caissons

Caisson is a term with several related meanings in engineering and military contexts. In civil engineering, a caisson is a large, watertight chamber used as a foundation or working space for underwater or poorly drained construction. In military terminology, a caisson is a wagon or box used to transport artillery ammunition; the word is also encountered in historical fortification contexts.

In construction, caissons come in two main forms. Box caissons are watertight, hollow boxes—often rectangular—that are sunk into place and then pumped dry to create a dry working space for excavation and construction of foundations. Open caissons are closed at the top but open at the bottom; they are lowered into position and water is pumped out to enable workers to excavate the surrounding soil from within. A pneumatic (or pressurized) caisson is an open caisson that is kept dry by maintaining a higher internal air pressure, allowing workers to continue excavation below the water table. These methods are commonly used for bridges, piers, harbor structures, and some tunnel and dam projects.

The military usage of the term refers to an ammunition wagon attached to artillery units; the caisson transports shells and sometimes holds equipment or supplies. The word’s origin lies in the French caisson, meaning a box or chest.

The term is also associated with decompression sickness, historically called caisson disease, which affected workers who spent extended periods inside pressurized caissons. Prevention includes staged decompression and careful pressurization management during ascent.

Overall, caissons describe either watertight work chambers for underwater construction or mobile containers for military ammunition, with related applications in foundation work and defense-related logistics.