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first time. Freshets in the river flooded it five times in one season,
without any damage whatever. The river was permitted to flow over
the top and into the pit without damage. At low stages the leakage
was from 200 to 500 gal. i^er min., thus requiring a small expense
for pumping.

Fig. 36 shows the lock walls under construction, the head of water
on the dam then being 26 ft.

In reference to that part of the paper under the heading "Stresses
in Steel Sheet-Pile Walls of Pockets", the nature of the material
used to fill the cellular-type pocket determines the method of calcu-
lating the stresses exerted by this filling on the steel sheet-piling
walls. This pressure is transferred to the walls of the steel sheet-
piling, and the walls are held intact by the strength of the interlock
of this sheet-piling in tension. In 1908 — previous to the construction
of the Black Rock coffer-dam — the various interlocks of steel sheet-
piling were tested physically for this value in tension per linear inch
of interlock, and it was established that the Lackawanna steel sheet-
piling develoi^ed an ultimate strength of 9 700 lb. per lin. in., which
was then thought to be sufficiently high to allow such piling to be
used for any depth then thought practicable for cellular pockets in
a coffer-dam. The maximum stress developed by filling the pockets
of the Black Rock coffer-dam, and to be resisted by the interlocks
of the steel sheet-piling, was computed for the contractors by J. C.
Meem, M. Am. Soc. C. E., and found to be 5 600 lb. per lin. in. of
interlock.

Several articles were written by engineers on the stress that would '
be developed in the interlocks of the piling caused by the filling in




Fig. 32. — CONSTSUCTING THE Blaine Coffer-Dam, Havana, Cuba.




Fig. 33. — The Bow of the Maine, within the Coffee-Dam.
Water at Elevation — 10 Feet.



DISCUSSION ON COFFEE-DAM FOE 1 OOO-FOOT PIEE 565

the pockets of the Maine coffer-dam, one writer* placing it as high Mr.
as 7 400 lb. per lin. in. of interlock. '^^^ "^^°"

By using the formula adopted to find this tension stress, and
making the same assumptions as Mr. Staniford, that the maximum
pressure exists at a point 58 ft. below the top of the filling, and also
that the weight of the river mud is 80 lb. per cu. ft., with a natural
slope of 3^ to 1, the result is 5 480 lb. per lin. in. of interlock. This
figure, as will be seen, agrees substantially with Mr. Staniford's 5 200
lb. per lin. in.

In determining the formula to use in this calculation, the hori-
zontal thrust from the material within the pocket was assumed to
act perpendicular to the longitudinal piling walls; also, the corre-
sponding unit pressure at any given depth was assumed to be dis-
tributed uniformly along the longitudinal walls between the transverse
walls of pockets. Therefore, under these conditions, the longitudinal
piling wall approaches the curve of a parabola with the maximum
tension at the point of support, or at the Y-pile, and its value is



t



^ V Z* — 16 ^2 p
12



when t = the tension, in pounds per linear inch, in the longitudinal
and transverse walls, for the reason that the Y-pile is fabricated with
angles of 120°, the components are equal, therefore the result is equal
to one of them.

p = the pressure, in pounds per square foot, against the longi-
tudinal walls at the depth considered;
h = the ordinate at the center line of the pocket, in feet ;
I = the distance between transverse walls, in feet.

The suggestion has been made that possibly a single wall of steel
sheet-piling would have answered for the cut-off wall in a coffer-dam
of this type. A thoughtful analysis of this idea will lead to its abandon-
ment almost immediately; first, for the reason that this type of con-
struction, as Mr. Staniford's design shows, requires a rock fill on
the inside for stability. Therefore, though it is not impracticable,
it is not good engineering to attempt to drive a single wall of steel
sheet-piling through this rock fill for such a depth. Again, assuming
this cut-off wall to have been constructed before any fill had been
placed: this would mean that an assembled single wall, from 60 to 70
ft. in height, would have to stand it vertically, and be secured only
by resting on bed-rock and being supported for about one-third of

* Engineering News, December 29th, 1910.



566



DISCUSSION ON COFFER-DAM FOR 1 000-FOOT PIER



Mr.
Boardman.



-Ii9-




li"x vjj'a.W.-^^




Fig. 35. — Coffer-Dam at Browns Lauding for Lock No. 2, Cape Fear River.




Fig. 36. — Coffer-Dam at Browns Landing. Lock Walls Under Construction.



DISCUSSION OX COFFER-DAM FOR 1 000-FOOT PIER 569

its height in harbor silt. Any one who has assembled a wall of this Mr.
kind, without first establishing secure supports for it, knows the diffi- ^°^''



Online LibraryAmerican Society of Civil EngineersTransactions of the American Society of Civil Engineers (Volume 81) → online text (page 46 of 167)