Difference between cofferdams and caissons

Cofferdams and caissons are both structures that are used when undertaking construction works in areas submerged in under water.

The main difference between the two is that a cofferdam is a temporary structure which is removed after completion of the work, whereas a caisson is built to remain in place as part of the completed structure.

The aim of a cofferdam is to be as watertight as possible to create a dry area in which to carry out the required works, or at least to limit water ingress to a safe level that can be pumped away.

A caisson is typically a box-like structure made of materials such as timber, steel, masonry and reinforced concrete. It may be constructed onshore then floated to the required location, where it is sunk into place, enabling access to the bed to undertake works.

The type of structure being built will determine whether a caisson or a cofferdam is most suitable. The two principal factors influencing the choice are the ground conditions and the depth to which the work is to be taken. In general, cofferdams are suitable for depths of up to 18 m below high water level, while caissons are more suitable for greater depths.

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