What Is Caisson Foundation?
Caisson is a watertight structure which it is generally made up of timber, steel, and reinforced cement concrete (R.C.C) and constructed in connection with the excavation for the foundation of piers, bridges, and dock structures etc.
Basically, the word Caisson means a box-type structure. It is a large diameter foundation which is generally adopted in the underwater Construction.
It is a Prismatic Hollow box for the cylinder which is constructed on the ground and then sunk into the ground at the desired depth and filled with the concrete to form a foundation for the Structure.
Caisson Foundation is widely used in the construction of piers of the bridges and the foundations of The structures which are constructed in the rivers or water bodies.
The main functions of caissons are soil or water retention and transfer of vertical and horizontal loads into the subsoil strata.
Advantages of Caissons Foundation
- Excavation and the process of concreting can be done in dry conditions.
- Caissons are good in the lateral as well as axial loading capacity.
- Caisson is highly adaptable in different site conditions.
- Caissons are economical as compared to traditional foundation.
- Caisson can be inserted at any place.
- It is strong enough to withstand the moments and loads applied to the structure.
- Caisson does not require pile caps, because it is filled with concrete.
- Caissons have a large cross-sectional area which helps to resist the scouring effect.
- Caissons can be extended up to the large depths in the ground.
Disadvantages of Caisson Foundation
- The Inspection and cleaning at the bottom of the Caisson are difficult.
- There is a lack of skilled workers which are required for the construction of the Caisson Foundation.
- Caissons can not be construed in the contaminated sites.
- Caissons are required to be constructed under the supervision of the Experts.
- There may be a risk to the workers to get affected by the caisson diseases. So the proper health control is essential.
- The limit for the depth of the penetration of the Caisson is up to 35 m.
Application of Caisson Foundation in Civil Engineering
- It is used in the construction of piers of the bridges in the water bodies.
- Caisson is also used in the construction of large and Multi-Floor Buildings.
- Caissons provide a platform for installation of machineries and pumps below the ground level.
- Pneumatic Caissons are used in the construction of railway bridges.
- Caissons are also used to provide access to the shaft and tunnels.
Types of Caisson Foundation
The Shape and the size of the Caisson is depended upon the structure for which it is constructed. Caissons are classified mainly into three types.
- Open Caisson.
- Box Caisson.
- Pneumatic Caisson.
#1. Open Caisson
Open Caisson is a box-type hollow structure which is generally open at both top and bottom side. Open Caisson is suitable for the sandy soils and the soil having soft strata where a solid or firm stratum is not available.
Open Caisson is constructed in reinforced cement concrete (R.C.C) and middle portion filled with sand or gravel and the lower portion is filled with the concrete which acts as a concrete plug.
The Construction of the Open caisson is generally preferred in the dry condition or dewatered area.
If it is not possible on the site to construct the Caisson in the dry conditions then it is constructed in the barges or slipways.
According to the shape of the Open Caisson are classified as
- Single Wall Open Caisson
- Cylindrical Open Caisson
- Open Caissons with Dredging Wells.
#2. Box Caisson
The Box Caisson is the same as the open Caisson. The only difference between open and box Caisson is that box Caisson has its bottom side closed.
It is made up of timber, steel, and reinforced cement concrete ( R.C.C). Box type of Caisson foundation is generally used where the bearing stratum of the soil is available at the shallow depth.
#3. Pneumatic Caisson
Pneumatic Caisson is open at the bottom side and closes at the top side. Pneumatic caisson is specially used at the place where it is not possible to construct well.
It is suitable where the depth of water is more than 12 m. In the construction process of the pneumatic Caisson, the compressed air is used to remove water from the working chamber and the foundation work is carried out in the dry condition.
The process of the Construction of the pneumatic Caisson is the same as the open Caisson. The Quality Control in the Pneumatic Caisson is good because the work is done in the dry conditions. Pneumatic Caissons are more expensive as compared to the Open Caisson.
Construction Method of Caisson Foundation
There are various methods by which Caissons are installed into the ground.
Prefabrication of Caisson
- Firstly, the caissons are generally prefabricated near the Construction site or at any other suitable location.
- After prefabrication of the Caissons, they are tested for the water tightness.
- In the tests, the Caissons are check for any leakages and, if found they are repaired immediately.
Transportation of the Caisson
- Then the Caissons are transported to the required location.
- The transportation of the Caisson should be done by following all precautionary measures.
Positioning and Immersion of Caissons
- It is very important that Caisson should be properly positioned at the desired place.
- The final positioning of a caisson is achieved with the help of tug boats or with cables from floating equipment and anchors or dead-man beds.
- When the caisson is ready, the soil is dug from the work chamber which causes the caisson to subside into the ground.
- Traditionally the excavation is done by the hands but nowadays the sand pumps and other machinery are used for the excavation.
- The excavated soil is removed with the help of a hydraulic transport system.
- The Caisson is subsided under its own weight.
- When the caisson has reached the desired depth, then the work chamber is completely filled with the Concrete. This prevents further sinking of the Caisson.
- After the immersion of the Caisson, they are ballasted.
- The main function of the dump of rubble at the side of the caisson is to prevent it to slide away and helps to maintain maximum stability.