Caisson Foundation

A caisson foundation, also known as a drilled shaft or drilled pier foundation, is a type of deep foundation used in civil engineering and construction. Caissons are cylindrical or rectangular structures that are drilled or excavated into the ground to support structures like bridges, buildings, or towers. They are particularly suitable for projects where shallow foundations may not provide sufficient support.

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Caisson foundation is one type of watertight structure that may have different shapes, 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.

There are different types of caisson foundations like open caisson, box caisson, and pneumatic caisson.

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:

Dry
Dewatered Construction
Artificial Island

Caisson Foundation Construction Process — Open Caissons

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.

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.

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.

Uses:

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 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.

Construction Material:

Following are the material used for the construction of the caisson foundation.

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:
Circular
Rectangular
Square
Octagonal

Floating of Caissons

Floating Caisson Foundation — **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.

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Caisson Foundation | Open Box & Pneumatic Caisson

Caisson Foundation