Caissons

A caisson is a structure used in construction
particularly underwater projects like bridge or dam construction. It is constructed to provide a dry, protected environment for workers and
construction materials

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

  For deep-water construction, a pneumatic 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

For getting out the consumed material and excavation ashes, the workers use muck tube connected to the surface. A crane at the surface removes the soil with a clam shell bucket