Caisson Foundation - Types, Advantages and Disadvantages

Caisson Foundation

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A caisson foundation is a watertight retaining structure made up of wood, steel or reinforced concrete.

It is manufactured as a hollow box or cylinder box, which sinks at the preferred depth and fills with the concrete to forms a foundation.

Caisson foundation is often used in the construction of bridge piers and other structures that require foundations under rivers and other water bodies.

Caissons are similar to pile foundations, however, constructed using a unique methodology.

They are used, when soils with sufficient bearing strength are found under the surface with weak materials like filler or peat.

This foundation is a type of deep foundation built above the ground level, which sinks to the required depth by digging or scouring the material.

Cassion foundation definition:

Caissons are formed by raising a deep hole in the ground after which it is filled with concrete; steel reinforcement is commonly used for a length portion of the caisson.

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If a geotechnical engineer finds the soil suitable for carrying structure loads, the caisson is either immersed in bedrock (called “rock caissons”) or inserted deep into the underlying soil strata.

They are usually “bored” at the bottom to spread the load over the wide area, special drilling bits are used to remove the soil for these “belled caissons”.

Caisson foundation carries the weight of construction at their lower end, often bell-shaped.

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Functions of Caisson Foundation:

The foundation system and the soil beneath the construction complicatedly prevent vertical shifting.

When the load is applied, most of the soil settles and it becomes difficult for structure.

When there is a differential settlement the components of structure settle at different rates, causing cracks, affecting the structural integrity.

In some uncommon cases, the soil may become swollen, pushing the structure upward and leading to similar issues.

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Therefore, the foundation system together with the soil supports the structure.

Types of Caisson Foundation:

Types of Caisson Foundation

Open caisson:

The caisson is open at the top and bottom, they are hollow chambers.

There is a cutting edge at the bottom of the caisson. The shafts sink into place by removing soil from inside the shaft (chamber) until the bearing reaches the stratum.

Typically, an open caisson sinks that penetrate it through a dry or dewy, construction area or an artificial island.

A synthetic island of sand forms to elevate the ground surface above the water level.

Thus a dry area is obtained to submerge the caisson; the dimensions of the sand island should be sufficient to supply a working space around the caisson.

In this case, it is not possible to immerse the caisson in the dry; it is a built-in spillage or barge and is transported to its final location by floating, incorrect bottoms for this purpose.

Guide piles usually involve dipping the first few lifts of the caisson. The submerge ends through open water and then penetrates the soil.

An open caisson usually produces wood, metal, reinforced concrete for construction and foundation of bridges, open caissons are also called “wells”.

Pneumatic Caisson:

Pneumatic caisson is required when the soil enclosed cannot be excavated satisfactorily.

It also works when there is a possibility of a steady flow of water or difficult obstructions occurring during submergence.

Pneumatic caissons are open at the bottom & closed at the top.

At the bottom of the pneumatic caisson is a working chamber, which maintains compressed air for the stress required to prevent the entry of water and dust into the chamber.

The manufacturing procedure of pneumatic caisson is similar to that of the open caissons, also the working chamber remains airtight.

The ultimate load-carrying capability and slicing-edge are additionally like e open caisson foundation.

Floating Caisson:

Floating caissons are boxes with large holes open at the top and closed at the bottom. This caisson floats at the place where it is kept.

They fill the site with sand, ballast, dry concrete and gravel.

A floating caisson does not penetrate in the soil like the open and pneumatic caisson, remains on the flat bearing surface.

The load-carrying capacity is revealed as resultant aspect friction at the bottom resistance.

The caisson sinks to its final position, it is filled with sand or gravel. A solid cap is formed on top to bear the structural load.

To prevent rip-off below, a rip-tear is pouring across the base.

Generally, these caissons are of R.C.C or metal. The formation of the caisson within the plan may be round, square, rectangular or elliptical.

It usually consists of several cells formed by diaphragm division. A caisson design as a vessel, it must sink in steady water and be supplied with a proper internal strut.

Box caissons:

Box cases include containers with 4 sides and a bottom. The top of these boxes is open & the concrete ground is built at the bottom.

The finished box caissons are placed in the original location and the pre-prepared targets are positioned.

Once a suitable location decided, the entire void is filled with concrete.

Box caissons are more cost-effective when it is not possible to assemble on site. This type of caisson is required, when the economic system is required.

The excavated caissons are simple because the caissons are placed within an excavation site and it is normally cylindrical and then filled with concrete.

Excavated Caissons:

 This caisson is placed within the excavated space. These are usually cylindrical and then filled with concrete.

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Construction Method:

The following steps can be adopted to construct a caisson foundation.

1. We should first determine the place for the establishment of the caisson, it is 3.7 meters pre-cast.

2. With the help of the towboat, the caisson is taken to the construction site and mooring cables tied to the caisson.

3. Concrete is used to make the slips and the area becomes heavy and submerged in water.

4. Mooring cables are used to hold caissons in place.

5. When the caisson lastly touches the bottom of the river, the mooring cables are eliminated.

6. Finally, the cap is inserted.

Also, read our other article: Types of Foundation, Pier and Beam Foundation

When caisson is required:

  • When the soil contains massive boulders, which obstructs the penetration of the pile.
  • Also, when a big sub-construction is required underneath or beneath the rear bed to offer resistance to the forces which might be dangerous due to non-permanent objects.
  • When the foundation is subjected to a big lateral load.
  • Also, when the depth of water level in river and sea is more.
  • When there are river forces involved in load compositions.
  • Caissons are the most popular when it is necessary to hold the load at the end.
  • Caissons are suitable when current groundwater levels are aggressive flows.

Pile vs Caissons vs Pier:

Pile FoundationCaisson Pier Foundation
A pile foundation is a long cylinder strong material such as concrete. Caisson is water level structures made of wood, steel or reinforced concrete. A pier foundation is a collection of the large-diameter cylindrical columns to support the superstructure
The loads are transferred by means of vertical timber, concrete or steel. It built above the ground level and then sink into the ground. It transfer load through bearing only.
Pile foundation is required to resist bigger loads i.e. bridge or flyover. Caisson is required when a big lateral load. Pier foundations are required to resist the small load.
Pile foundation is required to resist bigger loads i.e. bridge or flyover. Caisson doesn’t have a footing. Pier has a footing.

Advantages of caisson foundation:

  1. It is Economical.
  2. The pile reduces the need for a cap.
  3. Less noise and vibration which construction.
  4. Easily familiarize to different site conditions.
  5. High axial and lateral loading capacity.

Disadvantages of caisson foundation:

  1. Highly sensitive to manufacturing processes.
  2. Not good for polluted places.
  3. Lack of building experience.
  4. Lack of qualified inspectors.

Frequently Asked Questions:

What are caisson foundations?

A caisson foundation is a watertight retaining structure made up of wood, steel or reinforced concrete.

What is the purpose of a caisson?

The purpose of caisson is to provide stability to the structure.

Where is caisson foundation used?

Caisson is used as the foundation for bridges piers and abutments in rivers, seas, lakes, breakwaters & other shore construction works. 

Are caissons still used?

Yes, caissons are used to complete new construction.

How much do caissons cost?

$15,000 as a minimum charge for 200 LF of reinforced concrete filled caissons.

Conclusion:

Caissons provide an alternative means to achieve adequate installation at intermediate depths to critical depths in both ground and water environments.

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