Definition - What does Caisson mean?
A caisson, in the context of underwater construction, is a watertight structure used for underwater construction. A caisson is typically used in the construction of bridges, dams, hydraulic elevators, and ship repair to ensure a dry working environment. Caissons are prefabricated before installation for use and allowed to sink under their own weight. They are similar to pile foundations except that they are used when the floor of the body of water has a suitable bearing strength such as bedrock. For softer materials caissons are bell shaped in order to disperse the building load.
Trenchlesspedia explains Caisson
There are four types of caissons: box, compressed air, open, and monolithic caissons. Box, open, and monolithic caissons are typically open to atmospheric pressure at the top. Compressed air, or pneumatic caissons, use positive air pressure in order to keep water and mud out of the working environment. All types usually sink under their own weight until bedrock or another stable material is found, but some caissons may also have ballasts added to ensure the structure does not float or shift.
Care needs to be taken when workers exit from a pneumatic caisson to avoid caisson disease. This disease is similar to decompression sickness and occurs when a person re-enters atmospheric conditions too quickly. This condition can result in severe illness or death.