- French word “Caisson” means box, and it can be open at either ends or at one end.
- Such box with close bottom can be prefabricated on shore and then floated to the desired site, and is allowed to seat on prepared base.
- In case of caisson with both ends open is allowed to sink due to its massive dead load and also by removing soil inside the box.
- Such box when filled with sand, becomes very massive and is capable of resisting horizontal trust due to waves, or impart of ship or due to slender heavy wind etc.
- When depth of water is high, pile being slender, is unable to resist heavy thrust. In such cases caisson foundation gives better solution.
Types of Caissons:
- There are three types of Caissons :
- Box caisson
- The open caisson
- Pneumatic caisson
- This is a water tight vessel which is open at the top and closed at the bottom.
- The box could be made out of timber, reinforced concrete or steel.
- The box is constructed on the river bank.
- Then it is floated and towed to the position.
- It is then filled with dead load and sunk at the sight where it serves as a foundation.
- The box type caisson is suitable for shallow depth foundation and where the actuating forces are not considerable.
Advantages of Box Caisson:
- Box caisson can be provided in case other types prove costly or are not feasible.
- It can be extended upto large depths.
- It can be constructed on shore and floated to the place of installation.
- Essentially it is cylindrical structure made of brick R.C.C. or steel constructed at site with open bottom and top.
- The structure is penetrated at the site and when it reaches the required location, rich flowable concrete is provided to serve on plug/seal.
- When seal is completely cured, water in caisson is pumped out and then, the bottom is closed, and interspace is filled with suitable material such as murum sand etc. and then the top is also closed by concrete.
- On the top either pier or abutment is then constructed.
Advantages of open caissons:
- The caisson can be constructed to great depths.
- The construction cost is relatively low.
- This type of caisson is more suitable when the level of water in the river is sufficiently high and it is required to go very deep in order to get suitable strata having requisite bearing capacity.
- This type is essentially well with open bottom and closed top.
- In between these two, air pressure is maintained so that water does not get in excavation and concreting work can go on with ease.
- The working chamber at the bottom of the caisson is kept dry by forcing out water under air pressure.
- Air locks are provided at the top.
- The caisson is sunk as the excavation proceeds.
- Upon reaching its final depth, the working chamber is filled with concrete.
- For movement of labourer working bottom one separate passage is provided.
- For transport of material excavated muck etc. separate passage is provided.
Advantages of Pneumatic Caisson:
- Control over the work and preparation of foundation for the sinking of caisson are better since the work is done in the dry.
- The caisson can be sunk vertically as careful supervision is possible.