emailed the good people at Coastworks (they are the Co carrying out this part of the project) on Friday and they replied first thing this morning- I was very impressed with speed of response, and its a "caisson"..
umm? ok so checked this out in an online dictionary, and here you have it..
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 caissonShallow caissons may be open to the air, while deep caissons to penetrate soft mud may be 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.
So we can all sleep now..
November 9th, 2009, 10:05 PM
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