Types Of Caisson Foundation:
The foundation constructed by sinking a caisson to rest on firm base under water and then filling the same with concrete or stone masonry to act as a pier is known as caisson foundation. This type of underwater foundation is mainly constructed in sandy soils and where the depth of water is more.
Type Of Caissons:
The various types of caissons used in constructing caisson foundations under water are given below:
1. Box caissons,
2. Open caissons,
3. Pneumatic caissons.
1. Box Caissons:
The caisson in the form of an air tight vessel without top, floated to the site and sunk to rest on a firm base, is called box caisson. This type of caisson is made of timber, steel or reinforced concrete.
2. Open Caisson:
The caisson which is without its top and bottom is called open caisson. This type of caisson is like a well with vertical sides only. It may be square, rectangular, oval or of any other suitable shape. Open caissons are usually made of RCC or of steel plates, riveted or welded together.
3. Pneumatic Caisson:
The caisson which is open at the bottom and close at the top and sunk by means of utilizing compressed air is known as pneumatic caisson.
Pneumatic caisson consists essentially of the working chamber, the shaft, and an air lock.
This type of caisson is usually made of n inner and outer skin plates with steel trusses or girders to form a box like structure. It is provided with a cutting steel edge at its bottom to facilitate sinking in the soil. The working chamber, provided in the caisson, is about 3 to 4 m deep and is made air tight by providing an air tight roof. It is meant for working of workmen during sinking process. Access to the working chamber is made through the shaft and air lock.The air pressure to be used in the working chamber depends upon the depth. The maximum pressure allowed at a depth of 35 m is 3.75 km/cm2. A normal workman can work for eight hours under a pressure of 1.3 kg/cm2. For greater pressure, the working hour is to be reduced accordingly.