William Macfarland Patton.

A treatise on civil engineering online

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the cylinder proper sank below the water. Within the masor
shell the inner-lining cylinder was built up, which acted as a coff<
dam and protected the masonry backing. When the crib fina!
rested on its proper foundation-bed, the dredging-tube was fill
with Portland cement concrete, deposited under water, up to
point about 50 feet below the top of the pier. The water ^
then pumped out. Above this Louisville cement concrete "w
used to within 5 feet of the coping; on which was placed Po:
land cement concrete.

The pier consisted of a facing of sandstone masonry filled
with concrete backing. The body of the pier is 38 feet in diai
eter ; two courses of coping with projections gave a diameter of
feet on top.

To a depth of 50 feet below low water the material pass
through was fine river sand mixed with clay. A number of hea
logs were encountered, the removal of which greatly interfered wi
and retarded the sinking. Below this depth the material was cle^
coarse sand and occasional bowlders. This material continued tc
depth of about 6 feet above the rock, where large bowlders we
encountered. At this level the sinking was stopped and filling t:
dredge-chamber commenced, the structure resting on this layer
bowlders at a depth of 115.9 feet below low water, the depth to sol



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CONSTRUCTION.



511




Fio. 195. (c) \

Fio. io«.(d)



Cl|»imb«B



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FOUNDATIONS FOR TOWER BRIDGE.



£ being about 120 feet below low water. The rate of einkin
from li to 2 feet in 24 hours. The height of the masonry pie
only 18^ feet. There was required 5792 cubic yards of coi
e, and 376 cubic yards of stone masonry.
The time occupied in the sinking covered a period of aboi
nonths, including delays and interruptions from logs, floods, an
severity of the winter. The final completion required nearly
ith more.

The water-jets facilitated both the sinking and keeping th
son level and in position.

En Pig. 195(a) is shown half plan and horizontal projection (
>, and in Fig. 195(6) half elevation and half vertical section. I
, 195(c) is shown a perspective view of top and bottom of tt
nder, cutting edge, and inside dredging-cylinder, and in Fij
{d) a vertical section through completed cylinder drawn to
3h smaller scale.

THE TOWER BRIDGE, LONDON, ENGLAND.

i2db. There are many novel and interesting features in the d(
\ and construction of the Tower Bridge, across the River Thame
i location necessitated either an open span or one so high abo^

water as not to obstruct navigation; but owing to the very lo
ks on both sides of the river the necessary cost of approach!
3tically precluded the use of a high-level bridge, independent!
ts great inconvenience for the trafl&c. A general design an
ation of the bridge is shown in Fig. 377. The bridge consisi
hree spans — two shore spans each 270 feet long, and a chanm
Q 200 feet long; above which, and at the proper height to gi^
icient clearance, a fixed footway is constructed between tl
n towers. The main river towers and smaller shore towers ai
ie of steel columns covered with masonry, and in addition mi
ry abutments and anchorages.

Foundation-beds. — The material underlying the bed of tl
sr is the well-known London clay. Experiments were mac
ietermine the bearing power of the material. A trial cylii
, sunk into the clay, began to settle under a weight of 6
3 per square foot. Allowing for skin friction and buoyanc;
mated on the usual data, the actual pressure on the foundatioi

does not exceed 3000 pounds per square foot. These were n<
en into consideration, and the dimensions of the piers and caii



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CONSTRUCTION. 513



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514 TOWER BRIDGE, LONDON.

sons were made such that the entire weight of structure and loac
should not be more than 4 tons per square foot of base. Th
Memphis Bridge has a total of 5 tons per square foot on a compac
clay resembling the London clay, and about 1 J tons after deduct
ing friction and buoyancy. As the middle span was a bascule span
operated by powerful machinery, it was important to preclude pos
sibility of any settlement. This construction required both lonj
and thick piers for the river towers, independent of the dimensioni
required to reduce the unit pressure on the base. Owing to th(
necessity of a number of openings or hollow chambers, required fo]
the free movement of the counterpoise arms, and the proper work
ing of machinery, gearing, etc., the main river piers were some
what irregular in section, the general length of the pier propei
being about 185 feet and width 70 feet, of the caisson proper
90 feet wide and 195 feet long, and extreme bottom dimensioni
100 feet wide and 204 feet long. Sections and plans are showi
in Fig. 195 (rt), (6), and (c). There is no record of a single
caisson of as great dimensions as those given above; the caisson
of the East River Bridge were very large, the largest havin|
bottom dimensions of 172 x 102 feet. A single caisson could hav(
been used, but, for whatever reason, it was determined in this cas
to use a number of small steel caissons or cribs, and to sink then
by dredging the material from the inside. Twelve small caisson
were used; four of these 28 X 28 feet square were used on eacl
longitudinal face, and at each end two of a triangular section
These were sunk in juxtaposition around the outline of the pien
enclosing a central space. Plans of the square and triangula
caissons are shown in Fig. 195 (e) and (/); also a cross-sectioi
through the structure, showing one caisson sunk to its full depth
and another in process of sinking, with the outside and insid
staging used in sinking the caissons, is shown in Fig. 195(^)
The positions of these caissons are shown in Fig. 195(^). A par
elevation and vertical section of one of the square caissons i
shown in Fig. 195(A). Also, details are shown in figures (t), [k]
(1), and {m).

The caissons were built up, as the sinking progressed, of plate
and angle-irons. The lowest section, 19 feet in height, constitute<
the caisson proper, and was to be left permanently at the bottom c
the structure. The remaining portion above was simply intende
for a temporary coffer-dam, and was subsequently removed; othei



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coNSTRUcnoNV ^ or y 515



wise the general construction is the same as in any iron and steel
caisson or crib.

The small caissons were spaced with clear intervals between
them of 2i feet. This is indicated in Fig. 195 (e) and (/). On the
adjacent faces of any two caissons angle-irons were riveted, as
shown. These formed grooves or channels into which piles were
subsequently driven, thereby making a tight joint between two ad-
jacent caissons, in order to permit of the removal of the adjacent
faces so as to secure a monolithic structure throughout: The de-
tails of these angle-iron grooves and closing piles are shown in



Fig. 195. (n)

Pig. 195(i). This portion of the construction will be again referred
to under Sinking and Excavating. In order to control and regu-
late the sinking of the several caissons, a system of stagings were
erected around, and also a central staging in the space to be en-
closed by caissons. This staging is shown in plan in Fig. 195(;0,
and in section in Fig. 195(.^). The lower sections of the caissons
were built on platforms placed a little distance above low water.
Two pairs of trussed girders were then placed on top of the staging,
and above the section of the caisson. Rods made of links about h
feet long, and joined with pins, were used to suspend the- caisson
from the girders. These rods were 2^ inches in diameter. The



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CONSTRUCTION. 517



work to 16 inches, and when the water was permanently excluded
the rate of progress was as much as 3} feet per day.

Concreting, — After sealing up around and under the shoe the
riyets holding the side facing the enclosed central portion to the
ends of the caisson were cut, and to prevent the adhesion of the
mortar to that side boards were placed against it. In addition
boxes were placed against this side, and the concreting was com-
menced and carried up to within a few inches of the fifth or top
frame of the permanent casing or caisson. The boards and boxes
were built up as the concreting progressed. The purpose of the
boxes was to leave recesses or dovetails, so that when the inner side
of tbe caisson was removed and the enclosed central space filled
with concrete it would tooth or bond into the concrete already
placed in the caissons proper.

The lower section or permanent portion of the caisson was
19 feet high. When the concrete reached the third frame of this
portion, boards and boxes were placed against those sides of the
caisson facing the adjoining one, and for the same purposes,
as it was intended to remove three sides of the caissons only,
leaving the outside shell of iron, it was important that the
many distinct columns of concrete should be toothed or bonded
into each other. Thus each of the twelve caissons was sunk and
filled with concrete to the top of the permanent caisson, giving as
many isolated columns of concrete around the outer face of the
structure. These columns were separated by the iron sides of the
caisson and 2| feet of clear and open space between them. Before
these adjacent sides could be removed it was necessary to make a
water-tight connection between the caissons. It was to accomplish
this that the grooves were formed by angle-irons on the outside
edges of the caisson. Before driving piles in these grooves the
caissons were well tied together in order to prevent the wedging
action of the piles from spreading them apart, and they were
properly braced as shown in Pig. 195(m), in order to prevent any
distortion. Piles were then driven in the grooves, as shown in sec-
tion in Pig. 195(i). This, then, gave water-tight compartments of
about 28 X 2i feet between the caissons. The water was pumped out
and the material dredged out by hand, two men working at a time,
the material being lifted out in buckets. When the proper depth
was reached concrete was placed in these spaces up to the top of
the third frame of the permanent caisson. The iron sides of the
caisson were then removed, and concrete filling connecting and



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CONSTRUCTION. 619



timber and concrete or of iron and concrete. For a depth of only
40 feet below low water the method of sinking, whether by dredg-
ing in an open crib or by compressed air, wonld not be a matter of
much moment, other than from considerations of economy; but
probably the pneumatic process would have been adopted.

The aggregate area of the bottom of the tower caissons is about
33,000 square feet, and depth sunk below low-water surface 40.G
feet.

The aggregate area of the bottom of the five caissons for the
Memphis Bridge is about 67,000 square feet, and average depth
sunk below low water 70 feet. These were sunk in two years and
five months through 40 to 50 feet of sand and clay.

The large caisson of the Washington Bridge, New York, 104.8
X 54.4 feet, was sunk 40.6 feet in six months through sand, gravel,
and rock, and 10,400 cubic yards of masonry completed in nine
months.

In both of the above cases the pneumatic caisson was used.
The large timber crib of the Poughkeepsie Bridge, 100 x 60
feet and 104 feet high, was sunk by open dredging through 53 feet
of water and 82 feet of mud and mixed sand and clay in about
three months, and the caisson sunk onto this crib and the masonry
completed to a point 149 feet above the cutting edge of the crib,
ready for the steel towers, in about three months more. (See Eng,
News, Jan. 18, 1894.)

Greater depths have been reached by this open-crib process than
by any other. The great difficulty lies in the danger of meeting
obstacles, such as wrecks of old ships, barges, logs, etc., which re-
tard the progress and may result in entirely stopping the work.
The more serious objection is the necessity of depositing the con-
crete under water. Whether this is done through iron or timber
tubes, or in specially designed buckets which only open when the
bottom is reached, the cement will inevitably be separated to a
greater or less extent from the sand and broken stone. The result-
ing product is at best uncertain. The only remedy known to the
author is to allow the cement to take an initial set before lowering
it through the water. It has, however, been used to a great extent
when the depth below the water surface is over 100 feet, as this
is considered the limit under the pneumatic process, next to be
described.

The splaying or spreading out at the bottom has always been
considered necessary in sinking such cribs or caissons. Mr. Ander-



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CONSTRUCTIOX. 621



be a simple section of the main shaft, and to keep it always at the
top.

481. The necessary air-pressnre varies with the depth of water,
or rather the depth of the cutting edge of the caisson below the
water surface. A pressure of one atmosphere or 15 pounds — more
accurately, 14.7 pounds — to the square inch will support a column
of water in vacuo 34 feet in height, or about 1 pound pressure
for every 2^ feet = 27 inches. If, then, we wish to support a col-
umnr of water 68 feet in height but not in vacuo, it would be neces-
sary to provide a pressure at bottom of 45 pounds, — 15 pounds to
balance the air-pressure on top and 30 pounds to balance the
weight of the column of water.

432. Only using the double walls, omitting the tubes and cover-
ing the enclosed space between the walls with an iron roof com-
posed of strong eye-beams placed transversely at intervals of 4 or 5
feet, to the under side of which a full layer of ^-inch plate iron is
riveted. Figs. 190-194 will represent the construotion of a typical
iron caisson. The roof would be usually built at the top of the
lower wedge-shaped section, along AB, Fig. 190, the height above
the bottom being usually from 7 to 9 feet. The iron walls above
this roof may or may not be used. The writer believes it is always
advisable to use them. Practice on this point differs. The con-
struction of iron caissons will not be further discussed.

433. The following drawings. Figs. 196, 197, show the construc-
tion of two typical timber caissons, with air-locks, pipes, shafts,
cribs, and coffer-dams used in connection with them.

The vacant enclosed space A is the working chamber; BB are
the walls of the working chamber; open or solid timber work rest-
ing on BB is called the roof or deck of the caisson, as shown in
CCDD, The timber and concrete work above the roof is not an
essential part of the caisson. The masonry can be commenced
directly on top of the roof or deck; this requires the sinking of the
caisson to be regulated by the rapidity with which the masonry can
be built, which may interfere with the progress of the work. In
case of accident the entire masonry work might be submerged,
causing both trouble and expense in removing and keeping the
water out so as to resume the work. The piers of two of the most
important structures in this country, namely, the East River sus-
pension bridge and the St. Louis steel-arch bridge over the Missis-
sippi River, were constructed in this manner.

434. The cribs, as shown in the drawings above the roof, are



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623 TYPES OF PNEUMATIC CAISSONS.



is



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C0N8TBUCTI0N. 52S



built of timber, and the spaces between are filled with concrete.
Whether the cribs shall be constructed with solid outside walls, and
solid partition walls built as shown in the drawings, the partitions
being carried up solid for five or more courses, then shifted in posi-
tion, alternating in number 2 and 3 or 3 and 4 according to the size of
the crib as shown in Figs. 197-199, by which the concrete is formed
into one practically solid and homogeneous mass; or whether the
crib shall be open-work timbers simply crossing each other in courses
at right angles, thereby dividing the concrete into a series of small
columns but poorly connected by horizontal layers of 12 inches in
thickness, in addition to the necessarily great difficulty of filling
properly under and around so many timbers, and the further danger
of the concrete being exposed to moving water before time has
elapsed for the setting of the mortar, as seen in Figs. 196, 198 — is
simply a difference of opinion and practice. Large and important
structures have been built on both designs. And whether the walls
of the working-chamber shall be constructed as in the drawings,
the same remarks apply.

In either case the timber and concrete cribs are more economical,
can be built more rapidly, and hence their tops can be kept more
certainly above the water than when the masonry is constructed
direct on the caisson.

The caissons and cribs shown in Figs. 196-198 are from the
designs of Mr. Geo. S. Morison, one of the most prominent engi-
neers and bridge-builders in this country, and have, consequently,
the sanction of a high authority. They have been used in many
important bridges. The one illustrated was used in the Cairo
Bridge across the Ohio River, near its junction with the Mississippi
River. Fig. 196 is part longitudinal and vertical section. The
drawing shows about two thirds of the full length. The bulk of
the timber used is 12 X 12 inches in cross-section. The sloping
walls of the working-chamber are formed of 17 X 17'inch timbers.
All parts of the structure are thoroughly bolted with screw and
drift bolts, and well braced. These caissons were shod with iron
plates f inch in thickness and 36 inches high, to protect the cutting
edge.

The greatest immersion in water was 90.27 feet, and penetration
in sand 86.42 feet. The masonry was started 10 feet below the bed
of the river, and on top of the crib.

The two designs are shown side by side. Only a part of each is
shown, about two thirds. In order to show air-locks, shafts, and pipes



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524 TYPES OF CBIB8 AND COFFER-DAMS.

in place. The omitted portion is identical in construction with
the parts shown.

486. The caissons and cribs shown in Figs. 197-199 are from the
designs of the author, and have been used in many important foun-
dations, notably those of the Susquehanna River Bridge, B. & 0.
Ry., at Havre de Grace. Fig. 197 shows part longitudinal and
vertical section and part elevation through coffer-dam, crib, and

UoRizoNTAX. Section of Coffeb-dam.
Fig. 200.



OF BRACEd.ANO OOWCRCTE IN PLACE. FROM. FlO. 197. BRACE* A»D COHCHfJt, FROM Ftt-W?.

Fig. 199. Fig. 19a

caisson; Fig. 199 shows horizontal projection of crib; and Pig. 200
horizontal projection of coflfer-dam, showing one set of the system
of cross-bracing adopted.

All timbers, excepting the planks 3 x 12 inches, are 12 X 12
inches in cross-section for crib and coffer-dam. All timbers in
caisson were also 12 x 12 inches, except the outside vertical pieces,
which were 12 x 14 inches in cross-section, and the lining plank on
the inside of the air or working-chamber. In some of the larger
caissons the cross-braces were 16 X 16 inches square. The whole
was thoroughly bolted with screw and drift bolts. The drift-bolts



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C0K8TBUCTI0N. 525



in the solid timber roof were 1 inch square or round and 22 inches
long, spaced 5 feet intervals over each course.

In the largest caisson the deck was composed of eight courses of
timber, alternating in direction longitudinally, transversely, and
diagonally. Each course was bedded on a layer of cement mortar,
and spaced from i to ^ inch intervals, which was filled with grout
before laying the course above it. The joints between the planks
forming the interior lining of the roof and walls of the working
chamber were thoroughly calked with oakum. Oakum was also
wrapped around the ends of all bolts reaching into the interior
against which the nut and washer were pressed hard. The joints
on the outside of the caisson were calked, but not so compactly.
The walls of the crib were also calked sufficiently to make them
▼ater-tight. This was done that the concrete in the crib might
always be ladd in the dry. Outside vertical plank was spiked to the
crib, mainly to hold the oakum in place; it also reduced somewhat
the frictional resistance of the material below the bed of the river.

436. It was originally intended that the top of the crib should
only reach to the bed of the stream. Owing, however, to the great
inclination of the bed-rock, and the time and expense required to
blast it to a level or horizontal surface, it was decided to stop sink-
ing the caisson when it rested on or near the higher points of the
rock; therefore the top of the cribs reached from 5 to 13 feet above
the bed, and were from 5 to 35 feet below the water surface. The
masonry was commenced a little before the top of the crib sank be-
low the water surface. No coffer-dam was used on the first crib, the
masonry being built up as the* caisson sank. The risks and delays
caused by this method determined the use of coffer-dams on all
other caissons.

437. The coffer-dams were built with caps, «ills, and vertical
posts framed together, and resting on top of the crib. These frames
were sheeted with two courses of 3-inch plank, the first placed di-
agonally, the outer course horizontally, as shown in section and
elevation in Fig. 197, and in plan Fig. 200. Crosspieces were
placed over the tops and held down to the cribs by long iron rods,
having hooks at the lower ends catching hold of eye-bolts fastened
to the cribs. When these bolts were unhooked the sides and ends
of the dam could be pulled apart and removed. This was done
after the caissons rested on rock, and the masonry was built well
above the water. Fig. 200 shows plan of coffer-dam and system of
interior bracing adopted.



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526 AIB-LOOKS, PIPES AND SHAFTS.

A longitudinal truss (see Fig. 197) was used to stiffen the roof.
The design of the cutting edge provided broad surfaces for bear^
ing on the material, thereby supporting the caisson when desired,
and enabling the sinking to be regulated. The truss could be used
for the same purpose. At the same time the men had easy access to
the cutting edge. The design of the walls was very strong and
well connected, and supported by the deck of the caisson. No iron
shoe or plates were used to protect the cutting edge, experience
amply proving that they are not necessary. It may, however, be
wise to use them.

438. Air-locks are used for the passage of men and material,
either from the outside to inside of the caisson, or the reverse,
without the withdrawal or escape of more than one lockful of
compressed air for each such passage. They may be single or
double. Double air-locks were used in the Cairo Bridge and others



Fig. 201.



constructed by Mr. Morison, and Fig. 201 shows the single air-
lock used by the writer in other caissons described. A description
of the working of the single air-lock will render the understanding
of the other easy by a simple examination of the drawing.



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CONSTRUCTION. 627



439. ABaaming that the air-chamber and shaft contain com-
tain compressed air, and that the lower door is closed and held
against its bearings by the compressed air, the upper door of the
lock is in this case open. The men enter the air-lock from above
and close the upper door, opening at the same time a valve placed
in the lower door, or the one of a pair in the double lock, and the
compressed air rushes into the lock; its pressure now holds the
upper door shut. The air will continue to flow into the lock
until the pressure is the same as in the air-chamber. The lower
door now opens readily, as the pressure is the same on both sides.
The men then descend into the air-chamber. In coming out
they pass into the air-lock through the lower and open door,
closing this behind them; then, opening a valve in the upper door,
the compressed air rapidly escapes from the air-lock, and continues
to do so until it is reduced to the atmospheric pressure on the out-
side when the upper door opens and the men pass out.

In the double lock *the doors open sideways or horizontally in-
stead of vertically, as above described for the single lock. The prin-
ciple is the same: one of each pair of doors is shut when the otlier
is open. Two independent locks are thus formed, each opening at



Online LibraryWilliam Macfarland PattonA treatise on civil engineering → online text (page 44 of 145)