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 Caissons:
- Box Caissons
- Excavated Caissons
- Floating Caissons
- Open Caissons
- Pneumatic Caissons
- Sheeted Caissons
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:
- Minimizes pile cap needs
- Slightly less noise and reduced vibrations
- Easily adaptable to varying site conditions
- High axial and lateral loading capacity
Disadvantages of Caissons:
- Extremely sensitive to construction procedures
- Not good for contaminated sites
- Lack of construction expertise
- Lack of Qualified Inspectors
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.
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:
- After some initial form work and concrete pours, the cutting edge is floated to the breakwater by towboat and fastened to the caisson guide. Concrete is placed (poured) into steel forms built up along the perimeter of the box. With every concrete placement, the box becomes heavier and sinks into the water along the caisson guide.
- Forms are also built inside the box around the air domes and concrete is placed in between. The resulting open tubes above the air domes are called dredge wells.
- When the caisson finally touches the river bottom, the air domes are removed and earth is excavated through the long dredge well tubes, as shown in the animation below. The caisson sinks into the river bottom. Excavation continues until the caisson sinks to its predetermined depth.
- As a final step, concrete is placed (poured) into the bottom 30 feet of the hollow dredge wells and the tops are sealed.
Straight Shaft Drilled Piers (Caissons)
- Used in moderate to high swelling soils. (This is one of the most effective foundation designs for use in sites that contain expansive soils.)
- Purpose is to attain required penetration into zone where there is little or no seasonal moisture variation. Current standard of care in the area is a minimum penetration of 6 feet into bedrock and minimum length of 16 feet. Dead loads should be as high as practical. This design requires relatively long spans between piers and more reinforcing in grade beam.
- Caissons into bedrock
- Friction Piers into stiff clays
- End Bearing Belled Piers
- Appropriate Voiding – Should be constructed with void material of appropriate strength and thickness
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.