Caisson (engineering) القيسونات

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.


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 the height of the water in the muck tube must be carefully regulated to prevent unnecessary overpressure or low pressure which could allow 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. The Brooklyn Bridge was built with the help of caissons, and several workers were either killed or permanently injured by caisson disease during its construction, including the designer's son, Washington RoeblingCaissons have also been used in the installation of hydraulic elevators where a single-stage ram is installed below the ground level.

PILE TESTS

Pile tests are necessary to:
determine the expected settlement of the pile at working load or at some multiple thereof;
determine the ultimate bearing capacity of the pile; and/or
check the pile integrity (structural soundness).
In this unit, we will illustrate and describe some of the methods of testing piles. The tests described include both the static and the dynamic methods.
Methods of Testing Piles
There are many methods of testing piles. These methods can be grouped into three categories:
the static load principle,
the dynamic load principle, and
other tests.
For tests based on the static load principle, the load applied to the test pile is static, that is, not moving. Two types of tests based on the static load principle are:
the maintained load test (ML), and
the constant rate of penetration (CRP) test.
For tests based on the dynamic load principle, the load applied is dynamic, that is, moving or vibrating. Three types of tests based on the dynamic load principle are:
the driving formula test,
the Case-Goble test, and
the steady state dynamic response test.
Other tests include:
the sonic test, and
the coring test.

HALFAYA BRIDGE 30-MAY-2009