Pneumatic Caisson

Published: | Updated: August 24, 2020;

Definition - What does Pneumatic Caisson mean?

A pneumatic caisson is a watertight box or cylinder-like structure that is closed at the top and open at the bottom, resting on the bed of the waterbody. They are used for underwater construction of foundations for bridge piers, abutments in rivers, and foundations for large multi-story buildings. They are designed to keep water out of the construction zone and act as a seal that keeps the inside of the caisson dry for workers to carry out work safely.

The inside of the caisson is kept dry by using compressed air to force water out of the structure. This process creates an airtight working chamber where construction activities, such as excavations, can be carried out safely. Pneumatic caissons are ideal for challenging situations where it is not possible to carry out wet ground excavations in the open. However, this method is complex, time-consuming, and relatively expensive when compared to other types of caissons.

Trenchlesspedia explains Pneumatic Caisson

Pneumatic caissons consist of an airtight chamber at the bottom where compressed air is supplied to prevent water from entering. Soil can be excavated inside the chamber, and foundation work can be carried out in dry conditions similar to how it is done on the ground surface. Pneumatic caissons are closed at the top but open at the bottom.

The working chamber is constructed similar to an open caisson and arranged to be filled with concrete to aid in the sinking. To connect the working chamber with the outside air, the shafts have airlocks that provide a means for workers to enter and exit, and materials to be transported and removed.

Sinking the Caisson

First, a concrete structure with wedge-like cutting shoes underneath is built at ground level. The shoes are high enough to allow workers to work under the structure after it is completed. The caisson is sunk by removal of the soil under it with water cannon and sand pumps. When the caisson reaches the water table, air pressure is increased to allow workers to continue working in dry conditions. Workers exit and enter through an airlock. The caisson is positioned at the required depth using bentonite lubrication and internal and external ballast as required. Once the correct position of the caisson is achieved, the work area is filled with concrete, and the foundation of the structure is completed.

Some Benefits of Pneumatic Caissons

  • The surrounding groundwater and soil is least affected as the compressed air fed into the working chamber is controlled to be equal to the pressure of groundwater

  • Pneumatic caissons can be sunk in many different soil conditions

  • No temporary earth retaining structures are required as the caissons themselves become the final underground structure

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