Dams of the following thickness have proved perfectly secure:—
| Depth of water. | Thickness. |
|---|---|
| 6 feet. | 3 feet. |
| 10 feet. | 5 feet. |
| 15 feet. | 8 feet. |
| 20 feet. | 12 feet. |
| 25 feet. | 14 feet. |
The best form for a large coffer-dam is circular, or elliptical; as the pressure is thus resisted more equally in all places than when there are flat sides and angles in the plan.
To keep the dam dry while the work is going on, pumps are rigged along one side of the dam the lower ends of which are placed in a trench or well which drains the bottom.
The piers of the Victoria bridge at Montreal, (Canada,) are put down by coffer-dams. Some of the piers being in but few feet of water, and upon a rocky bottom, which did not admit of the driving of piles; the dams for such were built in sections, floated to the site and anchored.
FOUNDATION BY CAISSON.
282. In deep water the coffer-dam becomes very expensive, on account of the size and length of the piling, and the quantity of bracing required. In such cases recourse is had to the caisson; which is simply a box in which the masonry is built, and afterwards sunk to the proposed site. The manner of putting down a piece of masonry by caisson will best be shown by an example.
Suppose we wish to sink a pier thirty feet long, twenty feet high, and six feet wide, in twenty feet of water.
Let the caisson bottom be of two courses of square 12 × 12 timbers, fastened strongly at right angles to each other. Let the courses of masonry be two feet thick. Assume the weight of a cubic foot of stone as one hundred and sixty lbs., a cubic foot of wood at thirty, and of water sixty-two lbs. per foot.
Every floating body will sink until it has displaced a quantity of water equal to its own weight.