalt becomes hard.
Definitions of Bituminous Materials. The most recently
adopted definitions of the bituminous materials employed in road
construction are:
Native Bitumen. Native bitumen is a mixture of native or
pyrogenous hydrocarbons and their non-metallic derivatives, which
may be gases, liquids, viscous liquids, or solids and which are soluble
in carbon disulphide.
104 HIGHWAY CONSTRUCTION
Artificial Bitumen. Artificial bitumen is produced by the
destructive distillation of pyrobitumens and other substances of
an organic nature; the bitumens so produced are commonly known
as tars, the word tar being compounded with the name of the material
which has been subjected to the process of destructive distillation,
thus designating its origin, as, coal tar, oil tar, etc.
Bituminous. Bituminous refers to that which contains bitumen
or constitutes a source of bitumen.
Emulsions. Emulsions are oily substances made mixable with
water through the action of a saponifying agent or soap.
Fixed Carbon. Fixed carbon is the organic matter of the
residual coke obtained upon burning hydrocarbon products in a
covered vessel in the absence of free oxygen.
Fluxes. Fluxes are fluid oils and tars which are incorporated
with asphalt and semi-solid or solid oil and tar residuums for the
purpose of reducing or softening their consistency.
Residuums, Residual Petroleum, or Residual Oils. These are
heavy viscous residues produced by the evaporation or distillation
of crude petroleums until at least all of the burning oils have been
removed.
Bituminous Cement. The bituminous cements or binders
are prepared from (1) coal-, oil-, and water-gas tars; (2) asphaltic
petroleums; (3) asphalt; and (4) combinations of asphalt and the
residues of distillation from asphaltic petroleums.
Coal- Tar Binder. Coal-tar binder is the residue obtained by
the distillation of the crude tar produced in the manufacture of
illuminating gas and the manufacture of coke for metallurgical
purposes; the required consistency is obtained by removing part
or all of the contained oils. Owing to the difference in the tem-
peratures employed in the two producing processes, the constituents
of the tar, while identical in their characteristics, differ in their
amount; the most marked difference being in the free carbon con-
tent, of which coke-oven tar has the least.
Oil-Gas Tar. Oil-gas tar is produced in the manufacture of
gas from oil. The tarry residue is rather varied in character and
is prepared for use by distillation; it usually contains a large amount
of free carbon.
Water-Gas Tar. Water-gas tar is produced in the manufac-
HIGHWAY CONSTRUCTION 105
ture of carbureted water gas for illuminating purposes and results
from the petroleum product employed for carbureting. It is a
thin oily liquid containing a large percentage of water. It is pre-
pared for road use by mechanical dehydration and distillation.
It has a strong gassy odor when applied to the road, but this dis-
appears in a few days.
Asphaltic Petroleums. Asphaltic petroleums are native petro-
leums which yield asphalts upon reduction. They are used in the
crude state, or after the illuminating and other oil constituents
have been removed by cracking or blowing.
Asphalt. Asphalt is the name by which the native semi-solid
and solid bitumen is known. Asphalt is the most permanent type
of binder and has been used for many years in the construction of
sheet-asphalt pavements. The semi-solid or tarry varieties are
called "malthas" and are the ones generally employed as a binder
for broken-stone roads; the solid variety is used almost exclusively
for street pavements. Rock asphalt or bituminous rock is the name
given to a great variety of sandstone rocks more or less saturated
or cemented by maltha or hard asphalt. In referring to rock asphalts
it is customary to add the name of the locality where they are found,
as "Kentucky rock asphalt", "Italian rock asphalt", etc. The
semi-solid varieties are used in the natural state or after the water
and volatile hydrocarbons have been removed by heating. The
hard varieties are prepared by softening with a suitable flux.
Combinations of Asphalt and Distillation Residues. Com-
binations of asphalt and the residue from the distillation of tars
and petroleums are made by adding either a refined maltha or a
pulverized solid asphalt; the mixing being accomplished by the
injection of compressed air through suitably formed nozzles.
Tests for Bituminous Materials. The bituminous materials
are subjected to certain tests for the. purpose of ascertaining their
chemical and physical properties. The results of the tests are
used in specifications to secure the furnishing of the desired quality
of material and to control the processes of manufacture; also to
form a record by which the behavior of the materials under similar
and dissimilar service conditions can be compared. The complex
character of the materials requires a suitably equipped laboratory
for the application of the tests.
106 HIGHWAY CONSTRUCTION
The tests are determinations of (1) amount of water contained;
(2) materials soluble in water; (3) homogeneity at a temperature
of 77 Fahrenheit; (4) specific gravity; (5) consistency or vis-
cosity measured by a standard penetration machine; (6) ductility,
or the distance the material can be drawn out before breaking;
(7) toughness, or resistance to fracture under blows in an impact
machine ; (8) melting or softening point, measured by a thermometer
while the temperature is raised through a water or oil bath; (9)
distillation the products yielded at different temperatures during
continuous distillation in a suitable flask or retort are caught and
weighed; (10) amount of free carbon a sample is dissolved in carbon
bisulphide, the solution filtered, and the insoluble residue weighed;
(11) amount of fixed carbon a sample is placed in a platinum
crucible and heated until the emission of flame and smoke ceases,
then is allowed to cool and the residue is weighed; and the difference
between the weight of the sample and the residue is the fixed carbon;
(12) paraffine the presence of paraffine is determined by treating
a sample with absolute ether, freezing the mixture, filtering the
precipitate, evaporating and weighing the residue; (13) amount
of bitumen contained found by weighing a sample of the dried
material, by adding carbon bisulphide to dissolve the bitumen,
and by drying and weighing the residue after the extraction is
completed. The loss is the amount of bitumen soluble in
carbon bisulphide. A sample also is treated with naphtha and
the character of the residue is noted as to whether it is sticky
or oily.
Concrete Pavements
Construction Methods. Several methods with many varia-
tions are employed for the construction of concrete pavements.
The principal ones are: (1) grouting method, the construction
being commonly called "concrete macadam"; (2) mixing method;
(3) block or cube method.
Grouting Method. In this method the stone is spread upon
the foundation and lightly rolled, after which a mixture of one
part of cement and three parts of sand in the dry state is spread over
the stone and swept into the interstices by brooms, then sprinkled
with water and rolled; more cement and sand are spread, sprinkled,
and rolled; the operation is repeated until the interstices are filled.
HIGHWAY CONSTRUCTION 107
A variation of this method, known as the Hassam paving, is made "
by spreading the stone, ranging in size from 1| inches to 2J inches,
and rolling it to a thickness of 4 inches, then filling the interstices
with a grout composed of one part cement and three parts of sand
mixed with water in a mixing machine, from which it flows over
the surface, the machines being drawn along the roadway for this
purpose; rolling is proceeded with at the same time and sufficient
grout is applied to fill the interstices. On the foundation so prepared
a wearing surface is formed; the stone is spread in the same manner
as in the first course; the grout, composed of one part cement and
two parts sand, mixed with sufficient water to make it very fluid, is
applied by flowing over the surface of the compacted stone. The
surface is finished by applying a thick grout composed of one part
each cement, sand, and pea-sized trap rock.
Mixing Method. In this method the ingredients are com-
bined into concrete by either hand or machine mixing; the con-
crete is deposited in place in one or two courses, the former being
called "one-course" pavement and the latter "two-course" pave-
ment. In the one-course method, the concrete mixed in the pro-
portions of one part cement, one and one-half parts sand, and three
parts stone is spread upon the prepared natural soil foundation
and tamped to a thickness of about 6 inches.
In the two-course work the concrete mixed in the proportions
of one part cement, two and one-half parts sand, and five parts
stone is spread upon the prepared natural-soil foundation and com-
pacted by rolling or tamping to the required thickness. On its
surface, and before the cement has set, the wearing surface of about
2 inches in thickness is placed and tamped to the required contour.
The mixtures used for the wearing surface vary, being composed
of sand and cement, or of sand, cement, and small broken stone.
The wearing surface of the Blome pavement is composed of one
part cement and one and one-half parts of aggregate, which is
made up of 50 per cent J-inch, 30 per cent J-inch, and 20 per cent
-tV-inch granite screenings. The surface is formed into 4J-inch
by 9-inch blocks by cutting grooves \ inch wide and \ inch deep
by means of special tools.
Materials. The materials used in the construction of concrete
pavements should be selected with care. The stone should be a
108 HIGHWAY CONSTRUCTION
hard tough rock, free from dust and dirt, and graded so as to reduce
voids to the minimum. The sand should be free from loam, clay,
vegetable and organic matter, and should grade from coarse to
fine. The cement should be of a quality to meet the standard
tests. The water should be clean and free from organic matter,
alkalies, and acids. Rapid drying of the concrete should be pre-
vented by covering it with a canvas which is kept moistened with
water for several hours; after its removal the surface should be
covered with sand or earth which is to be kept moist for a period
of two weeks. Improperly mixed or constructed concrete pave-
ment will wear unevenly, crack, and rapidly become very defective.
Expansion Joints. To provide for the expansion and con-
traction of the concrete under changes of temperature, expansion
joints are formed at intervals ranging from 15 to 50 feet. The
edges of the joints are protected from injury by angle irons, and
the space between them, about J inch, is filled with a bituminous
cement which extends the full depth of the concrete. When the
concrete is laid between curbs longitudinal joints from f inch to
IJ inches wide, filled with bituminous cement, are formed along
the curb.
Reinforced-Concrete Pavement. Concrete pavements rein-
forced with steel in the form of woven-wire, Fig. 70, expanded metal,
and round bars are constructed in two courses, the reinforcement
being placed between the foundation course and the wearing surface.
Concrete with Bituminous Surface. In this type the surface
of the concrete pavement, constructed by either the grouting or
mixing method, is covered with a bituminous cement made from
either asphalt, coal tar, or a mixture of both.
Block or Cube Pavement. In' this type of pavement, blocks
or cubes of concrete are molded in a machine or cast in molds.
The blocks are stacked and allowed to season for three months,
during which time they are wet twice a day. They are laid by
hand on a sand cushion spread upon the foundation, then are brought
to a firm bearing and uniform surface by rolling with a light roller.
The surface is covered with a layer of sand or sandy loam which
is broomed and flushed by water into the joints and the rolling is
repeated; after which the surface is covered with a layer of sand,
and the traffic then admitted.
HIGHWAY CONSTRUCTION
109
A variation from the methods described is made in the patented
pavement "rocmac". This is composed of broken stone cemented
by silicate of lime, obtained by mixing powdered carbonate of lime
with a solution of silicate of soda and sugar. The silicate of lime
mortar is spread upon the foundation to a depth of about 2 inches,
over which the broken stone is distributed to such a depth as will
give, when compacted, a depth of about 4 inches. It is rolled and
sprinkled with water until the mortar flushes to the surface, and
Fig. 70. Laying Reinforced Concrete Road. Woven Wire Fabric in Foreground Ready
to Be Placed between Upper and Lower Coat
Courtesy of Municipal Engineering and Contracting Company, Chicago
then is covered with a layer of stone screenings and finally opened
to traffic.
MAINTENANCE AND IMPROVEMENT OF ROADS
Repair and Maintenance of Broken=Stone Roads. These
terms frequently but erroneously are used interchangeably. Repair
means the restoring of a surface so badly worn that it cannot be
maintained in good condition. A well-maintained road should not
require repairs for a considerable length of time. The maintenance
of a road is the keeping of it, as nearly as practicable, in the same
condition as it was when constructed.
Good maintenance comprises: (1) constant daily attention
to repair the ravages of traffic and the elements; (2) cleansing to
110 HIGHWAY CONSTRUCTION
remove the detritus caused by wear, the horse droppings, and
other refuse; and (3) application of water or other dust layer.
When the surface of a water-bound broken-stone road requires
to be renewed, it is loosened and broken up by scarifying, the new
stone spread, rolled, watered, and bound in the same manner as in
new construction; or the old surface is cleansed from dust and
other matter by sweeping and washing, and the new stone spread
upon it, compacted and finished as in new construction.
The resurfacing of water-bound roads with a bituminous con-
struction is becoming common. The methods employed are the
same as heretofore described under "bituminous macadam".
Systems of Maintenance. Several systems for maintaining
roads are in use, the one yielding the best results being that which
provides for the continuous employment of skilled workmen. The
men so employed become familiar with the peculiarities of the
sections in their charge and with the best way to deal with them.
Efficient maintenance requires that the surfaces be kept smooth
so that surface water may flow away rapidly" and that the injury
caused by traffic on uneven surfaces may be avoided; that incipient
ruts, hollows, and depressions be eliminated by cutting out the area
involved in the form of a square or rectangle and filling with new
material; that dust and horse droppings be removed; that loose
stones be removed; that gutters be clear so the rain water may be
removed quickly; that ditches and culverts be cleaned out in advance
of the spring and fall rains; that weeds and grass be removed from
the earth shoulders, and that these and the dust sweepings be not
left on the sides of the road to be redistributed, but be removed
immediately and disposed of in such manner as will not cause injury;
that bridges be examined and repaired at least twice a year.
Improvement of Existing Roads. The improvement of existing
roads may be divided into three branches: (1) rectification of align-
ment; (2) drainage; (3) improvement of the surface.
The first of these consists in the application of the principles
which have been laid down for the location, etc., of new roads and
will include straightening the course by eliminating unnecessary
curves and bends ; improving the grade either by avoiding or cutting
down hills and by embanking valleys; increasing the width where
necessary, and rendering it uniform throughout.
HIGHWAY CONSTRUCTION 111
The second, or drainage, consists in applying the principles laid
down for the drainage of new roads, and in constructing the works
necessary to give them effect.
The third, or improvement of the surface, consists in improving
the surface by any of the methods previously described and that
the funds available will permit.
Value of Improvement. The improvement of roads is chiefly an
economical question relating to the w T aste of effort and to the saving
of expenditure. Good roads reduce the resistance to locomotion,
and this means reduction of the effort required to move a given load.
Any effort costs something, and so the smallest effort costs the least,
and therefore the smoothest road saves the most money for every-
one who traverses it with a vehicle.
Cost of Improvement. Before undertaking any improvement
generally it is required to know the cost of the proposed improve-
ment and the benefits it will produce. In the improvement of roads
the amount of money that may be expended profitably for any
proposed improvement may be calculated with sufficient accuracy
by obtaining first the following data: (1) the quantity and quality
of the traffic using the road; (2) the cost of haulage; (3) plan and
profile of the road; and (4) character and cost of the proposed
improvements. From the data ascertain the total annual traffic and
the total annual cost of hauling it. Next, calculate the annual cost
of hauling the given tonnage over the road when improved. Then
the difference between the two costs will represent the annual inter-
est on the sum that may be expended in making the improvement.
For example, if the annual cost of haulage over the existing road is
$10,000 and the cost for hauling the same tonnage over the improved
road will be $7000, the difference, $3000, with money at 6 per cent
per annum, represents the sum of $50,000 that logically may be
appropriated to carry out the improvement.
Traffic Census. The direction, character, and amount of traffic
using a road is obtained by direct observation during different
seasons of the year. As a preliminary to observing the traffic it is
usual to determine the weight of the vehicles; this is done by weigh-
ing typical vehicles and by establishing an average weight for each
type. The traffic is classified according to the motive power as
horse-drawn vehicles and motor vehicles. Each of these classes is
112
HIGHWAY CONSTRUCTION
TABLE XI
Traffic Census
Average Hours per Day ; for
Taken at
By
.Days
EMPTY VEHICLES
LOADED VEHICLES
Nov. to
March
Aug. to
Oct.
Nov. to
March
Aug. to
Oct.
Pleasure
Horse < / T - , ,
[Light
[ Commercial < Medium
(Heavy
fMotorcycles
Pleasure j Runabouts
[Touring cars
Motor
(Light
Commercial j Medium
( Heavy
divided into pleasure and commercial traffic, the latter class being
subdivided into loaded and non-loaded vehicles. The number of
horses to a vehicle in horse-drawn traffic and the speed of motor
vehicles may be noted. A summary of data is suggested in Table XL
The observations are made from 6 a. m. to 6 p. m., during a period
of seven days each month, with occasional observations, from 6 p. m.
to 6 a. m. or for the entire 24 hours if the amount of traffic requires it.
The weight of the traffic is ascertained by multiplying the
number of each kind of vehicle by the average weight established
for that type.
HIGHWAY CONSTRUCTION
PART II
CITY STREETS AND HIGHWAYS
The first work requiring the skill of the engineer is the laying
out of town sites properly, especially with reference to the future
requirements of a large city, where any such possibility exists.
Few if any of our large cities were so planned. The same principles,
to a limited extent, are applicable to all towns or cities. The topog-
raphy of the site should be studied carefully, and the street lines
adapted to it. These lines should be laid out systematically, with a
view to convenience and comfort, and also with reference to econ-
omy of construction, future sanitary improvements, grades, and
drainage.
Arrangement of City Streets. Generally, the best method of
laying out streets is in straight lines, with frequent and regular inter-
secting streets, especially for the business parts of a city. When
there is some centrally located structure, such as a courthouse, city
hall, market, or other prominent building, it is very desirable to have
several diagonal streets leading thereto. In the residence portions
of cities, especially if on hilly ground, curves may replace straight
lines with advantage, by affording better grades at less cost of grad-
ing, and by improving property through avoiding heavy embank-
ments or cuttings.
Width of Streets. The width of streets should be proportioned
to the character of the traffic that will use them. No rule can be
laid down by which to determine the best width of streets; but it
may be said safely that a street which is likely to become a com-
mercial thoroughfare should have a width of not less than 120 feet
between the building lines the carriage-way 80 feet wide, and the
sidewalks each 20 feet wide.
In streets occupied entirely by residences a carriage-way 32 feet
wide will be ample, but the width between the building lines may be
114
HIGHWAY CONSTRUCTION
as great as desired. The sidewalks may be any amount over 10 feet
which fancy dictates. Whatever width is adopted for them, not
more of it than 8 feet need be paved, the remainder being occupied
with grass and trees.
Street Grades. The grades of city streets depend upon the
topography of the site. The necessity of avoiding deep cuttings or
high embankments which seriously would affect the value of adjoin-
ing property for building purposes, often demands steeper grades
than are permissible on country roads. Many cities have paved
streets on 20 per cent grades. In establishing grades through unim-
proved property, they usually may be laid with reference to securing
the most desirable percentage within a proper limit of cost. But
when improvements already
have been made and have been
located with reference to the
natural surface of the ground,
the matter of giving a desirable
grade without injury to adjoin-
ing property frequently is one
of extreme difficulty. In such
cases it becomes a question of
a
Fig. 71. Diagram Showing Arrangement of
Grades at Street Intersections
how far individual interests shall
be sacrificed to the general good.
There are, however, certain con-
ditions which it is important to
bear in mind: (1) That the
longitudinal crown level should be sustained uniformly from street
to street intersection, whenever practicable. (2) That the grade
should be sufficient to drain the surface. (3) That the crown levels
at all intersections should be extended transversely, to avoid forming
a depression at the junction.
A rrangements of Grades at Street Intersections . The best arrange-
ment for intersections of streets when either or both have much
inclination is a matter which requires careful consideration and
upon which much diversity of opinion exists. No hard or fast rule
can be laid down; each will require special adjustment. The best
and simplest method is to make level the rectangular space aaaaaaaa,
Fig. 71, with a rise of one-half inch in 10 feet from AAAA to B,
HIGHWAY CONSTRUCTION
115
placing gulleys at AAAA and the catch basins at ccc. When this
method is not practicable, adopt such a grade (but one not exceeding
2J per cent) that the rectangle AAAA shall appear to be nearly
level; but to secure this it must have actually a considerable dip in
the direction of the slope of the street. If steep grades are con-
tinued across intersections, they introduce side slopes in the streets
thus crossed, which are troublesome, if not dangerous, to vehicles
turning the corners, especially the upper ones. Such intersections
W* 1
IlklU
M<y<L_ki
^ ^.<?-?
>{p<3./d kj
1 *^ 1
I 1
! 46 75^
I 1
^ ] (jO'
1 9^ ' !
( \50.3 95.96 |>
95.96 _/|
,5^'
1Z
JZ'
^' /"
s
^
1
N
^
t
^ 5ft?'
5\- e
\\ll.90 31. IK
;
50^ Vj
' Q^
1 |
M ~^
tfi !
t- A !
fa. 72
r
Fig. 72. Diagram Showing Arrangement of Intersections for Steep Grades in
Duluth, Minnesota
are especially objectionable in rainy weather. The storm water
will fall to the lowest point, concentrating a large quantity of water
at two receiving basins, which, with a broken grade, could be divided
among four or more basins.
Fig. 72 shows the arrangement of intersections in steep grades
adapted for the streets of Duluth, Minnesota. From this it will be
seen that at these intersections the grades are flattened to 3 per
cent for the width of the roadway of the intersecting streets, and that
the grade of the curbs is flattened to 8 per cent for the width of the
116
HIGHWAY CONSTRUCTION
intersecting sidewalks. Grades of less amount on roadway or side-
walk are continuous. The elevation of block corners is found by
adding together the curb elevations at the faces of the block corners,
and 2J per cent of the sum of the widths of the two sidewalks at the
corner, and dividing the whole by two. This gives an elevation
equal to the average elevation of the curbs at the corners, plus an
average rise of 2J per cent across the width of the sidewalk.
"Accommodation summits" have to be introduced between
street intersections in two general cases: (1) in hilly localities, to
avoid excessive excavation; and (2) when the intersecting streets
are level or nearly so, for the purpose of obtaining the fall necessary
for surface drainage.
The elevation and location of such a summit may be calculated
as follows: Let A, Fig. 73, be the elevation of the highest corner;
Fig. 73. Diagrams for Calculating "Accommodation Summits" between
Street Intersections
B, the elevation of the lowest corner; D, the distance from corner
to corner; and R, the rate of the accommodation grade. The
elevation of the summit is equal to
DXR+A+B
The distance from A or B is found by subtracting the elevation of
either A or B from this quotient, and dividing the result by the rate
of grade. Or the summit may be located mechanically by specially
prepared scales. Prepare two scales divided to correspond to the
rate of grade; that is, if the rate of grade be 1 foot per 100 feet, then
one division of the scale should equal 100 feet on the map scale.
HIGHWAY CONSTRUCTION 117
These divisions may be subdivided into tenths. One scale should
read from right to left, and one from left to right.
To use the scales, place them on the map so that their figures
correspond with the corner elevations; then, as the scales read in
opposite directions, there is of course some point at which the oppo-
site readings will be the same. This point is the location of the
summit, and the figures read off the scale give its elevation. If the
difference in elevation of the corners is such as not to require an
intermediate summit for drainage, it will be apparent as soon as
the scales are placed in position.
When an accommodation summit is employed, it should be
formed by joining the two straight grade lines by a vertical curve,
as described in Part I. The curve should be used both in the crown
of the street and in the curb and footpath.
Where the grade is level between intersections, sufficient fall for
surface drainage may be secured without the aid of accommodation
summits, by arranging the grades as shown in Fig. 74. The curb is
Curb Level
_. ~ ' Bottom of Gutter , .
Fig. 74. Diagram Showing Arrangement of Grades to Avoid
"Accommodation Summits"
set level between the corners; a summit is formed in the gutter; and
receiving basins are placed at each corner.
Transverse Grade. In its transverse grade the street should be
level ; that is, the curbs on opposite sides should be at the same level,
Fig. 75. Street with Unequal Transverse Grade but with Level Street
and the street crown rise equally from each side to the center. But
in hillside streets this condition cannot be fulfilled always, and
opposite sides of the street may differ as much as 5 feet. In such
cases the engineer will have to use his discretion as to whether he
shall adopt a straight slope inclining to the lower side, thus draining
118
HIGHWAY CONSTRUCTION
the whole street by the lower gutter, or adopt the three-curb method
and sod the slope of the higher side.
In the improvement of old streets with the sides at different
levels, much difficulty will be met, especially where shade trees have
1
Fig. 76. Street with Unequal Transverse Grade Inclined so as to Drain by Lower Gutter
to be spared. In such cases, recognized methods have to be aban-
doned, and the engineer will have to adopt methods of overcoming
the difficulties in accordance with the conditions and necessities of
each particular case. Figs. 75, 76, and 77 illustrate several typical
Fig. 77. Street with Unequal Transverse Grade with Three Curbs and Higher Slope Sodded
arrangements in the cases of streets where the opposite sides are at
different levels.
Transverse Contour or Crown. The reason for crowning a
pavement i.e., making the center higher than the sides is to
provide for the rapid drainage of the surface. The most suitable
form for the crown is the parabolic curve, which may be started at
the curb line, or at the edge of the gutter adjoining the carriage-way,
Fig. 78. Method of Obtaining Transverse Contour or Crown of a Road
about one foot from the curb. Fig. 78 shows this form, which is
obtained by dividing the abscissa, or width from the center of the
street to the gutter, into ten equal parts, and by dropping perpen-
diculars at each of these divisions, the lengths of which are deter-
mined by multiplying the rise at the center by the square of the
HIGHWAY CONSTRUCTION
119
TABLE XII
Rise of Pavement Center above Gutter for
Different Paving Materials
PAVING MATERIAL
PROPORTIONS OF RISE AT
CENTER TO WIDTH OF
CARRIAGE-WAY
Wood blocks
Stone blocks
Brick
Asphalt
1 : 100
1 :80
1 :80
1 :80
successive values of the abscissas. The amounts thus obtained can
be added to the rod readings; and the stakes, set at the proper dis-
tance across the street, with their tops at this level, will give the
required curve.
The amount of transverse rise, or the height of the center above
the gutters, varies with the different paving materials; smooth pave-
ments requiring the least, and rough ones and earth the greatest
rise. The rise is generally stated in a proportion of the width of the
carriage-way. The most suitable proportions are shown in Table XII.
Drainage of Streets. Sub-Foundation Drainage. The sub-
foundation drainage of streets cannot be effected by transverse
drains, because of their liability to disturbance by the introduction
of gas, water, and other pipes.
Longitudinal drains must be depended upon entirely; they may
be constructed of the same materials and in the same manner as road
drains. The number of these longitudinal drains must depend upon
the character of the soil. If the soil is moderately retentive, a single
row of tiles or a hollow invert placed under the sewer in the center of
the street generally will be sufficient; or two rows of tiles may be
employed, one placed outside each curb line. If, on the other hand,
the soil is exceedingly wet and the street very wide, four or more
lines may be employed. These drains may be permitted to dis-
charge into the sewers of the transverse streets.
Surface Drainage. The removal of water falling on the street
surface is provided for by collecting it in the gutters, from which it is
discharged into the sewers or other channels by means of catch basins
placed at all street intersections and dips in the street grades.
Gutters. The gutters must be of sufficient depth to retain all the
water which reaches them and prevent its overflowing on the foot-
120 HIGHWAY CONSTRUCTION
path. The depth should never be less than 6 inches, and very
rarely need be more than 10 inches.
Catch Basins. Catch basins are of various forms, usually circu-
lar or rectangular, built of brick masonry coated with a plaster of
Portland cement. Whichever form is adopted, they should fulfill
the following conditions :
(1) The inlet and outlet should have sufficient capacity to
receive and discharge all water reaching the basin.
(2) The basins should have sufficient capacity below the outlet
to retain all sand and road detritus, and prevent its being carried
into the sewer.
(3) They should be trapped so as to prevent the escape of
sewer gas. (This requirement frequently is omitted, to the detri-
ment of the health of the people.)
(4) They should be constructed so that the pit can be cleaned
out easily.
(5) The inlet should be constructed so as not to be choked
easily by leaves or debris.
(6) They must offer the least possible obstruction to traffic.
(7) The pipe connecting the basin to the sewer should be
freed easily of any obstruction.
The bottoms of the basins should be 6 or 8 feet below the street
level; and the water level in them should be from 3 to 4 feet lower
than the street surface, as a protection against freezing.
The capacity and number of basins will depend upon the area
of the surface which they drain.
In streets having level or light longitudinal grades, gullies may
be formed along the line of the gutter at such intervals as may be
found necessary.
Catch basins usually are placed at the curb line. In several
cities, the basin is placed in the center of the street, and connects to
inlets placed at the curb line. This reduces the cost of construction
and cleaning, and removes from the sidewalk the dirty operations of
cleaning the basins.
Catch basins and gully pits require cleaning out at frequent
intervals; otherwise the odor arising from the decomposing matter
contained in them will be very offensive. No rule can be laid down
for the intervals at which the cleaning should be done, but they must
HIGHWAY CONSTRUCTION 121
be cleaned often enough to prevent the matter in them from putre-
fying. There is no uniformity of practice observed by cities in this
matter; in some, the cleaning is done but once a year; in others, after
every rain-storm; in still others, at intervals of three or four months;
while in a few cities the basins are cleaned out once a month.
FOUNDATIONS
The stability, permanence, and maintenance of any pavement
depend upon its foundation. If the foundation is weak, the surface
soon will settle unequally, forming depressions and ruts. With a
good foundation, the condition of the surface will depend upon
the material employed for the pavement and upon the manner of
laying it.
The essentials necessary to the forming of a good foundation are :
(1) The entire removal of all vegetable, perishable, and yielding
matter. It is of no use to lay good material on a bad substratum.
(2) The drainage of the subsoil wherever necessary. A per-
manent foundation can be secured only by keeping the subsoil dry;
for, where water is allowed to pass into and through it, its weak spots
will be discovered quickly, and settlement will take place.
(3) The thorough compacting of the natural soil by rolling with
a roller of proper weight and shape until there is formed a uniform
and unyielding surface.
(4) The placing on the natural soil so compacted of a thickness
of an impervious and incompressible material sufficient to cut off all
communication between the soil and the bottom of the pavement.
The character of the natural soil over which the roadway is to be
built has an important bearing upon the kind of foundation and the
manner of forming it; each class of soil will require its own special
treatment. Whatever its character, it must be brought to a dry and
tolerably hard condition by draining and rolling. Sand and gravels
which do not hold water, present no difficulty in securing a solid and
secure foundation; clays and soils retentive of water are the most
difficult. Clay should be excavated to a depth of at least 8 inches
below the bottom of the finished covering; and the space so excavated
should be filled in with sand, furnace slag, ashes, coal dust, oyster
shells, broken brick, or other materials which are not absorbent of
water excessively. A clay soil or one retaining water may be cheaply
122 HIGHWAY CONSTRUCTION
and effectually improved by laying cross drains with open joints at
intervals of 50 or 100 feet. These drains should be not less than
18 inches below the surface, and the trenches should be filled with
gravel. They should be 4 inches in internal diameter, and should
empty into longitudinal drains.
Sand and planks, gravel and broken stone successively have
been used to form the foundation for pavements; but, although emi-
nently useful materials, their application to this purpose always has
been a failure. Being inherently weak and possessing no cohesion,
the main reliance for both strength and wear must be placed upon
the surface covering. This covering usually (except in case of sheet
asphalt) composed of small units, with joints between them varying
from | inch to 1| inches posesses no elements of cohesion; and
under the blows and vibrations of traffic the independent units or
blocks will settle and be jarred loose. On account of their porous
nature, the subsoil quickly becomes saturated with urine and sur-
face waters, which percolate through the joints; winter frosts upheave
them; and the surface of the street becomes blistered and broken up
in dozens of places.
Concrete. As a foundation for all classes of pavement (broken
stone excepted), hydraulic-cement concrete is superior to any other.
When properly constituted and laid, it becomes a solid, coherent mass,
capable of bearing great weight without crushing. If it fail at all, it
must fail altogether. The concrete foundation is the most costly,
but this is balanced by its permanence and by the saving in the cost
of repairs to the pavement which it supports. It admits of access to
subterranean pipes with less injury to the neighboring pavement
than any other, for the concrete may be broken through at any
point without unsettling the foundation for a considerable distance
around it, as is the case with sand or other incoherent material; and
when the concrete is replaced and set, the covering may be reset at
its proper level, without the uncertain allowance for settlement
which is necessary in other cases.
Thickness of Course. The thickness of the concrete bed must
be proportioned by the engineer; it should be sufficient to provide
against breaking under transverse strain caused by the settlement of
the subsoil. On a well-drained soil, 6 inches will be found sufficient;
but in moist and clayey soils, 12 inches will not be excessive. On
HIGHWAY CONSTRUCTION 123
such soils a layer of sand or gravel, spread and compacted before
placing the concrete, will be found very beneficial.
The proportions of the ingredients required for the manufacture
of concrete are ascertained by measuring the voids in each ingredient.
The strongest concrete will be produced when the volume of cement
is slightly in excess of that required to fill the voids in the sand, and
the volume of the combined cement and sand exceeds by about 10
per cent the volume of the voids in the stone or other material used
for the aggregate. Concrete frequently is mixed in the arbitrary
proportions of 1 part of cement, 3 parts of sand, and 6 parts of stone,
and although the results have been satisfactory, the proportions may
not be the most economical.
The ingredients of the concrete should be thoroughly mixed
with just sufficient water to produce a plastic mass, without any
surplus water running from it. After mixing, the concrete should
be deposited quickly in place, and brought to a uniform surface and
thickness by raking, then tamped until the mortar flushes to the
surface, then left undisturbed until set. The surface of concrete
laid during dry, warm, weather should be protected from the drying
action of the sun while the initial setting is in progress. This may
be accomplished by sprinkling with water as frequently as the rate
of evaporation demands or by covering it with a layer of damp
sand, straw, hay, or canvas. During freezing weather it is customary
to suspend the laying of concrete for the reason that alternate freez-
ing and thawing disintegrate it.
Measuring Voids in the Stone and Sand. The simplest method
for measuring the voids and one sufficiently accurate for the manu-
facture of concrete is the "pouring method' 5 in which a suitable
vessel of known capacity (usually one cubic foot) is filled with the
material, in which it is desired to ascertain the voids. Water then
is poured into the vessel until its surface is flush with the surface of
the material. The water is measured, and its amount is considered
to equal the total of the voids.
STONE=BLOCK PAVEMENTS
Stone blocks commonly are employed for pavements where
traffic is heavy. The material of which the blocks are made should
possess sufficient hardness to resist the abrasive action of traffic, and
124
HIGHWAY CONSTRUCTION
TABLE XIII
Specific Gravity, Weight, Resistance to Crushing, and Absorptive
Power of Stones
MATERIAL
SPECIFIC
GRAVITY
WEIGHT
(Ib. per
cu. ft.)
RESISTANCE
TO CRUSHING
(Ib. per sq. in.)
PERCENTAGE
OF WATER
ABSORBED
Min.
Max.
Min.
Max.
Min.
Max.
Min.
Max.
Granite
2.60
2.80
163
176
12,000
35,000
0.066
0.155
Trap
2.86
3.03
178
189
19,000
24,000
0.000
0.019
Sandstone
2.23
2.75
137
170
5,000
18,000
0.410
5.480
Limestone
1.90
2.75
118
175
7,000
20,000
0.200
5.000
Brick, paving
1.95
2.55
10,000
20,000
sufficient toughness to prevent them from being broken by the
impact of loaded wheels. The hardest stones will not give neces-
sarily the best results in the pavement, since a very hard stone
usually wears smooth and becomes slippery. The edges of the
block chip off, and the upper face becomes rounded, thus making the
pavement very rough.
The stone sometimes is tested to determine its strength, resist-
ance to abrasion, etc. ; but, as the conditions of use are quite different
from those under which it may be tested, such tests are seldom
satisfactory. However, examination of a stone as to its structure,
the closeness of its grain, its homogeneity, porosity, etc., may assist
in forming an idea of its value for use in a pavement. A low degree
of permeability usually indicates that the material will not be
greatly affected by frost. For data see Table XIII.
Materials. Granite. Granite is employed more extensively for
stone-block paving than is any other variety of stone; and because
of this fact, the term "granite paving" is generally used as being
synonymous with stone-block paving. The granite employed should
be of a tough, homogeneous nature. The hard, quartz granites
usually are brittle, and do not wear well under the blows of horses'
feet or the impact of vehicles; granite containing a high percentage
of feldspar will be injuriously affected by atmospheric changes; and
granite in which mica predominates will wear rapidly on account
of its laminated structure. Granite possesses the very important
property of splitting in three planes at right angles to one another,
HIGHWAY CONSTRUCTION 125
so that paving blocks may readily be formed with nearly plane faces
and square corners. This property is called the rift or cleavage.
Sandstones. Sandstones of a close-grained, compact nature
often give very satisfactory results under heavy traffic. They are
less hard than granite, and wear more rapidly, but do not become
smooth and slippery. Sandstones are generally known in the
market by the name of the quarry or place where produced as
"Medina", "Berea", etc.
Trap Rock. Trap rock, while answering well the requirements
as to durability and resistance to wear, is objectionable on account
of its tendency to wear smooth and become slippery; it is also diffi-
cult to break into regular shapes.
Limestone. Limestone usually has not been successfully em-
ployed in the construction of block pavements, on account of its
lack of durability against atmospheric influences. The action of
frost commonly splits the blocks; and traffic shivers them, owing
to the lamination being vertical.
Cobblestone Pavement. Cobblestones bedded in sand possess
the merit of cheapness, and afford an excellent foothold for horses;
but the roughness of such pavements requires the expenditure of a
large amount of tractive energy to move a load over them. Aside
from this, cobblestones are entirely wanting in the essential requisites
of a good pavement. The stones being of irregular size, it is almost
impossible to form a bond or to hold them in place. Under the
action of the traffic and frost, the roadway soon becomes a mass of
loose stones. Moreover, cobblestone pavements are difficult to
keep clean, and very unpleasant to travel over.
Belgian=Block Pavement. Cobblestones were displaced by
pavements formed of small cubical blocks of stone. This type of
pavement was laid first in Brussels, thence imported to Paris, and
from there taken to the United States, where it has been widely
known as the "Belgian-block" pavement. It has been largely used
in New York City, Brooklyn, and neighboring towns, the material
being trap rock obtained from the Palisades on the Hudson River.
The stones, being of regular shape, remain in place better than
cobblestones; but the cubical form (usually 5 inches in each dimen-
sion) is a mistake. The foothold is bad; the stones wear round; and
the number of joints is so great that ruts and hollows are quickly
126 HIGHWAY CONSTRUCTION
formed. This pavement offers less resistance to traction than cobble-
stones, but it is almost equally rough and noisy.
Granite=Block Pavement. The Belgian block gradually has
been displaced by the introduction of rectangular blocks of granite.
Blocks of comparatively large dimensions were employed at first.
They were from 6 to 8 inches in width on the surface, from 10 to 20
inches in length, with a depth of 9 inches. They merely were placed
in rows on the subsoil, perfunctorily rammed, the joints filled with
sand, and the street thrown open to traffic. The unequal settlement
of the blocks, the insufficiency of the foothold, and the difficulty of
cleansing the street, led to the gradual development of the latest
type of stone-block pavement, which consists of narrow, rectangular
blocks of -granite, properly proportioned, laid on an unyielding and
impervious foundation, with the joints between the blocks filled
with an impermeable cement.
Experience has proved beyond doubt that this latter type of
pavement is the most enduring and economical for roadways sub-
jected to heavy and constant traffic. Its advantages are many,
while its defects are few.
Advantages.
(1) Adapted to all grades.
(2) Suits all classes of traffic.
(3) Exceedingly durable.
(4) Foothold, fair.
(5) Requires but little repair.
(6) Yields but little dust or mud.
(7) Facility for cleansing, fair.
Defects.
(1) Under certain conditions of the atmosphere, the surface of
the pavement becomes greasy and slippery.
(2) The incessant din and clatter occasioned by the movement
of traffic is an intolerable nuisance; it is claimed by many physicians
that the noise injuriously affects the nerves and health of persons
who are obliged to live or do business in the vicinity of streets
so paved.
(3) Horses constantly employed upon it soon suffer from the
continual jarring produced in their legs and hoofs, and quickly
wear out.
HIGHWAY CONSTRUCTION 127
(4) The discomfort of persons riding over the pavement is very
great, because of the continual jolting to which they are subjected.
(5) If stones of an unsuitable quality are used for example,
those that polish the surface quickly becomes slippery and exceed-
ingly unsafe for travel.
Blocks. Size and Shape. The proper size of blocks for paving
purposes has been a subject of much discussion, and a great variety
of forms and dimensions are to be found in all cities.
For stability, a certain proportion must exist between the depth,
the length, and the breadth. The depth must be such that when the
wheel of a loaded vehicle passes over one edge of the upper surface
of a block, the block will not tend to tip up. The resultant direction
of the pressure of the load and adjoining blocks always should tend
to depress the whole block vertically ; where this does not happen, the
maintenance of a uniform surface is impossible. To fulfill this re-
quirement, it is not necessary to make the block more than 6 inches
deep.
Width. The maximum width of blocks is controlled by the
size of horses' hoofs. To afford good foothold to horses drawing
heavy loads, it is necessary that the width of each block, measured
along the street, shall be the least possible consistent with stability.
If the width be great, a horse drawing a heavy load, attempting to find
a joint, slips back, and requires an exceptionally wide joint to pull
him up. It is therefore desirable that the width of a block shall not
exceed 3 inches; or that four blocks, taken at random and placed
side by side, shall not measure more than 14 inches.
Length. The length, measured across the street, must be
sufficient to break joints properly, for two or more joints in line lead
to the formation of grooves. For this purpose the length of the
block should be not less than 9 inches nor more than 12 inches.
Form. The blocks should be well squared, and must not taper
in any direction; sides and ends should be free from irregular pro-
jections. Blocks that taper from the surface downwards (wedge-
shaped) should not be permitted in the work; but if any are allowed,
they should be set with the widest side down.
Manner of Laying Blocks. The blocks should be laid in parallel
courses, with their longest side at right angles to the axis of the
street, and the longitudinal joints broken by a lap of at least 2 inches,
128
HIGHWAY CONSTRUCTION
Quite 'r Formed of 3 Rows of
Blocks, Set Longitudinally
Figs. 79 and 80. The reason for this is to prevent the formation
of longitudinal ruts, which would happen if the blocks were laid
lengthwise. Laying blocks obliquely and "herringbone" fashion has
been tried in several
cities, with the idea that
the wear and formation
of ruts would be reduced
by having the vehicle
cross the blocks diago-
nally. The method has
failed to give satisfactory
Cross Section
Fig. 79. Section Showing Method of Laying Ston<
Block Pavement
results; the wear was
irregular and the foothold defective; the difficulty of construction
was increased by reason of labor required to form the triangular
joints; and the method was wasteful of material.
The gutters should
be formed by three or
more courses of block,
laid with their length
parallel to the curb.
At junctions or inter-
sections of streets, the
blocks should be laid
/.I- r 1 T v~s_y -I
-^
pr-S '.'.'.'
~
ii iii
~
i i i _
F"la ga ing
ii iii
'
-
i i i
i i i i i
^
i ' i ' , ' i ' , ' ,
~
3
LiiiJaJrid
.5
Plan
Fig. 80.
Plan of Stone-Block Pavement Showing Method
of Laying Blocks
diagonally from the cen-
ter, as shown in Fig. 81. The reasons for this are: (1) to prevent
the traffic crossing the intersection from following the longitudinal
joints and thus forming depressions and ruts; (2) laid in this manner,
the blocks afford a more secure foothold for horses turning the
corners. The ends of the diagonal blocks where they abut against
the straight blocks, must be cut to the required bevel.
The blocks forming each course must be of the same depth, and
no deviation greater than J inch should be permitted. The blocks
should be assorted as they are delivered, and only those correspond-
ing in depth and width should be used in the same course. The
better method would be to gage the blocks at the quarry. This
would lessen the cost considerably; it would avoid also the incon-
venience to the public due to the stopping of travel because of the
rejection of defective material on the ground. This method undoubt-
HIGHWAY CONSTRUCTION
'129
edly would be preferable to the contractor, who would be saved the
expense of handling unsatisfactory material; and it also would leave
the inspectors free to pay more attention to the manner in which the
work of paving is performed.
The accurate gaging of the blocks is a matter of much impor-
tance. If good work is to be executed, the blocks, when laid, must be
in parallel and even courses; and if the blocks are not gaged accurately
to one uniform size, the result will be a badly paved street, with the
courses running unevenly. The cost of assorting blocks into lots of
uniform width, after delivery on the street, is far in excess of any
Fig. 81. Diagram Showing Method of Laying Stone Blocks at
Intersection of Streets
additional price which would have to be paid for accurate gaging at
the quarry.
Foundation. The foundation of the blocks must be solid and
unyielding. A bed of hydraulic-cement concrete is the most suitable,
and its thickness must be regulated according to the traffic; the
thickness, however, should not be less than 4 inches, and need not be
more than 9 inches. A thickness of 6 inches will sustain traffic of
600 tons per foot of width.
Cushion Coat. Between the surface of the concrete and the base
of the blocks, there must be placed a cushion coat formed of an
incompressible but mobile material, the particles of which readily
will adjust themselves to the irregularities of the bases of the blocks
130 HIGHWAY CONSTRUCTION
and transfer the pressure of the traffic uniformly to the concrete
below. A layer of dry, clean sand 1 inch to 2 inches thick forms an
excellent cushion coat. Its particles must be of such fineness as to
pass through a No. 8 screen; if the sand is coarse and contains peb-
bles, it will not adapt itself to the irregularities of the bases of the
blocks; hence the blocks will be supported at a few points only, and
unequal settlement will take place when the pavement is subjected
to the action of traffic. The sand also must be perfectly free from
'moisture, and artificial heat must be used to dry it if necessary.
This requirement is an absolute necessity. There should be no
moisture below the blocks when laid; nor should water be allowed
to penetrate below the blocks; if such happens, the effect of frost
will be to upheave the pavement and crack the concrete.
Where the best is desired without regard to cost, a layer of
asphaltic cement \ inch thick may be substituted for the sand, with
superior and very satisfactory results.
Laying Blocks. The blocks should be laid stone to stone, so that
the joint may be of the least possible width; wide joints cause
increased wear and noise, and do not increase the foothold. The
courses should be commenced on each side and should be worked
toward the middle; and the last stone should fit tightly.
Ramming. After the blocks have been set, they should be well
rammed down; and the stones which sink below the general level
should be taken up and replaced with a deeper stone or brought to
level by increasing the sand bedding.
The practice of workmen invariably is to use the rammer so as
to secure a fair surface. This does not give the result intended to be
secured, but brings each block to an unyielding bearing. The result
of such a surfacing process is to produce an unsightly and uneven
roadway when the pressure of traffic is brought upon it. The
rammer used should weigh not less than 50 pounds and have a
diameter of not less than 3 inches.
Fillings for Joints. All stone-block pavements depend for their
waterproof qualities upon the character of the joint filling. Joints
filled with sand and gravel of course are pervious. A grout of lime
or cement mortar does not make a permanently waterproof joint; it
becomes disintegrated under the vibration of traffic. An impervious
joint can be made only by employing a filling made from bituminous
HIGHWAY CONSTRUCTION 131
or asphaltic material; this renders the pavement more impervious
to moisture, makes it less noisy, and adds considerably to its
strength.
Bituminous Cement for Joint Filling. The bituminous materials
employed are: (1) coal tar having a specific gravity between 1.23
and 1.33 at 60 degrees Fahrenheit, a melting point between 120 and
130 degrees Fahrenheit, and containing not over 30 per cent of free
carbon. (2) asphalt, either natural or artificial, entirely free from
coal tar or any product of coal-tar distillation, and containing not
less than 98 per cent of pure bitumen soluble in carbon bisulphide.
Of the total amount soluble in carbon bisulphide, 98.5 per cent must
be soluble in carbon tetrachloride. The penetration, when tested
by the Dow method, must be not greater than 110, at 115 degrees
Fahrenheit, and at 77 degrees Fahrenheit must range between 25
and 60. The specific gravity at 60 degrees Fahrenheit must not be
more than 1.00.
The mode of applying the coal-tar filler is as follows : After the
blocks are laid, gravel heated to about 250 degrees Fahrenheit
is spread over the surface and swept into the joints until they are
filled to a depth of about 2 inches. The blocks then are rammed.
The coal-tar filler heated to a temperature between 250 and 300
degrees Fahrenheit is poured into the joints until they are about
half filled, hot gravel is swept in until it reaches to within | inch
of the surface, and hot filler is then poured in until it is flush with
the surface of the blocks; after this sufficient hot gravel is applied
to the joints to conceal the filler.
In applying the coal-tar filler it is essential that both the gravel
and filler are heated sufficiently. Otherwise the filler will be chilled
and will not flow to the bottom of the joint, but will form a thin layer
near the surface, which under the action of frost and the vibration
of traffic, will be cracked and broken up quickly; the gravel will
settle, and the blocks will be jarred loose, causing the surface of
the pavement to become a series of ridges and hollows. The filler
should not be applied during a rainfall or while the blocks are wet
or damp, for such a condition would prevent the filler from adhering
to the blocks. The asphalt filler is heated to a temperature between
400 and 450 degrees Fahrenheit and poured into the joints until
they are entirely filled.
132
HIGHWAY CONSTRUCTION
Hydraulic-Cement Filler is composed of equal parts of Portland
cement and sharp sand mixed with clean fresh water to a suitable
consistency. The joints between the blocks are filled to a depth
of 2 inches with gravel, and the blocks are rammed, after which
the filler is poured into the joints until they are filled flush with the
surface of the blocks. In dry weather the blocks should be mois-
tened by sprinkling with water before applying the filler. After
the filler has taken its initial set, the whole surface of the pavement
is covered with a layer of sand about J inch thick and if the weather
is dry and warm it is sprinkled with water daily for three days.
Traffic is not permitted to use the pavement until at least seven
days after completion.
Stone Pavement on Steep Grades. Stone blocks may be
employed on all practicable grades, but on grades exceeding 10
per cent, cobblestones
afford a better foothold
than blocks. The cob-
blestones should be of uni-
form length, the length
being at least twice the
breadth say stones 6
inches long and 2| inches
to 3 inches in diameter.
These should be set on a concrete foundation, laid stone to stone,
and the interstices filled with cement grout or bituminous cement;
or a bituminous-concrete foundation may be employed and the
interstices between the
stones may be filled with
asphaltic paving cement.
Should stone blocks be
preferred, they must be
laid, when the grade ex-
ceeds 5 per cent, with a
serrated surface, by either
of the methods shown in
Figs. 82 and 83. The method shown in Fig. 82 consists in slightly
tilting the blocks on their bed so as to form a series of ledges or
steps, which will insure a good foothold for horses' hoofs. The
Fig. 82.
Laying Stone Pavement on Steep Grades by
Tilting Blocks
Fig. 83. Laying Stone Pavement on Steep Grades by
Separating Blocks and Filling with Grout
HIGHWAY CONSTRUCTION
133
method shown in Fig. 83 consists in placing between the rows of
stones a course of slate, or strips of creosoted wood, rather less
than 1 inch in thickness and about 1 inch less in depth than the
blocks; or the blocks may be spaced about 1 inch apart, and the
joints filled with a grout composed of gravel and cement. The
pebbles of the gravel should vary in size between f inch and f inch,
BRICK PAVEMENTS
A brick pavement consists of vitrified bricks laid on a suitable
concrete foundation, Fig. 84.
Qualifications of Brick. The qualities essential to a .good
paving brick are the same as for any other paving material, viz,
hardness, toughness, and ability to resist the disintegrating effects
Vitrified Brick-
Fig. 84. Section Showing Method of Laying Vitrified Brick Pavement
of water and frost. These qualities are imparted to the brick by
a process of annealing, through which the clay is brought to the
point of fusion, and the heat then gradually reduced until the kiln
is cold.
Composition. The material from which is made the majority
of the brick used for paving is a shale. Shales are indurated clays
with a laminated structure and the appearance of slate, and occur
in stratified beds. The average composition of the shales that
have proved satisfactory for the manufacture of paving brick is
shown in Table XIV.
An excess of silica causes brittleness; or an excess of alumina
causes shrinking, cracking, and warping. Iron renders the clay
fusible and makes the brick more homogeneous. Lime in the form
of silicate is valuable as a flux, but in the form of carbonate it will
134
HIGHWAY CONSTRUCTION
TABLE XIV
Average Composition of Shales for Paving-
Brick Manufacture
CONSTITUENTS
PROPORTIONAL
PART
(per cent)
(Non-Fluxing)
Silica .
Alumina
Water and loss on ignition
Moisture
56.0
22.0
7.0
2.0
(Fluxing)
Sesquioxide of iron
Lime
Magnesia
Alkalies
7.0
1.0
1.0
4.0
Total
100.0
decrease the strength of the brick; at a high temperature it is changed
into caustic lime, which, while rendering the clay more fusible, will
absorb moisture upon exposure to the weather and thus cause the
brick to disintegrate. Magnesia exerts but little influence on the
character of the brick. The alkalies in small quantities render
the clay fusible.
Color. The color of the clay is of no practical importance;
it is due to the presence of the metallic oxides and organic substances.
Iron produces bricks which are either red, yellow, or blue, according
to the quantity present and the degree of heat; some organic sub-
stances produce a blue, bluish-gray, or black color.
The color of the brick is governed partly by the color of the
clay, by the temperature of burning, by the kind of fuel used, and
by the sand that is used to prevent the brick from sticking to the
dies or to each other in the kiln.
Manufacture. In the manufacture of the brick, the shale is
crushed usually in dry paws and then passed through a 4-mesh or
an 8-mesh screen. The screened material is mixed with water in
a pug mill to the required consistency. The finer the material
is crushed and the more thoroughly it is worked or tempered in
the mill, the more uniform and better the brick is.
The plastic clay, in the "stiff-mud" process, as it leaves the pug
mill is forced by an auger through a die which forms a bar of stiff
HIGHWAY CONSTRUCTION 135
clay of the desired dimensions, and this is cut by an automatic cutter
into bricks of the size required. The bricks then, in some factories,
are repressed in a die, during which the edges of the brick are rounded
and the lugs, grooves, and trade-mark stamped on the sides. When
repressing is not practiced, the bar of clay as it comes from the pug
mill is cut by wires, the brick being called "wire-cut lug" brick.
The bricks, made by either method, are placed in a heated
chamber to dry, this requiring from 18 to 60 hours according to the
clay, temperature, and plant arrangement. When dry the bricks
are stacked in the kiln, which is usually of the down-draft type with
furnaces built in the outer w r alls. The bottom of the kiln is perfo-
rated to allow the gases to pass through to the flues placed below the
floor and connected to the chimney. The heat from the furnace
passes upward into the kiln, then downward through the bricks
into the flues and thence to the chimney. At the beginning of the
burning the heat is applied slowly to drive off the contained water
without cracking the bricks. When the dryness of the smoke
indicates the absence of moisture in the bricks, the fires are gradually
increased until the temperature throughout the kiln is from 1500
to 2000 degrees Fahrenheit, this temperature being maintained from
seven to ten days. The kiln then is closed, the fires are drawn, and
the bricks are allowed to cool. This part of the process is called
annealing, and to produce a tough brick requires from seven to ten
days. The cooled bricks are sorted into different lots; the No. 1
paving bricks are generally found in the upper layers in the kiln.
Sizes. Two sizes of bricks are made: one size measuring 8JX
2| X 4 inches weighing about 7 pounds and requiring 58 to the square
yard. The other, measuring 8 J X 3 J X 4 inches and frequently called
"blocks", weighs about 9J pounds and requires 45 to the square
yard.
Characteristics. The characteristics of brick suitable for paving
are : not to be acted upon by acids shale bricks not to absorb more
than 2 per cent nor less than ^ of 1 per cent of their weight of water,
and clay bricks not to absorb more than 6 per cent of their weight
of water (the absorption by a shale brick of less than | of 1 per cent
of its weight of water, indicates that it has been overburned) ; when
broken with a hammer, to show a dense close-grained structure? free
from lime, air holes, cracks, or marked laminations; not to scale,
136 HIGHWAY CONSTRUCTION
spall, or chip, when quickly struck on the edges; hard but not
brittle.
Tests for Paving Brick. To ascertain if brick possesses the
required qualities they are subjected to three tests: (1) abrasion
by impact (commonly called the "rattler" test); (2) absorption;
(3) cross breaking.
The Rattler Test. The rattler is a steel barrel 28 inches long and
28 inches in diameter, the sides formed of 14 staves fastened to two
cast-iron heads furnished with trunnions which rest in a cast-iron
frame. It is provided with gears and a belt pulley arranged to
revolve at a rate of from 29 J to 30 J revolutions per minute. The
material employed to abrade the brick is spherical balls of cast iron,
the composition of which is: combined carbon, not less than 2.50 per
cent; graphitic carbon, not more than 0.10 per cent; silicon, not more
than 1 per cent; manganese, not more than 0.50 per cent; phos-
phorus, not more than 0.25 per cent; sulphur, not more than 0.08
per cent. Two sizes of balls or shot are used, the larger being 3.75
inches in diameter when new and weighing about 7J pounds, the
smaller being 1.875 inches in diameter and weighing 0.95 pounds.
A charge consists of ten large shot with enough small shot to make
a weight of 300 pounds. The shot is used until the large size is worn
to a weight of 7 pounds and the small shot is worn to a size that will
pass through a circular hole If inches in diameter made in a cast-
iron plate J-inch thick.
The brick to be tested are subjected to a temperature of 100
degrees Fahrenheit for three hours. Ten bricks are weighed and
placed in the rattler with a charge of spherical shot, and the rattler
is revolved for 1800 revolutions. The bricks then are taken out,
pieces less than 1 pound in weight are removed and the balance
weighed. From the weights before and after rattling the percent-
age of loss is calculated. The loss ranges from 16 per cent to 40 per
cent. Brick to be used under heavy traffic should not lose more than
22 per cent, and for light traffic not more than 28 per cent.
Absorption Test. The absorption test is made on five bricks
that have been through the rattler test. They are weighed, and
are immersed in water for 48 hours, then are taken out and weighed,
with the surplus water wiped off. From the weights before and after
immersion the percentage of water absorbed is calculated.
HIGHWAY CONSTRUCTION 137
Cross-Breaking Test. This test is made by placing a brick edge
on supports 6 inches apart. The load is applied at the center of
the brick, and is increased uniformly until fracture occurs. The
average of the result on ten bricks is used in computing the modulus
OTJ77
of rupture, R=r^-', in which IF is the average breaking load in
pounds, L the length between supports in inches, b the breadth,
and d the depth in inches.
Brick=Pavement Qualifications. Advantages. The advantages
of brick pavement may be stated as follows:
(1) Easy traction.
(2) Good foothold for horses.
(3) Not disagreeably noisy.
(4) Yields but little dust and mud.
(5) Adapted to all grades.
(6) Easily repaired.
(7) Easily cleaned.
(8) But slightly absorbent.
(9) Pleasing to the eye.
(10) Expeditiously laid.
(11) Durable under moderate traffic.
Defects. The principal defects of brick pavements arise from
lack of uniformity in the quality of the bricks, and from the liability
of incorporating in the pavement bricks too soft or too porous a
structure, which crumbles under the action of traffic or frost.
Foundation. A brick pavement should have a firm foundation.
As the surface is made up of small, independent blocks, each one
must be supported adequately, or the load coming upon it may force
it downwards and cause unevenness, a condition which conduces
to the rapid destruction of the pavement. Several forms of founda-
tion have been used such as gravel, plank, sand, broken stone,
and concrete. The last mentioned is the best.
Sand Cushion. The sand cushion is a layer of sand placed
on top of the concrete to form a bed for the brick. Practice regard-
ing the depth of this layer of sand varies considerably. In some
cases it is only | inch deep, varying from this up to 3 inches. The
sand cushion is very desirable, as it not only forms a perfectly
true and even surface upon which to place brick, but also makes the
138 HIGHWAY CONSTRUCTION
pavement less hard and rigid than would be the case were the brick
laid directly on the concrete.
The sand is spread evenly, sprinkled with water, smoothed,
and brought to the proper contour by screeds or wooden templets,
properly trussed and mounted on wheels or shoes which bear upon
the upper surface of the curb. Moving the templet forward levels
and forms the sand to a uniform surface and proper shape.
The sand used for the cushion coat should be clean and free from
loam, moderately coarse, and free from pebbles exceeding J inch
in size.
Manner of Laying. The bricks should be laid on edge or on
one flat, as closely and compactly as possible, in straight courses
across the street, with the length of the bricks at right angles to
the axis of the street. Joints should be broken by at least 3 inches.
None but whole bricks should be used, except in starting a course
or making a closure. To provide for the expansion of the pavement,
both longitudinal and transverse expansion joints are used, the
former being made by placing a board templet J-inch thick
against the curb and abutting the brick thereto. The transverse
joints are formed at intervals varying between 25 and 50 feet, by
placing a templet or building lath f-inch thick between two or three
rows of brick. After the bricks are rammed and ready for grouting,
these templets are removed, and the spaces so left are filled with
coal-tar pitch or asphaltic paving cement. The amount of pitch
or cement required will vary between 1 and 1J pounds per square
yard of pavement, depending upon the width of the joints. After
25 or 30 feet of the pavement is laid, every part of it should be rammed
with a rammer weighing not less than 50 pounds and the bricks
which sink below the general level should be removed, sufficient
sand being added to raise the brick to the required level. After
all objectionable brick have been removed, the surface should be
swept clean, then rolled with a steam roller weighing from 3 to 6
tons. The object of rolling is to bring the bricks to an unyielding
bearing with a plane surface; if this is not done, the pavement
will be rough and noisy and will lack durability. The rolling should
be executed first longitudinally, beginning at the crown and working
toward the gutter, taking care that each return trip of the roller
covers exactly the same area as the preceding trip, so that the second
HIGHWAY CONSTRUCTION
139
passage may neutralize any careening of the brick. due to the first
passage.
The manner of laying brick at street intersections is shown in
Fig. 85.
Joint Fillings. The character of the material used in filling
the joints between the brick has considerable influence on the success
Fig. 85. Method of Laying Bricks at Street Intersections
and durability of the pavement. Various materials have been
used such as sand, coal-tar pitch, asphalt, mixtures of coal tar
and asphalt, and Portland cement, besides various patented fillers,
as "Murphy's grout", which is made from ground slag and cement.
Each material has its advocates, and there is much difference of
opinion as to which gives the best results.
The best results seem to be obtained by using a high grade of
Portland cement containing the smallest amount of lime in its
140 HIGHWAY CONSTRUCTION
composition; the presence of the lime increasing the tendency of
the filler to swell through absorption of moisture, causing the pave-
ment to rise or to be lifted away from its foundation, and thus
producing the roaring or rumbling noise so frequently complained of.
The Portland-cement grout, when uniformly mixed and care-
fully placed, resists the impact of traffic and wears well with brick.
When a failure occurs, repairs can be made quickly, and, if made
early, the pavement will be restored to a good condition. If, how-
ever, repairs are neglected, the brick soon loosen and the pave-
ment fails.
Fig. 86. Grout Box Used in Laying Brick Pavement
Courtesy of National Paving Brick Manufacturers Association, Cleveland, Ohio
The office of a filler is to prevent water from reaching the founda-
tion, and to protect the edges of the brick from spalling under
traffic. In order to meet both of these requirements, every joint
must be filled to the top, and must remain so, wearing down with
the brick. Sand does not meet these requirements. Although at
first making a good filler, being inexpensive and reducing the liability
of the pavement to be noisy, it soon washes out, leaving the edges
of the brick unprotected and consequently liable to be chipped.
Coal tar and the mixtures of coal tar and asphalt have an advantage
in rendering a pavement less noisy and in cementing together any
breaks that may occur through upheavals from frost or other causes;
HIGHWAY CONSTRUCTION 141
but, unless made very hard, they have the disadvantage of becoming
soft in hot weather and flowing to the gutters and low places in
the pavement, there forming a black and unsightly scale and leaving
the high parts unprotected. The joints, thus deprived of their filling,
become receptacles for water, mud, and ice in turn; and the edges
of the brick are broken down quickly. Some of these mixtures
become so brittle in winter that they crack and fly out of the joints
under the action of traffic.
The Portland-cement filler is prepared by mixing 2 parts of
cement and 1 part of fine sand with sufficient water to make a thin
grout. The most convenient arrangement for preparing and dis-
tributing the grout is a water-tight wooden box carried on four
wood wheels about 12 inches in diameter, Fig. 86. The box
may be about 4 feet wide, 7 feet long, and 12 inches deep, furnished
with a gate about 8 inches wide, in the rear end. The box should
be mounted on the wheels with an inclination, so that the rear end
is about 4 inches lower than the front end.
Following are the successive operations of placing the filler:
The cement and sand are placed in the box, and sufficient water is
added to make a thin grout. The grouting box is located about
12 feet from the gutter, the end gate opened, and about 2 cubic
feet of the grout allowed to flow out and run over the top of the
brick (care being taken to stir the grout while it is being dis-
charged), Fig. 87. If the brick are very dry, the entire surface of
the pavement should be wet thoroughly with a hose before applying
the grout; if not, absorption of the water from the grout by the
bricks will prevent adhesion between the bricks and the cement
grout. The grout is swept into the joints by ordinary bass brooms.
After a length of about 100 feet of the pavement has been covered
the box is returned to the starting point, and the operation is
repeated with a grout somewhat thicker than the first. If this second
application is not sufficient to fill the joints, the operation is repeated
as often as may be necessary to fill them. If the grout has been
made too thin, or the grade of the street is so great that the grout
will not remain long enough in place to set, dry cement may be
sprinkled over the joints and swept in. After the joints are filled
completely and inspected, allowing three or four hours to intervene,
the completed pavement should be covered with sand to a depth
7\
HIGHWAY CONSTRUCTION
143
of about J inch, and the roadway barricaded, and no traffic allowed
on it for at least ten days.
The object of covering the pavement with sand is to prevent
the grout from drying or settling too rapidly; hence, in dry and
windy weather, it should be sprinkled from time to time. If coarse
sand is employed in the grout, it will separate from the cement
during the operation of filling the joints, with the result that many
joints will be filled with sand and very little cement, while others
will be filled with cement and little or no sand; thus there will be
Fig. 88. Coal-Tar Heating Tank
Courtesy of Barber Asphalt Paving Company,
Philadelphia, Pennsylvania
many spots in the pavement in which no bond is formed between
the bricks, and under the action of traffic these portions quickly
will become defective.
The coal-tar filler is best applied by pouring the material from
buckets, and brooming it into the joints with wire brooms; and in
order to fill the joints effectually, it must be used only when very
hot. To secure this condition, a heating tank on wheels is necessary,
Fig. 88. It should have a capacity of at least 5 barrels, and be kept
at a uniform temperature all day. One man is necessary to feed
the fire and draw the material into the buckets; another, to carry
HIGHWAY CONSTRUCTION 145
the buckets from the heating tank to a third, who pours the material
over the street. The latter starts to pour in the center of the street,
working backward toward the curb, and pouring a strip about 2 feet
in width. A fourth man, with a wire broom, follows immediately
after him, sweeping the surplus material toward the pourer and in
the direction of the curb. This method leaves the entire surface
of the pavement covered with a thin coating of pitch, which
immediately should be covered with a light coating of sand, the
sand becoming imbedded in the pitch. Under the action of traffic,
this thin coating is worn away quickly, leaving the surface of the
bricks clean and smooth, Fig. 89.
Tools Used by Hand in the Construction of Block Pavements.
The principal tools required in constructing block pavements com-
prise hammers and rammers of varying sizes and shapes, depending
on the material and size of the blocks to be laid; also crcnvbars, sand
screens, and rattan and wire brooms. Cobblestones, square blocks,
and brick require different types both of hammer and rammers
for adjusting them to place and for forcing them to their seats.
A cobblestone rammer, for example, is usually made of wood (gener-
ally locust) in the shape of a long truncated cone, banded with
iron at top and bottom, weighing about 40 pounds, and having
two handles, one at the top and another on one side. A Belgian-
block rammer is slightly heavier, consisting of an upper part of
wood set in a steel base; while a rammer for granite blocks is still
heavier, comprising an iron base with cast-steel face, into which
is set a locust plug with hickory handles. For laying brick, a
wooden rammer shod with cast iron or steel and weighing about 27
pounds is used. A light rammer of about 20 pounds weight, consist-
ing of a metallic base attached to a long, slim, wooden handle, is
used for miscellaneous work, such as tamping in trenches, next to
curbs, etc.
Concrete=Mixing Machine. Where large quantities of concrete
are required, as in the foundations of improved pavements, concrete
can be prepared more expeditiously and economically by the use of
mechanical mixers, and the ingredients will be mixed more thoroughly
than by hand. Thorough incorporation of the ingredients is an
essential element in the quality of a concrete. When mixed by
hand, however, the incorporation is rarely complete, because it
146 HIGHWAY CONSTRUCTION
depends upon the proper manipulation of the hoe and shovel. The
manipulation, although extremely simple, is rarely performed by
the ordinary laborer as it should be unless he is watched constantly
by the overseer.
Several varieties of concrete-mixing machines are in the market,
all of which are efficient and of good design. A convenient portable
Fig. 90. Smith Concrete Mixer on Truck with Gasoline Engine,
Power Charger, and Water Tank
Courtesy of T. L. Smith Company, Milwaukee, Wisconsin
type is illustrated in Fig. 90. The capacity of the mixers ranges
from 5 to 20 cubic yards per hour, depending upon size, regularity
with which the materials are supplied, speed, etc.
Gravel Heaters. A special type of oven usually is employed
for heating the gravel used for joint filling in stone-block pavements.
These heaters are made in various sizes, a common size being 9 feet
long, 5 feet wide, and 3 feet 9 inches high.
HIGHWAY CONSTRUCTION 147
WOOD=BLOCK PAVEMENTS
Wood-block pavements, Fig. 91, are formed of rectangular blocks
measuring from 3J to 4 inches wide, 5 to 10 inches long, and 4 inches
deep, impregnated with creosote, or other preservative, laid in a
bed of Portland-cement mortar spread upon a concrete foundation,
with the joints between the blocks filled with either Portland-cement
grout, or a bituminous filler.
The wood used is obtained from the long-leaved yellow pine
(pinus palustrus), lob-lolly pine (pinus taeda), short-leaved pine
(pinus echinata), Cuba pine (pinus heterophylla), black gum (nyssa
sylvatica), red gum (liquidambar styrraciflua), Norway pine (pinus
resinosa), or tamarack (larix laricina).
The wood should be cut from sound trees, free from cracks,
snakes, and knots.
The great enemy of
wood pavement is decay
due to a low form of
plant life called fungi.
The fungi attack the
wood from the outside,
and if the wood is in the
right Condition for the Fig 91> Section Showing Foundation for Wood-Block
spores to grow, they ulti-
mately will penetrate the entire structure of the wood. There
are three classes of fungi: one which attacks all parts of the
wood structure; another which attacks the cellulose; and a third,
which is the most common, and attacks only the lignin the name
of the many organic substances that are incrusted around the cellu-
lose, and which with the latter constitute the essential part of woody
tissue here the fungi dissolve the lignin and the cellulose to make
food for their development. Heat, air, and moisture are necessary
to the existence of the fungi; without any one of these elements
they cannot live. To destroy the fungus life and preserve wood
from decay many processes have been devised; the one that seems
to meet the requirements better than any other is the process of
creosoting.
Creosoting. This process consists in impregnating the wood
with the dead oil of tar, called "creosote", from which the ammonia
148 HIGHWAY CONSTRUCTION
has been removed. Its effect on the wood is to coagulate the albu-
men and thereby prevent its decomposition, also to fill the pores of
the wood with a bituminous substance which excludes both air and
moisture, and which is obnoxious to the lower forms of animal and
vegetable life.
The coal-tar creosote oil is used without admixture or adultera-
tion with other oils or tars. Its characteristics are : specific gravity,
1.03 to 1.08, at a temperature of 100 degrees Fahrenheit; contain
not more than 5 per cent of tarry matter, nor more than 2 per cent
of water, and not more than 8 per cent of tar acids, 99 per cent to
be soluble in hot benzol ; when subjected to distillation at gradually
increasing temperatures up to 400 degrees Fahrenheit not more than
5 per cent of distillate shall pass over, at 450 degrees not more than
35 per cent, and up to 600 degrees Fahrenheit not more than 80 per
cent; after complete distillation not more than 2 per cent of coke
shall remain; upon sulphonating a sample of the total distillate, the
residue shall not exceed 1 per cent.
For applying the creosote to the wood, several methods are
followed. The one in most favor for paving blocks is the "pressure
process", which essentially consists in: (1) steaming the w T ood for
the purpose of liquefying the sap and other substances contained in
the interfibrous spaces; (2) creating a vacuum for the purpose of
removing the liquefied substances; (3) injecting the creosote under
pressure.
The operation is performed in metal cylinders called "retorts",
6 or more feet in diameter and of any desired length, usually about
100 feet. The load of blocks, called a "charge", is placed upon
metal cars called "buggies" and is run into the retort cylinder, the
ends of which then are hermetically closed w r ith "heads" or doors.
Steam, at a gage pressure varying from 15 pounds to 45 pounds per
square inch, is admitted to the retort (in some plants a vacuum is
first created) and the pressure maintained for several hours. When
the operator considers that the steaming has been continued a suffi-
cient length of time, the products of condensation are removed from
the retort through a blow-off cock in the bottom; when this is accom-
plished an air exhaust, or vacuum pump is put in operation, and a
vacuum of from 20 inches to 26 inches is created and maintained
for about one hour, at the end of which time the creosote is allowed
HIGHWAY CONSTRUCTION 149
to flow into the retort until it is filled. A pressure pump then is
started to force the creosote into the retort until the pressure reaches
100 pounds to 150 pounds per square inch. This pressure is main-
tained until the required amount of creosote has been injected in
the wood, then the surplus is drawn off, the heads opened, and the
charge withdrawn.
The amount of creosote injected into the wood varies from 10
pounds to 22 pounds per cubic foot of wood. The amount is deter-
mined primarily by measuring the tanks and is verified by testing
sample blocks. A sample block is bored entirely through in the
direction of the fiber with an auger 1 inch in diameter, the hole
being located midway between the sides and about J the length of
the block from one end. The borings are collected, thoroughly
mixed, and the quantity and ratio of creosote to wood in the borings
determined by extracting the creosote completely with carbon
bisulphide.
The condition of the wood at the time of the treatment, is prefer-
ably dry and free from an excess of water. After treatment, and
until used, the blocks during dry weather should be sprinkled fre-
quently with water to prevent drying and cracking. The treated
blocks are sometimes subjected to tests to determine the resistance
to wear when saturated with water, the resistance to compression
and impact, and to ascertain the amount of water the wood will
absorb.
Laying the Blocks. The surface of the concrete foundation is
cleansed from dust and dirt by sweeping, then sprinkled with water.
Upon the cleaned surface a cushion coat is formed, by spreading a
layer of sand 1 inch thick, Fig. 92, or a mortar composed of 1 part
Portland cement and 2 parts sand, mixed with sufficient water to
form a stiff paste (the practice of using a mixture of cement and
sand slightly moistened with water produces a defective pavement).
The blocks are set upon the cushion coat with the fiber vertical,
Fig. 93, in straight, parallel courses at right angles to the axis of the
street, except at intersections where they are set at an angle of 45
degrees with the axis of the street. They are laid so as to have the
least possible width of joint (wide joints hasten the destruction of
the wood by permitting the fibres to broom and wear under traffic) .
Blocks in adjoining courses break joint by at least 3 inches. At the
150
HIGHWAY CONSTRUCTION
Fig. 92. Spreading Sand Foundation for Wood Blocks in LaSalle Street, Chicago
Courtesy of Engineering News, New York City
Fig. 93. Laying Wood Blocks in LaSalle Street, Chicago
Courtesy of Engineering News, New York City
HIGHWAY CONSTRUCTION
151
Fig. 94. Wood-Block Pavement Being Hammered and Rolled, Preparatory to Putting in Filler
Courtesy of Engineering News, New York City
Fig. 95. Spreading Sand Filler on Wood-Block Pavement
Courtesy of Engineering News, New York City
152 HIGHWAY CONSTRUCTION
curb it is customary to place one or two rows of blocks with the
length parallel to the curb and f inch therefrom.
After the blocks are laid they are brought to a uniform surface
by ramming with hand rammers or rolling with a light steam roller,
Fig. 94. When laid upon a mortar cushion, the rolling or ramming
must be completed before the mortar sets.
In some cases the cushion coat is omitted, the surface of the
concrete freed from dust by dry sweeping is covered with a thin
coat of a bituminous cement and the blocks laid directly upon it.
Sometimes, the side and one end of each block, when it is about to
be set in place, are dipped in the same bituminous material that is
used to cover the concrete, the blocks are placed in contact and the
surface is covered with a thin coating of the bituminous material,
this being covered with a layer of sand or fine gravel.
After the blocks have been brought to a uniform surface, the
joints are filled with either fine sand, cement grout, or a bituminous
cement, Fig. 95. When sand is used, it should be fine and dry,
spread over the surface of the pavement, and swept about until the
joints are filled. Cement grout is made of equal parts of Portland
cement and fine sand mixed with water to the required consistency.
It is spread over the surface of the blocks and swept into the joints
until they are filled. The surface of the pavement then is covered
with sand, and the grout is allowed to set for about seven days
before traffic is admitted. The bituminous filler is composed of
coal-tar pitch, asphalt, or combinations of these, and other ingre-
dients. The filler is applied hot in the same manner as described
under brick pavement. To provide for the expansion of the blocks
the joint next the curb is filled with bituminous filler.
Qualifications of Wood Pavements. Advantages. The advan-
tages of wood pavement may be stated as follows:
(1) It affords good foothold for horses.
(2) It offers less resistance to traction than stone, and slightly
more than asphalt.
(3) It suits all classes of traffic.
(4) It may be used on grades up to 5 per cent.
(5) It is moderately durable.
(6) It yields no mud when laid upon an impervious foundation.
(7) It yields but little dust.
HIGHWAY CONSTRUCTION 153
(8) It is moderate in first cost.
(9) It is not disagreeably noisy.
Defects. The principal objections to wood pavement are :
(1) It is difficult to cleanse.
(2) Under certain conditions of the atmosphere it becomes
greasy and very unsafe for horses. This may be remedied by cover-
ing the surface with a thin layer of fine sand or gravel; a similar
treatment will absorb the oil which exudes during warm weather.
(3) It is not easy to open for the purpose of gaining access to
underground pipes, it being necessary to remove rather a large
surface for this purpose, which has to be left a little time after being
repaired before traffic again is allowed upon it.
ASPHALT PAVEMENTS
Sheet=Asphalt Pavement. Sheet asphalt is the name used to
describe a pavement having a wearing surface composed of sand
graded in predetermined proportions, of a fine material or filler,
and of asphalt cement, all incorporated by mixing in a mechanical
mixer, and laid upon a concrete foundation, the surface of the latter
being covered with a thin layer of bituminous concrete called a
"binder".
Asphalt Cement. This is prepared from solid bitumen, refined
and fluxed with (1) the residuum from paraffine petroleum; (2) the
residuum from asphaltic petroleum; (3) a mixture of paraffine and
asphaltic petroleum residuums; (4) natural malthas, or is prepared
from (5) solid residual bitumen produced in the distillation of
asphaltic petroleums, and fluxed with residuum oil produced from
the same material.
Refined asphalt is that freed from the combined water and
accompanying inorganic and organic matter. By comparatively
simple operations the several varieties of asphalt may be separated
from their impurities. Two methods are employed for refining; one
using steam and the other direct fire. In both methods the asphalt
is placed in tanks and slowly heated until thoroughly melted, and
during the melting the mass is agitated by a current of either air or dry
steam. The method of using steam is superior to the fire method.
In the latter method there always is danger of overheating, in addi-
tion to the formation of coke and the cracking of the hydrocarbons.
154 HIGHWAY CONSTRUCTION
The varieties of asphalt known as gilsonite and grahamite, which
are practically pure bitumen, do not require refining, but they are
used to a very small extent in paving.
The greater part of the solid bitumen used for paving in the
United States is obtained from the \Yest Indies and South America.
The more extensively used being that found at Trinidad, W. I., and
at Bermudez, Venezuela. The asphalts known by the trade names
' 'California'' and "texaco" are produced by refining asphaltic oils,
and may or may not require to be fluxed.
Fluxes are fluid oils and tars which are mixed with asphalt to
produce a desired consistency. The refined asphalt is melted and
the flux previously heated added to it, in the proportion required to
produce the desired consistency. The mixture of asphalt and flux
is agitated either by mechanical means or by a blast of air until the
materials are thoroughly incorporated and the desired consistency
is obtained.
Sand. The sand should be siliceous and so free from organic
matter, mica, soft grains, and other impurities, that these will not
amount to more than 2 per cent of its volume.
Fine Material or Filler. This consists of any sound stone,
usually limestone or sand, pulverized to such fineness that the whole
will pass the No. 50 sieve, and not more than 10 per cent will be
retained on the No. 100 sieve, and at least 70 per cent will pass the
No. 200 sieve. Portland cement sometimes is used instead of the
pulverized stone.
The paving composition is prepared by heating the ingredients
separately to a temperature between 300 and 350 degrees Fahren-
heit, then incorporating them by mixing in a mechanical mixer.
The hot sand is measured into the mixer, followed by the hot filler;
these two materials are thoroughly mixed together, and the hot
cement is added in such a way as to distribute it evenly over the
mixed sand and filler; the mixing then is continued until the materials
form a uniform and homogeneous mass, with the grains of sand
completely covered with cement. A typical mixture is: sand 100
pounds; filler 17.5 pounds; bitumen in asphalt cement 17.5 pounds.
The proportions of the ingredients in the paving mixture are
not constant, but vary with the climate of the place where the
pavement is to be used, the character of the sand, and the amount
HIGHWAY CONSTRUCTION 155
and character of the traffic that will use the pavement. The ranges
are indicated in the following data:
Data for Asphaltic Paving
Asphaltic Paving Mixture.
CONSTITUENTS
Asphalt cement
Sand
Stone dust
PER CENT
12 to 15
70 to 83
5 to 15
Weight of Material. A cubic yard of the prepared material weighs about
4500 pounds. One ton of refined asphaltum makes about 2300 pounds of
asphalt cement, equal to about 3.4 cubic yards of surface material.
Wearing Surface per Cubic Yard of Material.
THICKNESS AREA
(inches) (sq. yd.)
21 12
2 IS
11 27
Laying the Pavement. The hot paving mixture is hauled to
the street and dumped at a place outside of the space m which it
spholttin.
inder l in.
Concrete 5 in.
Fig. 9f>. Section of Aephalt Pavement Showing Layers
is to be laid. It then is thrown into place with hot shovels, and
spread with hot rakes uniformly to such a depth as will give the
required thickness when compacted. The finished thickness
varies between 1| inches and 2 inches. The reduction of thickness
by compression is about 40 per cent generally. Before the mixture
is spread, the surfaces of curbs and street fittings that will be in
contact with it are painted with hot asphalt cement.
The pavement is constructed in two forms: (1) The paving
mixture is laid directly upon the surface of the concrete foundation;
156 HIGHWAY CONSTRUCTION
(2) the surface of the concrete foundation is covered with a coat
of asphaltic concrete, Fig. 96, called the "binder course", the object
of which is to unite more securely the wearing surface to the foun-
dation. This it does by containing a larger percentage of cement,
which, if put in the surface mixture, would render it too soft. The
binder is composed of sound, hard stone broken to pass a IJ-inch
screen, sand, pulverized stone, and asphalt cement, mixed in the
desired proportions. A typical mixture is: stone 100 pounds;
sand 40 pounds; stone dust 8 pounds; bitumen in asphalt cement
8 pounds.
The paving composition is compressed by means of rollers
and tamping irons, the latter being heated in a fire contained in an
iron basket mounted on wheels. These irons are used for tamping
such portions as are inaccessible to the roller, namely, gutters,
around manhole heads, etc.
Two rollers are sometimes employed; one, weighing 5 to 6
tons and of narrow tread, is used to give the first compression;
and the other, weighing about 10 tons and of broad tread, is used
for finishing. The rate of rolling varies; the average is about 1
hour for 1000 square yards of surface. After the primary com-
pression, natural hydraulic cement, or any impalpable mineral
matter, is sprinkled over the surface, to prevent the adhesion of
the material to the roller and to give the surface a more pleasing
appearance. When the asphalt is laid up to the curb, the surface
of the portion forming the gutter is painted with a coat of hot
cement.
Although asphaltum is a poor conductor of heat, and the cement
retains its plasticity for several hours, occasions may and do arise
through which the composition before it is spread has cooled; its
condition when this happens is analogous to hydraulic cement which
has taken a "set", and the same rules which apply to hydraulic
cement in this condition should be respected in regard to asphaltic
cement.
If the temperature of the air at the time of hauling is below
70 degrees Fahrenheit the wagons carrying it are covered with
canvas or other material to prevent the loss of heat. The tem-
perature when delivered at the place where it is to be used must
not be less than 280 degrees Fahrenheit.
HIGHWAY CONSTRUCTION 157
Two methods are followed in laying an asphalt pavement
adjoining street railway tracks: (1) a course of granite-block or
brick paving is laid between the rail and the edge of the asphalt;
(2) the asphalt is laid directly against the rail, which, if its tem-
perature is below 50 degrees Fahrenheit, is heated by suitable
apparatus to a temperature of about 60 degrees Fahrenheit
immediately before the asphalt is laid.
Foundation. A solid, unyielding foundation is indispensable
with all asphaltic pavements, because asphalt of itself has no
power of offering resistance to the action of traffic, conse-
quently it nearly always is placed upon a bed of hydraulic-
cement concrete. The concrete must be set thoroughly and
its surface dry before the asphalt is laid upon it; if not, the
water will be sucked up and converted into steam, with the
result that coherence of the asphaltic mixture is prevented, and,
although its surface may be smooth, the mass is really honey-
combed, so that as soon as the pavement is subjected to the action
of traffic, the voids or fissures formed by the steam appear on
the surface, and the whole pavement is broken up quickly.
Qualifications of Asphalt Pavements. Advantages. These
may be summed up as follows :
(1) It gives easy traction.
(2) It is comparatively noiseless under traffic.
(3) It is impervious.
(4) It is easily cleansed.
(5) It produces neither mud nor dust.
(6) It is pleasing to the -eye.
(7) It suits all classes of traffic.
(8) There is neither vibration nor concussion in traveling
over it.
(9) It is laid expeditiously, thereby causing little inconvenience
to traffic.
(10) Openings to gain access to underground pipes are easily
made.
(11) It is durable.
(12) It is repaired easily.
Defects. These are as follows:
(1) It is slippery under certain conditions of the atmosphere.
158
HIGHWAY CONSTRUCTION
The American asphalts are much less so than the European, on
account of their granular texture derived from the sand. The
difference is very noticeable; the European are as smooth as glass,
Granite Curb\
Vitrified BricK
"a
3
:". Sand , .;^.
fi
Asphalt
Binder
J
r
^S^A:^^^', ||jj
Sand and Cement
I.;.'. \i..... -v;
i
r v- < . -/ V- ," r<rt>-
< \?' ^.'^ : <} 77- : :
^.Vl''. 7 -
-I
.1
Fig. 97. Section of Asphalt Pavement Showing Use of Vitrified Brick to Form Gutter
while the American resemble fine sandpaper. The slipperiness
can be decreased by heating the surface of the pavement with a
surface heater, then applying a layer of coarse sand and rolling
it into the surface.
(2) It will not stand constant moisture, and will disintegrate
if sprinkled excessively.
(3) Under extreme heat it is liable to become so soft that it
will roll or creep under traffic and present a wavy surface; and
under extreme cold there is danger that the surface will crack and
become friable.
(4) It is not adapted to grades steeper than 2J per cent,
although it is in use on grades up to 7.30 per cent.
Asphalt
Brick Gutter
Curb
Fig. 98. Plan of Asphalt Pavement Showing Use of Vitrified
Brick to Form Gutter
(5) Repairs must be made quickly, for the material has little
coherence, and if, from irregular settlement of foundation or local
violence, a break occurs, the passing wheels- rapidly shear off the
sides of the hole, and it soon assumes formidable dimensions.
HIGHWAY CONSTRUCTION 159
Although pure asphaltum is impervious absolutely and insoluble
in either fresh or salt water, yet asphalt pavements in the continued
presence of water are quickly disintegrated. Ordinary rain or
daily sprinkling does not injure them when they are allowed to
become perfectly dry again. The damage is most apparent in
gutters and adjacent to overflowing drinking fountains. This
defect has long been recognized, and various measures have been
taken to overcome it, or at least reduce it to a minimum. In some
cities, ordinances have been passed, seeking to regulate the sprinkling
of the streets; and in many places the gutters are laid with stone
or vitrified brick, Figs. 97 and 98, while in others the asphalt is
laid to the curb, a space of 12 to 15 inches along the curb being
covered with a thin coating of asphalt cement.
Failure of Asphalt Pavement. The failure of asphalt pavement
is due to any one, or a combination, of the following causes:
(1) Unsuitable Materials. The asphalt may be changed so
by natural causes as to possess little or no cementing power. The
fluxing agent may form only a mechanical instead of a chemical
union w r ith the asphalt, or its character may be such as to render
the asphalt brittle, in which condition it easily is broken up under
traffic. The sand may be graded improperly, either too coarse or
too fine, or contain loam, vegetable matter, or clay.
(2) Improper Manipulation. The crude asphalt may have
been refined at too high a temperature, which reduces or destroys
the cementing property. The cement may be of improper con-
sistence, of insufficient quantity, or inadequately mixed. If the
cement is too hard, the pavement will have a tendency to crack
during cold weather; and if too soft, it will push out of place and
form w r aves under traffic. The quantity of cement needed varies
with the character of the sand a fine sand requires more cement
than a coarse one, and the proportion of cement must be varied to
suit the sand. When the ingredients are mixed inadequately, the
cement and the particles of sand are not brought into intimate
contact. Free oil or an excess of asphalt in the binder, making
it too rich, is liable to work up and be absorbed by the wearing
surface, and thus cause it to disintegrate. The mixture may be
chilled while being transported from the plant to the street. There
may be separation of the cement and sand, for if the distance from
160 HIGHWAY CONSTRUCTION
the plant to the street is great and there is any delay, some of the
cement may work to the bottom of the load, and when it is dumped,
there will be fat and lean spots, both of which are injurious. The
paving mixture may be laid upon a damp or dirty foundation.
There may be inadequate compression, for the importance of
thorough compaction is not appreciated always and this portion
of the work is slighted frequently.
(3) Natural Causes. All materials in nature continually
are undergoing change due to the action of the elements, and to
this asphalt is not an exception. Subjected to the action of heat,
all bitumens grow harder, and when the maximum degree of
hardness is attained, natural decay sets in so that under the com-
bined action of the elements, the material gradually rots and
disintegrates.
(4) Defective Foundation. By unequal settlement a weak
foundation will cause cracks and depressions in the surface which
will enlarge speedily under traffic. A porous foundation will per-
mit the ground water to rise, by capillary action, to the underside
of the wearing surface, where by freezing it will cause cracks and
thus provide access for surface water; non- watertight connection
between curbs and street fittings also furnishes a path for surface
water to reach the underside of the wearing surface, where the
presence of water causes rapid decay.
(5) Other Causes. Illuminating gas, escaping from leaking
pipes under the pavement causes disintegration of the asphalt.
Contraction, caused by the decrease in cementing power through
aging of the asphalt, results in cracks. Due to an excess of fluxing
material, there may be rolling and waving of the pavement under
traffic. Injury may be caused by fires made upon the pavement,
or by oil droppings from motor vehicles.
Sheet asphalt pavement usually is constructed under a contract
that provides for its maintenance during a period of years (five
or ten) by the contractor. Such a contract stipulates that the
condition of the pavement at the expiration of the maintenance
or guarantee period shall be as follows: Surface free from depres-
sions exceeding f -inch deep, when measured between any two points
4 feet apart on a line conforming substantially to the original con-
tour of the pavement. Free from cracks. Contain no disintegrated
HIGHWAY CONSTRUCTION 161
material. Thickness not reduced more than f inch. Foundation
free from cracks and settlement.
Rock Asphalt Pavement. This is the name applied to pave-
ment made from the limestones and sandstones found naturally
impregnated or cemented with bitumen. They are prepared for
use by crushing and heating, and are used in their natural con-
dition or mixed with other materials. Deposits are found in many
parts of the United States and Europe. In Europe, rock asphalt
is the material most extensively used for paving, under the name
"asphalte". The European rock asphalts are impregnated very
uniformly with from 7 per cent to 14 per cent of asphalt, and readily
compact into a hard, smooth pavement which in frosty latitudes
becomes very slippery. The American rock asphalts are impreg-
nated irregularly with from 5 per cent to 30 per cent of asphalt.
Their use for paving is limited, chiefly owing to the cost of trans-
portation.
Asphalt Blocks. Formation. Asphalt paving blocks are formed
from a mixture of asphaltic cement and crushed stone in the pro-
portion of 8 or 12 per cent of cement to 88 or 92 per cent of stone.
The materials are heated to a temperature of about 300 Fahrenheit,
and mixed while hot in a suitable vessel. When the mixing is
complete, the material is placed in molds and subjected to heavy
pressure, after which the blocks are cooled suddenly by plunging
into cold water. The usual dimensions of the blocks are 4 inches
wide, 3 inches deep, and 12 inches long.
Foundation. The blocks usually are laid upon a concrete
foundation with a cushion coat of sand about J-inch thick. They
are laid with their lengths at right angles to the axis of the street,
and the longitudinal joints should be broken by a lap of at least
4 inches. The blocks then are rammed with hand hammers, or
are rolled with a light steam roller, the surface being covered
with clean, fine sand; no joint filling is used, as, under the action
of the sun and traffic, the blocks soon become cemented.
The advantages claimed for a pavement of asphalt blocks
over those for a continuous sheet of asphalt are: (1) that they can
be made at a factory located near the materials, whence they can
be transported to the place where they are to be used and can be
laid by ordinary paviors, whereas sheet pavements require special
162
HIGHWAY CONSTRUCTION
machinery and skilled labor; (2) that they are less slippery, owing
to the joints and the rougher surface due to the use of crushed stone.
minr mi
Fig. 99. Rake and Smoothing Irons Used in Laying Asphalt Pavement
Courtesy of Barber Asphalt Paving Company, Philadelphia, Pennsylvania
Another Form. Another form of asphalt block, known as the
"Lueba" block, consists of a block 8J inches long, 4J inches wide,
^ and 4 inches thick, with the lower 3 inches
'^EJ JB composed of Portland-cement concrete cov-
%^H^^^ ered with 1 inch of natural-rock asphalt;
| the two materials being joined under heavy
m^ hydraulic pressure. The blocks are laid on
a concrete foundation and the joints be-
^^| lIpP tween them are filled with hydraulic-cement
grout.
Tools Employed in Construction of
Asphalt Pavements. The tools used in lay-
ing sheet-asphalt pavements comprise hand
rammers iron, rakes, smoothing irons, Fig.
99; pouring pots, Fig. 100; hand rollers,
either with or without a fire pot, Fig. 101 ;
Fig. 100. Pouring Pots Used 3 . .
with Asphalt Pavements and steam rollers, with or without pro vision
Courtesy of Barber Asphalt PI,' .IP n T^ i rvr rni
Paving company for heating the front roll, rig. 102. Ihese
HIGHWAY CONSTRUCTION
1G3
rollers are different in construction, appearance, and weight from
those employed for compacting broken stone. The difference is
due to the different character of the work required.
Fig. 101. Hand Rollers Used in Laying Asphalt Pavements
Courtesy of Barber Asphalt Paving Company, Philadelphia, Pennsylvania
Fig. 102. Small Road Roller Used in Laying Asphalt Pavements
Courtesy of Barber Asphalt Paving Company, Philadelphia, Pennsylvania
The principal dimensions of the 5-ton roller are as follows;
Front roll or steering wheel, diameter 30 to 32 inches
Real roll or driving wheel, diameter 48 inches
Front roll, width 40 inches
Rear roll, width 40 inches
Length, extreme 14 feet
Height, extreme. . 7 to 8 feet
Water capacity 80 to 100 gallons
Coal capacity 200 pounds
164 HIGHWAY CONSTRUCTION
MISCELLANEOUS PAVEMENTS
Under this head will be described briefly the most notable
examples of pavements devised as substitutes for the recognized
standard types, and sometimes used where good materials are not
available, or where insufficient funds prevent their purchase, and
in some cases for the purpose of utilizing waste products.
Burnt Clay. In the Mississippi Valley, during the dry season,
the clay is cut from the roadway to a depth of about 2 feet, and
piled so as to form enclosures about 15 feet in diameter and 2 feet
high. After remaining so for about ten days to dry out, a fire is
made in the inclosure, more dry clay placed on top and the burning
proceeded with. The burnt clay after cooling, is relaid upon the
road, and then, being of a thoroughly porous nature, settles into a
dry, solid layer.
Straw. Clay roads have been improved by shaping and harrow-
ing the road, then applying a layer of wheat straw, which is
moistened with water, and cut and mixed with the clay by a disk
harrow. More straw is added and the operation repeated, then com-
pacted with a steam roller. The treatment is applied twice a year.
Oyster=Shell. The shells are spread on the road previously
shaped and rolled. They crush readily and, possessing a high
cementing quality, bind together to form a compact, smooth road
surface, but owing to their softness, they are quickly ground to
powder which is carried away readily by wind and rain water.
Chert. The siliceous material found overlying the red sand-
stone, which forms the covering of the red hematite iron ore in
some of the Southern States, is used for both street and road paving.
It is laid directly upon the earth surface or upon a prepared foun-
dation, sprinkled, and compacted in the same manner as w^ater-
bound macadam.
Slag. The slag produced in the manufacture of iron and steel
is used in various ways for paving. (1) It may be crushed to
the desired sizes and used in the same manner as broken stone,
laid in one or two courses, sprinkled and rolled. In some cases,
a binder composed of quicklime is used; in others, a waste sulphite
liquor is mixed with the water used for moistening it before rolling;
and in others, it is mixed with coal-tar or other bituminous cement
HIGHWAY CONSTRUCTION 165
and formed into a pavement in the same manner as bituminous
macadam. The pavement called "tarmac", a large amount of
which has been used in England, is composed of slag, coal tar,
rosin, and Portland cement. (2) The slag may be formed into
blocks by casting in molds, which are used in the same manner as
stone blocks. In this form they are called "scoria" or "slag" blocks.
Clinker. Where crematories are employed for the destruction
of garbage about 33 per cent of the material remains after burning,
in the form of clinker. This is broken up and ground to a fine
powder, mixed with either a hydraulic or a bituminous cement, and
pressed into blocks and slabs.
Petrolithic. Petrolithic paving is made by applying a bitu-
minous oil to earth, sand, gravel, clay, or loam roads. The soil is
plowed to a depth of at least 6 inches, pulverized by harrowing,
and sprinkled with water. The bituminous oil is applied in one
or two coats at the rate of 1 gallon per square yard, the oil and soil
are mixed and compacted by a roller weighing 5000 pounds, the
surface of which is studded with spikes having a flat head measuring
2X3 inches, and on which account it is' named a "sheep's-foot"
roller. In operation, the spikes or feet are forced into the loose
soil and compress or pack it from the bottom upwards. After a
thorough mixing and tamping, the surface is shaped with a road
grader and rolled with a roller of the ordinary form.
Kleinpflaster. Kleinpflaster is the name given to a stone pave-
ment used in Germany for exceptionally heavy traffic, and used also
in England, under the name "durax". It is made of 3-inch cubes
of hard stone, cut by machinery, and laid in small segments of
circles. The stones are laid as close as possible and the joints are
filled with hydraulic-cement grout or bituminous filler.
Iron. Several experiments have been made with iron for
paving, but, while eminently durable, it was rough, noisy, and
slippery, and its use either alone or combined with other materials
has been abandoned.
Trackways. Formed of stone slabs, brick, concrete blocks,
steel, and other materials, trackways have been constructed at
various times for the purpose of reducing the resistance to traction.
Their use on an extensive scale, however, has been abandoned
except in Italy, Spain, and Germany.
166 HIGHWAY CONSTRUCTION
National Pavement. National pavement is composed of pul-
verized clay, loam, or ordinary soil, heated and mixed with liquid
bitumen. The mixture is spread to a depth of 2 or 3 inches upon
the surface of the compacted and drained natural soil and is com-
pressed by a power roller.
Fibered Asphalt Pavement. Fibered asphalt pavement is com-
posed of wood fiber, obtained as a waste product from the process
of extracting tannin and asphalt. The fiber is heated and mixed
with a predetermined quantity of asphalt. The hot mixture is run
into molds forming small blocks which are shipped to the place of
use. The blocks are there heated to a temperature of 275 F. in a
traveling heater that moves along the roadway and from which the
hot mixture emerges in a continuous stream 18 inches wide and is
deposited on the previously prepared foundation to a depth of 4
inches. After spreading, it is compressed to a thickness of 2 inches
with a power roller.
Westrumite. Westrumite is an asphalt cement temporarily
liquefied by emulsification. It is mixed cold with broken stone in
an ordinary concrete mixer, spread on the foundation, and com-
pacted by rolling. The evaporation of the vehicle leaves the asphalt
cement as the binder.
MISCELLANEOUS STREET WORK
FOOTPATHS
A footpath or walk is simply a road under another name a
road for pedestrians instead of one for horses and vehicles. The
only difference that exists is in the degree of service required; but
the conditions of construction that render a road well adapted to
its object are very much the same as those required for a walk.
The effects of heavy loads such as traverse carriage-ways are
not felt upon footpaths; but the destructive action of w r ater and
frost is the same in either case, and the treatment to counteract
or resist these elements as far as practicable, and to produce per-
manency, must be the controlling idea in each case, and should
be carried out upon a common principle. It is not less essential
that a walk should be well adapted to its object than that a road
should be; and it is annoying to find it impassable or insecure and
HIGHWAY CONSTRUCTION 167
in want of repair when it is needed for convenience or pleasure.
In point of economy, there is the same advantage in constructing
a footway skilfully and durably as there is in the case of a road.
Width. The width of footwalks (exclusive of the space occu-
pied by projections and shade trees) should be ample to accommodate
comfortably the number of people using them. In streets devoted
entirely to commercial purposes, the clear width should be at least
one-third the width of the carriage-way; in residential and suburban
streets, a very pleasing result can be obtained by making the walk
one-half the width of the roadway, and by devoting the greater
part to grass and shade trees.
Cross Slope. The surface of footpaths must be sloped so that
the surface water will readily flow to the gutters. This slope need
not be very great; J inch per foot will be sufficient. A greater slope
with a thin coating of ice upon it, becomes dangerous to pedestrians.
Foundation. As in the case of roadways, so with footpaths,
the foundation is of primary importance. Whatever material may
be used for the surface, if the foundation is weak and yielding, the
surface w r ill settle irregularly and become extremely objectionable,
if not dangerous, to pedestrians.
Surface. The requirements of a good covering for sidewalks are :
(1) It must be smooth but not slippery.
(2) It must absorb the minimum amount of water, so that it
may dry rapidly after rain.
(3) It must not be abrased easily.
(4) It must be of uniform quality throughout, so that it may
wear evenly.
(5) It must neither scale nor flake.
(6) Its texture must be such that dust will not adhere to it.
(7) It must be durable.
Materials. The materials used for footpaths are as follows:
stone, natural and artificial; wood; asphalt; brick; tar concrete; and
gravel.
Stones. Of the natural stones, sandstone (bluestone) and
granite are employed extensively. The bluestone, when well laid,
forms an excellent paving material. It is of compact texture,
absorbs water to a very limited extent, and hence soon dries after
rain; it has sufficient hardness to resist abrasion, and wears well
168 HIGHWAY CONSTRUCTION
without becoming excessively slippery. Granite, although exceed-
ingly durable, wears very slippery, and its surface has to be rough-
ened frequently.
Slabs, of whatever stone, must be of equal thickness throughout
their entire area; the edges must be dressed true to the square
for the whole thickness (edges must not be left feathered as shown
Fig. 103. Faulty Joint in Stone Sidewalk
in Fig. 103) ; and the slabs must be bedded solidly on the foundation
and the joints filled with cement mortar. Badly set or faultily
dressed flagstones are very unpleasant to walk over, especially
in rainy weather; the unevenness causes pedestrians to stumble,
and rocking stones squirt dirty water over their clothes.
Wood. Wood has been used largely in the form of planks;
it is cheap in first cost, but proves very expensive from the fact that
it lasts but a comparatively short time and requires constant repair
to keep it from becoming dangerous.
Asphalt. Asphalt forms an excellent footway pavement; it
is durable and does not wear slippery.
Brick. Brick of suitable quality, well and carefully laid on a
concrete foundation, makes an excellent footway pavement for
residential and suburban streets of large cities, and also for the main
streets of smaller towns. The bricks should be good qualities of
paving brick (ordinary building brick are unsuitable, as they soon
wear out and are broken easily). The bricks should be laid in
parallel rows on their edges, with their lengths at right angles to
the axis of the path.
Concrete. Concrete or artificial stone is used extensively as
a footway paving material. Its manufacture is the subject of
several patents, and numerous kinds are to be had in the market.
When manufactured of first-class materials and laid in a sub-
stantial manner, with proper provision against the action of
frost, artificial stone forms a durable, agreeable, and inexpensive
pavement.
HIGHWAY CONSTRUCTION 169
Concrete walks are formed in one or two courses. In one-course
work, the concrete is laid to a depth of 4 inches and tamped until
sufficient mortar flushes to the surface to permit the forming of a
smooth surface. In two-course work, the concrete for the base
is spread and tamped to a depth of 3 inches, the top or surface
course is spread upon the base before the latter has begun to set.
The top course has a thickness of about 1 inch, and it is tamped
and its surface is brought to the required plane by a straightedge
and by troweling. Expansion is provided for by transverse joints
extending the full depth of the concrete. The joints are placed
4 feet apart and are formed by placing across the side forms a J-inch
thick metal dividing strip, which is removed before the cement
sets so that the edges of the joint may be smoothed and rounded
with a suitable tool.
The area to be covered by the walk is excavated to a minimum
depth of 8 inches, or to such greater depth as the nature of the ground
may require to secure a solid foundation. The surface of the ground
so exposed is compacted by ramming, and a drainage course is
formed of broken stone, gravel, or steam-plant cinders, thoroughly
compacted by ramming, and its surface is brought to a plane parallel
to and 4 inches below the finished surface of the concrete. In
some situations it may be necessary to connect the drainage course
with the sewers, street drains, or side ditches, for the purpose of
furnishing an outlet for standing water; this is done by the use of
3-inch drain pipe placed where required.
The forms of steel or wood should be made substantially, and
left in place until the concrete is set hard.
Concrete walks fail from the use of improper materials and
defective workmanship, insufficient expansion joints, heaving and
cracking by frost, due to imperfect drainage, displacement and
cracks, due to settlement of the drainage course this latter
being frequent when cinders are used, as in time they are liable
to decompose and shrink in volume and thus allow the con-
crete to settle. In two-course \vork failure may be in respect
to flaking of the surface by the action of water and frost entering
between and separating the courses. The concrete should not be
laid during freezing weather, nor should frozen materials be used
in the work.
170
HIGHWAY CONSTRUCTION
CURBSTONES AND GUTTERS
Curbstones. Curbstones are employed for the outer side of
footways, to sustain the covering and to form ^the gutter. Their
upper edges are set flush with the footwalk pavement, so that the
water can flow over them into the gutters
Fig. 104. Typical Section Showing Stone Curb Eight Inches Thick
The disturbing forces which the curb has to resist are: (1) The
pressure of the earth behind it, which is frequently augmented by
piles of merchandise, building materials, etc. This pressure tends
to overturn it, break it transversely, or move it bodily on its base.
(2) The pressure due to the expansion of freezing earth behind
Fig. 105. Typical Section Showing Stone Curb Five Inches Thick
and beneath the curb. This force is most frequent where the side-
walk is sodded partly and the ground accordingly is moist. Suc-
cessive freezing and thawing of the earth behind the curb will occasion
a succession of thrusts forward, which, if the curb be of faulty
design, will cause it to incline several degrees from the vertical.
HIGHWAY CONSTRUCTION 171
(3) The concussions and abrasions caused by traffic. To withstand
the destructive effect of wheels, curbs are faced with iron; and a
concrete curb with a rounded edge of steel has been patented and
used to some extent. Fires built in the gutters deface and seriously
injure the curb. Posts and trees set too near the curb, tend to
break, displace, and destroy it.
The use of drain tiles under the curb is a subject of much dif-
ference of opinion among engineers. Where the subsoil contains
water naturally, or is likely to receive it from outside the curb lines,
the use of drains is of decided benefit; but great care must be exer-
cised in jointing the draintiles, lest the soil shall be loosened and
removed, causing the curb to drop out of alignment.
The materials employed for curbing are the natural stones
as granite, sandstone (bluestone), etc.; artificial stone fire clay,
and cast iron.
The dimensions of curbstones vary considerably in different
localities and according to the width of the footpaths; the wider the
path, the wider should be the curb. However, it should be never
less than 8 inches deep, nor narrower than 4 inches. Depth is
necessary to prevent the curb's turning over toward the gutter. It
never should be in smaller lengths than 3 feet. The top surface
should be beveled off to conform to the slope of the footpath. The
front face should be hammer-dressed for a depth of about 6 inches,
in order that there may be a smooth surface visible against the
gutter. The back for 3 inches from the top also should be dressed,
so that the flagging or other paving may butt fair against it. The
end joints should be cut a true square the full thickness of the stone
at the top, and so much below the top as will be exposed; the remain-
ing portion of the depth and bottom should be squared roughly,
and the bottom should be fairly parallel to the top. (See Figs. 104
and 105.)
Combination Curb and Gutter. Concrete curb and gutter
combined is constructed by placing the concrete in suitable forms.
The concrete should be handled so as to prevent the separation
of the stone and mortar, and when placed should be tamped well
to bring the mortar to the surface and make complete contact with
the forms. The corner formed by the top and face surfaces is
rounded to a radius of about 1J inches; sometimes this corner is
172
HIGHWAY CONSTRUCTION
formed of a steel bar put in place before the concrete is laid and
anchored by metal strips spaced about 3 feet apart. Expansion
joints are formed at distances of 10 or 12 feet. The remarks made
under concrete walks regarding foundation, drainage, failure, etc.,
apply also to concrete curbs.
STREET CLEANING
The cleaning of streets is practiced for the purpose of protecting
the health of the neighboring residents and for the comfort of the
users. It is of comparatively recent development, and is ren-
dered possible only by the use of hard pavements. The materials
Fig. 106. Typical Machine Street Sweeper
Courtesy of Acme Road Machinery Company, Frankfort, New York
to be removed from the streets consist of animal droppings, material
worn from the pavement, materials dropped from vehicles, waste
from building construction, miscellaneous materials swept from
houses, stores, and factories, and the accumulation of snow during
winter.
Cleaning Methods. The local conditions and character of the
traffic and pavement determine the methods to be employed and
the intervals for cleaning the streets. The methods employed are:
sweeping, either by hand or by machine brooms; and flushing with
water the work being performed either during the day or the
night, by large gangs at night, and by means of a patrol system
during the day. Fig. 106 shows one of the machine sweepers used.
HIGHWAY CONSTRUCTION
TABLE XV
Rate and Cost of Street Cleaning
173
PAVEMENT
APPROXIMATE
SURFACE SWEPT
PER MAN
(sq. yds. per hr.)
APPROXIMATE
DIRT FROM DAILY
SWEEPING
(cu. yds. per
1000 sq. yds.)
AVERAGE COST
PER EACH
CLEANING
(cents per sq.
yd.)
(Wet)
(Dry)
(Mia.)
(Max.)
Asphalt
1000
1200
.007
.040
.0030
Granite-block
750
1000
.015
.024
.0050
Macadam (water-bound)
700
.100
.350
.0106
Wood
.070
.200
.0070
Brick
.0034
In the hand-cleaning method by day patrol, each man is fur-
nished with a push broom, shovel, and can carrier in which to place
the refuse, and has a certain section of street to clean each day.
The day patrol sometimes is supplemented by a large gang working
during the night. When machine brooms are employed they usually
are operated at night and are supplemented by the patrol system
during the day. As to which is the most economical, it will depend
upon the cost of labor and the condition of the pavements; on pave-
ments covered with ruts and depressions machine brooms are
ineffective.
The approximate costs of the various methods of street cleaning
per 1000 sq. yds. are:
Sweeping (hand) $0.281
Sweeping (machine) . 317
Flushing (hand-hose) . 319
Flushing (machine) . 721
The average cost of supervision varies from .011 cent to 34
cents per mile.
The amount of surface cleaned by a machine broom depends
upon the width of the broom, the power of the horses or other
motive power, gradient, and condition of the surface. The wider
the broom the less will be the cost. The average speed of travel
is about 1J miles per hour.
In Table XV are indicated the amount of surface which an
average man will sweep per hour, depending upon the condition
of the pavement dry, wet, or muddy; relative amount of dirt
HIGHWAY CONSTRUCTION 175
produced by the different pavements, if swept daily; and the average
cost of cleaning different pavements.
Removal of Snow. The methods employed for keeping roads
and streets passable during the period of snowfall varies according
to the climatic conditions. In localities subject to heavy falls
of snow, and continuous low temperature that retards the melting
of the snow until spring, two methods are followed: (1) a narrow
track is opened by a snowplow, through the center of the road,
the snow being formed into long, narrow heaps on each side;
(2) the snow is not removed, but is compacted by rolling with a
light-weight wood or metal roller, Fig. 107. In localities having
light falls and in the larger cities, the snow is pushed by plows or
rotary brooms toward the gutters from where it is loaded into
vehicles, hauled to a natural waterway and dumped, or in the
absence of this it is placed in vacant lots and in some cases it is
disposed of by dumping into the sewers through the manholes,
but this must be done carefully, as there is liability of choking the
sewer by the snow's consolidating. Light falls may be disposed
of by the application of a stream of water to the surface of the
street thereby washing the snow into the sewer. Many machines
have been devised for melting the snow by the application of steam,
hot air, etc., but none of them have been successful economically.
In some cities the snow is melted by an application of rock salt
which produces a thawing action when mixed with the snow by
the traffic, the slushy mixture so formed is swept to the gutters
by machine brooms and washed into the sewers by a stream of
water from the hydrants. Objection is made to this method on
account of the intense cold produced and its injurious effect upon
the feet of pedestrians and on the hoofs of horses.
In order to cause the minimum of inconvenience to traffic it
is necessary that the snow be removed from the streets as quickly
as possible, therefore, it is customary, before the arrival of winter,
to lay out the method and organization required and to make arrange-
ments for the quick mobilization of the force needed for its removal.
To accomplish this the city is divided into districts, in each of which
there is established a headquarters and depot stocked with the
necessary tools to execute the work in that district, and to which
the laborers report when the snow commences to fall.
176 HIGHWAY CONSTRUCTION
Street Sprinkling. Streets and roads are sprinkled with water
for the purpose of abating dust and cooling the air. While water-
bound macadam and earth surfaces must be sprinkled to abate
the dust, a stone-block, brick, asphalt, or wood pavement will not
require sprinkling if thoroughly cleansed and kept clean. On
unclean and badly maintained pavements, sprinkling with water
as usually performed converts the fine dust into a slime which renders
all smooth pavements slippery, and in warm weather it becomes a
prolific breeding place for disease germs, it clings to the feet and
clothing of pedestrians, and, with its accompanying germs, is carried
into buildings and dwellings.
The average cost of sprinkling per square yard is $0.009.
The systems followed for executing the work of street cleaning,
snow removal, and sprinkling are: (1) by contract where the con-
tractor furnishes all the tools and labor; (2) by contract for the
labor only, the city furnishing the tools and machinery ; (3) by the
city, with its own staff and machinery.
SELECTING THE PAVEMENT
The problem of selecting the best pavement for any particular
case is a local one, not only for each city, but also for each of the various
parts into which the city is imperceptibly divided; and it involves so
many elements that the nicest balancing of the relative values for each
kind of pavement is required in arriving at a correct conclusion.
In some localities, the proximity of one or more paving materials
determines the character of the pavement; while in other cases a
careful investigation may be required in order to select the most
suitable material. Local conditions always should be considered;
hence it is not possible to lay down any fixed rule as to what material
makes the best pavement.
Qualifications. The qualities essential to a good pavement
may be stated as follows :
(1) It should be impervious.
(2) It should afford good foothold for horses and adhesion
for motor vehicles.
(3) It should be hard and durable, so as to resist wear and
disintegration.
(4) It should be adapted to every grade.
HIGHWAY CONSTRUCTION 177
(5) It should suit every class of traffic.
(6) It should offer the minimum resistance to traction.
(7) It should be noiseless.
(8) It should yield neither dust nor mud.
(9) It should be cleaned easily.
(10) It should be cheap.
Interests Affected. Of the above requirements, numbers (2),
(4), (5), and (6) affect the traffic and determine the cost of haulage
by the limitations of loads, speed, and wear and tear of horses and
vehicles. If the surface is tough or the foothold bad, the weight
of the load a horse can draw is decreased, thus necessitating the
making of more trips or the employment of more horses and vehicles
to move a given weight. A defective surface necessitates a reduction
in the speed of movement and a consequent loss of time; it increases
the wear of horses, thus decreasing their life service and lessening
the value of their current services ; it also increases the cost of main-
taining vehicles and harness.
Requirements, numbers (7), (8), and (9), affect the occupiers
of adjacent premises, who suffer from the effect of dust and noise;
they also affect the owners of said premises, whose income from
rents is diminished where these disadvantages exist. Numbers
(3) and (10) affect the taxpayers alone first, as to the length of
time during which the covering remains serviceable; and second,
as to the amount of the annual repairs. Number (1) affects the
adjacent occupiers principally on hygienic grounds. Numbers (7)
and (8) affect both traffic and occupiers.
Problem Involved in Selection. The problem involved in the
selection of the most suitable pavement consists of the following
factors: (1) adaptability; (2) desirability; (3) serviceability; (4) com-
parative safety; (5) durability; (6) cost.
Adaptability. The best pavement for any given roadway will
depend altogether on local circumstances. Pavements must be
adapted to the class of traffic that will use them. The pavement
suitable for a road through an agricultural district will not be suit-
able for the streets of a manufacturing center ; nor will the covering
suitable for heavy traffic be suitable for a pleasure drive or for a
residential district.
General experience indicates the relative fitness of the several
178 HIGHWAY CONSTRUCTION
TABLE XVI
Resistance to Traction on Different Pavements
TRACTIVE RESISTANCE
Lb. per Ton
Fraction of the Load
Sheet-asphalt
Brick
30 to 70
15 to 40
eV to - 3 \,
Cobblestone
50 to 100
40 tO 2 ] o
Stone-block
30 to 80
eV to &
Rectangular wood-block
30 to 50
eV to 4*0
Round wood-block
40 to 80
s 1 ,) to 2 ' 5
materials as follows: for country roads, suburban streets, and pleasure
drives broken stone; for streets having heavy and constant traffic
rectangular blocks of stone, laid on a concrete foundation, with
the joints filled with bituminous or Portland-cement grout; for
streets devoted to retail trade, and where comparative noiselessness
is essential asphalt, wood, or brick. More recent experience
indicates that concrete, when properly laid and reinforced at neces-
sary points, may be employed to advantage for any pavement,
both as base and as wearing surface.
Desirability. The desirability of a pavement is its possession
of qualities which make it satisfactory to the people using and
seeing it. Between two pavements alike in cost and durability,
people will have preferences arising from the condition of their
health, personal prejudices, and various other intangible influences,
causing them to select one rather than the other in their respective
streets. Such selections often are made against the demonstrated
economies of the case, and usually in ignorance of them. Whenever
one kind of pavement is more economical and satisfactory to use
than is any other, there should not be any difference of opinion
about securing it, either as a new pavement or in the replacement
of an old one.
The economic desirability of pavements is governed by the
ease of movement over them, and is measured by the number of
horses or pounds of tractive force required to move over them a
given weight usually 1 ton. The resistance offered to traction
by different pavements is shown in Table XVI.
HIGHWAY CONSTRUCTION 179
Serviceability. The serviceability of a pavement is its quality
of fitness for use. This quality is measured by the expense caused
to the traffic using it namely, the wear and tear of horses and
vehicles, loss of time, etc. No statistics are available from which
to deduce the actual cost of wear and tear.
The serviceability of any pavement in great measure depends
upon the amount of foothold afforded by it to the horses provided,
however, that its surface be not so rough as to absorb too large a
percentage of the tractive energy required to move a given load
over it. Cobblestones afford excellent foothold, and for this reason
are largely employed by horse-car companies for paving between
the rails; but the resistance of their surface to motion requires
the expenditure of about 40 pounds of tractive energy to move
a load of 1 ton. Asphalt affords the least foothold ; but the tractive
force required to overcome the resistance it offers to motion is
only about 30 pounds per ton.
Comparative Safety. The comparison of pavements in respect
to safety, is the average distance traveled before a horse falls. The
materials affording the best foothold for horses are as follows,
stated in the order of their merit:
(1) Earth, dry and compact.
(2) Gravel.
(3) Broken stone (macadam).
(4) Wood.
(5) Sandstone and brick.
(6) Asphalt.
(7) Granite blocks.
, ^ Durability. The durability of pavement is that quality which
determines the length of time during which it is serviceable, and
does not relate to the length of time it has been down. The only
measure of durability of a pavement is the amount of traffic tonnage
it will bear before it becomes so worn that the cost of replacing it
is less than the expense incurred by its use.
As a pavement is a construction, it necessarily follows that there
is a vast difference between the durability of the pavement and the
durability of the materials of which it is made. Iron is eminently
durable; but, as a paving material, it is a failure.
The durability of a paving material will vary considerably with
180
HIGHWAY CONSTRUCTION
TABLE XVII
/ Terms of Life of Various Pavements
MATERIALS
TERMS OF LIFE
(Years)
Granite-block
12 to 30
Sandstone
6 to 12
Asphalt
Wood
10 to 14
7 to 15
Limestone
1 to 3
Brick
5 to 15
Macadam
5 to ?
the condition of cleanliness observed. One inch of overlying dirt
will protect the pavement most effectually from abrasion, and
prolong its life indefinitely. But the dirt is expensive; it injures
apparel and merchandise, and is the cause of sickness and discom-
fort. In the comparison of different pavements, no traffic should
be credited to the dirty one. The life or durability of different
pavements under like conditions of traffic and maintenance, may
be taken as shown in Table XVII.
Cost. First cost or the cost of construction, is largely controlled
by the locality of the place, its proximity to the particular material
used, and the character of the foundation. The question of cost
is the one which usually interests taxpayers, and is problably the
greatest stumblingblock in the attainment of good roadways. The first
cost usually is charged against the property abutting on the highway
to be improved. The result is that the average property owner
always is anxious for a pavement that costs little, because he must
pay for it, not caring for the fact that cheap pavements soon wear
out and become a source of endless annoyance and additional expense.
Thus false ideas of economy usually have stood, and undoubtedly
always will stand to some extent, in the way of realizing that the
best is the cheapest.
The pavement which has cost the most is not always the best;
nor is that which cost the least the cheapest; the one which is truly
the cheapest is the one which makes the most profitable returns
in proportion to the amount expended upon it. No doubt there
is a limit of cost to go beyond which would produce no practical
benefit; but it always will be found more economical to spend enough
HIGHWAY CONSTRUCTION 181
to secure the best results, and this always will cost less in the long
run. One dollar well spent is many times more effective than one-
half of the amount injudiciously expended in the hopeless effort
to reach sufficiently good results. The cheaper work may look as
well as the more expensive, for a time, but very soon may have
to be done over again.
Economic Benefit. The economic benefit of a good roadway
is comprised in the following: its cheaper maintenance, the greater
facility it offers for traveling, thus reducing the cost of transporta-
tion; the lower cost of repairs to vehicles, and less wear of horses,
thus increasing their term of serviceability and enhancing the value
of their present service ; the saving of time ; and the ease and comfort
afforded to those using the roadway.
Relative Economies. The relative economies of pavements
whether of the same kind in different condition, or of different
kinds in like good condition are determined sufficiently by summing
their cost under the following headings of account:
(1) Annual interest upon first cost and sinking fund.
(2) Annual expense for maintenance.
(3) Annual cost for cleaning and sprinkling.
(4) Annual cost for service and use.
(5) Annual cost for consequential damages.
Interest on First Cost. The first cost of a pavement, like any
other permanent investment, is measurable for purposes of comparison
by the amount of annual interest on the sum expended. Thus,
assuming the worth of money to be 4 per cent, a pavement costing
$4 per square yard entails an annual interest loss or tax of $0.16
per square yard.
Cost of Maintenance. Under this head must be included all
outlays for repairs and renewals which are made from the time
when the pavement is new and at its best to a time subsequent,
when, by any treatment, it is put again in equally good condition.
The gross sum so derived, divided by the number of years which
elapse between the two dates, gives an annual average cost for
maintenance.
Maintenance means the keeping of the pavement in a condition
practically as good as when first laid. The cost will vary con-
siderably depending not only upon the material and the manner in
182 HIGHWAY CONSTRUCTION
which it is constructed, but upon the condition of cleanliness
observed, and the quantity and quality of the traffic using the
pavement.
The prevailing opinion that no pavement is a good one unless,
when once laid, it will take care of itself, is erroneous; there is no
such pavement. All pavements are being worn constantly by
traffic and by the action of the atmosphere; and if any defects
which appear are not repaired quickly, the pavements soon become
unsatisfactory and are destroyed. To keep them in good repair,
incessant attention is necessary, and is consistent with economy.
Yet claims are made that particular pavements cost little or nothing
for repairs, simply because repairs in these cases are not made,
while any one can see the need of them.
Cost of Cleaning and Sprinkling. Any pavement, to be con-
sidered as properly cared for, must be kept dustless and clean.
While circumstances legitimately determine in many cases that
streets must be cleaned at daily, weekly, or semiweekly intervals,
the only admissible condition for the purpose of analysis of street
expenses must be that of like requirements in both or all cases
subjected to comparison.
The cleaning of pavements, as regards both efficiency and cost,
depends (1) upon the character of the surface; (2) upon the nature
of the materials of which the pavements are composed. Block
pavements present the greatest difficulty; the joints can never
be perfectly cleaned. The order of merit as regards facility of cleans-
ing, is: (1) asphalt, (2) concrete, (3) brick, (4) stone, (5) wood,
(6) macadam.
Cost of Service and Use. The annual cost for service is made
up by combining several items of cost incidental to the use of the
pavement for traffic for instance, the limitation of the speed of
movement, as in cases where a bad pavement causes slow driving
and consequent loss of time ; or cases where the condition of a pave-
ment limits the weight of the load which a horse can haul, and so
compels the making of more trips or the employment of more
horses and vehicles; or cases where conditions are such as to
cause greater wear and tear of vehicles, of equipment, and of
horses. If a vehicle is run 1500 miles in a year, and its main-
tenance cost $30 a year, then the cost of its maintenance per
HIGHWAY CONSTRUCTION 183
mile traveled is 2 cents. If the value of a team's time is, say
$1 for the legitimate time taken in going 1 mile with a load,
and in consequence of bad roads it takes double that time, then
the cost to traffic from having to use that mile of bad roadway is
$1 for each load. The same reasoning applies to circumstances
where the weight of the load has to be reduced so as to neces-
sitate the making of more than one trip. Again, bad pavements
lessen not only the life service of horses, but also the value of their
current service.
Cost for Consequential Damages. The determination of conse-
quential damages arising from the use of defective or unsuitable
pavements, involves the consideration of a wide array of diverse
circumstances. Rough-surfaced pavements, when in their best
condition, afford a lodgment for organic matter composed largely
of the urine and excrement of the animals employed upon the road-
way. In warm and damp weather, these matters undergo putre-
factive fermentation, and become the most efficient agency for
generating and disseminating noxious vapors and disease germs,
now recognized as the cause of a large part of the ills afflicting
mankind. Pavements formed of porous materials are objectionable
on the same, if not even stronger, grounds.
Pavements productive of dust and mud are objectionable,
and especially so on streets devoted to retail trade. If this par-
ticular disadvantage be appraised at so small a sum per lineal foot
of frontage as- $1.50 per month, or 6 cents per day, it exceeds the
cost of the best quality of pavement free from these disadvantages.
Rough-surfaced pavements are noisy under traffic and insuffer-
able to nervous invalids, and much nervous sickness is attributable
to them. To all persons interested in nervous invalids, this damage
from noisy pavements is rated as being far greater than would
be the cost of substituting the best quality of noiseless pavement;
but there are, under many circumstances, specific financial losses,
measurable in dollars and cents, dependent upon the use of rough,
noisy pavements. They reduce the rental value of buildings and
offices situated upon streets so paved offices devoted to pursuits
wherein exhausting brain work is required. In such locations,
quietness is almost indispensable, and no question about the cost
of a noiseless pavement weighs against its possession.
184
HIGHWAY CONSTRUCTION
TABLE XVIII
Comparative Rank of Pavements
CHARACTERISTICS
VARIETY
Asphalt (sheet)
Asphalt (block)
Concrete
Value
Macadam (bituminous)
Qualities
(per
Macadam (water-bound)
cent)
Brick
Granite
Sandstone
Wood)
Low tractive resistance
20
20.C
19.0
18.0
19.0
ll.G
18.0
12.C
14.0
20.0
Service on grades
10
3.0
3.0
7.0
4.0
8.0
9.0
10.0
10.0
2.0
Non-slipperiness
5
1.5
2.5
4.0
2.5
4.5
3.5
3.5
5.0
2.0
Favorableness to travel
5
5.0
4.5
3.5
4.0
4.5
3.5
3.5
4.0
4.5
Sanitariness
10
10.0
9.0
7.0
8.0
3.0
8.0
6.0
7.0
9.0
Noiselessness
3
2.5
2.5
2.0
2.5
2.5
1.5
1.0
1.5
3.0
Minimum dust
3
2.5
2.5
2.0
2.0
1.0
2.0
1.5
2.0
2.0
Ease of cleaning
5
5.0
5.0
3.5
4.0
1.0
3.5
1.5
1.5
5.0
Acceptability
4
3.5
3.5
2.5
3.0
1.5
2.5
2.0
2.5
4.0
Durability
15
7.5
8.5
6.0
3.0
1.5
10.0
15.0
14.0
11.5
Ease of maintenance
5
3.5
4.0
3.0
3.0
2.5
4.0
4.5
5.0
5.0
Cheapness (first cost)
10
4.5
4.0
5.0
7.5
10.0
4.0
3.0
3.5
3.0
Low annual cost
5
1.5
2.5
3.0
3.5
1.0
4.5
5.0
5.0
5.0
Totals. . . .
100
70.0
70.5
66.5
66.0
52.0
74.0
68.5
75.0
76.0
Approximate first cost
(dollars per sq. yd.)
2.30
2.65
1.85
1.35
1.00
2.65
3.25
3.00
3.45
When an investigator has done the best he can to determine
such a summary of costs of a pavement, he may divide the amount
of annual tonnage of the street traffic by the amount of annual
costs, and know what number of tons of traffic are borne for each
cent of the average annual cost, which is the crucial test for any
comparison, as follows:
(1) Annual interest upon first cost and sinking fund $
(2) Average annual expense for maintenance and renewal . .
(3) Annual cost for custody (sprinkling and cleaning) ......
(4) Annual cost for service and use
(5) Annual cost for consequential damages
Amount of average annual cost
Annual tonnage of traffic
Tons of traffic for each cent of cost
Gross Cost of Pavements. Since the cost of a pavement
depends upon the material of which it is formed, the width of the
HIGHWAY CONSTRUCTION 185
roadway, the extent and nature of the traffic, and the condition
of repair and cleanliness in which it is maintained, it follows that
in no two streets is the endurance or the cost the same, and the
difference between the highest and lowest periods qf endurance
and amount of cost is very considerable.
Comparative Rank of Pavements. In Table XVIII is given
the rank of the various pavements in percentage, prorated from
the values assigned in the first column to the desired qualities.
The pavement ranking first in any given quality is given the
full value for that quality, the others grading down from this
value according to the extent to which they possess the desired
quality. An examination of the table shows macadam to be the
cheapest; least durable, and most difficult to maintain and cleanse;
rather favorable to travel; comparatively low in sanitariness; and
high in annual cost. While the table may be used as an aid in
determining the most suitable pavement according to the factors
that are susceptible of a numerical value, the values assigned must
be modified by local conditions; first cost will necessarily vary in
different localities, and certain factors will be more important in
one locality than another.
Specifications. A specification or detailed description of the
various works to be carried out always is attached to a contract,
and is prepared before estimates are called for. The prominent
points that are essential to the production of a specification that
will fulfill its purpose properly, are: (1) description of the work;
(2) extent of the work; (3) quality of the materials; (4) tests for
the materials; (5) delivery of the materials; (6) character of the
workmanship; (7) manner of executing the work.
Attention to these points and a clear and accurate description
of each detail (leaving nothing to be imagined) not only will
contribute materially to the rapid and efficient execution of the
work, but will avoid any future misunderstanding. Every item
of the work should be allotted a separate clause, for confusion
must ensue when a single clause includes descriptions of several
matters.
As a rule it is undesirable to insert in specifications any dimen-
sions or weights that can be shown on the drawings. However,
when it is necessary to insert them, words should be used instead
186 HIGHWAY CONSTRUCTION
of numerals; the use of numerals, and particularly decimal numbers,
should be avoided, as there is a risk of having them set up incorrectly
by the typesetter and overlooked in the proofreading. When a
numeral is used it should be followed by the word or words indicat-
ing the numeral, placing the numeral in parenthesis.
Brevity, so far as it is consistent with completeness, should
prevail, but the word "et cetera" should be excluded rigidly, and
the matters covered by it should be defined clearly. Neither should
important points of the work be dismissed with the direction that
"the w r ork shall be done to the satisfaction of the engineer". A
direction of this kind usually implies that the engineer does not
know what he wants, and therefore leaves the matter to the superior
knowledge of the contractor an attitude not very creditable to
the former. The only really legitimate use of this phrase is in a
general clause referring to the whole of the work.
The specifying of impracticable sizes of materials must be
avoided as it causes unnecessary discussion and frequently leads
to a charge for "extras".
A clause or phrase permitting the furnishing of alternative
materials or workmanship should be excluded, because such per-
mission affords ground for dispute and difference of opinion. On
the other hand, specifying that certain articles manufactured by
a particular firm shall be used should be avoided, as it suggests
unfairness on the part of the engineer, and may create the idea
that his selection is not without profit to himself.
With regard to the actual methods of carrying out the work,
the contractor should not be tied to any particular means of effecting
the required end, unless special circumstances require it, for, pro-
vided the materials and workmanship are satisfactory, it is better
to allow the contractor to use his own discretion as to the manner
of producing the required result.
While the standard and proper tests for the materials always
should be stipulated, yet if they are carried to an extreme degree,
as frequently happens, they defeat their own object. When it
becomes impossible to carry out certain unreasonable demands,
the alternative is to evade them as much as possible; and it must
be borne in mind that the more stringent the demand, the greater
the difficulty in enforcing it.
HIGHWAY CONSTRUCTION 187
Contracts. A good, clear, and comprehensive contract is a
difficult thing to write, but it should be "common sense" from
beginning to end, and should be the joint production of both engi-
neering and legal ability, neither sacrificing the one feature to the
other.
The stipulations of the contract form the legal part of the
document and are distinct from the technical description of the
work to be done. The essential points are: (1-) time of commence-
ment; (2) time of completion; (3) manner and times of payment;
(4) prices for which the work is to be performed; (5) measurements;
(6) damages for noncompletion; (7) protection of persons and
property during the prosecution of work; (8) such special stip-
ulations as may be required for the particular work that is being
contracted for.
It should be borne in mind that the contract and specifications
when duly signed by the parties interested, is a legal document,
which must be produced in court in the event of a dispute arising,
therefore, it is of the utmost importance that it be written clearly
in simple language, the clauses being arranged in logical sequence,
and the descriptions made exact and complete without being need-
lessly verbose.
High-sounding phrases, and duplication of statements or infor-
mation, should be avoided as tending to confusion. Specifications
are seldom judged by literary standards of excellence, therefore,
words may be repeated again and again if they express the meaning
of the writer more clearly and forcibly than an alternative would do.
In the case of a lengthy contract and specification, a complete
index with the clause and page numbers will be found an aid to
finding quickly any required subject; cross references may some-
times be introduced with advantage.
INDEX.
INDEX
Asphalt pavements 153
asphalt blocks 161
failure ~ 159
foundation 157
laying 155
qualifications 157
rock asphalt 161
sheet-asphalt 153
tools employed in constructing __ __ 162
Axle friction - - 11
B
Bituminous-macadam 99
amiesite 103
asphaltic petroleums 105
bitulithic. . 103
bituminous materials, definitions of 104
bituminous materials, tests for 106
cement, bituminous 104
construction 99
features 99
rock asphalt 103
Brick pavements 133
brick, qualifications of good 133
brick-pavement, qualifications of 137
fillings, joint 139
foundation 137
hand tools used 145
heaters, gravel 146
laying, manner of 138
machine, concrete-mixing 145
sand cushion 137
test _ 136
Broken-stone road 85
compacting stone 93
construction 85
macadam and telford roads, suppression of dust on 95
quality 88
rock, testing 89
shape and size of stonea 92
species of stone _ 91
spreading stone 92
thickness of stones. _ 92
INDEX
^ PAGE
City streets and highways _ 113
arrangement 113
asphalt 153
brick _ 168
cleaning 172
drainage 119
foundations ^ 121
grades 114
pavement, miscellaneous 164
selecting 176
stone-block 123
street work, miscellaneous 166
transverse contour or crown 118
width .' 113
wood 147
City streets, foundations for 121
concrete 122
Concrete pavements 106
bituminous surface, with 108
block or cube 108
construction 106
joints, expansion 108
materials 1 107
reinforced-concrete 108
Country roads and boulevards 1
location 1 12
maintenance and improvement 110
methods, preliminary construction 35
nature-soil roads 74
vehicles, resistance to movement of 1
Country roads and boulevards, drainage of 38
ends from weather, protection of 42
fall, for 40
gutters . 43
hillside roads 45
location 39
materials 40
outlets 43
road gutters, inner and outer , 45
side ditches 43
silt 42
soils, nature of 39
springs in cuttings, treatment of 44
water breaks 46
Culverts 46
arch i 53
box.. 52
INDEX 3
Culverts (Continued) PAGE
design, factors in 46
earthenware pipe 49
functions i__ 46
iron pipe 51
short span bridges used as 53
E
Earthwork 55
classification 57
cuts and fills, balancing 55
embankments, methods of forming 58
prosecution of 58
shrinkage in 57
slopes, side _ 55
tools 60
G
Grade problems 33
establishing a grade 34
level stretches 34
minimum '__ 33
undulating 33
Gravel roads 83
laying gravel 84
preparation of gravel 83
repair 85
L
Location of roads 12
instruments 14
object of 13
points to consider 13
reconnoissance 13
selection, final 22
elements entering into 22
grade problems 33
gradient 31
location, final 30
mountain roads, treatment of 26
profile, construction of ._ 31
repose, angle of 32
roads, alignment of 27
typical cases, treatment of 23
survey, preliminary 15
bridge site 22
features ^._ 15
map ,. 20
memoir _ 22
topography _ 15
4 INDEX
M PAGE
Miscellaneous pavement 164
burnt clay 164
chert 164
clinker 165
iron 165
kleinpflaster 165
oyster-shell 164
petrolithic 165
slag 164
straw 164
trackways 165
Miscellaneous street work 166
curbstones and gutters 170
footpaths 166
foundation 167
materials ,. 167
slope, cross 167
surface 167
width _' - 167
Mountain roads, treatment of 26
N
Nature-soil roads 74
earth 74
sand 77
sand and gravel soils, application of oil to 78
sand-clay 77
R
Roads, maintenance and improvement of 110
broken-stone __ 109
improvement of existing roads 110
systems 110
traffic census _ 111
Roads with special coverings 79
foundations 79
materials 79
preparation __: 80
thickness _-- 79
types 81
road covering, elements of 79
surfaces, wearing 82
bituminous-macadam 99
broken-stone 85
classification 82
concrete 106
function.. -- 82
INDEX 5
Roads with special coverings PAGE
surfaces, wearing (Continued)
gravel - --- 83
thickness 82
S
Selecting pavement - 176
benefit, economic 181
contracts - 187
cost of pavements, gross _ - 184
economics, relative 18 1
interests affected 177
problem involved in 177
qualifications 176
rank of pavements, comparative 185
specifications 185
Stone-block pavements 123
Belgian-block 125
blocks, dimensions of 127
blocks, manner of laying 127, 130
cobble-stone 125
cushion coat 129
foundation 129
granite-block 126
joints, filling for 130
materials 124
granite 124
limestone 125
sandstones 125
trap rock 125
ramming 130
stone pavement, steep grades on _ L 132
Street cleaning 172
methods 172
snow, removal of 175
sprinkling 176
Streets, drainage of 119
catch basins 120
gutters 119
surface 119
T
Table
different road materials, proportionate rise of center to width of car-
riageway 37
effects of grades a horse can draw on different pavements . 9
grades, methods of designating 32
gross loads for horse on different pavements 9
life of various pavements, terms of . ............. 180
6 INDEX
Table (Continued) ' PAGE
loaded vehicles over inclined roads, data for 11
paving-brick manufacture, average composition of shales for 134
rank of pavements, comparative rank of 184
resistance due to gravity on different inclinations _* 5
resistance to traction on different pavements 178
resistance to traction on road surfaces 2
rise of pavement center above gutter for different paving materials 119
stones, specific gravity, weight, resistance to crushing, and absorptive
power of 124
street cleaning, rate and cost of 173
traction power of horses at different velocities 7
tractive power with time, variation of 8
traffic census 112
wind pressure for various vehicles _ 12
V
Vehicles, resistance to movement of 1
air, resistance of 12
friction, axle 11
power and gradients, tractive 7
traction, resistance to _ 1
W
Wood-block pavements 147
blocks, laying 149
creosoting 147
qualifications 152
LD 21-100m-7,'33
3 C 5550
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