Caisson Foundation - Types, Mechanism, Pros & Cons Explained

As a cornerstone of modern site engineering, a caisson serves a vital role in establishing steady foundations for different structures such as bridges, skyscrapers, and other buildings. The caisson foundation offers a potential solution for conditions where soil might be too weak to support heavy loads. It provides a dry area at a site that is typically underwater. Read on to learn more about this vital foundation type.

What is a Caisson Foundation?

Caisson is a water-retaining structure and pier foundation system used as a bridge pier to construct dams or repair ships. A prefabricated hollow box or cylinder is sunk into the ground to some depth and then filled with concrete, which helps create a dry, sturdy area below water level by keeping water out, making it suitable for foundations in aquatic environments. This feature is critical in supporting heavy loads and providing stability where other types of foundations may not be feasible. The caisson remains in its original position and becomes an integral part of the structure.

Importance of the Caisson Foundation

The foundation acts as a large, hollow box or cylinder that is sunk to a desired depth and then filled with concrete, giving it the strength needed to support significant loads and prevent water from seeping into the construction area. This dry, reinforced space ensures long-term stability and reduces the risk of shifting due to soil movement. While compared to pile foundations, caissons offer distinct advantages and features that make them suitable for certain applications.

Mechanism of Caisson

A caisson is an impermeable box with no bottom that may be submerged in water while maintaining a dry workspace. Water cannot enter the caisson when it gets submerged because of trapped air. To counteract this, a platform is built above, with a tube leading to the caisson, through which compressed air is pumped. This equalises the pressure inside and outside, keeping water from seeping in.

Once the caisson is stabilised, workers enter and start excavating the soil at the bottom. However, removing soil could allow water to flood in, so a water-filled pipe is installed. Air pressure from both the caisson’s interior and the external environment holds the water in place within the pipe. Workers lower a bucket through the pipe to collect soil, which is then lifted to the surface. This process allows workers to dig progressively deeper into the riverbed, ensuring the caisson remains stable and watertight throughout.

Types of Caisson Foundations

Here is an overview of the different types of Caisson foundations:

Box Caissons: Watertight, heavy boxes that open at the top are called box caissons. After being floated to the building site, they are lowered and filled with brickwork or concrete.
Excavated Caissons: To stabilise the foundation, these cylindrical caissons are placed into a site that has already been dug and filled with concrete.
Floating Caissons: Also known as floating docks, floating caissons are prefabricated boxes with cylindrical cavities that are easy to transport and install.
Open Caissons: Similar to box caissons, open caissons have openings on both ends. They are utilised for deep manholes, tunnels, and jetties and are appropriate for soft terrain.
Pneumatic Caissons: Perfect for underwater construction, these are big, air-pressure-powered, waterproof boxes.
Compressed Air Caissons: These employ an air pressure and are intended for dry working environments when alternative techniques are less successful.
Monolithic Caissons: Compared to other caisson types, this monolithic caisson is the largest one of any other type.

Caisson Construction Process

The site must first be marked and secured to place a caisson foundation. After that, a cutting edge is added to the precast caisson on land to help with its controlled sinking. A towboat is used to transport the partially built caisson to the installation location once it is ready. The caisson is attached to a guiding structure at the site, which guarantees proper alignment as it is lowered gradually. After that, concrete is added gradually to give it weight, which causes the caisson to gradually sink until it reaches the required depth. The caisson can be precisely positioned in underwater foundations such as bridge piers and breakwaters thanks to the guide’s assistance in controlling this descent.

Around the air domes, forms are built inside the box, and concrete is poured between them to avoid getting in water due to soil transfer. The open space above the air domes is called dredge wells. The caisson sinks deeper when it reaches the riverbed because the air domes are taken off and dirt is dug up through the dredge wells. Until the desired depth is reached, excavation keeps going. Lastly, the foundation is finished by pouring concrete into the bottom part of the dredging wells and sealing the tops.

Advantages of the Caisson Foundation

For underwater foundations, caissons offer affordable solutions.
They are easier to build and clean than any other type of foundation.
Caisson effectively handles both horizontal and vertical loads.
They can be altered according to the specific site requirements.
They are easy to handle and install.
They don’t require additional pile caps.

Disadvantages of the Caisson Foundation

Precise and complicated engineering techniques are required to build a caisson foundation.
Old structures cannot be renewed with this technique.
Structural engineers and geotechnical engineers are quite limited in this field to design this type.
Workers in underground structures may face hazardous diseases like caisson disease due to decompression.
Qualified inspectors are required, but not easily available for inspecting this type of foundation.

Conclusion

Caisson foundation stands as a significant part of civil engineering for its ultimate support and structural integrity for bridges, docks, etc. It offers permanent support for the structure that is going to be built above it. The word caisson defines a box-like structure. In short, construction work that needs high structural support, especially underwater, may use this caisson foundation for its strength and stability.