In the construction of bridge piers and deep-water docks, it is necessary to provide the workers a water-free environment to excavate the foundations. In shallow water and on dry land, coffer dams and open-air caissons are sufficient. For deep water, an enclosed caisson is necessary.

The caisson is a water tight compartment that is open at the bottom. (The earliest caissons were shaped like cylinders, open at both ends.) Workers descend to the caisson through a shaft, or man-lock. As they excavate the bottom, the material is transported to the surface through the excavation shaft, or lock. As the excavating continues, the massive caisson sinks deeper into the soft bed until the bedrock, or any other desired depth, is reached. The downward edges of the caisson walls are wedge-shaped to aid in its downward penetration of the bottom.

Pressurized caissons were used by Isambard K. Brunel in the...