Cofferdam

(civil engineering) A temporary damlike structure constructed around an excavation to exclude water.
(naval architecture) A void between two bulkheads designed to separate two adjacent liquid-containing compartments.

A temporary, wall-like structure to permit dewatering an area and constructing foundations, bridge piers, dams, dry docks, and like structures in the open air. A dewatered area can be completely surrounded by a cofferdam structure or by a combination of natural earth slopes and cofferdam structure. The type of construction is dependent upon the depth, soil conditions, fluctuations in the water level, availability of materials, working conditions desired inside the cofferdam, and whether the structure is located on land or in water (see illustration). An important consideration in the design of cofferdams is the hydraulic analysis of seepage conditions, and erosion of the bottom when in streams or rivers.

Types of cofferdams for use on land: (a) cross-braced sheet piles, (b) cast-in-place concrete cylinder; and in water: (c) cross-braced sheet piles, (d) earth dam.

Where the cofferdam structure can be built on a layer of impervious soil (which prevents the passage of water), the area within the cofferdam can be completely sealed off. Where the soils are pervious, the flow of water into the cofferdam cannot be completely stopped economically, and the water must be pumped out periodically and sometimes continuously.

A nautical application of the term cofferdam is a watertight structure used for making repairs below the waterline of a vessel. The name also is applied to void tanks which protect the buoyancy of a vessel.