A Cofferdam is a temporary enclosure built in pairs, or within a body of water and constructed to hold onto water and soil allowing the surrounding area to be pumped out, excavated, and dried out. Cofferdams are generally used in the construction of bridge piers, as well as other support structures surrounded by water. Cofferdams walls are most commonly formed from sheet piles which are held up by walers, cross braces, and internal braces. Cofferdams are usually dismantled after the ultimate work is completed.
A Cofferdam can essentially be constructed in nearly any desired form. Nevertheless, for realistic reasons of cost reduction most Cofferdams are circular or rectangular. Generally, circular Cofferdams have the benefit that they present a more open excavation over the total plan area when bracing is supplied solely by the ring beams. Yet, installation of sheet piles in circular cofferdams involves stricter construction management. Rectangular cofferdams are easier to construct, but usually require more intricate internal bracing.