I08 DOCK ENGINEERING.
Combining all these elements, we have for equilibrium
M0=M1+M2+ M3.
The exact distribution of stress being indeterminate, a very large factor of
safety is essential.
J’he stress in the internai tie-rods can only be adequately covered by
assuraing the clay to be in a fluid condition and exerting a pressure propor-
tionate to its specific gravity.
Fig. 66 shows a cofferdam as employed in dock construction at Liver-
pool. It was straight in plan between its extreme abutments for a total
length of 260 feet, divided into 15-feet bays by cross diaphragms of 3-inch
planking, thus obviating the use of internal tie-rods. The height was
38 feet and the bottom and top internal widths 20 feet and 12 feet respec-
tively. It derived some additional support from raking shores not shown
in the figure. A dam of this type can be constructed, maintained, and
removed at a cost ranging from £35 to £50 per foot run, much depending
upon the nature of the site and the duration of the work.
Fig. 67. —Cofferdam at Hull.
Fig. 67 shows a cofferdam used at the Alexandra Dock, Hull. It was
segmental in form, with a radius of 255| feet and a length of 461 feet. The
piles were driven vertically, enclosing a space 5 feet wide. Five sluice
openings were formed to allow the tide to flow in and out until the
completion of the dam.*
* Hurtzig on “The Alexandra Dock, Hull,” Min. Proc. Inst. C.E., vol. xcii.