Caisson

Caisson, in civil engineering, is a structure much employed in the foundation of the piers of bridges or quays in deep running water. It consists of a strongly built casing of woodwork or metal, forming an enclosure that may be floated to the proper position over the site of the pier, and sunk by careful admission of water through a sluice. When settled in position, the work of building up the foundations of the pier may be carried on within the caisson undisturbed by the flow of water. Excavation is usually effected inside by means of a hollow metal column with an open-bottomed chamber at its base, within which the men work under compressed air. The caissons of the Forth Bridge were 70 feet in diameter, and reached a depth of 89 feet below the water-level. Often the caisson is simply filled up with concrete, with or without a brickwork lining. In shipbuilding, a caisson is a sort of hollow pontoon, which can be sunk under a ship, pumped out, and re-floated with the ship on it, The term is also applied to a case containing explosives, and formerly a submarine or subterranean mine: and to a hulk-shaped vessel made to fit into, and to block up, the opening of a dock. The caisson having been pumped dry, is floated into position and then filled with water, whereupon it settles tightly into its bed, and constitutes a nearly water-tight door to the dock.