CAISSON, when employed as a military term, denotes an am munition wagon or chest; in architecture, a sunk panel or coffer in a ceiling, or in the soffit of an arch or a vault. In civil engineer ing, the word is used in connection with a variety of hydraulic works. An early use is as an equivalent of camel; a floating air tight tank, formerly constructed of timber but now always of steel, capable of being sunk to the required depth by the admis sion of water, which is afterwards pumped out or evacuated by air pressure, thus imparting additional buoyancy to any craft to which it may be secured. This contrivance is the prototype of the pontoon and of the floating-dock (q.v.). It is employed, in various forms, in connection with salvage operations. See SALVAGING.
More commonly caisson implies a case or enclosure employed for keeping out water during the construction of foundations and other works in water-bearing strata, at the side of or under rivers and in the sea bed. There are three general types of caisson con struction:— 0) Box caissons, employed when no sinking operation through strata is required, are open at the top and closed at the bottom. The sides of the caisson, when it is sunk in position on a prepared bed, emerge above the water-level, and it serves as a shell for the building of a foundation. Such caissons are built of timber (espe cially in America), of steel, and of reinforced concrete or of com binations of these materials. They are often employed in building the solid superstructures of rubble mound breakwaters (q.v.) ; and, since about 1905, reinforced concrete caissons have been used in the construction of quay walls. In a few cases, e.g. at Dublin, they have been sunk on a dredged and levelled natural bottom, but more usually are founded on a prepared bed of rubble stone or concrete as at Marseille. When sunk and filled with concrete these large boxes form monolithic masses, often considerably ex ceeding I,000 tons in weight (see DocK). A large dry dock at Le Havre, opened in 1927, was built in and upon a steel box caisson framework, which was floated into position over the sit( of the dock, previously dredged to the required depth, and they( sunk in place.
(2) Open caissons. One variety, employed for bridge f ounda. tions when little or no sinking through strata is required, is oper at bottom and top and has no cutting edge. Caissons of thi: description, made of iron plates, were used for some of the foundation piers of the Forth bridge. Timber caissons are fre quently employed in America. Another form used for sinking through strata is provided with a cutting edge at the bottom. Rectangular iron plate caissons of this description were employed in constructing the foundations of the Tower bridge, London. Cylinder caissons, consisting of a shell of masonry, concrete, wood. steel or reinforced concrete, or of composite construction, and provided with some form of cutting edge, are commonly em ployed for bridge (q.v.) foundations and in sinking shafts in allu vial strata. They are sunk to the required depth by excavating the material within the cylinder and at the same time weighting it, or using a water-jet to assist the sinking process. The internal space is, after sinking, filled with masonry or concrete. The open "well" foundations of brickwork, used for centuries in India, and the concrete well monoliths (see DOCK), widely used in dock con struction, and sometimes for the foundations of bridge piers, are, strictly speaking, open caissons. The monoliths are built up in tiers on a steel cutting edge or curb and have two or more open vertical wells within which excavation is carried on by grab dredging or other means.
(3) Compressed-air caissons. Where foundations have to be carried down to a considerable depth in soft, water-bearing strata, such as running sand, or where hard material is met with, which cannot be readily excavated by the means permissible in open caisson working, the use of compressed air is often resorted to. Compressed-air working can in some cases, when necessary, be applied to the sinking of caissons, started by the open method, by the addition of the necessary airtight floor, air locks and shafts. The method of compressed-air sinking consists essentially in the provision of an airtight floor in the caisson, at a height which provides room for a working chamber between it and the cutting edge, in which men can work under compressed air and carry on the necessary excavation. A shaft and air lock afford access and exit for men and materials; and the air pressure is varied in ac cordance with the head of water. Compressed-air caissons are employed not only for bridge foundations but in the construction of quay walls and foundations of other structures in water-bearing strata. Movable compressed-air caissons, which, of ter the lowest portions of the foundations have been laid, are raised by screw jacks and moved into position for constructing the next section, have been used in some instances for bridge pier foundations and dock construction.
Floating, sliding and rolling caissons built of steel are often employed for closing dry-dock entrances. They are also used, but less frequently, in place of hinged gates at the entrances to locks and docks (q.v.).
See C. E. Fowler, Sub-Aqueous Foundations (3rd ed. 1914) ; H. S. Jacoby and R. P. Davis, Foundations of Bridges and Buildings (1925).
(N. G. G.)