Cofferdam

Coffer-dam, a casing of timber or iron fixed round the foundations of bridge-piers, sea and river walls, etc., during their construction, from the interior of which the water is removed, so that operations may go on below the water-level outside. It may be made of a double ring of wooden piers driven in side by side, the space between the two rows being filled up with clay or other impervious material. To resist the outside pressure of water, the interior of the coffer-dam is strongly stayed, and the piers are themselves well counter-braced. Stone may be used instead of timber, but sheet-iron casing is the more frequently employed. Wrought-iron caissons (q.v.) placed side by side, the joints between them being well caulked with felt or other such material, form a serviceable arrangement. The term is also applied to a watertight compartment built into the side of a warship, and filled with some water-excluding material, so that, if projectiles enter the vessel, very little water follows through the aperture.