COFFER-DAM, a dam formed to enable foundations to be laid in a site which is under water. The engineer surrounds the site with an embankment in such fashion as to form an en closure from which the water is excluded. Where the depth of water is small and the current slight, simple clay dams may be used, but in general coffer-dams consist of two rows of piles, the space between which is packed with clay puddle. The dam must be sufficiently strong to withstand the exterior pressure to which it is exposed when the enclosed space is pumped dry. See DAMS.