Caisson, in engineering construction, is a chest used in 'laying' the foundations of the piers of bridges, quays, and like structures, in deep and rapid rivers. It consists of a very strong platform of timber or of metal plates, to which the sides are attached. The site of the pier being levelled by dredging or otherwise, the caisson is brought over the spot, and moored in the proper position. Two or three of the lower courses of masonry are then built upon the platform of the caisson, and the water is slowly admitted by a sluice, in order to cause the caisson to settle into its place.
When the foundations are laid with concrete, the caisson may consist of a simple frame of wooden walls, floated into position to form an inclosure, into which the concrete can be shot, and can set undisturbed by the wash of the water. Compressed air is now very generally employed inside a metal column, in a chamber at the bottom of the column, where workmen are required for excavating. The column is open at the bottom, and the water is prevented from occupying the working chamber by the counterpressure of the air. Communication between the working chamber and the external atmosphere above is effected by means of what is aptly called an 'Air-lock' (q.v.), serving for the exit and entrance of the workmen and materials. The air in the lock is lowered to the pressure of the atmosphere before the chamber is opened for the passage of men or materials to the open air; and it is, on the contrary, raised to the pressure of the air in the working chamber before men and materials are admitted.
In excavating for the foundations of the Forth Bridge, the caissons were of very large dimensions, being 70 feet in diameter, the greatest depth reached varying from 71 feet to 89 feet below high-water, and from 39 feet to 43 feet into the bed of the Forth. The pneumatic process was in the main adopted (see BRIDGE).
The new Tay viaduct, of which Mr W. H. Barlow was the engineer, has main spans of 245 feet, each pier carrying which is formed of two iron cylinders, 23 feet in diameter, filled with brickwork and concrete, and sunk to depths varying from 20 feet to 30 feet into and resting upon sand, the depth of water at high tide being 23 feet. The weight borne by each superficial foot in the cylinders is estimated at 3 tons.
The bridge over the Ganges at Benares, with spans of 335 feet, has piers composed of single iron caissons of oval shape, 65 feet long, 28 feet broad, lined with brickwork and filled with concrete. They are sunk to a depth of about 100 feet.
One of the most remarkable instances of the sinking of foundations by means of iron caissons was exhibited in the erection of a graving dock at Toulon. Here, the caisson was 472 feet long by 134 feet wide, and 62 feet deep. It embraced the entire dock, which was built of masonry. The excavation was performed by the use of a compressed-air chamber in the bottom of the caisson, as in the Forth Bridge.