Caisson (engineering)

In geotechnical engineering, a caisson is a retaining, watertight 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. These are constructed such that the water can be pumped out, keeping the working environment dry. When piers are to be 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.

Schematic cross section of a pressurized caisson

Shallow caissons may be open to the air, whereas pneumatic caissons, which penetrate soft mud, are 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 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.

The caisson will be 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.

Caisson disease is so named since it appeared in construction workers when they left the compressed atmosphere of the caisson and rapidly reentered normal (decompressed) atmospheric conditions. It is caused by the same processes as decompression sickness in divers. Construction of the Brooklyn Bridge, which was built with the help of caissons, resulted in numerous workers being either killed or permanently injured by caisson disease during its construction, including the designer's son and Chief Engineer of the project, Washington Roebling.^[1]

Caissons have also been used in the installation of hydraulic elevators where a single-stage ram is installed below the ground level.

Caissons, codenamed Phoenix, were an integral part of the Mulberry harbours used during the World war II Allied invasion of Normandy.

Types

A diagram of an open caisson, devised by Jacques Triger, dated 1846

The four main types of caisson are box caisson, open caisson, compressed-air caisson and monolith caisson.

Box

Box caissons are prefabricated concrete boxes with sides and bottom and are set down on prepared bases. Once in place they will be filled with concrete to become part of the permanent works, for example the foundation for a bridge pier. One problem with box caissons is that hollow concrete structures float (see WWII concrete ships) and so they must be ballasted or anchored to prevent this until they can be filled with concrete. Adjustable anchoring systems combined with a GPS survey allow engineers to position a box caisson with pinpoint accuracy. Elaborate anchoring systems may be required in tidal zones.

Open

Open caissons are similar to box caissons except that they do not have a bottom face. They are suitable for use in soft clays (e.g. in some river-beds) but not for where there may be large obstructions in the ground. Open caissons 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. The open caissons may fill with water during sinking. The material is excavated by clamshell excavator bucket on crane. The caissons are sunk by self-weight, concrete or water ballast placed on top, or by hydraulic jacks. The leading edge of the caisson or "cutting shoe" is sloped out at a sharp angle (usually made of steel) to aid sinking in a vertical manner. The shoe is generally wider than the caisson to reduce friction and the leading edge may be supplied with pressurised bentonite slurry (it swells in water to stabilise settlement or fill depressions/voids). The formation level subsoil may still not be suitable for excavation or bearing capacity. The water in the caisson (due to high water table) balances the upthrust forces of the soft soils underneath. If dewatered, the base may "pipe" or "boil" and the caisson sink. To combat this problem piles may be driven from the surface. H-beam sections (typical column sections, due to resistance to bending in all axes) 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 is poured under the water known as a "tremie pour". This will act as a pile cap and resist the upward forces of subsoil once dewatered. The piles will act as bearing (transmitting load to deeper soils or friction along their surface length) and anchorage (resist floatation in the same manner).

Compressed-air

Compressed-air caissons have the advantage of providing dry working conditions which are better for placing concrete. They are also well suited for foundations for which other methods might cause settlement of adjacent structures.

Monolithic

Monoliths are, as their name suggests, larger than the other types but are similar to open caissons. They are often found in quay walls where resistance to impact from ships is required.

Boat lift caissons

The word caisson is also used as a synonym for the water-filled trough part of caisson locks, canal lifts and inclines in which boats and ships rest whilst being lifted from one canal elevation to another. This is the opposite of the caissons mentioned earlier; the water is retained on the inside of the caisson, not excluded from the caisson.

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.

See also

• Cofferdam, a temporary water excluding structure built in place, sometimes surrounding a working area as does an open caisson.

Patents

• U.S. Patent 123,002 -- Improvement in construction of sub-aqueous foundations

References

1. ^ 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. http://archive.rubicon-foundation.org/4028. Retrieved 2008-10-08.