caisson

Caisson is a term with several related meanings in engineering, medicine, and military terminology. It denotes a watertight chamber or enclosed container used for construction, transport, or storage.

In civil engineering, a caisson is a watertight, hollow structure sunk or built in place to create a dry working space for underwater foundations. Caissons may be open at the bottom (open caissons) or fully enclosed and pressurized (pneumatic caissons). They are sunk, sealed, and pumped dry to provide a foundation for piers, bridges, cofferdams, and offshore platforms. After the foundation is completed, the caisson is dewatered and often removed or integrated into the structure. The method requires careful control of ballast, soil conditions, and, in the pneumatic type, air pressure.

When workers used pressurized pneumatic caissons, they spent extended periods in a controlled atmosphere, which led to decompression sickness known as caisson disease upon ascent. Modern practice emphasizes staged decompression or alternative methods with reduced pressurization.

In military terminology, a caisson is a two-wheeled or four-wheeled wagon that carries artillery ammunition and sometimes gun equipment. It accompanies field guns and howitzers as part of the gun-team’s transport and supply train. Some later armored or mechanized versions exist, but the traditional image is a horse-drawn ammo wagon.

The word derives from French caisson, meaning a box, chest, or container.