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weight and strength to the pier, the backing is cut to


dimension, so that, when laid, the masonry of each course,
with the exception of the joints, forms one solid mass of

1558. Coping. The top course of masonry forms the
coping and should project 2 or 3 inches over the main body
of the pier. When the coping is of two courses, the projec-
tion is divided between them, and the outer edge of the top
course is beveled to an amount equal to the total projection.
The coping courses are usually dressed stone, which adds
greatly to the finish of the work.

1559. Pneumatic Caisson Foundations, River
beds of alluvium extending to great depth, exposed to the
scour of a constantly shifting channel, are not suited to pile
foundations. The Mississippi and Missouri rivers are stri-
king examples of this class. A firm bearing stratum of suf-
ficient thickness to support a foundation is often from 80 to
100 feet below the bed of the river. To meet these condi-
tions, caissons built of heavy timbers are sunk either to bed
rock or to a firm stratum of clay or gravel of such depth as
to insure permanent safety.

1560. Test Holes. After the bridge site has been
determined upon and before the bridge plan has been con-
sidered in detail, test holes are sunk on the center line at
intervals of from 100 to 200 feet, to determine the character
of the material forming the river bed. The results of these
examinations will largely determine the lengths of the
spans. The deeper the foundations the greater will be the
spans. Having determined the locations of piers, test holes
are sunk at each pier site, in sufficient number to afford
ample knowledge of the character of the material to be en-
countered in sinking the caisson. A complete record of
ea^ch test hole is kept, and a sectional profile platted, showing
the specification.

1561. Modes of Sinking Test Holes. Test holes
are sunk either by diamond drills or by driving wrought-
iron pipe. Piles are driven to support a platform, upon



which is placed the machinery used in sinking test holes.
Wrought-iron pipe is commonly adopted. It is cut in sec-
tions of from 6 to 10 feet. The thread on pipe and couplings
should be so cut that when coupled the ends of pipe will
abut and so prevent the stripping of thread which is liable
to result from the repeated blows of the driver. A steel
cap, shown in Fig. 468, is screwed to the top of the pipe to
receive the blows of the hammer.



1562. The Driver. The driver is constructed on
the principle of the pile driver. The hammer consists of a
section of an oak or other hard wood tree, from 9 to
12 inches in diameter, turned to a uniform size and fitted
at the sides with steel grooves, through which pass the
guides extending the full length of the leaders. An iron
ring is fastened in the top of the hammer to which is at-
tached the rope used in raising the hammer. This rope
passes through a common pulley with wooden sheave,
which is fastened with
rope to the head of the

The leaders are of
sufficient length to
allow the hammer a
drop of 4 feet after a
new .section has been
attached. A force of
4 or 5 men is required
for the efficient work-
ing of the machine. A
In sinking into the
earth, the pipe cuts a
section equal to the
inside area of the pipe.
At intervals of 6 or 8
feet in sinking, and
before an additional section is attached, the pipe is cleared
by a sand pump.


FIG. 470.




Coarse Gravel.

Fine Gravel.

Hard Clay.


1563. The Sand Pump. This pump consists of a
section of iron pipe of such size as will work freely inside
the pipe being driven. Fig. 469 shows longitudinal section
and plan of sand pump. The valve A consists of a ball of
iron which rests in a hollow seat and acts automatically.
The pump is lowered into the pipe by means of a rope
attached to the ball B. Water is poured into the pipe so
that the contents may be reduced to a fluid state. As the
descending pump strikes the surface of the water in the
pipe, the valve A is forced upwards and the water and sand
pass" through the hole C. The upper part of the valve
chamber D is ribbed, as shown at E. This arrangement
confines the valve and at the same time allows the sand
and water to pass into the chamber F. When small stones
and pebbles enter the pipe and are too large to pass
through the valve opening, they must be broken up by a
churn drill. The best form of drill is one with a cutting
edge or bit similar to that commonly used in steam drills,
shown in Fig. 470.

The drill A B is about 18 inches in length. The cutting
edge or bit A is in the form of a cross with equal arms. To
the end B an ordinary pipe coupling is attached. The body
of the drill is of gas pipe in sections, which are added as the
hole deepens. The bit must be kept sharp and the couplings
frequently examined that no stripping of thread occurs,
which might easily result in the loss of the drill and prevent
further sinking. Any change in the material removed from
the pipe is readily detected, and the depth of the stratum is
determined by measuring from the top of the pipe.

1564. Record of Test Holes. A good form for
keeping a record of test holes is given in Fig. 471, which
shows a sectional profile, giving the thickness and character
of each stratum passed through. The profile given in Fig.
471 is of a test hole driven in the bed of a river. After pass-
ing through different strata of sand and gravel, a stratum
of hard clay is encountered. After penetrating 9 feet into
this clay, any further sinking is unnecessary, since 9 feet of


hard clay will afford a foundation amply strong for any
ordinary bridge.

1565. Dimensions of Caisson. The depth of the
foundation stratum will affect the size of the caisson as the
faces of the pier are battered, increasing the size of the plan
as the depth increases.

For example, suppose the neat dimensions of a bridge pier
at the top are 6 feet by 30 feet, and all the faces batter at
$ inch to the foot. If the stratum upon which the caisson
is to rest is 93 feet below the top of the pier, and the height
of the caisson from cutting edge to deck is 13 feet 4 inches,
and the deck is to extend on all sides G inches outside of the
base of the pier, what will be the dimension of the caisson
floor ? As the pier faces batter at a rate of inch to the
foot, the increase in each dimension will be as many inches
as the pier is feet in height. The height of the pier is 93 ft.
4 in. 13 ft. 4 in. = 80 feet. We, therefore, add 80 inches,
or 6 feet 8 inches, to each dimension and we have for the
base of the pier, length 36 feet 8 inches, and width 12 feet
8 inches. The caisson deck, which projects 6 inches on all
sides beyond the pier base, will have a length of 37 feet
8 inches and a width of 13 feet 8 inches. The sides of the
caisson are battered on all sides to reduce the friction of the
earth against them during the progress of sinking. The
total batter on each side is 12 inches. This batter will give
to the base of the caisson the following dimensions, viz.,
length 39 feet 8 inches, and width 15 feet 8 inches. With
the general dimensions of the caisson floor as determined
above, the details may be modified to suit special conditions.

All caisson plans must meet certain requirements, viz. :
There must be adequate supply shafts for admitting men
and materials; air pipes for the compressed air, and a con-
crete shaft by means of which the concrete used in sealing
and filling the caisson may be conveyed from the top of the
masonry, where it is mixed, to the caisson chamber. Supply
shafts are of boiler iron and from three feet to four feet in
diameter, depending upon the size of the caisson. The



shafts are built in sections of from four to eight feet, the
connections being made by means of exterior flanges, which
are bolted together.

1 566. Air Locks. The shafts (usually two in num-
ber) are fitted with air locks, by means of which men and
materials pass from the outer air to
the caisson chamber, and vice versa,
without the escape of the compressed
air. The principle upon which the
air lock is constructed is explained in
Fig. 472.

A is the air lock leading to the
shaft B, which extends to the caisson
chamber. A person entering the cais-
son finds the outer door in the posi-
tion C. He first closes the air cock
J9, and swings the door shut, the door
taking the position E. He then opens
the air cock F, and the air in the lock
A receives the pressure of the air in
the caisson, forcing the door E firmly
against the casing, which is usually
fitted with a rubber gasket, making
an air-tight joint. The pressure
against the door G being removed, it

opens of itself, taking the position H. The person is then
in direct communication with the caisson chamber, descend-
ing to it by means of the ladder K. At the bottom of the
shaft is another door, which is closed when the air lock at
the surface is removed for adding another section to the
shaft. One of the shafts is used for admitting men and
tools, the other for removing material. The air lock used
in removing material is provided with a windlass, the axle
of which has air-tight bearings and extends through the
sides of the lock, being fitted with two cranks which are
operated by laborers. Most caissons are fitted with a sand
pipe, by means of which the pressure of the air in the


caisson chamber is utilized in blowing out the sand or any
fine material excavated in sinking.

1567. Plan of Caisson. A general plan of a timber
caisson is shown in Fig. 473, in which G represents the
plan; //"the longitudinal section, and K the cross-section.
The walls [7 and V, enclosing the caisson chamber, are
built of three courses of timber 12" X 12" square. The
outer and inner courses consist of superimposed horizontal
timbers extending the full length and width of the caisson.
The inner course of timbers is laid in an upright position,
and extends to within 1 foot of the top of the caisson deck.
The timbers in the walls are securely bolted together with
drift bolts, each bolt passing entirely through two timbers
and penetrating fully 6 inches into the third. As the tim-
bers are laid, they are poured with hot coal tar or pitch.
At frequent intervals, the horizontal layers of timber are
bolted to the upright timbers with screw bolts. The bolt
heads must be countersunk and the sockets filled with pitch.
Rubber washers are used to insure tight joints.

The cutting edge a is of \ inch boiler plate, 8 inches in
width, and backed by 4-inch oak plank. The walls above
the cutting edge increase in thickness with each course of
timber, attaining their full thickness of 3 feet in the third
course. When the caisson is of great size, longitudinal
division walls are built dividing the caisson chamber into
compartments. Openings are made in these walls to admit
of free communication between the various compartments.
The caisson shown in Fig. 473 has not sufficient breadth to
require any interior division walls. Struts Y of 12" X 12"
timber placed at intervals of about 8 feet insure lateral
stiffness, and 2-inch iron tie-rods Z fitted with turnbuckles
prevent the walls from spreading.

The deck consists of six courses of 12" X 12" timbers so
laid as to render the chamber as nearly air-tight as possi-
ble, and give the greatest possible stiffness and strength to
the structure. The first course A forms the ceiling of the
chamber, the timbers extending the entire width of the




caisson. A layer of zinc enclosed between two layers of
felt is laid over the entire ceiling course and ceiling, and
floated with pitch. The timbers are laid close, with joints
filled with pitch and fastened to the walls with heavy anchor
bolts. Course B is laid diagonally to course A and bolted
to it, a share of the bolts extending into the side walls.
The diagonals stop at 6 feet from the ends of the caisson.
The balance of the course is laid longitudinally, with the
alternate timbers passing between the uprights and extend-
ing to the outside sheathing of the caisson. Course C is
laid transversely; course D diagonally, the diagonal timbers
being at right angles to those in course B, and stopping at
6^ feet from the ends of the caisson, as in course B, and the
balance of the course laid longitudinally, as in that course.
Course R is laid transversely. Course F^ forming the deck
of the caisson, is laid transversely, and the masonry is
started upon it.

An adz is used to give to the outside walls their proper
batter. They are sheathed with 4-inch plank, tongued and
grooved, the joints of which are filled with either hot coal
tar or pitch. The sheathing affords a smooth outside sur-
face, which greatly reduces the friction of the earth against
the sides of the caisson. The timbers forming the inside
walls and ceiling of the caisson chamber are first thoroughly
calked and then covered with a layer of l-inch hemlock or
spruce. This surface is then covered with tarred paper and
a second layer of 1^-inch matched spruce boards, with leaded
joints. L and M are supply shafts L for admitting men
and tools, and M for removing excavated materials. N is
a shaft for admitting concrete for sealing. The small shaft
shown between L and N is an air pipe for conveying air
from the compressor to the caisson chamber. The pipes P,
Q, and R are sand pipes, by means of which sand and other
fine material encountered in sinking may be forced out ot
the chamber by compressed air.

The air lock connecting with shaft L is shown in plan at
5, and in elevation at T. It is fitted with exterior flanges,
which fit the flanges of the sections of the shaft L.


Ordinarily the air lock Tis used. When, however, the masonry
has reached the height of the air lock T, the air lock b at
the foot of shaft L is closed. The air lock T is then re-
moved, another section of shafting added, and the lock again
placed in position. The air lock b is then opened and the
door fastened to the caisson ceiling. The air lock for shaft
Mis placed within the caisson chamber at X. This shaft is
used in hoisting excavated material, which is placed in buck-
ets and raised by a windlass placed on the top of the masonry.
The buckets are filled and placed in the lock. Connection
with the caisson chamber is then cut off, and the bucket
hoisted to the surface.

The caisson is usually built near the shore, and when
completed it is floated to the pier site, where it is held in
position by strong hawsers fastened to cluster piles. The
masonry is then started on the caisson deck, and the pres-
sure of the air increased as the weight of the masonry causes
the caisson to sink. As the caisson approaches the bed of
the stream, it must be accurately located, so that when
grounded it will take the exact position prescribed for it in
the plan. Though of great weight, so long as the caisson
floats, its position may be readily changed, but, once
grounded, only a slight change of position is possible.

1 568. Sinking the Caisson. Once grounded in the
proper position, the sinking of the caisson should be pros-
ecuted with vigor. Since all excavated material must pass
through an air lock, the process of hoisting it to the surface
is necessarily slow.

After the enclosed area has been excavated to a depth of
from 12 to 18 inches, the cutting edge of the caisson is un-
dermined to an equal depth. The air pressure is then re-
laxed, and the weight of the caisson, together with its load
of masonry, causes it to sink until it again rests on a firm
footing. When the excavated material is sand, it is usually
removed from the chamber by the sand pipe. To effect this
a piece of flexible hose is attached at one end to the air pipe
near the ceiling. The other end is fitted with a shear valve.


The sand is shoveled into piles, and the hose brought into
direct contact with it. The valve is then opened, and the
air pressure forces the sand through the hose and air pipe
to the surface, where another piece of hose is attached, which
carries the sand outside the masonry. When rock is en-
countered it is broken by blasting, and removed in buckets
through the shaft. The* rock encountered in sinking the
caisson of the Washington bridge at New York was drilled
with an air drill, the compressed air being furnished by the
same planjt which supplied compressed air to the caisson.
Dynamite was used to break the rock. The caisson was
lighted by electricity generated by a small dynamo stationed
in the compressor house.

When the caisson is situated at a distance from the shore,
the compressor plant is placed on a boat securely anchored
at a short distance from the caisson.

1569. To Determine the Air Pressure in the
Caisson. The air pressure in a caisson must be sufficient
to resist two external forces the one due to the atmospheric
pressure and the other due to the pressure of the water.
The atmospheric pressure is taken at 15 pounds per square
inch. The pressure of. the water in pounds per square inch
is found by multiplying the depth by .434. The sum of the
two pressures will be the amount of the air pressure which
must be maintained in the caisson in order to exclude the

EXAMPLE. At a depth of 50 feet, what will be the working pressure
in a caisson ?

SOLUTION. .434x50 = 21.7; 21.7 + 15 = 36.7 Ib. Ans.

1570. Sealing the Caisson. When the caisson
reaches a secure foundation the process of sealing at once
follows. This process consists in filling the entire chamber
with concrete. The concrete is mixed on top of the pier and
conveyed to the caisson chamber through the concrete shaft.
This shaft or pipe is from 12 to 18 inches in diameter and
fitted at both top and bottom with an air-tight door. WhiJe
charging the pipe with concrete the bottom door is closed.


When the pipe is full the surface door is shut, the bottom
door opened, and the contents of the pipe is discharged with-
in the chamber. The concrete is then carried in wheelbar-
rows to the extremities of the chamber, which are first filled,
the concrete being forced into every cavity. The chamber
is completely filled from floor to ceiling, the space about the
concrete pipe and shaft being left until the last. When the
space has become too small to work in, the workmen leave
the chamber, and the remaining space is readily filled with
material from the top of the shaft.


1571. Pile Driving. There is no subject connected
with construction upon which there is so little accurate
knowledge. This is partly accounted for by the fact that
the material into which piles are driven lies below the sur-
face of the ground, and exact knowledge of it is difficult to

Nor will a knowledge of the material into which the piles
are driven enable the engineer to accurately measure the
forces which give to the pile its bearing power. .

The bearing power of a pile depends upon two things, viz. :
first, the strength of the pile considered as a column, and,
second, the friction of the ground against the sides of the pile.

1572. Pile-Driving Formulas. A number of for-
mulas for guiding engineers in pile work have been prepared
by eminent engineers. Most of these formulas are more or
less complicated. Some employ values which are difficult to
obtain and are not suited to practical constructors. The fol-
lowing formula, published by the " Engineering News," and
known as the Engineering News' formula, is very simple,
and can be safely followed under all circumstances:

in which L safe load in tons, pounds, or other units; iv =
weight of hammer in same unit; h = fall of hammer in feet;
5 = penetration of pile in inches at the last blow, and as-


sumed to be sensible at an approximately uniform rate
(and head of pile in good condition, i. e. , not split or broomed).
This formula gives a factor of safety of 6, i. e., the actual
load which the pile can safely carry is only of its total
bearing power, and is applicable to all forms of railroad con-
struction from an ordinary trestle to a drawbridge pier or
turntable foundation.

1 573. Methods of Driving. There are six methods
of driving piles.

First Method. Ordinary method, in which a hammer
weighing from 2,000 to 3,000 pounds or more is dropped
from a height of from 20 to 30 feet, falling free upon the
head of the pile. Intervals between blows, from 5 to 20

Second Met hod. The same as first, except that the ham-
mer is attached to rope which is slacked on the winding
drum, allowing the hammer to fall. This method permits
more rapid blows than the first method, but there is a loss of
from 20 to 40 per cent, of the force of the blow, caused by
the friction of rope on the drum and the hoisting sheave. It
also admits of deliberate deception on the part of the con-
tractor, who can check the fall of the hammer by the friction
brake, delivering blows of not half the force which the
amount of fall would indicate. This method is, however,
very convenient and fair if properly used.

Third Method. By \Vater Jet. In this a stream of
water under pressure is ejected at or near the point of the
pile, the water rising along the sides of the pile and remov-
ing nearly all the end and side resistance, so that the pile
sinks by its own weight, though sometimes extra pressure is
added. This method is specially adapted to compact sandy
soils, and is often efficacious where all other methods fail.

Fourth Method. By Direct Pressure of a Constant
Weight. This method is applicable to soils of a wet silty
nature (practically saturated with water).

This method is much employed in dock building at and in
the neighborhood of New York. The method is sometimes


known as pulling down piles. In the mud of the Hudson
river, it is almost impossible to drive a pile by ordinary
methods, and the process of pulling is employed by placing
part of the weight of a scow as an insistent weight upon the
pile, which sinks it into the mud.

Fifth Method. By \asmytli or Other Steam Pile

Drivers. In this the hammer weighs from 3,000 to 5,000
pounds. The fall is short, usually about 3 feet, but the blows
are correspondingly rapid, usually about 60 per minute.
Otherwise the principle is the same as Method 1.

Sixth Method. By Gunpo wder Pile Driver, In

this each blow is a double one, the first caused by the fall of
the hammer, and the second by the explosion of the powder
on the head of the pile, which in turn throws the hammer
upwards. By this method, there is scarcely any intermission
in the downward movement of the pile.

1574. The Striking Force of the Hammer. In

calculating the striking force of the hammer, the resistance
of the air and friction is not regarded. The leaders, i. e.,
the upright timbers between which the hammer works, are
supposed to be vertical, and the hammer, held, in place by
well lubricated guides, falls about as freely as though uncon-
fined. Thus, a 3,000-pound hammer falling a height of 20
feet will strike a blow of 3,000 X 20 = 60,000 ft.-lb.

1575. Interval of Time Between Blows. Blows
should be delivered at as nearly uniform intervals as possible,
and the driving continued until the pile is completely driven.
The effect of an interval of rest of even a few minutes is to
permit the ground to settle about the pile, thereby greatly
increasing its resistance to driving. This effect is most
marked in fine, soft, and wet earth, and least in coarse gravel
and sand. When driving in soft, wet soils, the penetration
from last blow should not be taken for value of S, but after
allowing an interval of rest, depending upon the action of
the material upon the piles, the mean penetration from
several blows should be taken.



1576. Effects of Broomed Heads. According to
best authorities, a broomed head will destroy from half to
three-quarters of the effect of a blow, even where the broom-
ing is not more than half-inch deep. To apply a formula,
it will be necessary to adz or saw off the head of the
pile so as to secure the full force of the hammer. Apply the
formula to several cases, the average result of which may be
depended upon.

1577. Effect of Driving with Hammer Attached

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