What Is Caisson Foundation:
Caisson foundation is one type of watertight structure that may have different shape-round, rectangular, etc., which is sunk through ground or water to exclude water and semi-fluid materials during the process of excavation of foundations and which is subsequently becomes an integral part of the structure.
Uses of Caisson Foundation:
The Following are uses of caisson foundation,
- Caisson foundation is mostly used in deep foundation construction where the foundation should be extended up to or below the river bed so as to obtain the proper stability.
- Caisson foundation is one type of good foundation that is constructed in connection with piers and abutments in rivers and lakes, bridges, breakwater dock structures, and also in case of shore protection, lamp house, etc.
- In case, when a good foundation is to be constructed underwater the use of a caisson is more preferable.
- When constructing any structure in river water, lake and depth of water are more, than caisson foundation structure is used.
- Caisson is used as a foundation for bridges piers, and abutments in rivers, seas, lakes, breakwaters, and other shore construction work.
- Caisson foundation also used for pump house construction in which is subjected to huge vertical as well as horizontal forces.
- It is also occasionally used for large and multi-story buildings and other structures.
Caisson Foundation Construction Material:
Following are the material used for the construction of caisson foundation.
Shapes of Caisson:
Caisson foundation mainly have two major shapes and other combination other shapes.
Shapes of Caisson foundation:
- Basic Shape
- Combination of Basic Shape Basic Shape:
Types of Caisson Foundation:
The main function of caisson foundation is to make space for easy construction of foundation under water.
Following are the types of caisson used,
- Open Caisson
- Box Caisson
- Pneumatic Caisson
1. Open Caisson Foundation:
An open caisson foundation is box types of construction that is generally open at the bottom and top of the box. These types of caisson are normally used for sandy soils or soft bearing stratum and where no firm bed is available at a higher depth.
According of shaped of open caisson, they are also classified in further types,
- Open Caisson
- Single Wall Open Caisson
- Cylindrical Open Caisson
- Open Caissons with dredging wells.
Construction of Open Caissons:
The sinking process can be done in following conditions:
- Dewatered Construction
- Artificial Island
In the method of artificial island, the island is constructed by raising the ground surface above water level temporarily for obtaining relatively dry area for the sinking process.
The size of Artificial island should be such that it can provide more working space all around the caisson.
- In case, dry condition is one possible on construction site, then caisson is built in barges or slipways. Then, it displaced to its final position by floating. For sinking the first few lifts guide piles are generally used. Remember that the Caissons sinking is directly done in open water.
- Dredging the soil around well makes caisson sinks by its own weight and the excavation process is done by dredging with the help of grab buckets. The soil left near the cutting edge can be removed by man power. Also, there are water jet on external wall of well to make easy run the sinking process more easily.
- The caisson is displaced to required depth, and then concrete seal cap is provided caisson is dewatered by the method of pumping.
Advantages of Open Caisson Foundation:
Following are the advantages of Open Caissons:
- Open Caisson can be constructed up to large depths.
- It construction cost is relatively less on bed level or lower side.
Disadvantages of Open Caisson Foundation:
Following are the disadvantages of open Caissons;
- Since the concrete placing is done underwater for concrete seal, it may not be satisfactory.
- If any obstruction of boulders or logs are encountered,
- Then the progress of work becomes slow.
- Inspection and cleaning at the bottom of Caisson are very hard and difficult hence not possible.
- The help of divers may be required for excavation near
- Haunches at the cutting edges.
2. Box Caisson Foundation:
Box caisson is almost the same as the open types of caissons, the only difference is that it is closed at the bottom. Box caisson is constructed on the ground surface and then it is launched in water by filling sand or gravel or concrete in the empty spaces.
Read More: Different Types of Foundation
3. Pneumatic Caisson Foundation:
The pneumatic types of caissons are open at the bottom and close at the top. These types of caisson foundations are specially used at the place where it is not possible to construct the well.
- It is used when the depth of water more than 12 m.
- In the construction of Pneumatic Caisson, to remove water from the working chamber compressed air is used and the foundation work is carried out in the dry condition.
- It can be constructed from timber, concrete, or steel.
Components of Pneumatic Caisson:
Following are the different components of pneumatic caisson,
1. Air Shaft:
- A passage which connects the working chamber and air lock is termed as ‘air shaft’
- This passage or air shaft is generally used to get out or to reach to the working chamber to ground surface.
- In case of too big caisson, the separate chamber of air shaft may be provided for workers and material.
Air Shaft is constructed from steel material. Any joint in the construction of air shaft is sealed by rubber gasket airlock is provided on each air shaft at the top. During the sinking process of the caisson, the air shaft is extended above the water level.
2. Working Chamber:
- Working Chamber is an airtight chamber which is made up of structural steel having a height of 3m.
- The air inside the working chamber is kept at a pressure just more than atmospheric pressure to prevent the entry of air and water into the chamber.
- Chamber’s external face is kept thick. The working chamber is generally made smooth and leak proof to reduce skin friction. For easy sinking process, cutting edges provided at the bottom.
3. Air Lock:
An Airtight chamber construction at the upper end of the air shaft above the water level made of steel material is called ‘Air Lock’. The airlock is provide the facility of easy entry and exit of workers or workmen from the caisson without releasing the air pressure in the working chamber.
- Air lock chamber generally consist of two air tight doors, one door opens into shaft and another door opens to the atmosphere. So, when workers enter into the airlock chamber from outside door, then pressure in the chamber is kept at atmospheric level.
- Then the air lock chamber pressure is increased gradually till it becomes equal to the working chamber. When pressure becomes equal then workmen is allowed to go into the air shaft. Reverse process is done when workmen comes out of the air shaft to air lock.
- Also, there is facility to send fresh air inside the shaft by opening fresh air valve which circulate fresh air for workers or workmen and which allows to work into the working chamber up to 2 hrs.
- Workers can only work for 2 hrs. inside the shaft.
4. Miscellaneous Equipment’s:
Different types of miscellaneous equipment’s used in pneumatic caisson are as follows:
- Air Compressors
These equipment are normally placed above the bed level. By using a compressed air pipe, can be utilized in the working chamber.
Advantages of Pneumatic Caisson Foundation:
Following are the various advantages of pneumatic caissons:
- Quality control is good because work is done in conditions.
- Soil bearing capacity tests can be possible in situ.
- There is an easy and direct facility is available to reach the bottom of the caisson, hence any obstruction can easily be removed.
- Concrete elements able to gain more strength due to dry conditions.
- Greater depth can be achieved in a bigger size project in the bedrock by using pneumatic caissons.
- There is no risk of sinking or settlement of adjoining structures because of no lowering of the groundwater table.
Disadvantages of Pneumatic Caisson Foundation:
Following are disadvantages of pneumatic caisson.
- Construction cost of pneumatic caisson is much higher compare to open caissons.
- During performing various constructional activities in a caisson, a proper care has to be taken; otherwise it may lead to fatal accidents.
- There is limit of depth below water table about 30 m to 40 m. Hence beyond 40 m depth, construction is not possible.
- There are more chances of caisson diseases to workmen working under high pressure.
- Labour cost is high.
Loads on Caissons:
Followings are the load that acts on the caisson:
- Dead load of super-structure
- Load of bearing pier
- Various horizontal forces
Following are the horizontal forces the acts on caisson,
- Wind force
- Earth Pressure
- Seismic load
- Centrifugal forces
- Water current forces
- Bracking and tractive efforts of the moving vehicles
Floating of Caissons:
In case when caisson are constructed away from site or off-shore, then these caisson are transported to the required location by means of floating, then it is called as floating of caissons. In short, It is method of displacing or transporting off site constructed caisson by floating method is called floating caisson.
Floating Caissons are also known as box caisson. Box caissons are casted on ground and cured; it is transferred in water and displaced to the site by floating for sinking operation.
After Casting the box Caisson, it is transported to the site by floating in water, then it is called as ‘floating’ Caisson.
- Caisson sinking process can be made faster by increasing the self-weight of caisson and it is raised by adding sand or gravel inside the caisson.
- Floating caisson is not provided with cutting edge as compared to the other caisson. They just rest on hard or level strata.
- In this way, load carrying capacity of caisson completely depends on resistance at the base since there is no skin friction on sides.
- A Concrete cap is provided at top of caisson to carry the loads uniformly from the super structure. To prevent scouring action at the base of floating caisson, rip-rap is provided around the base for better understanding.
Construction of Floating Caissons:
Construction method of floating caisson is as follows:
- Firstly, the Floating Caisson are constructed and cured on ground.
- After proper curing is done, it is floated to the desired location.
- For proper sinking of caisson the self-weight is increased by adding gravel or sand so as to facilitate proper sinking process.
- The caisson base location where is to be placed is excavated and levelled.
- After that the caisson is sunk to required location and depth.
- To prevent the scouting, the Rip-Rap is provided all around the base of the caisson.
- At the top of caisson concrete cap is casted, so as to carry the loads uniformly from superstructure.
- Floating caissons casted by using reinforced cement concrete or steel.
- In case, if caisson is too floated and placed in rough waters, internal strutting and diaphragm walls are used.
Read In Hindi: कैसन फाउंडेशन (Caisson Foundation) – इसके प्रकार और उपयोग
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