What is a Caisson Foundation?
A caisson foundation, also known as a pier foundation, is a type of deep foundation used to support structures such as bridge piers and concrete dams. It consists of a prefabricated hollow box or cylinder that is sunk into the ground to a desired depth and filled with concrete to form a foundation. This type of foundation is commonly used in situations where soil of sufficient bearing strength is found below weak surface layers like fill or peat.
Caisson foundations are similar in shape to pile foundations, but are installed using a different method. They can be floated to the job site and sunk into place, making them ideal for use in construction projects requiring a foundation beneath bodies of water. The construction process involves drilling a deep hole into the ground and filling it with concrete. Steel reinforcement is sometimes used for a portion of the length of the caisson.
Caissons can be drilled to bedrock or deep into the underlying soil strata, depending on the suitability of the soil to carry the building load. When caissons rest on soil, they are often bell-shaped 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 lower ends of the caisson foundations carry the building loads, which are often bell-shaped.
Functions of Caisson Foundation
The stability of a building depends on its foundation system and the quality of the soil it is built on. The foundation system is designed to prevent the building from moving vertically. However, most soils tend to settle when a load is placed on them. This can pose a problem when the building settles but the utilities, such as pipes and cables, do not settle at the same rate.
Differential settlement is a more critical issue than settlement. It occurs when different parts of the building settle at varying rates. This can result in cracks, which can potentially affect the structural integrity of the building. In rare instances, some soils may even swell and push the building upwards, causing similar problems. Therefore, it is essential that the foundation system works in conjunction with the soil to provide adequate support for the building.
Types of Caisson Foundations
Caissons are specialized structures used in construction projects, particularly in underwater settings. There are different types of caissons used for various purposes, including box caissons, excavated caissons, floating caissons, open caissons, and pneumatic caissons.
Box caissons are constructed using heavy timbers and are watertight boxes that are open at the top. They are usually transported to the intended location by floating and then sunk into place by constructing a masonry pier within them.
Excavated caissons, on the other hand, are placed within an excavated site and are cylindrical in shape. Once the caisson is in position, it is backfilled with concrete to complete the structure.
Floating caissons, also referred to as floating docks, are prefabricated boxes that come with cylindrical cavities. They are useful for constructing structures that require a waterborne base.
Open caissons are small cofferdams used for pier formation. They are placed in the desired location and then pumped dry before being filled with concrete.
Lastly, pneumatic caissons are large watertight boxes or cylinders that are mostly used in underwater construction. They are designed to withstand high water pressure and allow for construction activities to be carried out in a dry environment.
Advantages and Disadvantages of Caissons:
Advantages of Caissons:
In the field of economics, there is a focus on minimizing the need for pile caps. This can help reduce costs and make construction more efficient. Additionally, there is a desire to minimize noise and vibrations during construction. By doing so, projects can be completed with less disruption to the surrounding area.
One important consideration in construction is adaptability to varying site conditions. This is especially true in the case of foundations, where soil conditions can vary greatly from one location to another. An adaptable foundation system can help ensure that construction proceeds smoothly, regardless of the specific conditions encountered on a particular site.
Another important factor in foundation design is loading capacity. Foundations must be able to support not only the weight of the structure above, but also any lateral loads that may be applied to it. A foundation with high axial and lateral loading capacity can help ensure the stability and safety of the structure it supports.
Disadvantages of Caissons:
Construction procedures must be handled with extreme sensitivity when dealing with certain projects. For instance, certain projects may be unsuitable for contaminated sites due to potential risks. Additionally, a lack of construction expertise can pose a challenge for successfully completing a project, as can a shortage of qualified inspectors to oversee and ensure proper construction practices are followed. These factors can all impact the success and safety of a construction project, highlighting the importance of careful planning and thorough execution.
Drilled Pier Foundations
Drilled piers are a type of deep foundation system that involves the placement of fresh concrete and reinforcing steel into a drilled shaft. These shafts are constructed using rotary methods, either through the use of a self-contained drill unit or a crane-mounted drill unit. The hole is drilled through soil or rock until it reaches the desired bearing stratum. To prevent caving soils or water infiltration, temporary or permanent steel casings may be employed to maintain the sides of the drilled excavation.
Drilled shafts are capable of sustaining high axial and lateral loads, making them an ideal choice for structures that require such support, such as bridges and large buildings. These shafts typically have a diameter ranging from 18 to 144 inches and are known by various names, including caissons, bored piles, and drilled piers. They have gained global acceptance due to their cost-effectiveness and excellent performance as a deep foundation system.
Concrete Caissons
Caissons are typically used as a means of providing structural support for a variety of construction projects, such as foundation walls, porches, patios, monoposts, and other similar structures. These caissons are comprised of holes that are drilled into the earth, with diameters typically ranging from 10 to 12 inches. The holes are then embedded into bedrock, with a depth of around 3 to 4 feet.
To ensure the caisson’s structural integrity, two or more reinforcing bars, also known as rebar, are inserted into the hole and run the full length of it. Once the rebar is in place, concrete is poured into the hole, creating a strong foundation for the structure.
It’s important to note that caissons are designed to rest on an underlying stratum of rock or satisfactory soil. This is because if unsatisfactory soil exists, it could lead to problems with the stability of the structure being built. Therefore, caissons provide a reliable and effective solution for construction projects that require strong foundational support.
Caisson Construction Process
The construction process involves floating the cutting edge to the breakwater and securing it to the caisson guide. Steel forms are then erected along the box’s perimeter, and concrete is poured into them, gradually increasing the box’s weight and causing it to sink into the water alongside the caisson guide. Inside the box, forms are also built around the air domes, and concrete is poured in between them to create open tubes called dredge wells. Once the caisson reaches the river bottom, the air domes are removed, and the earth is excavated through the dredge well tubes. This process continues until the caisson sinks to its designated depth. Finally, concrete is poured into the dredge wells’ bottom 30 feet, and the tops are sealed.
Straight Shaft Drilled Piers (Caissons)
An effective foundation design for sites with expansive soils is required to achieve the necessary penetration into an area where moisture variation is minimal. The current standard of care mandates a minimum penetration of 6 feet into bedrock and a minimum length of 16 feet, with high dead loads preferred. For this design, longer spans between piers and additional reinforcement in the grade beam are necessary. Three possible foundation designs for such sites are caissons into bedrock, friction piers into stiff clays, and end bearing belled piers. In addition, appropriate voiding should be incorporated by using a void material with the appropriate strength and thickness.
A 24-meter diameter caisson shaft was constructed by connecting a series of 1.2-meter thick diaphragm wall panels. The purpose of building four of these caissons was to create a stable foundation for the main structure of a building tower.
Excavation work was carried out inside one of the caisson shafts using typical excavating machines, as shown in the accompanying photo. This process involved the removal of soil and debris from the interior of the shaft to prepare it for further construction.
