PNEUMATIC CAISSONS The Compressed-air Method.—A pneumatic caisson as ordinarily employed consists essentially of an air-tight box, or working chamber, open at the bottom, which may be filled with compressed air to keep back the water and permit the excavation of the soil from below the bottom of the caisson by men working in the compressed air. The working chamber is ordinarily at the bottom of a crib, constructed in a manner similar to an open caisson and arranged to be filled with concrete to aid in sinking. Shafts with air locks connect, the working chamber with the outside air, and provide means of entering the working chamber and transporting materials to and from it.
This method is frequently employed for foundations to depths within which men may safely work in the compressed air, about 110 feet below water surface, and is sometimes combined with the open caisson method, being used where obstructions may be met in sinking the open caisson, or for bedding a caisson which has been sunk by open dredging. The caissons are constructed of timber, metal, or concrete. The use of timber caissons has been quite common in the United States, although concrete seems to be coming into more general use, while in Europe, iron and steel are preferred.
The pneumatic s3-stem was first applied to large foundations in this country by Mr. Eads in the construction of the St. Louis arch bridge over the Mississippi in 1870, where a depth of 109 feet below water surface was reached. This was followed by the Brooklyn bridge foundations, in which the caissons were very large in plan and sunk to a depth of 7S feet. Since that time, pneumatic caissons have been used in a large number of structures, with rapid improve ment in the methods of handling the work, and in preventing inju rious effects upon men working in the compressed air.
Pneumatic caissons have been quite extensively used for heavy building foundations, particularly in New York City, where it has been found necessary to carry the foundations of high buildings through unstable materials to solid rock, without undermining older buildings on more shallow foundations. In such construction, separate caissons of small area are sunk for individual piers, although frequently two or more piers rest upon a single caisson. The layout of
a foundation for a heavy building is a matter requiring very careful study. The first instance in which this method was applied to a building in New York was in the _Manhattan Life Insurance Com pany's building by Kimball Thompson, architects, in 1S93.' Since that time the use of pneumatic caissons has become quite common.
206. Construction of materials and methods of construction of a caisson usually vary with its size and shape. Cylin der caissons are generally of steel, although concrete is now being used to considerable extent. The thickness required for the concrete walls usually prevent its use for cylinders less than about S feet in diameter. Large caissons of rectangular shape are frequently made of timber, although steel is sometimes used, while the use of concrete is increas ing rapidly.
The working chamber is surrounded by sloping sides, resting upon the cutting edges and widening to give support to the roof, which must be capable of carrying the load of filling used in sinking the caisson. In large caissons it is necessary to brace the side walls, bulkheads being sometimes built both transversely and longitudinally across the work ing chamber for the purpose. In some of the older caissons, when the masonry of the pier was built upon the roof of the working chamber, the roofs were made very thick. The roof of the large caisson for the Brooklyn bridge was 22 feet thick of solid timber; that of the Ilavre de Grace bridge was S feet thick. In others, heavy bulkheads were used to support thinner roofs.
In timber caissons, in addition to the walls of solid timber, plank sheeting is used on both inside and outside surfaces, being caulked carefully to make them air- and watertight. In met a1 caissons, also, the joints be leaded and caulked to withstand the interior air pressure and outside water pressure. When the con struction above the roof is of concrete, reinforcement near the bottom of the concrete may make it practically self-supporting, and the roof needs only sufficient strength to carry the concrete until it has hardened. The concrete may also be expected to exclude the water more effectively than caulking.