Caisson, << KAY son, >> in building, is a watertight chamber that is used in the construction of building foundations, bridges, tunnels, and other structures. Caissons provide an area in which construction crews can safely work underground or underwater. In addition, caissons may be filled with concrete and become part of the finished structure.
Most caissons have the shape of a cylinder or a box. The walls may be made of steel, concrete, or timber. All caissons are open at the bottom, where digging takes place. But they may be open or closed on top. The two main types of caissons are open and pneumatic.
An open caisson has an open top and bottom. The bottom edges, called cutting edges, are constructed so they can cut into the ground. The caisson sinks deeper into the ground as earth is removed beneath it.
A pneumatic caisson is closed at the top. It uses compressed air to keep water out of the working chamber and to provide oxygen for the workers. This type of caisson consists of two sections. The lower section, where the workers are, has cutting edges like those of open caissons. Concrete is poured into the upper section. Its weight helps drive the caisson deeper into the ground. Workers and materials move in and out of the lower section through a shaft. Pneumatic caissons are usually used to support bridge piers that are located in deep water.
Before workers enter a pneumatic caisson, they must step into a large airtight chamber called an air lock. The outer door is closed, and the air pressure in the air lock is gradually increased until it is the same as the pressure in the working chamber. When workers prepare to leave the caisson, they again pass through the air lock and the pressure is gradually reduced. If workers go through a change of pressure too quickly, they may develop bends, a painful condition also known as caisson disease. Bends can be fatal in some cases.
