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tion of about 6 hours, it had reached about 5 ft., the average during
the rise of the tide thus being 3 ft. The clear width of each sluiceway
was 7 ft. 8 in. To fill the dam to its capacity of about 8 000 000 cu.
ft., S hours were required. Practically no movement of the dam was
caused by the rewatering.

Removal of the Coffer-Dam.

Preliminary to pulling the sheet-piling of the dam, the fill in the
pockets and in the outshore embankment was removed to a depth of
about 10 ft. below low water, in order to relieve some of the pressure,
and jet pipes, operated by compressed air, were forced down along the
piling nearly their full depth, in order to break the hold of the mud
on the piles.

The plant for pulling the sheet-piling comprised a 25-ton and a
40-ton steam lighter for the main iwrtion of the dam, and stiff-leg
derricks for the returns extending inshore at the upper and lower ends
of the dam, equipped with inverted McKiernan-Terry steam hammers
operated by compressed air. The hammers were attached to the piles
with 3, 3J, and 4-in. pins, inserted in a single hole burned in each pile
about 2 ft. below the top. Various types of grips were also used, but
the most successful was a relatively light one furnished by the
Lackawanna Steel Company, which was rigged so that it could be
opened by pulling a trailing rope when the pile had been pulled out of
reach from the ground. This grip, which catches the pile at the top
of the web, has been used continuously by one lighter for all but the
hardest pulling.

The pulling was started where the piles were from 65 to 70 ft. long,
and considerable difficulty was experienced in taking out the first pile,
about a week being consumed in trying various piles before one was
extracted from the diaphragm of Pocket 28. The pulling of the dia-
phragms and outshore panels of Pockets 18 to 29 was then accom-
plished by driving them up with the hammer.

It was foxmd impossible, however, to start any of the piles next to
the rip-rap embankment by these means. A 100-ton lighter was



540 COFFER-DAM FOR 1 000-FOOT PIER

secured with the intention of using it, with an inverted hammer, to
break one of the inside arcs. After working 4 days and trying several
pockets, this rig, being unable to budge any of these inside piles, was
dispensed with. A heavy, guyed, A-frame was then erected on top of
the rip-rap embankment and rigged with a set of eight-sheave blocks,
with sixteen parts of I -in. line, through which the pull of the 25-ton
derrick was transmitted to the pile. A 64-ft. pile was thus drawn by
direct pull without the aid of the hammer. The line of piles adjacent
to the rip-rap having thus been broken, no further difficulty was
experienced in this direction, and the remaining piles were drawn by
the derrick aided by the steam hammer.

As a rule, all the piles came out straight and in a condition satis-
factory for use in other work.

After the removal of the sheet-piling, the filling, outshore earth
embankment, and inshore rip-rap embankment will be removed by
dredging.

Cost of Work.

Dredging. — The dredging was executed under two separate con-
tracts, first for the original contract, and then over the area of the
supplementary contract. Under the first contract, 271 882 cu. yd. were
removed, at a cost of 23^ cents per cu. yd. Under the supplementary
contract, 87 875 cu. yd. were removed, at a cost of 27 cents per cu. yd.,
making a total, for the removal of material by dredging down to solid
rock, of $87 618.52.

Coffer-dam, RocJc Excavation, Walls, etc. — After due advertise-
ment of the specifications and plans, based on an engineer's estimate
of $497 000, for the coffer-dam and walls of the pier, the Department
obtained thirteen bids, ranging from $750 000 to $487 000, the low
bid, which was awarded to Holbrook, Cabot and Eollins, Incorporated.

In order that the work might be extended southward to West 44th
Street, to comprise an area which it was not possible to include at
the time of the original preparation of the contract, a supplementary
contract was entered into with the same contractors at the same unit
prices, after receiving permission from the Board of Estimate and
Apportionment and the Board of Aldermen to extend the work in
this manner.

It was clearly demonstrated at that time that a saving of fully
$150 000 would be made by doing the work in connection with the



COFFER-DAM FOR 1 000-FOOT PIER 541

original coffer-dam, thereby avoiding the cost of building a new coffer-
dam at the south end of the work.

The total of the original and supplementary contracts for the work
of building the coffer-dam, excavating the rock, and constructing the
concrete walls for the inner part of the pier building, was $626 000.

The unit price for the removal of solid rock under the contract
was only $1.60 per cu. yd., but assuming that one-half the cost of the
coffer-dam should be charged against rock excavation and the other
half against the construction of the concrete walls and other items
within the coffer-dam, the actual cost for the removal of this solid
rock, was about $5.31 per cu. yd.

Conclusion.

First. — Instead of excavating this large quantity of subaqueous
rock by the "dry" method, the work might have been performed with-
out a coffer-dam by the ordinary method of rock blasting imder water,
or, in other words, the excavation of the rock "wet".

If it were possible to perform this work so as to obtain under
water vertical face walls to the exact limits of the slip, it would have
cost, under ordinary normal prices for this class of work, four or
five times as much as by the method adopted.

It is not possible to conceive, however, that the work could have
been done under water, for, as a matter of fact, it was very difficult
to do it with line drilling in the open, and further, even were it
possible to deposit concrete in a satisfactory manner at such depths,
part of the work in addition would have been very expensive.

The particular result obtained by the construction of this dam
was that, although all preliminary examinations, borings, soundings,
etc., were made when the whole territory was covered by filling of
all kinds, including, in places, cribwork, coal yards covered with coal,
buildings, etc., the total amount paid under the contract obtained
through unit prices has been kept close to the original estimates.

Second. — The whole river face of this dam, for a length of about
800 ft., under its full water head, was absolutely dry, all the pumping,
during the whole construction period having been done by one 12-in.
centrifugal pump. Furthermore, all this pumping would have been
avoided, although much more was expected, had it not been for the
presence of cribwork at the north side, and on account of the impossi-



542 COFFEE-DAM FOR 1 000-FOOT PIER

bility of driving two piles in the large cylinders at the south end,
because, in driving, a boulder or some other interference was encoun-
tered, rendering it impossible to drive these two piles home for a
distance of only about 1 ft.

Supervision.

The work was executed by the Department of Docks and Ferries,
E. A. C. Smith, Commissioner of Docks, and E. C. Harrison, First
Deputy Commissioner.

The design, preparation of plans and specifications, and the con-
struction was under the direction of the writer, as Chief Engineer,
assisted by E. T. Betts, M. Am. Soc. C. E., Deputy Chief Engineer;
Mr. T. F. Keller and Elias Cahn, Assoc. M. Am. Soc. C. E., Assistant
Engineers. The work in the field was under the direction of J. J.
Pemoff, M. Am. Soc. C. E., Assistant Engineer.

The contracting firm for the work was Holbrook, Cabot and
Eollins, Incorporated.

Acknowledgment is made by the writer of the skill and persever-
ance of the contractors, particularly of T. B. Bryson, Assoc. M. Am.
Soc. C. E., on the work. The writer's thanks are also due to Brig.-
Gen. William M. Black, M. Am. Soc. C. E., Chief of Engineers,
U. S. A., for advice in the preparation of the plans, and the support
of his assurance that this work could be accomplished successfully
provided proper construction methods were used.



DISCUSSION OX COFFER-DAM FOR 1 000-FOOT PIER 543

DISCUSSION



Frederic E. Harris,* M. Am. Soc. C. E. (by letter).— The author Mr.
has presented his subject so clearly and fully that extended comment ^^"'s-
is unnecessary.

On first reviewing the paper one is impressed particularly with
the large sum of money involved for the construction of only 220 ft.
of pier and the excavation of areas adjacent to the pier sufficient for
three slips. However, when it is considered that it was necessary to
limit the projection of the pier outshore, it will be realized that the
sum expended for the inshore construction was not exorbitant. It
must also be taken into account that the greater part of the steel
sheet-piling was salvaged and was available, therefore, for use on
other work.

In coffer-dam construction, as in other constructions of this class,
the unknown element confronts the engineer. Officers of the Corps
of Civil Engineers of the Navy have been forcibly impressed with
the hazards accompanying construction of this class, as there is little
room for doubt that most of the delays and failures in naval dry
dock construction are traceable to weak or faulty coffer-dam construc-
tion. When both the design and construction of a coffer-dam are
imposed entirely on the contractor, there is a strong temptation to
save on first cost and perhaps to take chances on additional and
unnecessary risk. The contractor is seldom to blame for this condition,
as he is often forced into an untenable position by the double com-
petition involved, first, in competitive bids in which his estimated
cost of the coffer-dam which he plans is a part; and, second, in
which the saving that he may believe he can make on the coffer-dam
determines his profit, or, worse still, a decrease in his loss.

The writer has been much interested in the construction work
described, having discussed it quite frequently with Mr. Staniford,
Messrs. Holbrook and Bryson, of the Holbrook, Cabot and Rollins
Company, and the Commissioner of Docks and Ferries, Mr. R. A. C.
Smith, and has been influenced largely by Mr. Staniford's able and
conservative engineering plans, as shown by this work, to depart from
the precedent heretofore established in the plans and specifications
for naval dry docks. This departure, although it has not taken the
exact form of designing the coffer-dam required for the work, has,
nevertheless, laid down and specified a minimum coffer-dam require-
ment that would be acceptable to this extent, attempting, as Mr.
Staniford did in the work described, to minimize the risk and safe-
guard against the probability of failure, with its attending losses of
money and time.

* Washington, D. C.



544 DISCUSSION ON COFFEE-DAM FOR 1 000-FOOT PIER

Mr. The fact that the coffer-dam described by the author held and

' served most satisfactorily the purpose for which it was constructed,
is evidence of the soundness of the judgment used in its design.

The writer is particularly interested in the tests made to secure
information on the behavior of the rip-rap embankment under various
conditions of pressure. The following are the results of computations
made from the data in the paper :

In Test No. 1, assuming the head to be 3 ft. 6 in., the active pres-
sure was found to be 360 lb. ; the pressure at impending motion, 1 455
lb. ; and actual sliding on the bottom 2 800 lb., assuming the same
coefficient of friction on the bottom as that of the material itself,
and no friction on the board to which the force is applied. These
assumptions may account for the figures given being slightly larger
than those obtained in the test.

In Test No. 2, the head assumed was 2 ft. 9 in. The computed
active pressure found was 250 lb.; the computed impending motion,
970 lb. ; and for sliding, 1 460 lb., assuming a plane of rupture through
the toe and no friction on the board to which the force is applied.

In Test No. 5, the head was assumed to be IJ times the distance
of the application of the force from the top. At impending motion,
the computed force was found to be 500 lb. and the sliding on the
plane through the toe to be 680 lb.

From observations of the curves shown in Fig. 13, it is believed
that the initial movement was probably purely local, and that the
impending motion began at the initial points of the curves shown.
The agreement of the computations with the critical points of the
curves is remarkable, and partly verifies such an assumption.

Mr D. A. Watt,* M. Am. Sqc. C. E. (by letter).— It would be of

interest to know whether any piles were found, during the removal
of the pockets, which had been driven out of their interlock with
the next piles. In the writer's experience, this is by no means uncom-
mon where the driving is hard, for instance, with closing piles, although
no indication of the break may be apparent at the time. Thus, when
removing the coffer-dam for the Maine at Havana, more than one
case was found where one pile at one-quarter or one-third of its length
from the end had broken away from its neighbor. Fortunately, the
breaks were all below the level of the harbor bottom, which doubtless
was the reason the cylinders had not burst, as the surrounding mate-
rial (mud and clay) prevented the embedded piling from spreading.
The piles had been driven from 30 to 35 ft. into this material. During
the construction of the coffer-dam one of the cylinders broke open,
due to the closing pile being driven out of the interlock, but in this

* Albany, N. Y.



Watt



DISCUSSION ON COFFER-DAM FOR 1 000-FOOT PIER 545

case the weak point appeared to be above the harbor bottom, where Mr.
there was no outside material to counteract the strain.

Similar conditions were found in the later steel-pile-pocket
coffer-dam at Troy, N. Y., where the piles were driven into river gravel.
The penetration was not more than 8 or 9 ft., as the gravel was
compact; yet, when removing the piling, several cases were found
where the interlock had driven out. As with the Maine coffer-dam,
the breaks appeared to be below the surface of the bottom, and no harm
resulted.

The saturation of the filling in coffer-dams of this type, referred
to by the author as one of the sources of internal pressures, appears
to exist at a higher level than is usually assumed, although varying
naturally with the porosity of the filling. The filling used for the
Maine cylinders varied from a sandy to a heavy clay, and in all cases
the saturation line was very high. It began at about mean tide level
on the outer faces and sloped downward at about 5 horizontal to 1
vertical. The material, therefore, was dry only to an average of
about 5 ft. below mean tide; the remainder stayed in a fluid condition.
Efforts to lower the saturation level and to drain the filling with long
perforated pipes were useless; the pipes would fill up in a few hours
after being pumped out, and even near the surface these did not seem
to affect visibly the adjacent seepage. This high and constant line of
saturation was noteworthy because the total seepage into the coffer-
dam was extremely small (about J cu. ft. per sec), and there was as
much opportunity for the water to leak out of the cylinders into the
inclosure as to leak into them from the bay. In the Troy coffer-dam,
with the pockets filled with sand and gravel, the water line was natu-
rally much steeper. Judging from the height of the seepage appearing
on the inside faces, it was about 2 horizontal to 1 vertical.

The writer visited the 46th Street coffer-dam during its construc-
tion, and again after it had been in use for some months. Mr. Stam-
ford and the others responsible for the work are to be congratulated
on their success in handling a very difficult problem.

C. A. Wentworth,* M. Am. Soc. C. E. — The Society has received Mr.
a valuable contribution in this paper, describing an advanced step in ^° ""^^
coffer-dam construction to meet peculiar and difficult conditions. The
tests made in preparing for this work show a foresight which is too
often omitted. A comparatively small expenditure for borings and
tests at the outset of a difficult piece of foundation work frequently
saves much larger expenditures later. The entire responsibility of
determining the method to be used in carrying out a given design is
too often left entirely to the contractor, instead of considering the
method and final structure desired as parts of the same construction,
both of which are essential to a completed whole.

* Wayne, Pa.



546 DISCUSSION ox coffer-dam for 1 000-FOOT PIES

The relative amount of yielding in the rip-rap embankment of
the large coffer-dam confirms the results of the small-scale tests
remarkably well, and the description in the paper gives all the points
necessary for use in applying this method to similar work. The diffi-
culties encountered in the construction were surprisingly small, con-
sidering the magnitude of the work; and there is evidence of careful
preparation in laying out and carrying on the work which is a credit
to both the engineers and the contractor.

A coffer-dam of the open type, without cross-bracing, must fulfill
two functions: First, it must provide a cut-off wall sufficiently tight
to hold back the water and prevent it from opening channels, through
or under the dam, which might endanger its stability or cause a seepage
flow which could not be cared for by the pumps with reasonable expense.
This does not mean that an absolutely tight cut-off is necessary, and,
in fact, a water-tight cut-off is seldom obtained. It does mean, however,
that large leaks or seepage must be stopped.

The second function which a dam must fulfill is to provide sufficient
stability to resist the total pressure of the water. This stability
depends on two factors: The total mass, and the internal cohesion of
the mass, or its resistance to deformation. In the coffer-dam inider
discussion, the cut-off consisted of a double row of steel sheet-piles,
interlocked and driven to rock, and sealed with a clayey mud. This
cut-off was ideal as a stop for percolating water. The rip-rap furnished
— in the most compact form of any loose material — the necessary mass,
with the maximum stability attainable, short of a coherent structure.

In this case there was a coffer-dam requiring a cut-off wall 6S ft.
in height. Engineers must expect to encounter these problems more
frequently in the future, and the methods used must meet the con-
ditions. The methods applicable in any given case should be governed
by local conditions and the cost of available material. Wlaen this work
was started, 2 years ago, the cost of steel piling was low, and contractors
were seeking places for the cheap disposal of rip-rap from the subway
excavations. These two factors undoubtedly had much to do with the
type of coffer-dam selected. A third factor which defined the limits
of the work was the rock bottom, which made an unyielding support,
both for the steel cut-off wall and the rock fill. A variation of any
one of these three factors, such as a high cost for the first two or a
different character of bottom, might have made some other type
advisable. Each of these factors should be given its true weight in
considering the use of similar construction in other work.

If the same work were to be done again, with the present high
price of steel, and rip-rap being more expensive, some other means
might well be considered, such as the use of open concrete caissons
sunk by the dredging method and weighted by filling with mud, or



DISCUSSION OX COFFEE-DAM FOR 1 OOO-FOOT PIER 547

timber caissons similarly sunk and weighted. These two types would Mr.
represent coherent structures in which the minimum quantity of mate-
rial is required to resist the pressure, and their use in this or any
other case would be entirely a question of relative costs.

Another dam which should be considered is the ordinary timber
sheet-pile and earth-fill type. Such a dam would necessarily have to
be placed a greater distance from the bottom of the excavation in
order to provide room for the greater mass and flatter slopes required,
and also to provide a longer distance for the travel of seepage which
might pass a cut-ofl wall not carried to rock. The latter method would
not meet the restricted conditions at the north and south sides of
the coffer-dam under discussion, and, in fact, the general method of
construction was varied at these points to suit local conditions, large
circular caissons being used at the southwest corner, and a single line
of sheeting along part of the north side.

These alternates are pointed out in order to call attention to
methods which might well be considered in connection with a similar
problem under different local conditions.

The paper suggests some thoughts which may not be out of place
in this discussion. Harbor development in the United States is now
at a point where new methods and types of construction will be required
to meet the increased draft of ocean vessels, and to provide for a
greater permanence in the design of docks and bulkheads than has
prevailed in the past. Timber has been cheap and of excellent quality,
and has served its purpose well, for coffer-dam and dock work, for
depths of water up to about 30 ft.

Most of the docks in the United States are built of timber, usually
untreated. The life of work of this class is short, unless entirely
submerged in waters free from marine worms, but it has had the
advantage of cheapness and rapid construction, and these have been
the deciding factors in a rapidly growing and changing community.
In the teredo-infested waters of southern and Pacific ports, creosoted
timber and other forms of protection have been a necessity for wooden
piles, and, even with such protection, the increased cost and decreased
life of dock structures built of this material have made the need of
more permanent materials desirable.

For permanent docks in depths of 40 ft. or more, which are now
required for the largest ocean vessels, timber is getting beyond its
reasonable limit, and the use of concrete or masonry walls or piers,
or supporting dock structures by concrete caissons, will probably be
the next step forward to meet these new conditions of permanency
and depth. These methods are suitable, not only in waters where the
teredo is not found, but also for most conditions encovmtered where
a permanent dock is required.



548 DISCUSSION ON COFFEE-DAM FOE 1 000-FOOT PIEE

Mr. Concrete piles have been used in many places infested by the

Wentworth. ^gj.g,jQ^ \^■^^^ ^]^gy cannot be considered as permanent in waters subjected
to ice and frost action, and their permanency in tropical waters is
questionable, due to the greater chemical activity of the dissolved
alkaline salts in the warm water and the great rapidity with which
the steel reinforcement disintegrates when the necessarily thin pro-
tecting layer of concrete is broken or punctured.

No engineering work is immutable, and permanency, when applied
to harbor work, can only be considered as relative; but it is evident
that a new era has been reached in constructing works of this kind,
especially in ports like New York. It is a pleasure, therefore, to see
the substantial design of the dock walls constructed inside the coffer-
dam for the 46th Street Pier, and it is hoped that this work may be
a precedent for future construction.
Mr. Thomas H. Wiggin,* M. Am. Soc. C. E. — The author very modestly

'^^'°' calls this an unusual coffer-dam. A stronger term might very properly
have been used, as will be agreed to, the speaker thinks, by any one
who has had experience with such work. The construction itself, after
the coffer-dam was completed, and even the methods of building the
coffer-dam, were not in themselves so bold, because, though requiring
great skill and ingenuity on account of unusual dimensions, they
involved methods of pile-driving and excavation which are sufficiently
well standardized; but the conception of backing so high and so large
a coffer-dam with a mass of rip-rap of comparatively small thickness
was certainly one which required boldness, care, study, and sureness,
in order to warrant a construction, in that limited space, which did
not give opportunity for increasing the strength of the backing, in
case it should be found insufficient.

It seems to the speaker that the experiments undertaken to discover
the action such a structure might take, deserve more emphasis than



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