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Caisson (Pier) Foundation: Types, Construction and Advantages

Caisson Foundation also known as the Pier Foundation has been used as the bridge pier during the construction process of the concrete dam or during the repairing of the ships. This watertight retaining structure has been used in those bridge construction processes in which a foundation beneath the river or other water bodies is required. Caisson Foundation has the ability to be floated to the job site and sunk into the place.

The Caisson Foundation has been used in case the soil of adequate bearing strength has been found below the surface layer of the weak materials including peat or fill. The Caisson Foundation is a deep foundation constructed above ground level.

The Caisson Foundation has been constructed by auguring a deep hole into the ground and filling the hole with concrete material. The load of the construction structure has been carried out at its lower ends with the help of the caisson foundation which is sometimes bell-shaped.

The construction procedure of the Caisson Foundation

The construction process of the caisson foundation has been done in several steps which include:

• After doing the initial formwork and pouring the concrete on the construction structure, the cutting edge of the foundation was floated to the breakwater with the help of the towboat which helps to fasten to the guide of the caisson foundation. After that, concrete has been poured into the steel forms which have been built up along with the box perimeter. After placing the concrete, the box becomes heavier and can sink into the water along with the caisson guide.
• Inside the box, forms have been also built around the air domes and concrete has been poured in between.
• In the case of the Caisson Foundation finally touching the bottom of the river, the air domes have been removed as a result, the earth is excavated with the help of the long dredge well tubes and the Caisson Foundation sinks into the bottom of the river. The excavation process has been continued until the caisson has been sunk into the predetermined depth.
• At the last and final step of the caisson foundation construction process, concrete has been poured into the 30-foot bottom of the hollow dredge wells and the top has been sealed.

Advantages of Caisson Foundation

There are numerous advantages of using a caisson foundation in construction units which include:

• Caisson foundation can be extended to large depths.
• The cost of constructing a caisson foundation is relatively lower on the bed level or the lower side.
• The caisson foundation can be adopted at different site conditions.
• The caisson foundation also helps to minimize the need for pile caps.
• There is less noise and less vibration during the construction process of the caisson foundation.
• The caisson foundation also has high axial and lateral loading capacity.

Types of Caisson Foundation

The caisson foundation has been classified into several types which are:

Open Caisson

This type of caisson foundation is the type of box caisson. On the basis of the shapes of this type of foundation, this foundation has been classified into three types which are:

Single wall open caisson

There is no top or bottom for this type of box-like structure during the construction procedure. This type of open caisson consists of vertical walls.

Cylindrical open caisson

This type of open caisson is a cylindrical shell which is made up of masonry, timber, steel or reinforced concrete shod along with a cutting edge. This foundation has been sunk by excavating the soil in the shell. In this case, the thickness of the caisson should be adequate so that this foundation can sink under its own weight in case the inside soil has been dredged out.

Open caisson with dredging wells

This type of open caisson is the deepest foundation which is used for abutments, bridge piers, or other types of similar structures. This type of open caisson foundation is rectangular or square in plan which is subdivided into smaller sections. The outside walls and the inside divider walls of this caisson have been made up of reinforced concrete.

Box Caisson

This is another type of caisson foundation which is similar to the open caisson foundation though this type of foundation has been closed at the bottom. This type of caisson foundation has been cast and cured on the ground when it is required.

This type of caisson foundation has been launched into the water and towed to the site for sinking. By filling with gravel, sand or concrete inside the empty space of the caisson, this caisson can be sunk. The place must be levelled in which the base of the caisson has been rested. The natural bottom soil of the river bed has been dredged out to some depth and filled with sand for having a levelled base. This sand layer helps to distribute the load uniformly over the soil below and thus helps to avoid tilting of the caisson foundation.

Pneumatic Caisson

This caisson foundation type has been opened at the bottom and closed at the top in the meantime of the construction process. From the caisson chamber, the water has been excluded by means of the compressed air. These types of large watertight boxes are used for underwater construction purposes. During the construction of the Pneumatic caisson foundation, compressed air was used to remove the water from the chamber which helps to carry out the foundation work in a dry condition.