cofferdam

1. N a watertight structure, usually of sheet piling, that encloses an area under water, pumped dry to enable construction work to be carried out. Below a certain depth a caisson is required 围堰; 一种防水建筑物，通常由钢板桩筑成防水层，将水下某一区域围住，并将结构内水排干以便可以施工，在水下一定深度时，需要佩戴潜水箱