caisson

Hollow cylindrical or boxlike structure, usually of reinforced concrete, sunk into a riverbed to form the foundations of a bridge.

An open caisson is open at the top and at the bottom, where there is a wedge-shaped cutting edge. Material is excavated from inside, allowing the caisson to sink. A pneumatic caisson has a pressurized chamber at the bottom, in which workers carry out the excavation. The air pressure prevents the surrounding water entering; the workers enter and leave the chamber through an airlock, allowing for a suitable decompression period to prevent decompression sickness (the so-called bends).