COFFERDAM

In Architecture, and Bridge-building,acase of piling, water-tight, fixed in the bed of a river for the purpose of laying the bottom dry for a space large enough to build the pier on. Coffer-dams are formed in various ways, either by a single inclosure or a double onef with clay or chalk rammed in between the two to prevent the water from coming through the sides. They are also made either with piles only, driven close together, and sometimes notched or dovetailed into one another; or [s. 258]