Caisson

Caisson, concrete or steel box, open at both ends, used in civil engineering when excavation or construction must be carried out underwater, particularly in sinking foundations for bridges. Caissons are sunk in the required position and built up to keep the top above water. As the soil is dredged from the bottom, the caisson sinks. It is eventually filled with concrete and acts as part of the foundation of the bridge. Sometimes a pressurized, watertight chamber is built at the bottom of the caisson for workers.