The large prefabricated box or cylindrical type structures that can be sunk through soft ground or water and then filled with concrete thus forming a foundation are called as Caissons.The caisson is generally a box like structure with an open bottom and open top some times. The structure keeps water out of the construction area while its open bottom allows workers to place foundations and piers in the sea bed or riverbed. An open caisson can be used in shallow water; its open top allows light and air to enter from above the water line.
caisson has a closed top; pressurized air is pumped in, and personnel enter and leave through an airlock. Both types have sharp inclined lower edge, which allows the caisson to be deeply embedded in the ground.
Before putting the caisson in its place, engineers look for stable seabed like rock. But if they don’t find this layer they make artificial layer to provide safety to the caisson. In pneumatic caissons, an airlock allows access to the chamber and the pressurized air flow must be constant to ensure regular air changes for the workers and prevent excessive inflow of mud or water at the base of the caisson.
- Caissons are similar to pile foundations but installed using different ways.
- These are actually deep foundations constructed above ground level & sunk into earth to the required level by excavating material within the caisson.
- Caissons also provide dry space to carry out other construction works.