COFFER-DAM.
281. In founding in water from five to twenty-five feet deep, a contrivance called a “coffer-dam,” is sometimes used. It is formed by driving a double or triple row of piles around the foundation; which rows are made water tight, either by tongued and grooved square piles, or by round piles, to which is fastened a sheathing of plank. The space between the courses of piling is emptied of water and packed closely with clay or other material impervious to water. The interior of the dam is then pumped dry and the masonry laid as on dry land. The thickness of the dam depends upon the depth of water; the pressure upon the lower part being of course much greater than that at the upper. If it was considered as a mass resisting by its weight, overthrow from the pressure of the water, the thickness would be easily calculated. Thus, if the water is twenty feet deep the whole hydrostatic pressure upon each lineal foot of the dam is 20 × 1 × 10 × 62½ = 12,500 lbs.; and as the weight of water increases in the order of the terms of an arithmetical progression, as also the pressure, it may be expressed by the elements of a triangle, of which the height is the depth; and as the centre of gravity of a triangle is at two thirds of the height from the vertex, the pressure may be regarded as concentrated at one third of the depth from the bottom; and the leverage of the above 12,500 lbs. is
20
3 = 6.67 feet;
and the overthrowing force is 83,375 lbs. The resisting force of a clay dam twenty feet high and ten feet thick, would be
20 × 10 × 110 × 10
2 = 110,000 lbs.
Determining the thickness thus, would make the dam, when in deep water, very thick; and it is generally best to brace the inside against the ground, and when the masonry will admit, against that.
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.