Cofferdam is a temporary walled enclosure used in construction. It creates a space that protects workers and excavation sites from water and cave-ins. Cofferdams are used in building such structures as bridge piers, dams, and foundations. Simple cofferdams used in shallow water can be mounds of earth or sandbags. Cofferdams used in deeper water or deep excavations are usually made of interlocking steel sheets driven into the ground. Sometimes, they are made of closely spaced columns of wood, concrete, or metal. Such cofferdams must be braced to resist horizontal pressure from earth, water, and other sources. They may be made fairly watertight by packing clay behind the sheets or posts.
