caisson
Caisson
caisson
[′kā‚sän]caisson
Caisson
a structure that forms a water-free working chamber under water or in water-saturated ground by forcing compressed air into the chamber.
Caissons are usually built on the surface and sunk into the ground as the earth is removed under the effect of their own weight and the weight of the superstructure. The caisson can be lowered from dry land, from an artificial or silted island, or from the surface of the water. The basic operation in lowering the caisson consists of dredging the soil and delivering it to the surface. Rocky or hard clay soils are removed by blasting or by pneumatic tools. The dredging of scourable sandy or clay soils can be done with hydraulic equipment. The soil is washed out with water jets and removed from the caisson water-jet elevators. The use of hydraulic equipment makes possible substantial reductions in the number of persons working in the caisson, decreases the danger of the work, lowers the consumption of compressed air, accelerates construction, and reduces costs.
A compressor station continuously supplies compressed air to the caisson, maintaining the required air pressure. Safety measures must be taken, according to the air pressure in the working chamber, to prevent the occurrence of caisson disease among the workers. The length of the working day is carefully regulated, as is the compression time (that is, the transition from atmospheric pressure to the working pressure) and the decompression time (the reverse process).
Caissons today are made from reinforced concrete. The bottom of each of the sidewalls (consoles) ends in a steel footing that cuts into the soil while the caisson is being lowered. There are shaft openings in the ceiling; above these are the shaft pipes themselves and an airlock device that provides for the delivery of men and materials from the compressed-air zone to the atmospheric-pressure zone and back. After the foot of the caisson has reached the planned level, the working chamber is completely or partially filled with concrete or sand. When the operating loads are small or when the foundation is strong and under little strain, the working chambers are left unfilled.
Caissons were once widely used in building bridge foundations. Today, however, they have for the most part been replaced by new types of deep supports and driven pilings. Caissons are used in sinking submersible structures into the ground; these are relatively small, deep underground structures, the major portion of which is erected earlier on the surface. The caisson method is used in building pumping stations, water intakes, tunnels, and deep foundation pits for industrial buildings. The method has certain drawbacks, including dangerous conditions, high costs, and the comparatively shallow depth of submersion. In a number of instances these drawbacks have limited its use.
A detachable caisson is sometimes used for underwater work that does not involve sinking the structure into the ground (chiefly repair and reconstruction work in hydraulic engineering construction). A detachable caisson is a metal or reinforced-concrete open-bottom box (an air bell) that is lowered to the bottom. Communication with the caisson is carried out through vertical shafts that rise above water level.
The device that partly removes the water from around the hull of a ship during repair or inspection is also called a caisson, in this instance a wooden or metal box whose inner side is shaped to fit the hull area to be drained. After the water has been pumped out of the caisson, the caisson is forced tight against the hull by the pressure of the surrounding water. The use of a caisson makes it unnecessary in many instances to bring the ship into dry dock.
REFERENCES
Zingorenko, G. I., and N. A. Silin. Gidromekhanizatsiia kessonnykh rabot. Moscow, 1949.Khalizev, E. P. Vybor optimal’nogo rezhima raboty gidromekhanizatsionnykh ustanovok ν kessonakh. Moscow, 1957.
Pravila bezopasnosti pri proizvodstve rabot pod szhatym vozdukhom (Kessonnye raboty), 2nd ed. Moscow, 1960.
E. P. KHALIZEV
Caisson
a hollow, cylindrical shell (frequently circular) that is sunk into the earth. Caissons are used mainly for the installation of deep supports that transmit pressure to lower, stronger strata of the soil and for the construction of compartments that are buried in the earth. Caissons are mainly made from precast or cast reinforced concrete. The walls are vertical and smooth or stepped, with an inside taper toward the bottom, which facilitates sinking the caisson into the earth.
The soil within a caisson that is being sunk is removed by excavators, grab buckets, or water-jet pumps. When the planned level has been reached, the internal cavity of the caisson is completely filled with concrete (when erecting a support) or is partially filled to form a bottom resting on the soil and to seal off the underground compartment thus created, to prevent infiltration of water. The best practice is to sink caissons to depths of 20–25 m (especially in waterlogged soils). The diameter of a caisson can be up to 80 m. To provide rigidity in large caissons, the internal cavities are divided into sections by partitions.
Caissons are sunk in loose soils or sand by means of vibration equipment; in clayey soils, “thixotropic jackets” may be used. In this method a clay mortar is pumped in between the walls of the caisson and the surrounding soil; the mortar acts as a lubricant while the caisson is being sunk and subsequently acquires strength, particularly upon the addition of cement.
A shortcoming of a caisson is that it may deviate from the vertical while being sunk. In this case the soil is washed away from underneath on one side or extra loading is applied from the top.
M. V. MALYSHEV