Cofferdam

A cofferdam is a temporary watertight enclosure built to dewater a work area so foundations, quay walls, or lock structures can be built in the dry. Common forms are double-wall or cellular sheet-pile cofferdams, braced single-wall sheet-pile boxes, and earth-fill bunds, designed to resist hydrostatic head, soil pressure, and any unbalanced fill. The enclosure is pumped out, the structure built, then the cofferdam is removed or left as part of the works. Stability checks cover sliding, overturning, and base heave or piping under the differential head, with the retained water level and the design flood governing wall height. It differs from a caisson, which is a permanent gravity unit, not a temporary enclosure.