Types of Pile Foundation

 Pile Foundation

A shallow foundation is usually provided when the soil at a shallow depth has adequate capacity to support the load of the superstructure. However, in situations where the soil at shallow depths is poor, in order to transmit the load safely, the depth of foundation has to increased till a suitable soil stratum is met. In view of increased depth, such foundations are called deep foundations. Piles, Piers and Wells are example of deep foundation.

What is Pile foundation?

A pile is a relatively small diameter shaft, which is driven or installed into the ground by suitable means. The piles are usually driver in groups to provide foundations for structures. The pile groups may be subjected to vertical loads, horizontal loads or a combination of vertical and horizontal loads.

This piles are usually made of concrete and steel which are used to support the structure and transfer the load at desire depth either by end bearing or skin friction.

Where pile foundation are used?

·         Where the soil at shallow depth is compressible.

·         Where ground water table is high.

·         When heavy and non-uniform loads from superstructure are imposed.

·         Where soil excavation is not possible up to desire depth due to poor soil condition.

·         Where there is canal or river near the structure.

·         Where it becomes impossible to keep the foundation trenches dry by pumping or any other method due to heavy inflow of seepage.

Types of Pile foundation

1.       Base on load transfer

2.       Method of installation

3.       Types of material

Based on load transfer

·         End bearing pile

·         Friction pile

·         Sheet pile

·         Fender Pile

·         Tension pile

·         Under-reamed pile

·         Compaction pile

·         Anchor pile

·         Batter pile

1)    End bearing Pile:

If a bedrock or rock like material is present at a site within a reasonable depth, piles can be extended to the rock surface. In this case, the ultimate bearing capacity of the pile depends entirely on the underlying material; thus, the piles are called end or point bearing piles. In most of these cases the necessary length of the pile can be fairly well established.

Instead of bedrock, if a fairly compact and hard stratum of soil is encountered at a reasonable depth, piles can be extended a few meters into the hard stratum.

2)     Friction Pile:

In these types of piles, the load on pile is resisted mainly by skin/friction resistance along the side of the pile (pile shaft). Pure friction piles tend to be quite long, since the load-carrying. Capacity is a function of the shaft area in contact with the soil. In cohesion less soils, such as sands of medium to low density, friction piles are often used to increase the density and thus the shear strength.

When no layer of rock or rocklike material is present at a reasonable depth at a site, point/end bearing piles become very long and uneconomical. For this type of subsoil condition, piles ate driven through the softer material to specified depth.

3)      Sheet Pile:

This type of pile 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 foundation trenches.

• For isolation of foundation from adjacent soils.

• For confinement of soil and thus increase the bearing capacity of the soil.

4)      Fender Pile:

Fender piles and dolphins are used to protect water front structure from impact of any floating object or ship.

5)      Tension Pile:

Tension piles are also called uplift piles. These piles are used to anchor down the structures subjected to uplift due to hydrostatic pressure.

6)      Compaction Pile:

These piles are used to compact loose granular soil to increase its bearing capacity. Compaction piles do not carry load and hence they can be of weaker material. Sand piles can be used as compaction piles.

7)      Batter Pile:

Batter pile shall be used to transfer inclined load and horizontal forces.

8)      Anchor Pile:

These piles are used to provide anchorage against horizontal pull from sheet piling.

9)      Under-reamed Pile:

In area where black cotton soil could cause structural instability there under- reamed piles are used. It is a special type of bored pile which is provided with a bulb/pedestal at the end. The under reamed pile is constructed by making a hole in the ground by means of a hand-operated auger. An under reamed is then lowered in the cleaned hole. The under reamer is pressed down and rotated. Under pressure, the blades open up and due to rotary action, the soil is cut and falls in the bucket. When the bucket is full, the under reamer is pulled and cleaned. After the enlarged end is formed, the reinforcement cage is lowered and concreting is done.

The usual size of such piles are 150 to 200 mm shaft diameter, 3 to 4m long. The diameter of the under reamed portion is usually 2 to 3 times the shaft diameter.

Based on method if installation

1)      Driven or displacement Piles:

Driven piles may be of concrete, steel or timber. These piles are driven into the soil by the impact of hammer. Boring is not required for this type of piles. When a pile is driven into granular soils its densities the soil and increases strength of soil. But when a pile is driven in saturated clay, the soil instead of being compacted gets remoulded with reduction in strength.

When these piles are driven into the granular soils, they displace the equal volume of soil. This helps in compaction of soil around the sides of piles and results in the densification of soil. The piles which compact the soil adjacent to it is also called as compaction pile.

2)      Driven Cast-in-situ Pile:

It is a type of driven pile. They are constructed by driving a steel casing in to the ground. The hole is then filled with concrete by placing the reinforcement and the casing is gradually lifted.

3)      Bored or Replacement Pile:

Bored piles are constructed in pre-bored holes either using a casing or by circulating stabilizing agent like betonies slurry. The borehole is then filled with concrete after placing the reinforcement. The advantage of board pile is that there is no damage due to handling and driving which is common in driven piles.