A Caisson is a watertight structure used for retaining water in order to work on the foundations for bridges, piers, and other structures. Generally speaking, the water is pumped out to create a completely dry work environment. Some caissons can be open-air caissons, whereas others may use compressed air to keep the mud and water out. Installation involves pushing it into the mud, until it reaches clay or another solid foundation. Due to the pressures inside of caissons, there are many risks to the workers.
There are four main types of caissons:
Box caissons are put into place and filled with concrete, and may make up the foundation for other structures. These caissons must be anchored to allow them to remain in the proper position until they have been filled with concrete.