The word caissons is derived from the French word caisse meaning a box. In civil engineering, a caisson is defined as a structure which is sunk through ground or water to exclude water & semi fluid material during the process of excavation of foundations & which subsequently becomes an integral part of the substructure.

A Caisson
Uses of Caissons:
Following are the uses of caissons:
  1. To reach the hard bearing stratum for transferring the load coming on supports for bridge piers & building columns
  2. To serve as an impervious core wall of earth dams, when placed adjacent to each other
  3. To provide an access to deep shaft or a tunnel
  4. To provide an enclosure below water level for installing machinery, pumps etc.
Cofferdam and Caisson:
The main difference between a cofferdam & a caisson is that the former is a temporary structure while the latter forms the part of the permanent work. Following factors are to be considered while making a choice between cofferdam & caisson for a particular foundation work:

1)      A cofferdam becomes uneconomical in cases where the plan area of the foundation work is small as compared to the depth of water. Under such circumstances, a caisson would prove to be the most suitable.
2)      At places where cofferdam cannot be dewatered successfully, caissons are used. This may be due to the following reasons:

a. depth of water,
b. nature of soil to be penetrated, and
c. permeability of soil below foundation level,

3)      The process of constructing a cofferdam is greatly simplified in cases of soils which allow easily the driving of sheet piles. The caissons, on the other hand, are useful where obstructions of boulders would prevent the successful driving of the sheet piles.

4)      For heavy foundation works which are to be provided at a depth of about 12 to 15 metre below the level of standing water surface, caissons would prove to be more economical than cofferdams.

Materials Used for the Construction of Caissons:
The common materials which are usually employed for the construction of a caisson are as follows:
1)      Cast-iron
2)      Reinforced cement concrete
3)      Steel
4)      Timber.
Pneumatic Caisson

Classification of caissons:
There are four basic types of caisson namely:
1)      Box Caissons
2)      Open Caissons
3)      Monolithic Caissons
4)      Pneumatic Caissons