Caisson

kas'son, a substructure used in building the piers of bridges. If an engineer desires to build a pier in water, in midstream, for instance, there are several devices for getting a secure foundation. It will not do to begin building on the mud of the river bottom, for the pier would be almost certain to settle to one side and become unsafe, if it did not fall. One method of reaching a deep foundation below water and mud is the employment of a caisson.

A caisson may be likened to an enormous, water-tight tub, built bottom upward where it is desired that the pier shall stand. The caisson may be built on land and towed to place, or it may be built on the spot. In either case, the bottom must be enormously strong--for the bridge builder proceeds to build his pier on the caisson. As the caisson sinks into the oozy bed of the river, air is forced into it from above by a powerful pump. In this way the chamber, as the space below the bottom is called, is prevented from filling with water and mud. Workmen in this chamber labor with pick, shovel, and wheelbarrow, excavating so as to allow the caisson to sink deeper and deeper. The earth is sent up through a central shaft. A sand pump is used for the purpose. Air pressure is depended upon to keep water out; or indeed, water may be pumped out as is done in digging a well. As fast as the caisson sinks the masonry above is built up so as to be above the level of the water. When the caisson reaches bed rock, or clay that may be depended upon to support the pier, the chamber and shaft are filled, usually with concrete, and the work is complete.

The shaft through which workmen go up and down, and through which the excavated material is removed, is provided with air locks. The air pressure in the chamber is necessarily very great. On coming out of the caisson workmen remain for a time in an anteroom, an air lock, under pressure midway between that of the caisson and the outer air. Notwithstanding this precaution workmen are prostrated, not infrequently bleeding at the nose, with pain in the stomach, and neuralgia. Sometimes paralysis sets in and death may occur.

Bridge caissons may be of enormous size. A caisson built in constructing the bridge over East River, New York, was 102 feet in diameter. It was sunk to a foundation 172 feet below the surface of the river.

See Cofferdam