Foundations provide support to the structure and transfer the loads from the structure to the soil. But the layer at which the foundation transfers the load shall have an adequate bearing capacity and suitable settlement characteristics. There are several types of foundations depending on various considerations such as-
- Total load from the superstructure.
- Soil conditions.
- Water level
- Noise and vibrations sensitivity
- Available resources
- Time-frame of the project
Broadly speaking, foundations can be classified as shallow foundations and deep foundations Shallow footings are usually used when the bearing capacity of the surface soil is adequate to carry the loads imposed by a structure. On the other hand, different types of deep foundations are usually used when the bearing capacity of the surface soil is not sufficient to carry the loads imposed by a structure. So, the loads have to be transferred to a deeper level where the soil layer has a higher bearing capacity. A pile foundation is one of the deep foundation types.
What is Pile Foundation?
A Pile foundation, a kind of deep foundation, can be defined as a slender column or long cylinder made of materials such as concrete or steel which are used to support the structure and transfer the load at desired depth either by end bearing or skin friction.
Pile foundations are deep foundations. They are formed by long, slender, columnar elements typically made from steel or reinforced concrete, or sometimes timber. A foundation is described as ‘piled’ when its depth is more than three times its breadth.
Foundation piles are usually used for large structures and in situations where the soil at shallow depth is not suitable to resist excessive settlement, resist uplift, etc.
When to Use Pile Foundation
Often the question rises are the situations where pile foundations are suitable. Following are the situations when using a pile foundation system can be beneficial:
- When the groundwater table is high foundation pilings are the best solution-
- Heavy and un-uniform loads from the superstructure are imposed.
- Other types of foundations are costlier or not feasible.
- When the soil at a shallow depth is compressible.
- When there is the possibility of scouring, due to its location near the river bed or seashore, etc.
- When there is a canal or deep drainage system near the structure.
- When soil excavation is not possible up to the desired depth due to poor soil conditions.
- When it becomes impossible to keep the foundation trenches dry by pumping or by any other measure due to heavy inflow of seepage.
Whenever one of the above conditions occur (where pile foundations are suitable for), the foundation engineer has to choose a foundation for the structure among different types of pile foundation.
Types of Pile Foundation
Based on Function or Use
- Sheet Piles
- Load Bearing Piles
- End bearing Piles
- Soil Compactor Piles
Based on Materials and Construction Method
- Timber Piles
- Concrete Piles
- Steel Piles
- Composite Piles
Classification of Pile Foundation Based on Function or Use
As we can see in the following diagram, there are five pile types based on the uses and functions of piling foundations.
1. Sheet Piles
This type of piles is mostly used to provide lateral support. Usually, they resist lateral pressure from loose soil, the flow of water, etc. They are usually used for cofferdams, trench sheeting, shore protection, etc. They are not used for providing vertical support to the structure. They are usually used to serve the following purpose-
- Construction of retaining walls.
- Protection from river bank erosion.
- Retain the loose soil around the foundation trenches.
- For isolation of foundation from adjacent soils.
- For confinement of soil and thus increase the bearing capacity of the soil.
2. Load Bearing Piles
This type of building pile foundation is mainly used to transfer the vertical loads from the structure to the soil. These load-bearing pile foundations transmit loads through the soil with poor supporting property onto a layer that is capable of bearing the load-
Depending on the mechanism of load transfer from pile to the soil, load-bearing piles can be further classified as flowed-
3. End Bearing Piles
In this type of piles foundation, the loads pass through the lower tip of the pile- The bottom end of the end-bearing piles rests on a strong layer of soil or rock. Usually, the pile rests at a transition layer of a weak and strong slayer. As a result, the pile acts as a column and safely transfers the load to the strong layer.
4. Friction Pile
The Friction pile transfers the load from the structure to the soil by the frictional force between the surface of the pile and the soil surrounding the pile such as stiff clay, sandy soil, etc. Friction can be developed for the entire length of the pile or a definite length of the pile, depending on the strata of the soil. In friction piles, generally, the entire surface of the pile works to transfer the loads from the structure to the soil.
5. Soil Compactor Piles
Unlike other pile foundation types, this type of pile does not carry any direct loads. This type of piles is driven at placed closed intervals to increase the capacity of soil by compacting.
Types of Piles Based on Materials and Pile Foundation Construction Method
Displacement piles and Non-displacement or Replacement piles. Piles that cause the soil to be displaced vertically and radially as they are driven to the ground are known as Displacement piles. In the case of Replacement piles, the ground is bored and the soil is removed and then the resulting hole is either filled with concrete or a pre- cast concrete pile is inserted. On the basis of materials of pile foundation construction and their installation process load-bearing piles can be classified as follows:
- Timber Piles
2. Treated with Preservative
- Concrete Piles
1. Pre-cast Piles
2. Cast-in-place Piles
- Steel Piles
1. I-Section Piles
2. Hollow Piles
- Composite Piles
1. Timber Piles
Timber piles are the types of piled foundations that are placed under the water level. They last for approximately about 30 years. They can be rectangular or circular in shape. Their diameter or size can vary from 12 to 16 inches. The length of the pile is usually 20 times the top width.
They are usually designed for 15 to 20 tons. Additional strength can be obtained by bolting fish plates to the side of the piles
Advantages of Timber Piles
- Timber piles of regular size are available.
- Easy to install-
- Low possibility of damage.
- Timber pile footings can be cut off at any desired length after they are installed.
- If necessary, timber piles can be easily pulled out.
Disadvantages of Timber Piles
- Piles of longer lengths are not always available
- It is difficult to obtain straight piles if the length is short-
- It is difficult to drive the pile if the soil strata are very hard.
- Spicing of timber piles is difficult.
- Timber or wooden piles are not suitable to be used as end-bearing piles.
- For the durability of timber piles, special measures have to be taken. For example- wooden piles are often treated with preservatives.
2. Concrete Piles
Pre-cast Concrete Pile
The precast concrete pile foundation is cast in a pile bed in horizontal form if they are rectangular in shape. Usually, circular piles are cast in vertical forms- Precast piles are usually reinforced with steel to prevent breakage during their mobilization from the casting bed to the location of the foundation. After the pil are cast, curing has to be performed as per specification. Generally curing period for pre-cast piles is 21 to 28 days.
Cast-in-Place Concrete Piles
This type of pile footing is constructed by boring soil up to the desired depth and then, depositing freshly mixed concrete in that place and letting it cure there cast in situ concrete pile foundation is constructed either by driving a metallic shell to the ground and filling it with concrete and leaving the shell with the concrete or the shell is pulled out while concrete is poured. Usually, round piles are used in cast-in situ piling.
3. Steel Piles
Steel piles may be of l-section or hollow pipe. They are filled with concrete. The size may vary from 10 inches to 24 inches in and the thickness is usually 3/4 inches. Because of the small sectional area, the piles are easy to drive. They are mostly used as end-bearing piles-
Advantages of Steel Piles
- They are easy to install.
- They can reach a depth compared to any other type of pile foundations-
- Can penetrate through the hard layer of soil due to the less cross-sectional area.
- It is easy to steel piles
- Can carry heavy loads.
Disadvantages of Steel Piles
- This piling type is prone to corrosion.
- Has a possibility of deviating while driving.
- Comparatively expensive.