Caisson construction, also known as caisson foundation, is a water retaining structure that is mainly used in construction projects that take place underwater. Caisson foundations are prefabricated cylinders or hollow boxes that are sunk to the desired depths beneath oceans, rivers and other water bodies. They are then filled with concrete to create a solid foundation for the project, serving as an anchor. Structures such as concrete dams and bridge piers generally require a caisson foundation as a base.
While somewhat similar to pile foundations, caissons are installed using a different method. The soil of adequate bearing strength is found 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 is essential during construction for the purpose of providing stability to the structure. It helps to distribute the weight of structures evenly as the ground shifts under their weight. Caissons are generally constructed of concrete, steel, or wood. As with any construction project, the material to be used will depend on the function of the caisson and the type of caisson being constructed.
The Caisson construction process
Caisson construction services include a whole procedure of pre-construction, during construction and post-construction. On completion of the caisson system, there is a provision of engineered drawings to make the process appear wholesome. Following are the step included in the caisson method when building a caisson foundation:
- 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 finally touches the river bottom, the air domes are removed and earth is excavated through the long dredge well tubes, as shown in the animation below. 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 mentioned are some of the common instances for caisson construction:
- When the soil contains large boulders, which obstruct penetration of piles
- 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
- Depth of water level in the river and sea 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
Requirements for Caisson Construction
There are a few key factors to consider during the construction and transfer of caissons. Depending on if it’s a woord, steel, or concrete caisson construction, structural conditions and methods of installation used, there are some valid concerns held by the designers and engineers involved in the process. These include issues related to the construction, component design and installation of the caissons. Durability, stability, soil nature, scour depth, and the level of the water table are some of them.
Various requirements for caisson construction are involved for the purpose of sinking, filling and sealing. Important factors such as the length of the reinforcement bars and the height of the concrete lifts need to be decided in consultation with the construction contractors. In order to ensure the optimum facilitation of operations, the on-site supervisory staff must maintain the highest standards when it comes to the quality of materials used. Moreover, they must be specially trained to handle the challenging situations faced by the personnel during the sinking process of caisson construction. This will not only make the construction process faster but also ensure safety of the workforce at all times. As it is the case with anything in construction, caisson construction needs to be done with great precision and accuracy for successful results.
It is essential for contractors and engineers to keep the following requirements for caisson construction in mind. These involve the strength of the concrete mix design, the concrete shaft cover, and most importantly the quality of the formwork and concrete during caisson construction. Depending on the compliance and quality, the friction resistance to the soil can significantly increase or decrease during the sinking process.
Types of Caissons
There are different types of caisson foundations such as open, pneumatic, box, excavated and floating caissons. Open caissons are generally constructed to the depth of 50 metres and are open at both ends. Pneumatic caissons are open at the bottom but closed at the top for caisson construction at depths of over 100 metres. As the name suggests, excavated caissons are used at excavation sites, while floating caissons are prefabricated boxes with cylindrical cavities.
Advantages and Disadvantages of Caissons
There are some major benefits of using caisson construction for underwater projects. They are cost-effective to construct as compared to their alternatives, and also prove effective at deep depths of water bodies. They also have a high load capacity and can easily adapt to different environments and site conditions.
On the other hand, there are also a few disadvantages to caisson construction. They can be sensitive to gruelling construction procedures and are not considered good for use at contaminated sites. Moreover, only a few construction companies possess the requisite expertise and qualified inspectors needed for the supervision of caisson construction.
Caissons are considered to be essential building blocks during construction. Hence, it is necessary to maintain the highest standards during caisson construction, by ensuring that they are built and installed by a competent team of professionals.