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 subsequently becomes an integral part of the structure.
Uses of Caisson Foundation
The Following are uses of the 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 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 is also used for pump house construction 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 the caisson foundation.
- Reinforced Concrete
Shapes of Caisson
Caisson foundations mainly have two major shapes and other combinations of other shapes.
Shapes of Caisson foundation:
- Basic Shape
- Combination of Basic Shape Basic Shape:
Types of Caisson Foundation
The main function of the caisson foundation is to make space for the easy construction of a foundation underwater.
Following are the types of caisson used,
- Open Caisson
- Box Caisson
- Pneumatic Caisson
1. Open Caisson Foundation
An open caisson foundation is a box type 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 where no firm bed is available at a higher depth.
According to the shape of an open caisson, they are also classified into 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 under the following conditions:
- Dewatered Construction
- Artificial Island
In the method of an artificial island, the island is constructed by raising the ground surface above water level temporarily for obtaining a relatively dry area for the sinking process.
The size of the Artificial island should be such that it can provide more working space all around the caisson.
- In case, dry condition is possible on the construction site, then the caisson is built in barges or slipways. Then, it is 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 the well makes the 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 manpower. Also, there is a water jet on the external wall of the well to make easy run the sinking process more easily.
- The caisson is displaced to the required depth, and then the concrete seal cap is provided caisson is dewatered by the method of pumping.
Advantages of Open Caisson Foundation:
The following are the advantages of Open Caissons:
- Open Caisson can be constructed up to large depths.
- Its construction cost is relatively less on the 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 is 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 places 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 a 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 a pneumatic caisson,
1. Air Shaft:
- A passage that connects the working chamber and air lock is termed an ‘air shaft’
- This passage or air shaft is generally used to get out or to reach the working chamber to the ground surface.
- In case of a too big a caisson, a
- 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 the air shaft is sealed by a rubber gasket airlock 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 that 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 leakproof to reduce skin friction. For an easy sinking process, cutting edges are 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 an ‘Air Lock’. The airlock is provide the facility for easy entry and exit of workers or workmen from the caisson without releasing the air pressure in the working chamber.
- Air lock chambers generally consist of two airtight doors, one door opens into a shaft and another door opens to the atmosphere. So, when workers enter the airlock chamber from the outside door, the pressure in the chamber is kept at the atmospheric level.
- Then the airlock chamber pressure is increased gradually till it becomes equal to the working chamber. When the pressure becomes equal then workmen are allowed to go into the air shaft. The reverse process is done when workmen come out of the air shaft to an airlock.
- Also, there is a facility to send fresh air inside the shaft by opening a fresh air valve which circulates fresh air for workers or workmen and which allows working into the working chamber for up to 2 hrs.
- Workers can only work for 2 hrs. inside the shaft.
4. Miscellaneous Equipment:
Different types of miscellaneous equipment used in pneumatic caisson are as follows:
- Air Compressors
This equipment is 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 available to reach the bottom of the caisson, hence any obstruction can easily be removed.
- Concrete elements are 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 the lowering of the groundwater table.
Disadvantages of Pneumatic Caisson Foundation
The following are the disadvantages of a pneumatic caisson.
- The construction cost of pneumatic caissons is much higher compared to open caissons.
- During performing various constructional activities in a caisson, proper care has to be taken; otherwise, it may lead to fatal accidents.
- There is a limit of depth below the water table of 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.
- Labor cost is high.
Loads on Caissons:
The followings are the load that acts on the caisson:
- The dead load of super-structure
- A load of bearing pier
- Various horizontal forces
Following are the horizontal forces that act on a caisson,
- Wind force
- Earth Pressure
- Seismic load
- Centrifugal forces
- Water current forces
- Tracking and tractive efforts of the moving vehicles
Floating of Caissons
In case when caisson is constructed away from the site or off-shore, then these caissons are transported to the required location by means of floating, then it is called floating of caissons. In short, a method of displacing or transporting off-site constructed caisson by a floating method is called a floating caisson.
Floating Caissons are also known as box caissons. Box caissons are cast on the 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 the water, then it is called a ‘floating’ Caisson.
- The Caisson sinking process can be made faster by increasing the self-weight of the 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, the load-carrying capacity of the caisson completely depends on resistance at the base since there is no skin friction on the sides.
- A Concrete cap is provided at top of the caisson to carry the loads uniformly from the superstructure. To prevent scouring action at the base of a floating caisson, a rip-rap is provided around the base for better understanding.
Construction of Floating Caissons
The construction method of the floating caisson is as follows:
- Firstly, the Floating Caisson is constructed and cured on the ground.
- After proper curing is done, it is floated to the desired location.
- For proper sinking of a caisson, the self-weight is increased by adding gravel or sand so as to facilitate the proper sinking process.
- The caisson base location where is to be placed is excavated and leveled.
- After that, the caisson is sunk to the required location and depth.
- To prevent the scouting, the Rip-Rap is provided all around the base of the caisson.
- At the top of the caisson concrete cap is cast, so as to carry the loads uniformly from the superstructure.
- Floating caissons are cast 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.
In conclusion, a caisson foundation is a type of deep foundation used to provide stability and support in challenging soil and water conditions. We have explored the different types of caisson foundations, including open caissons, pneumatic caissons, and drilled caissons. However, they come with disadvantages like the need for specialized equipment and complex construction processes. Caisson foundations find application in bridge piers, wharves, offshore structures, and high-rise buildings. Construction involves drilling or excavating a shaft and filling it with concrete.
What is a caisson foundation?
A caisson foundation is a type of deep foundation that is used to transfer the load of a structure to deeper, more stable soil or rock layers. It consists of large, watertight, cylindrical structures that are sunk into the ground and filled with concrete or other suitable materials.
What are the applications of caisson foundation?
Caisson foundations are commonly used in various construction projects, including:
Bridges and overpasses: Caisson foundations provide support for bridge piers and abutments, allowing for the safe transfer of loads from the bridge structure to the ground.
Piers and wharves: Caisson foundations are used to support the vertical and horizontal loads of piers, wharves, and marine structures.
High-rise buildings: Caisson foundations are suitable for tall buildings where deep soil layers are needed to support the weight and lateral forces of the structure.
Offshore structures: Caissons are used to support offshore oil rigs, wind turbines, and other marine structures where stability and load-bearing capacity are critical.
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