The purpose of using a caisson in construction is to provide a temporary structure from which earth, water, and other materials can be removed and into which concrete or some other fill material can be placed. For example in the construction of a bridge it may be necessary to burrow into the soil at the bottom of a river until bedrock is reached. One way of doing this is to sink a caisson filled with compressed air into the river until it reaches the river bottom. Workers then can go into the caisson and dig soil out of the river bed until they come to bedrock. As they remove soil it can be transported upward out through the caisson. During this process the caisson continues to sink more deeply into the river bed until it reaches bedrock. At that point concrete may be poured into the caisson to form the lowest section of the new bridgepier.
Caissons may consist of a single unit looking like a tin can with both ends cut out. Or they may be subdivided into a number of compartments similar to a honeycomb. One factor in determining the shape of the caisson is the area it must cover. The larger the size of the caisson the more necessary it may be to subdivide it into smaller compartments.