COFFER-DAM, a hollow darn, constructed of a double range of piles, with clay rammed between, for the security and convenience of the workman while digging out and building the foundation of an ent•ance-lock to a dock, basin, or canal, when it cannot otherwise be laid dry.
In bridge-building, the term is applied to a ease of piling fixed in the bed of a river, without any bottom, for the pur pose of building a pier dry. Its sides must, therefore, reach above the level of the water, and, after it is fixed, the water must be pumped out by the engines, which, unless the work is very carefully done, must be kept constantly at work, to prevent leakage as much as possible. Coffer-dams are made either double, or single. In the double one, the space between the inner and outer rows of piles is rammed with clay or chalk ; the piles are driven as closely as practicable to each other by means of a pile engine, till fixed firmly into the earth ; sometimes they are grooved and tongued ; sometimes they are grooved in the sides, and fixed at a distance from ()tail other, with boards let into the grooves.
The first writer on the use of cofler-dams was Alberti, who, chap. vi., book '2. gives the following directions: "Make the foundation of your piers in autumn, when the water is lowest, having first raised an enclosure to keep off the water, which may be done in this manner : drive a double row of stakes close to that side of the row which is next to the intended pier, and fill up the hollow between the two rows with rushes and mud, ramming them together so hard, that no water can get through ; then, whatever you find within the enclosure—water, mud, sand, or whatever else is an hindrance to you—throw them out, and dig till you conic to a solid foundation." To this we may add, where the river is rapid and deep, and the lied of solid earth or clay, the coffer-dam must be constructed with three, four, five, or even six rows of piles, according to the rapidity and depth of the stream. Due care must also be taken to brace the sides well from fixed points, as well as to make the whole water-tight, by ramming in chalk or clay, as above directed.
Where the river is rapid and deep, and the lied of a loose consistence, though the sides be never so firm, the water w i11 ooze through the bottom in too great abundance to be taken out by the engines, recourse must therefbre be lied to a caisson. e CAISSON.
f 'flowing is a deseription of the large coffer-dam made at the New Houses of Parliament, for building the embankment, or river-wall. This dam was 1,•236 feet long. and 10 feet wide, constructed in the following manner :— a trench was first made by dredging in the bed of the river of the form of a segment of a circle, 27 feet wide, and 8 feet deep in the centre, to allow the piles to drive more easily ; two parallel rows of guide or main piles were then driven about 5 feet apart, leaving x width of 9 feet between them transversely : to these piles were fixed three tiers of waling of whole timbers, cut down and bolted together, one tier being fixed at the top on a level with high-water mark ; another level with the bed of the river : and the third mid way. The piles and waling were then bolted across with iron bolts 12 feet long, forming a carcase fin• the inner or sheet-piling ; the inner main piles being also firmly braced to resist the pressure at high water. Horizontal struts of whole timber, also, at the back of the brace piles abutted against other piles driven just within the inner edge of the foundation of the wall.
The piles were. 311 feet long, driven through the gravel, and 2 feet into the clay, the top of which is 28 feet below high water mark. AVithin the waling were two parallel rows of sheet-piling ; the outer or river-side of whole timbers—the inner or land-side of halt:timbers. After all the piles were driven, the gravel tii•ming the bed of the river between the piling was dug out down to the clay, and the space filled in with clay, and piddled. For the purpose of pupping out the water, a ten-horse power steam-engine was erected, which was kept at work night and day ; and considering the great extent of the dam, it is remarkably free from leakage. It occupied fourteen months in its construction.