04-08-2012, 02:30 PM
Caissons
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What are Caissons?
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.
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 from the edge of the workspace to a water-filled pit, connected by a tube to the surface.
Box Caisson
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 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 Caisson
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 tunneling, pipe jacking and other operations.
Advantages of Open Caissons
This type of Caisson can be extended up to large depths.
Cost of construction is relatively less on bed level or lower side.
Disadvantages of Open Caissons
Since the placing of Concrete is done for concrete seal under water, it may not be satisfactory.
Though cleaning and inspection at the bottom of caisson is very difficult and hence not possible.
The help of divers may be required for excavation near haunches at the cutting edges.
Pneumatic Caisson
A pneumatic caisson is essentially a large inverted box on which a pier is built, and inside of which work may be done because the water is forced out of the box by compressed air.
If an inverted tumbler is forced down into a bowl of water, the large air space within the tumbler gives some idea of the possibilities of working within the caisson.
If the tumbler is forced to the bottom of the bowl, the possibilities of working on a river bottom are somewhat exemplified. It is, of course, necessary to have a means of communication between this working chamber and the surface; and it is likewise necessary to have an air-lock through which workmen may pass.