In geotechnical engineering, a caisson ( // or //; borrowed from French caisson, from Italian cassone, meaning large box, an augmentative of cassa) is a watertight retaining structure  used, for example, to work on the foundations of a bridge pier, for the construction of a concrete dam,  or for the repair of ships.  Caissons are constructed in such a way that the water can be pumped out, keeping the work environment dry. When piers are being built using an open caisson, and it is not practical to reach suitable soil, friction pilings may be driven to form a suitable sub-foundation. These piles are connected by a foundation pad upon which the column pier is erected.
To install a caisson in place, it is brought down through soft mud until a suitable foundation material is encountered. While bedrock is preferred, a stable, hard mud is sometimes used when bedrock is too deep. The four main types of caisson are box caisson, open caisson, pneumatic caisson and monolithic caisson. 
A box caisson is a prefabricated concrete box (with sides and a bottom); it is set down on prepared bases. Once in place, it is filled with concrete to become part of the permanent works, such as the foundation for a bridge pier. Hollow concrete structures are usually less dense than water so a box caisson must be ballasted or anchored to keep it from floating until it can be filled with concrete. Sometimes elaborate anchoring systems may be required, such as in tidal zones. Adjustable anchoring systems combined with a GPS survey enable engineers to position a box caisson with pinpoint accuracy.[ citation needed]
An open caisson is similar to a box caisson, except that it does not have a bottom face. It is suitable for use in soft clays (e.g. in some river-beds), but not for where there may be large obstructions in the ground. An open caisson that is used in soft grounds or high water tables, where open trench excavations are impractical, can also be used to install deep manholes, pump stations and reception/launch pits for micro tunnelling, pipe jacking and other operations.[ citation needed]
A caisson is sunk by self-weight, concrete or water ballast placed on top, or by hydraulic jacks. The leading edge (or cutting shoe) of the caisson is sloped out at a sharp angle to aid sinking in a vertical manner; it is usually made of steel. The shoe is generally wider than the caisson to reduce friction, and the leading edge may be supplied with pressurised bentonite slurry, which swells in water, stabilizing settlement by filling depressions and voids. An open caisson may fill with water during sinking. The material is excavated by clamshell excavator bucket on crane.[ citation needed]
The formation level subsoil may still not be suitable for excavation or bearing capacity. The water in the caisson (due to a high water table) balances the upthrust forces of the soft soils underneath. If dewatered, the base may "pipe" or "boil",[ clarification needed] causing the caisson to sink. To combat this problem, piles may be driven from the surface to act as:
- Load-bearing walls, in that they transmit loads to deeper soils.
- Anchors, in that they resist flotation because of the friction at the interface between their surfaces and the surrounding earth into which they have been driven.
H-beam sections (typical column sections, due to resistance to bending in all axis) may be driven at angles "raked" to rock or other firmer soils; the H-beams are left extended above the base. A reinforced concrete plug may be placed under the water, a process known as Tremie concrete placement. When the caisson is dewatered, this plug acts as a pile cap, resisting the upward forces of the subsoil.[ citation needed]
A monolithic caisson (or simply a monolith) is larger than the other types of caisson, but similar to open caissons. Such caissons are often found in quay walls, where resistance to impact from ships is required.[ citation needed]
Shallow caissons may be open to the air, whereas pneumatic caissons (sometimes called pressurized caissons), which penetrate soft mud, are bottomless boxes sealed at the top and filled with compressed air to keep water and mud out at depth. An airlock allows access to the chamber. Workers, called sandhogs, move mud and rock debris (called muck) from the edge of the workspace to a water-filled pit, connected by a tube (called the muck tube) to the surface. A crane at the surface removes the soil with a clamshell bucket. The water pressure in the tube balances the air pressure, with excess air escaping up the muck tube. The pressurized air flow must be constant to ensure regular air changes for the workers and prevent excessive inflow of mud or water at the base of the caisson. When the caisson hits bedrock, the sandhogs exit through the airlock and fill the box with concrete, forming a solid foundation pier. 
A pneumatic (compressed-air) caisson has the advantage of providing dry working conditions, which is better for placing concrete. It is also well suited for foundations for which other methods might cause settlement of adjacent structures.[ citation needed]
Construction workers who leave the pressurized environment of the caisson must decompress at a rate that allows symptom-free release of inert gases dissolved in the body tissues if they are to avoid decompression sickness, a condition first identified in caisson workers, and originally named "caisson disease" in recognition of the occupational hazard. Construction of the Brooklyn Bridge, which was built with the help of pressurised caissons, resulted in numerous workers being either killed or permanently injured by caisson disease during its construction.  Barotrauma of the ears, sinus cavities and lungs and dysbaric osteonecrosis are other risks. 
- Caissons have also been used in the installation of hydraulic elevators where a single-stage ram is installed below the ground level.[ clarification needed]
- Caissons, codenamed Phoenix, were an integral part of the Mulberry harbours used during the World War II Allied invasion of Normandy. 
- Boat lift caissons: The word caisson is also used as a synonym for the moving trough part of caisson locks, canal lifts and inclines in which boats and ships rest while being lifted from one canal elevation to another; the water is retained on the inside of the caisson, or excluded from the caisson, according to the respective operating principle.[ citation needed]
- Structural caissons: Caisson is also sometimes used as a colloquial term for a reinforced concrete structure formed by pouring into a hollow cylindrical form, typically by placing a caisson form below grade in an open excavation and pouring once backfill is complete, or by drilling at grade, although this can be problematic with deep caissons, as unsupported excavations can collapse before the caisson form can be inserted. In this manner, the earth placed around the empty caisson form provides stability and strength, allowing concrete to be poured with fewer complications and with less risk of a form blowout. While, technically, only the form itself is actually a caisson, it is not uncommon for any below-grade cast concrete pillar to be referred to as, simply, a caisson.[ citation needed]
- Ventilation filtration systems: The word caisson is also used as a name for an airtight housing for ventilation filters in facilities that handle hazardous materials. The housing usually has an upstream compartment for a pre-filter element and a downstream compartment for a high-efficiency filter element. It may have multiple sets of compartments. The housing has gasketed access doors to allow for the change out of the filter elements. The housing is usually equipped with connection points used to test the efficiency of the filters and monitor changes in the differential pressure across the filter media.[ citation needed]
- Suction caisson – Open bottomed tube anchor embedded and released by pressure differential
- Air lock diving-bell plant – An underwater work support barge used at Gibraltar, a mobile barge-mounted engineering caisson used in the Port of Gibraltar
- Cofferdam – Barrier allowing liquid to be pumped out of an enclosed area, a temporary water-excluding structure built in place, sometimes surrounding a working area as does an open caisson.
- Offshore geotechnical engineering – A sub-field of engineering concerned with human-made structures in the sea, for information on geotechnical considerations.
- U.S. Patent 123,002 – Improvement in construction of sub-aqueous foundations
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- Wilson, Theodore Delavan, and Edward J. Reed. An outline of ship building, theoretical and practical. New York: J. Wiley & son, 1873. 383. Print.
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- Wallace, Mike (2018). Greater Gotham: A History of New York City from 1898 to 1919. New York: Oxford University Press. pp. 155–56. ISBN 9780195116359.
- Butler WP (2004). "Caisson disease during the construction of the Eads and Brooklyn Bridges: A review". Undersea Hyperb Med. 31 (4): 445–59. PMID 15686275. Archived from the original on 2011-08-22. Retrieved 2008-10-08.
- Stellman, Jeanne Mager. Encyclopaedia of occupational health and safety. 4th ed. Geneva: International Labor Office, 1998. 36.5. Print.
- Staff. "The story of the Mulberry harbours". arromanches-museum.com. Arromanches: Musée de Débarquement. Retrieved 1 June 2017.
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