What is a caisson, when is it used, and how is it constructed?

Caisson construction, or caisson foundation construction, is a water retaining structure that is mainly used in construction projects that take place underwater. Caissons are prefabricated cylinders or hollow boxes that are sunk to the desired depths in water bodies and filled with concrete to create a foundation for the project which serves as an anchor. A caisson is often required for structures such as concrete dams and bridge piers.

Caissons are similar to pile foundations, but are installed using a different method. Soil of adequate bearing strength needs to be identified below the surface and a deep foundation is sunk to the required level through material excavation. During the caisson method and construction, steel reinforcement is sometimes used for a portion of its length.

A caisson provides necessary stability to the structure. It helps distribute the weight of the structure evenly as the ground shifts under its weight. Caissons are generally constructed of concrete, steel, or wood. But as with any construction project, the choice of material will depend on the function and the type of the caisson.

The Construction process of a caisson

Caisson construction includes a whole procedure of pre-construction, during construction, and post-construction. The following are the steps included in the caisson construction method:

• After some initial formwork and concrete pour, the cutting edge is floated to the breakwater by a towboat and fastened to the caisson guide. Concrete is placed into steel forms built up along the perimeter of the box. With every concrete placement, the box becomes heavier and sinks into the water along with the caisson guide.

• Forms are also built inside the box around the air domes and concrete is placed in between. The resulting open tubes above the air domes are called dredge wells.

• When the caisson reaches the bottom, the air domes are removed and earth is excavated through the long dredge well tubes. The caisson sinks into the river bottom. Excavation continues until the caisson sinks to its predetermined depth.

• As a final step, concrete is placed into the bottom of the hollow dredge wells and the tops are sealed.

Conditions Requiring Caisson Construction

Below are some of the common instances where a caisson foundation construction is often required:

• When the soil contains large boulders obstructing penetration of piles

• When a massive substructure is required to extend to or below the rear bed to provide resistance against destructive forces due to floating objects and score etc.

• The foundation is subjected to a large lateral load

• The depth in the water body is high

• There are river forces included in the load compositions

• When the load is needed to carry at the end, caissons are preferred

• The present groundwater level is aggressive inflow, caissons are suitable

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