S.T.B.S. College of Diploma Engineering Seminar Topic – Caissons Guided By –

Caissons

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.

Caisson Schematic

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.

Types of Caissons There are four main types of caissons are, Box Caisson Open Well Caisson Pneumatic Caisson Monolithic Caisson

Types of Caissons

Shapes of Caissons Basic Shape Combination of Basic Shape Circular Rectangular Square Octagonal Double Circular Double Rectangular Double D Double Octagonal

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.

Open Caisson

Open Caisson

The open caissons may fill with water during sinking 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 pressurized Bentonite slurry.

The water in the caisson (due to high water table) balances the up thrust 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. 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.

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.

The process of sinking resembles in many points that described in the previous section. The caisson is built on shore, is launched, and is towed to its position. Sometimes, for the sake of economy(provided timber is cheap), that portion of the pier from the top of the working chamber to within a few feet below the low-water line, may be built as a timber crib and filled with loose stone or gravel merely to weight it down. This method is usually cheaper than masonry ; and the timber, being always under water, is durable.

Pneumatic Caisson

Components of Pneumatic Caissons Air Shaft Working Chamber Air Lock Man Lock Compressed Air Supply Tube

Advantages of Pneumatic Caissons Quality control is good because work is done in dry conditions. Concrete gain more strength due to dry conditions. For major projects, greater depths in bed rocks can be possible. There is no danger of settlement of adjoining structures because of no lowering of ground water table.

Disadvantages of Pneumatic Caissons Construction of pneumatic caissons is much expensive than open caissons. During working the various constructional activities, a proper care has to be taken, otherwise it may lead to fatal accidents. There is more chances of caisson diseases to workmen working under high pressure. Labor cost is high.

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