Caisson (from the Fr. caisse, the variant form "cassoon" being adapted from the Ital. casone), a chest or case. When employed as a military term, it denotes an ammunition wagon or chest; in architecture it is the term used for a sunk panel or coffer in a ceiling, or in the soffit of an arch or a vault.

In civil engineering, however, the word has attained a far wider signification, and has been adopted in connexion with a considerable variety of hydraulic works. A caisson in this sense implies a case or enclosure of wood or iron, generally employed for keeping out water during the execution of foundations and other works in water-bearing strata, at the side of or under rivers, and also in the sea. There are two distinct forms of this type of caisson: - (1) A caisson open at the top, whose sides, when it is sunk in position, emerge above the water-level, and which is either provided with a water-tight bottom or is carried down, by being weighted at the top and having a cutting edge round the bottom, into a water-tight stratum, aided frequently by excavation inside; (2) A bottomless caisson, serving as a sort of diving-bell, in which men can work when compressed air is introduced to keep out the water in proportion to the depth below the water-level, which is gradually carried down to an adequately firm foundation by excavating at the bottom of the caisson, and building up a quay-wall or pier out of water on the top of its roof as it descends.

An example of a caisson with a water-tight bottom is furnished by the quays erected alongside the Seine at Rouen, where open-timber caissons were sunk on to bearing-piles down to a depth of 9¾ ft. below low-water, the brick and concrete lower portions of the quay-wall being built inside them out of water (see Dock). At Bilbao, Zeebrugge and Scheveningen harbours, large open metal caissons, built inland, ballasted with concrete, floated out into position, and then sunk and filled with concrete, have been employed for forming very large foundation blocks for the breakwaters (see Breakwater). Open iron caissons are frequently employed for enclosing the site of river piers for bridges, where a water-tight stratum can be reached at a moderate depth, into which the caisson can be taken down, so that the water can be pumped out of the enclosure and the foundations laid and the pier carried up in the open air. Thus the two large river piers carrying the high towers, bascules, and machinery of the Tower Bridge, London, were each founded and built within a group of twelve plate-iron caissons open at the top; whilst four of the piers on which the cantilevers of the Forth Bridge rest, were each erected within an open plate-iron caisson fitted at the bottom to the sloping rock, where ordinary cofferdams could not have been adopted.

Where foundations have to be carried down to a considerable depth in water-bearing strata, or through the alluvial bed of a river, to reach a hard stratum, bottomless caissons sunk by excavating under compressed air are employed. The caisson at the bottom, forming the working chamber, is usually provided with a strong roof, round the top of which, when the caisson is floated into a river, plate-iron sides are erected forming an upper open caisson, inside which the pier or quay-wall is built up out of water, on the top of the roof, as the sinking proceeds. Shafts through the roof up to the open air provide access for men and materials to the working chamber, through an air-lock consisting of a small chamber with an air-tight door at each end, enabling locking into and out of the compressed-air portion to be readily effected, on the same principle as a water-lock on a canal. When a sufficiently reliable stratum has been reached, the men leave the working chamber; and it is filled with concrete through the shafts, the bottomless caisson remaining embedded in the work.

The foundations for the two river piers of the Brooklyn Suspension Bridge, carried down to the solid rock, 78 and 45 ft. respectively below high-water, by means of bottomless timber caissons with compressed air, were an early instance of this method of carrying out subaqueous foundations; whilst the Antwerp quay-walls, commenced many years ago in the river Scheldt at some distance out from the right bank, and the foundations of six of the piers supporting the cantilevers of the Forth Bridge, carried down to rock between 64 and 89 ft. below high-water, are notable examples of works founded under water within wrought iron bottomless caissons by the aid of compressed air. The foundations of the two piers of the Eiffel Tower adjoining the Seine were carried down through soft water-bearing strata to a depth of 33 ft. by means of wrought iron bottomless caissons sunk by the help of compressed air; and the deep foundations under the sills of the new large Florida lock at Havre (see Dock) were laid underneath the water logged alluvial strata close to the Seine estuary by similar means.

Workmen, after emerging from such caissons, sometimes exhibit symptoms of illness which is known as caisson disease (q.v.).

As in the above system, significantly termed by French engineers par caisson perdu, the materials of the bottomless caisson have to be left in the work, a more economical system has been adapted for carrying out similar foundations, at moderate depths, by using movable caissons, which, after the lowest portions of the foundations have been laid, are raised by screw-jacks for constructing the next portions. In this way, instead of building the pier or wall on the roof of the caisson, the work is carried out under water in successive stages, by raising the bottomless caisson as the work proceeds; and by this arrangement, the caisson, having completed the subaqueous portion of the structure, is available for work elsewhere. This movable system has been used with advantage for the foundations for some piers of river bridges, some breakwater foundations, and, at the Florida lock, Havre, for founding portions of the side walls.

Closed iron caissons, termed ship-caissons, and sliding or rolling caissons, are generally employed for closing graving-docks, especially the former (so called from their resemblance in shape to a vessel) on account of their simplicity, being readily floated into and out of position; whilst sliding caissons are sometimes used instead of lock-gates at docks, but require a chamber at the side to receive them when drawn back. They possess the advantage, particularly for naval dockyards where heavy weights are transported, of providing in addition a strong movable bridge, thereby dispensing with a swing-bridge across the opening.

The term caisson is sometimes applied to flat air-tight constructions used for raising vessels out of water for cleaning or repairs, by being sunk under them and then floated; but these floating caissons are more commonly known as pontoons, or, when air-chambers are added at the sides, as floating dry-docks.

(L. F. V.-H.)