What is caisson and their types?
The term caisson has been derived from the French word caisson, meaning box. It can be round or rectangular, which is sunk from the surface of either land or water to the desired depth. Caissons are used for bridge piers, abutments in rivers and lakes and other shore protection works. They are used to resist heavy vertical and horizontal loads and are used in the construction of large waterfront structures as pump houses.
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Comparison between pile, pier, and caisson:
|Pier foundation||Caisson||Pile foundation|
|Pier foundation is a type of deep foundation, which consists of a cylindrical column of large diameter to support and transfer large superimposed large superimposed loads to firm strata below.||Caissons are water-tight structures made up of wood, steel, or reinforced concrete built above the ground level and then sunken into the ground.||Pile foundation is a type of deep foundation, in which the loads are taken to a low level by means of vertical timber, concrete, or steel.|
|The types of pier foundations are masonry or concrete piers and drilled caissons.||The types of caissons are open pneumatic, monolithic, floating, excavated, etc.||The types of pile foundation are end-bearing piles, friction piles, compaction piles, anchor piles, tension or uplift piles, tension or uplift piles, sheet and batter piles, etc.|
|Pier is inserted down to the bedrock.||caisson is putting a box into underwater and pouring it with concrete.||Pile is a column of material driven by a pile driver.|
|Pier has a footing||Caisson doesn’t have a Footing.||The pile doesn’t have a footing.|
|Pier is typically dug out and cast in place using forms.||Caisson are driven Into surface condition.||Piles are driven into surface conditions.|
Classifications of Caisson foundation
Caissons may be classified into the following three categories:
1. Box caissons
2. Open caissons
3. Pneumatic caissons.
1. Box caissons.
Box caissons are open at the top and closed at the bottom. They can be made of steel. R.C.C. or timber. This type of caissons are built on land, then launched and brought to the site of the pier where they have to be sunk to the position. Box caissons can be used where,
(i) bearing stratum is available at shallow depth.
(ii) Loads are not heavy,
(iii) For sea wells and break waters.
Advantages of Box Caissons
The advantages of this caisson is its low cost of construction. used when construction of caissons at site would not be feasible or costly.
Disadvantages of Box caisson
The disadvantages of this type of caisson are that the bearing strata should be level or made level. This type is feasible only when suitable bearing stratum is available at shallow depth. Provision should be made for scour protection. The bearing stratum is often not compact.
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2. Open caissons.
It is a box of timber, steel or R.C.C. or masonry with both ends open. It is used for building well as well as bride foundations. Open caissons are called wells. Small caissons consist of one opening or well while large caissons contain a series of wells.
Advantages of Open caisson (well foundation)
The advantages of open caissons are their feasibility to be sunk to great depths. Their construction cost is relatively low.
Disadvantages of open caisson
The disadvantages of the open caissons are that the bottom of the caisson cannot be thoroughly cleaned and inspected. The concrete seal placed in water is not satisfactory and soil near the cutting edge may require hand excavation by diver. If obstructions like boulders or logs are encountered the construction gets slowed down.
3. Pneumatic caissons.
A pneumatic caisson is one which has a permanent or temporary roof near the bottom and its lower end is designed as a working chamber in which compressed air is forced to prevent the entry of water and thus permits excavation in dry conditions. The essential parts of pneumatic caissons are:
(1) Working chamber,
(2) Shaft and airlocks.
The working chamber may be made of timber, steel, or R.C.C. but the shaft is usually made of steel This type of caisson is composed of an inner and outer skin plate of steel with steel truss or girders as horizontal support to form a box-like structure.
For easy penetration in the soil, steel cutting edges are provided at the bottom. A working chamber of 3 to 4 meters high is formed by providing an air-tight roof. The access to the working chamber is kept through the shafts and airlocks.
When a man is required to go into the working chamber, he first enters into the airlock, the door of the air lock is closed and the pressure is increased equal to that of the working chamber. Now the door of the shaft is opened and the man enters into the working chamber. Similarly, at the time of coming out of the working chamber, the process is reversed.
The pneumatic caisson is constructed on the river bank and then launched and taken to the site of the pier. At the pier site, the concrete is filled in between the double walls and sunk into the river bed. Gene rally two shafts, one for workers, and the other for concrete delivery and refuse are provided as shown in Fig. 2.
After sinking the caisson into the river bed, the compressed air is forced in the working chamber to check the entry of water into the working chamber. Now the excavation is started in the river bed and the caisson is allowed to sink deeper. As the caisson sinks down to balance the pressure of outside water, the pressure of compressed air is increased and in this way, water is not allowed to enter the working chamber.
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When the caisson has reached the desired depth, the bottom is prepared for concreting, and concreting is done through the concrete shaft, forming a thick layer at the bottom that acts as a seal. Now the air pressure was released and shafts and air locks were removed from the caisson. The remaining portion is filled with concrete under atmospheric pressure, Formation of air pockets and development of shrinkage should not be allowed during the period of concreting and its setting.
Advantages Pneumatic Caissons
The advantages of pneumatic caissons are that all work can be done in dry and there is control overwork. The foundation is prepared better and the plumb ness of caissons is easier to control. The concrete placed in dry condition will have good and reliable quality. The obstruction from boulders and logs can readily be removed and excavation by blasting may be done if necessary.
Disadvantages of Pneumatic caissons
It has a high cost of construction. They cannot be sunk to depths greater than 35 m because the higher pressure below this depth cannot be resisted by the human body.
Bearing Capacity of Caissons
The bearing capacity of caissons is obtained in a similar manner as in the case with other foundations.
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