American Society of Civil Engineers.

Transactions of the American Society of Civil Engineers (Volume 81) online

. (page 45 of 167)
Online LibraryAmerican Society of Civil EngineersTransactions of the American Society of Civil Engineers (Volume 81) → online text (page 45 of 167)
Font size
QR-code for this ebook


the author gives them. They were, apparently, the only quantitative
proof of the design. The speaker made a few rough computations,
and would be interested if the author would include an outline of such
computations as were made by him to justify the proportions of the
design. For example, the prism of rock which supported the steel
walls on the side next the river, appears to have had a weight of about
200 000 lb. per lin. ft., and the water pressure against it appears to
have been approximately 100 000 lb. This would indicate the necessity
for a coefficient of friction of about 50% in order that the rock fill
should withstand successfully the inward pressure when the coffer-dam
was unwatered. The experiments appear to show that the coefficient
of friction at the ultimate strength of such a "granular" structure,
when subject to sliding pressure, would be considerably greater than

50%, perhaps half again as large.

• New York City.



EISCUSSIOX OX COFFER-DAM FOR 1 000-FOOT PIER 549

The margin, then, was apparently safe, but not large, and the Mr.



considerable movement of the structure indicated that just about the
right degree of conservatism was used in the proportioning. If the
structure had been a little lighter, it might have exhibited signs of
distress as the water was lowered inside, and, as there was little more
room for additional rip-rap inside, a determination of the next step
would have been a serious problem. No such distress occurred, how-
ever, and the design was justiiied, which must have been a source
of great gratification to the author and his assistants, and is reason
for admiration by the Engineering Profession.

The paper leaves little to be added, though its descriptions are
given with a minimum of words. There are a few points, however,
which might be explained in more detail. The first is the use of
wash-borings and steel-shod piles for the purpose of finding the position
of bed-rock. Under many circumstances, neither of those methods
would have been reliable. The speaker knows of areas where rock
has been located in great detail by wash-borings, but, on excavating,
no rock was found; and, similarly, with the driving of rods or piles,
it is difficult to know that one does not strike boulders. The speaker
presumes that the Dock Department was familiar with the general
nature of the shore, and felt confidence in the methods; otherwise,
that a few borings of another sort, that is, diamond drill or core
borings, would have been made in order to be sure that certain critical
points were well established.

It is noted that along the north side of the coffer-dam a single
row of steel sheet-piling was used. Of course, two rows give more
certainty of cut-off than one, but it would be interesting to have Mr.
Staniford explain whether a single row would not have done else-
where. The principal stability appears to have been derived from
the rock fill inside the steel piling, and it would seem to be possible
to use a single row and back it up so well with rip-rap as to make
the cellular construction unnecessary. The speaker visited the work
several times during construction, and noted particularly that the
cells of the steel piles along the south line were very much distorted
by the pressure. They were no longer cells, for some of them were
convexed toward the interior of the coffer-dam on both sides; that is,
they had simply been forced over by the action of the outside fill so
that they constituted not a row of cells, but two rows of piling with
certain cross-diaphragms. Considering the amount at stake, the
speaker is not at all surprised that two rows, or even more, should
have been thought desirable as a factor of safety; but, after the
experience, the point of view as to whether one row would be suffi-
cient for the future would be of interest.



Wiggin.



550 DISCUSSION ox COFFEK-DAM FOE 1 000-FOOT PIER

Mr. It is noted that the steel piling manufacturers have established

'^^'°' a strength of about 9 500 lb. per lin. in. for the interlock. This is an
important unit to remember in connection with such steel piling,
because it has been well established by tests, all giving substantially
the same figure, and it will be possible, in many cases, to use such
piling locked together as a sort of suspension for supporting pressures.

There are a few places in the paper where the derivation of certain
formulas, or, at least, a reference to their derivation, would add to
the completeness; for example, on page 505, there is a factor of 57%
which presumably comes from the Kankine sine formula for active
pressure. The formula on page 508, for coefficient of friction, would
be clearer for some explanation.

In one or two places, also, references to slopes are ambiguous, such
as a slope of 1.2 to 1, or 3 to 1. The wording leaves doubt as to whether
the horizontal is the larger or the lesser figure in the ratio.

In connection with this paper, mention might be made of some
steel piling work which the Board of Water Supply did on Staten
Island. The Narrows siphon is a 36-in., flexible-jointed, cast-iron pipe,
laid across the Narrows, from Brooklyn to Staten Island. On the
Staten Island shore, it had to be laid below the level of the bottom
of a future dock, and as the material for that dock had not been exca-
vated, it was necessary to carry a trench for a length of about 250 ft.
through a firm sandy material varying in depth from about 10 ft. at
the toe up to about 50 ft. near the shore, the maximum depth obtaining
for some distance. This pipe was laid under water, so that it was
not necessary to unwater the excavation, which, therefore, was not a
coffer-dam, but merely a sheeted trench with earth and water outside
and water alone inside.

The first design considered was the use of interlocking steel piles,
laid in straight lines, with wooden rangers and cross-bracing. That,
imdoubtedly, would have been successful, but it would have required
the placing by a diver of a good many pieces of timber for the rangers,
posts, and braces.

It was finally proposed to sheet the trench with interlocking steel
piles driven in arcs festooned between steel master-piles. The sheet-
piles carried the load as a suspension in a horizontal plane between
the master-piles, which were held apart by wooden braces. This design
differed from that first described in substituting suspension systems
for wooden rangers, an arrangement which incidentally eliminated the
bearing across grain of braces on rangers, which is a soiirce of weakness.

The sheet-piles were supplied by the U. S. Steel Company, and
were 12|-in. and 38 lb. per yd. The so-called master-piles were spaced
about 15 ft. apart along the trench; each consisted of a 12-in., 50-lb.,
I-beam, with a 12i-in. sheet-pile riveted to one flange and a 12-in.,
25-lb. channel to the other, to give it additional strength.



DISCUSSION OX COFFER-DAM FOE 1 000-FOOT PIER 551

Many of these sheet- and master-piles were about 65 ft. long. The Mr.
arcs of festoons were 15 ft. in radius, which, mider the assumptions '^^'°'
of earth pressure, caused a theoretical tension in these curves of about
3 000 lb. per lin. in., as compared with 9 500 lb., which was the supposed
strength of the interlock.

There was difficulty, as was expected, in driving the master-piles
exactly plumb and in correct position through the firm material. After
they were down, each one was carefully surveyed by locating two points
on it at the top as far apart vertically as practicable, and then pro-
jecting the line of these points to the position at the bottom. An
investigation was then made as to what curvature would really result,
making due allowance for the bending of the piles and inaccuracies
in the location measurements, and wherever the curvature to be
exjDected was less than about one-half that which was tried for
originally, another sheet-pile was put in the arc, thus increasing the
middle ordinate of the curve and reducing the stress.

T. Kennard Thomson,* M. Am. Soc. C. E. — Mr. Staniford deserves Mr.
the thanks and congratulations of the Engineering Profession for the '^'^°'^^°°-
marvelously successful solution of a most difficult engineering
problem. Commissioner Smith also deserves the thanks of every one
in New York City and State, as well as of many outside these limits,
for his courage, ingenuity, and success in securing for Manhattan
the 1 000-ft. docks for which there is such urgent demand.

The author's experiments are very interesting; but, as a rule, it
is almost impossible to make such experiments really conclusive, on
account of the great difficulty of allowing for all the conditions of
Nature. For instance, in a small model, a pile reprcoenting a rock
fill dumped into place would probably have a considerably smaller
percentage of voids than in the actual work, and some of these voids
might occur in such a way that the subsequent breaking of a few
stones might cause a sudden slide, with injurious results, especially
if the stones are coated with slime, which condition, also^ could not
be obtained in the model. Then, again, the formation of ice might
be disastrous, although difficult to make allowance for in the experi-
ment.

A cofFer-dam or retaining wall, supporting a fill, built of such pro-
portions as to be absolutely safe under most conditions, will fail com-
pletely under a load of semi-liquid material of the same weight and
height, due to the fact that it will act as a sort of ram. It is always
very difficult to estimate the effect of water on a fill, during or after
construction, as the cumulative effect is much greater in some mate-
rials than the hydraulic head would indicate.

• New York City.



Thomson.



552 Discussioisr on coffee-dam foe 1 000-foot piee

Mr. In sub-surface work or excavations, it is even much more difficult

to estimate the real pressures on a coffer-dam or retaining wall. The
materials have been placed, for the most part, by Nature, under great
pressures; and are often cemented together by clay or other natural
cements, and by roots, stones, etc., so that considerable excavation
can often be made without any coffer-dam or bracing. These con-
ditions cannot be reproduced in ordinary experiments, and, in actual
work, are often suddenly reversed by the action of springs, rains, etc.

Any experiments, therefore, which are intended to reduce the
sections of retaining walls, dams, or coffer-dams below the standards
adopted as the result of many experiments, theories, and actual
examples, should be checked with the utmost care and foresight.

The author refers to the use of manure for stopping leaks. A very
simple expedient, often used successfully in coffer-dam work, where
the leaks are small but persistent and troublesome, is to drop an occa-
sional shovelful of ashes or cinders in the water above the leak. This
would probably also remedy many leaks in concrete and masonry dams
and thus prevent much serious damage from ice forming in the
cracks and crevices.

Although too much credit cannot be given to Commissioner Smith
and Mr. Staniford for conceiving and constructing this greatly needed
improvement, one cannot help comparing it with a surgical operation,
for instance, for appendicitis. Both are expensive, and both are
absolutely necessary. In this case it was necessary to remove 12 acres
of valuable New York City real estate to make place for water.

The number of times such a surgical operation as the removal
of the appendix (or city real estate) can occur is limited, of course,
as far as the individual man or place is concerned, yet Manhattan
needs many more 1 000-ft. docks — in fact, all that can be obtained —
but the upper Hudson is not the most accessible place for them.

Six years ago, the speaker was much impressed with the wisdom
of the War Department in refusing to allow further encroachment
in the Hudson River, in spite of the very urgent need for such docks;
he has always felt that there is a correct solution for all problems,
if one only has the patience and ingenuity to find it.

In this case, when extension to the west was forbidden, and to
the north and east was impossible, the only direction left was the
south, it occurred to him that an extension might be made from the
Battery to Governor's Island. Such a course, however, would obviously
only accentuate a bad condition, for the City is too long and too
narrow as it is. Later, the idea occurred to him that the City might
be extended from the Battery 4 miles down the bay, with a width
of about I mile, and that a series of tunnels to Staten Island might



DISCUSSION ON COFFEE-DAM FOR 1 000-FOOT PIER 553

then be constructed. The City Hall woiild then be in the center of Mr.
the city, instead of at its southern end. Thomson.

The next step would be to form a new bay between Staten Island
and Sandy Hook, and introduce many other improvements, with which
most engineers are familiar. The net result of this plan would be
50 sq. miles of new real estate and 100 miles of additional water-front.
Some 6 or 7 sq. miles could be used for a free port between Staten
Island and Sandy Hook. This, alone, an eminent banker has declared,
would prove to be the salvation of New York.

Hamburg had to evict 16 000 people, as well as place drastic restric-
tions on many miles of river front; and even Bremen and Copenhagen
were much restricted in their efforts to secure enough water and land
space for their invaluable free ports. New York, however, can obtain
all the land required without interfering with any one's interests.

Charles S. Boardman,* M. Am. Soc. C. E. — The speaker wishes to Mr.
call especial attention to the design of this coffer-dam. Mr. Staniford ^o^'^'^^^.n.
claims that a precedent for the design is found in the coffer-dams
built by the United States Government at Black Rock Harbor, Buffalo,
N. Y., and for removing the wreck of the battleship Maine, at Havana,
Cuba. This is entirely true as to many details, such as the use of
long steel sheet-piling, especially in the Maine coffer-dam, for the mate-
rial in which each was constructed was harbor silt below about 35 ft.
of salt water. The foundation of the Maine coffer-dam was in clay,
but the steel sheet-piling for the 46th Street Pier was driven to
bed-rock. It is true, also, in comparing the Black Rock coffer-dam,
when it is considered that in that structure the steel sheet-piling was
driven to bed-rock, although through good sand, clay, and gravel.
Both the Black Rock and Maine coffer-dams differ in so many other
details that neither can be considered a precedent for the design of
the coffer-dam for the 46th Street Pier, New York City.

Steel sheet-piling has been used in cellular-form, gravity-type
coffer-dams, at Black Rock, Havana, Cape Fear River (Browns Land-
ing), N. C, and Troy, N. Y., but never before as a cellular-type
cut-off wall in a gravity-type earth and stone coffer-dam.

The form of the cell or pocket used by Mr. Staniford — with straight
partition walls and curved longitudinal walls — has been developed from
experience obtained at Black Rock and at Havana. It may be of
interest, therefore, to follow this thought more in detail, giving a
brief history of these two coffer-dams.

Mr. Staniford states:

"It was expected that the filling within the pockets [at Black
Rock and Havana] would make them stable against the external pres-

• Buffalo, N. Y.



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

Mr. sures, ■unaided, but in both cases it was found necessary to place an
Boardman. embankment of stone against the pockets, and in the case of Havana,
extensive bracing to the Maine was also placed, in order to stop the
continuous inward movement of the coffer-dam."

This statement is correct, but the stone embankment was placed
in each case for an entirely different reason than in the 46th Street
Pier, where it forms part of the original design of the coffer-
dam. This detail will also be referred to in a brief history of these
coffer-dams.

The Black Eock coffer-dam (Fig. 26) was constructed in 1908
and 1909, and consisted of seventy-seven pockets, each 30 ft. square,
formed of Lackawanna steel sheet-piling, in lengths ranging from
41 to 54 ft. and driven in straight walls. All the piling was di'iven
to bed-rock, making an average of 33 ft. of penetration through hard
materials. The maximum penetration was about 45 ft., at the extreme
west wall. Fig. 27 is a view, looking south, showing Pocket 'No. 40,
the northeast corner in the foreground. In the background may be
seen the City of Buffalo, the Niagara River, and the International
Bridge. This photograph gives a general idea of the location of the
ship lock in relation to the river.

The excavation was done by a dipper-dredge having an arm capable
of digging to about Elevation — 45 ft., leaving from 3 to 6 ft. of clay
and sand over the bed-rock.

The slopes of the materials forming the embankment by this
dredging method were not maintained as prescribed in the Govern-
ment engineer's plans, and, to re-establish these slopes, rock obtained
from other work on the ship canal was placed by dump scows at the
toe of the embankment and spread over its top by a dipper-dredge.
This rip-rap was placed on the side embankments, but not on the
end embankments of the coffer-dam, as will be seen in Fig. 28.

This figure also shows the elimination of the fill at the tops of the
pockets against the inside steel walls, which would have formed an
active pressure wedge against this wall. Fig. 29 shows the effect
produced on the steel sheet-piling wall by the clay filling in the pockets
when the embankment is low.

It is readily seen that this piling was thrown into tension at the
joints. The maximum curvature of the wall was at a point one-third
above the point of penetration, or the top of the embankment, the
middle ordinate of these walls at this point being about 3 ft., or about
one-tenth of the length of the jKicket.

Later, during construction at this end coffer-dam, the toe of the
embankment was robbed, causing a slide in the embankment, thus
lowering the point of support of the piling. This also created a curved
surface of rupture in the fill of the pockets. The steel sheet-piling



DISCUSSION ON COFFER-DAM FOR 1 000-FOOT PIER



555



Mr.
Boardman.





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

Mr. being deflected further, the result was that the maximum deflection
was at a point one-third of the height from the new point of support.
The steel channels broke in many of the pockets, the interlocks of
the piling in tension resisting this increased loading.

Before this coffer-dam was closed it was seen that the curved longi-
tudinal walls made the better design, and the four pockets, two at
each end, left for the passage of the contractor's plant, were constructed
in that way.

It was then found that in the fabricated tee-pile the tension strain
from these curved walls had a tendency to pull on the heads of the
rivets. The greatest movement inward of this coffer-dam at the
diaphragm walls, however, was only | in., measuring between the
tee-piles of opposite walls.

The coffer-dam around the battleship Maine, constructed by the
United States Government, under supervision of the "Maine Board",
composed of Col. W. M. Black (now Brig.-Gen.), Col. Mason M.
Patrick, and Maj. H. B. Ferguson, consisted of twenty cylinders,
each 50 ft. in diameter, with closing and connecting arcs of ten piles
on the outside wall. The cylinders and arcs were constructed of
Lackawanna 12| by |-in. straight-web sections, forty with 35-ft. lengths
and fifty with 25-ft. lengths, spliced to make sheet-piling 75 ft. long.

The original studies for this work show six cylinders, each 50 ft.
in diameter on each side of the coffer-dam and five cylinders, 40 ft.
in diameter, on the ends, but this was changed later to the plan
shown by Fig. 31.

It was planned to fill these cylinders with heavy clay, displacing
the 25 ft. of harbor silt which existed on this site below 35 ft. of
salt water and over soft clay at Elevation — 60. It was thought
that this heavy clay fill, when dropped, would sink to the clay bed,
thus raising or displacing the harbor silt and securing cylinders entirely
filled with clay.

Time did not permit waiting to deposit the clay by the dipper-
dredge, and the fill was made with a hydraulic dredge. This material
blanketed the existing harbor silt in the cylinders. When the coffer-
dam was ready to be unwatered, it was found impracticable to settle,
compact, or solidify this silt, nor could the water in it be withdrawn
or pumped out, and this fill, 25 ft. in height, remained in a semi-fluid
condition throughout the work.

For this reason the Maine coffer-dam was affected by the water
pressures after 15 ft. of water was pumped out, and even showed
slight movement with every high tide, the result being that the cylin-
ders gradually crept inward at the top, deflecting at Elevation 40 ft.
or more below the water level, or where this harbor silt existed as fill in
the cylinders. As the cylinders crept inward, the length of the center




Fig. 27. — Black Rock Coffer-Dam, Looking South. Pocket No. 40,
Northwest Coenee, in the Foeegeound.



■■■iiPHki


Li ^ -''^Jl''


^


-" «- /j»sia.s;«^


Pi/;:™^



Fig. 28. — Black Rock Coffee-Dam, Looking Noeth feom Southwest Corner.
Water 29 Ft. Below Noemal Level of Niagara River.




Fig. 29. — Black Rock Coffer-Dam. Inside Wall at South End. Clay
Puddle Movxng Piling to the Limit of Play in Interlock.




Fig. 30. — General View of Black Rock Coffer-Dam, Lookixg North, at
Completion of Concrete Lock Walls.



DISCUSSION ON COFFEE-DAM FOR 1 000-FOOT PIER



561



line of each became shorter, and they took an elliptical form, the jj^j.
area of contact between them increasing. Boardman.

This was further evidence that, as the diaphragm walls in the
Black Kock coffer-dam did not move, this movement of the Maine
coffer-dam cylinders tended to create diaphragm walls, and led to
the conclusion that, when further coffer-dam construction of this type
was required, the design could be much improved by adopting the
better parts of each, for the form or shape of the cells or pockets.
This is clearly exemplified in the plans for the 46th Street Pier
coffer-dam designed by Mr. Staniford as steel sheet-pile cellular
cut-off walls.




COFFER-DAM AROUND WRECK OF BATTLESHIP MAINE
Fig. 31.

The stone embankment of this coffer-dam was placed during
unwatering so as to maintain a balance of pressures and to take the
place of the weight of the water and mud removed from the coffer-dam.
The base of this stone embankment, being on harbor mud, could not
be depended on for a passive pressure in the same manner as the
stone embankment for the 46th Street Pier coffer-dam.

The United States Government engineers recognized and used the
best features of the Black Rock and Maine coffer-dams when designing
those for the lock and dams in the Cape Fear River, North Carolina.

Fig. 34 is a plan of the coffer-dam at Browns Landing for Lock
No. 2, and shows that three types of wall were used.

Pockets 1, 2, 3, 4, 5, 6, and 7 were of the square type used at
Black Rock, except that the 14 by |-in. section, having a much higher
section modulus (7.61), was used with the channel wales and steel
rods across the pockets, in order to maintain their form.



562 DISCUSSION" ox COFFER-DAM FOR 1 000-FOOT PIER

Fig. 35 shows this coffer-dam during construction. The river walls
were driven with two floating pile-drivers. A "pilot" pile of pine was
driven at • the corner of each pocket to support the template used in
driving the curved walls.

Pockets 8 to 29, inclusive, but excluding Panel 21, were of a com-
promise design, these walls being driven on the shore of the river in-
the original ground. The back wall was driven with the 14-in. arch-
web section, and the partition and front walls with the 12|-in. straight-
web section, the steel in the front wall being in tension.

Panels 21 and 30 each consist of a single wall, constructed after'
the design of bulkhead or dock walls, of steel sheet-piling with steel
channel wales and steel tie-rods to anchors. This work was carried
out most successfully, the concrete slab being placed under water
through a tremie, when the water was near the top of the coffer-dam.
The top of the coffer-dam was at Elevation -|- 32, and the finished
lock floor at Elevation + 1.0. The steel sheet-piling was in 49-ft. lengths.

This dam withstood a head of 30 ft. after it was pumped out the



Online LibraryAmerican Society of Civil EngineersTransactions of the American Society of Civil Engineers (Volume 81) → online text (page 45 of 167)