WHAT IS PNEUMATIC CAISSONS AND ITS ADVANTAGES.

 Pneumatic Caissons

A pneumatic caisson is a rigid, inverted box with open at the bottom and close at the top. A working chamber is provided at its bottom to keep the caisson free of water and mud by use of compressed air. The design of a pneumatic caisson is similar to that of an open caisson is similar to that of an open caisson in many respect. The ultimate load carrying capacity, the design of walls, concrete seal and cutting edge are similar to that of open caissons. Pneumatic caisson is specially used at the place where it is not possible to construct the well. It is suitable for the depth of the water more than 12 m. However following differences should be clearly noted.

1. Working Chamber

The working chamber is made up of mild steel. It is about 3 m high and consists of a strong roof at its top.The chamber is air tight and the air in chamber is kept at a specified pressure to prevent entry of water and soil into it. The wall of the chamber should be thick and leak proof. To keep the frictional resistance low the outside surface of the walls are made smooth.

2. Air Shaft

An air shaft is a vertical passage which connects the working chamber with an air lock at the top.The shafts are made up of steel tubes and the joints of the tubes are provided with rubber gaskets to make them leak proof.It provides an access to the working chamber for workmen and also used to transport the excavated materials to the ground surface. In large caissons two separate air shaft are provided one for the passage of workmen and other for the transportation of materials.

3. Air Lock

An air lock is a steel chamber provided at the upper end of the air shaft above water level.The purpose of provided an air lock is to permit the workmen and materials to go in or to come out of the caisson without releasing the air pressure in the caisson. The steel chamber of air lock is provided with two airtight doors, one of which opens to the shaft and the other opens to the outside atmosphere.

Advantages Of Pneumatic Caissons

• Excavation and pouring of concrete is done in the dry.
• The soil can be inspected and the soil samples can be taken if required.
• Bearing capacity of soil can be determined by conducting in-situ test in the working chamber.

Disadvantages Of Pneumatic Caissons

• It is very expansive.
• Large number of manual work required which increase the cost.
• The penetration depth below water table is limited to 30 m to 40 m.
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