cofferdam, temporary barrier for excluding water from an area that is normally submerged. Made commonly of wood, steel, or concrete sheet piling (see pile), cofferdams are used in constructing the foundations of dams, bridges, and similar subaqueous structures and for temporary drydocks. If double sheeting is utilized, the space between the sheets is usually filled with clay and gravel. When great strain or pressure is likely to be encountered, as in deep water, the pneumatic caisson is preferred to the cofferdam.
See L. White and E. A. Prentis, Cofferdams (2d ed. 1956).