Pile foundations can be categorized into three different types based on their construction methods: driven piles, cast-in-situ piles, and driven and cast-in-situ piles.
The first type, driven piles, are constructed by driving prefabricated piles into the ground using heavy machinery. These piles are typically made of steel, concrete, or timber, and are driven into the soil until they reach a firm layer that can support the load of the structure.
The second type, cast-in-situ piles, are created by drilling a hole into the ground and then filling it with concrete. This process is typically done using a drilling rig, and the hole is usually lined with a casing to prevent collapse during the casting process. Once the concrete has been poured, it is left to cure before the casing is removed.
The third type of pile foundation, driven and cast-in-situ piles, combines the two methods. This technique involves first driving a prefabricated pile into the ground and then casting concrete around it. This method provides the benefits of both techniques, as the prefabricated pile can be driven deeper into the soil than a cast-in-situ pile, while the concrete provides additional support and stability.
Types of Pile Foundations Based on Construction Method
1. Driven Pile Foundations
Pile foundations are essential components in the construction industry, made from a variety of materials including concrete, steel, and timber. The piles are manufactured before being transported to the construction site where they are installed using a pile hammer. When concrete piles are used, they are precast.
When driven into granular soils, the piles displace soil volumes equivalent to their size, facilitating compaction of the surrounding soil. As a result, the soil becomes denser, and the bearing capacity of the soil increases. Compaction piles refer to piles that compact the soil surrounding them.
However, soil types such as saturated silty soils and cohesive soils have poor drainage capabilities. When driven piles are installed through these soil types, they do not compact since the water must first be drained for the soil to be compacted. Consequently, stresses adjacent to the piles are supported solely by pore water, leading to an increase in pore water pressure and a decrease in soil bearing capacity.
2. Cast-in-situ Pile Foundations
Cast-in-situ piles are a type of concrete pile that is constructed by drilling holes in the ground to the required depth and then filling the hole with concrete. To ensure structural strength, reinforcements are also used in the concrete as per the specific requirements of the construction project.
Compared to drilled piers, cast-in-situ piles have a smaller diameter. These piles can either be straight bored or with one or more bulbs at intervals, which are known as under-reamed piles.
Under-reamed piles are particularly useful in situations where the soil is weak and unstable. The bulbs help to distribute the load of the structure evenly and reduce the risk of the pile sinking or tilting. In addition, the use of under-reamed piles can also help to minimize the overall number of piles needed for the construction project.
3. Driven and Cast-in-situ Piles
Driven and cast-in-situ piles combine the benefits of both driven and cast-in-situ piles. The installation process involves driving a steel shell with the help of a mandrel inserted into it. The shell has a diameter equivalent to that of the pile. Once the shell is driven, the mandrel is removed, and concrete is poured into the shell. The shell is made of reinforced thin sheet steel or pipes, such as the Armco welded pipes or common seamless pipes. Such piles are known as shell type piles.
Both types of piles have a conical tip at the bottom of the shell that can be separated from it. Driving the concrete out of the shell can form an enlarged bulb in both types of piles, such as in the Franki piles. In some cases, the shell remains in place, and the tube is concreted. This type of pile is commonly used in piling over water.
The shell-less type is formed by withdrawing the shell while the concrete is being placed. Regardless of the type, the bottom of the shell is always closed with a conical tip. Overall, driven and cast-in-situ piles are effective solutions for various construction projects, especially those that require piling over water.