Cofferdam

a contrivance resorted to by bridge builders. A typical cofferdam may be likened to a huge barrel without head or bottom. It is built around the spot where it is desired to erect a pier. Water and mud are pumped out, enabling workmen to enter and lay a proper foundation for the proposed pier. If the river, for instance, be of clay it may be possible to excavate to some depth to secure proper footing. In case it is necessary to go deep into the bottom for a footing the caisson is employed. Not infrequently an engine drives two tight circles of piles, one within the other, and fills the space between the circles of piling with clay rammed down close to make it water-tight. See Caisson