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Caisson (engineering)
In geotechnical engineering, a caisson (/ˈkeɪsən, -sɒn/; borrowed from French caisson 'box', from Italian cassone 'large box', an augmentative of cassa) is a watertight retaining structure. It is 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.
Caisson engineering has been used since at least the 19th century, with three prominent examples being the Royal Albert Bridge (completed in 1859), the Eads Bridge (completed in 1874), and the Brooklyn Bridge (completed in 1883).
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 box (with sides and a bottom); where required it is set down on a prepared base. Once in place, it is filled with ballast to become part of the works, such as the foundation for a bridge pier. Hollow concrete structures are generally less dense than water so a box caisson must be secured to prevent it from moving offsite until it can be filled with ballast. 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 and launch pits for microtunnelling, 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:
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Caisson (engineering)
In geotechnical engineering, a caisson (/ˈkeɪsən, -sɒn/; borrowed from French caisson 'box', from Italian cassone 'large box', an augmentative of cassa) is a watertight retaining structure. It is 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.
Caisson engineering has been used since at least the 19th century, with three prominent examples being the Royal Albert Bridge (completed in 1859), the Eads Bridge (completed in 1874), and the Brooklyn Bridge (completed in 1883).
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 box (with sides and a bottom); where required it is set down on a prepared base. Once in place, it is filled with ballast to become part of the works, such as the foundation for a bridge pier. Hollow concrete structures are generally less dense than water so a box caisson must be secured to prevent it from moving offsite until it can be filled with ballast. 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 and launch pits for microtunnelling, 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: