The Constructor

Caisson (Pier) Foundation – Types, Construction and Advantages

Caisson Foundation Construction

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What is a Caisson Foundation?

A caisson foundation also called as pier foundation is a watertight retaining structure used as a bridge pier, in the construction of a concrete dam, or for the repair of ships. It is a prefabricated hollow box or cylinder sunk into the ground to some desired depth and then filled with concrete thus forming a foundation. Caisson foundation is Most often used in the construction of bridge piers & other structures that require foundation beneath rivers & other bodies of water. This is because caissons can be floated to the job site and sunk into place. Caisson foundations are similar in form to pile foundations, but are installed using a different method. It is used when soil of adequate bearing strength is found below surface layers of weak materials such as fill or peat. It is a form of deep foundation which are constructed above ground level, then sunk to  the required level by excavating or dredging material from within the caisson. Caissons (also sometimes called "piers") are created by auguring a deep hole into the ground, and then filling it with concrete. Steel reinforcement is sometimes utilized for a portion of the length of the caisson. Caissons are drilled either to bedrock (called "rock caissons") or deep into the underlying soil strata if a geotechnical engineer finds the soil suitable to carry the building load. When caissons rest on soil, they are generally "belled" at the bottom to spread the load over a wider area. Special drilling bits are used to remove the soil for these "belled caissons". The caisson foundations carry the building loads at their lower ends, which are often bell-shaped.

Functions of Caisson Foundation

The foundation system of and the soils beneath the building prevent the complex from moving vertically. When a load is placed on soil, most soils settle. This creates a problem when the building settles but the utilities do not. Even more critical than settlement is differential settlement. This occurs when parts of your building settle at different rates, resulting in cracks, some of which may affect the structural integrity of the building. Conversely, in some rare instances soils may swell, pushing your building upwards and resulting in similar problems. Therefore, the foundation system must work in tandem with the soils to support the building.

Types of Caisson Foundations

Box caissons are watertight boxes that are constructed of heavy timbers and open at the top. They are generally floated to the appropriate location and then sunk into place with a masonry pier within it. Excavated caissons are just as the name suggests, caissons that are placed within an excavated site. These are usually cylindrical in shape and then back filled with concrete. Floating caissons are also known as floating docks and are prefabricated boxes that have cylindrical cavities. Open caissons are small cofferdams that are placed and then pumped dry and filled with concrete. These are generally used in the formation of a pier. Pneumatic caissons are large watertight boxes or cylinders that are mainly used for under water construction.

Advantages and Disadvantages of Caissons:

Advantages of Caissons:

Disadvantages of Caissons:

Drilled Pier Foundations

A drilled pier is a deep foundation system that is constructed by placing fresh concrete and reinforcing steel into a drilled shaft. The shaft is constructed by rotary methods using either a self-contained drill unit or a crane mounted drill unit. The hole is advanced through soil or rock to the desired bearing stratum. Temporary or permanent steel casings may be used to maintain the sides of the drilled excavation if caving soils or water infiltration becomes a problem. Drilled shafts can be used to sustain high axial and lateral loads. Typical shaft diameters range from 18 to 144 inches.  Drilled shafts (also called caissons, drilled piers or bored piles) have proven to be a cost effective, excellent performing, deep foundation system, that is utilized world-wide. Typically they are used for bridges and large structures, where large loads and lateral resistance are major factors.

Concrete Caissons

A 10" or 12" diameter holes are drilled into the earth and embedded into bedrock 3 to 4 feet. Usually used for the structural support for a type of foundation wall, porch, patio, monopost, or other structure. Two or more "sticks" of reinforcing bars (rebar) are inserted into and run the full length of the hole and then concrete is poured into the caisson hole. A caisson is designed to rest on an underlying stratum of rock or satisfactory soil and is used when unsatisfactory soil exists

Caisson Construction Process

Straight Shaft Drilled Piers (Caissons)

Fig:  A series of 1.2-metre thick diaphragm wall panels were joined to form a 24-metre diameter caisson shaft. Four of these caissons were built to provide a sound base for the foundation of the main structure of the building tower. The photo shows the excavation work using typical excavating machines inside one of the caisson shafts. Read More: Buoyancy Rafts or Hollow Box Foundations or Floating Foundations in Building Construction How to Protect Foundation Structures from Soils and Groundwater Attacks? Factors Affecting Selection of Foundation for Buildings Types of Deep Foundations used in Construction and Their Uses Types of Foundations for Buildings and their Uses in Construction
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