Pile Foundation When the soil at or near the ground surface is unable in bear the load of a superstructure, deep foundations are required to transmit the load to deeper strata.
The most common types of deep foundations are piles, piers, and caissons. A deep foundation is generally much more expensive than a shallow foundation.
It should be adopted only when a shallow foundation is not feasible.
A pile is a slender structural member made of steel, concrete, or wood. A pile is either driven into the soil or formed in-situ by excavating a hole and feeling it with concrete.
Necessity of pile foundation:
Pile foundation is used in following conditions:
1.When the strata at or just below the ground surface is highly compressible and very weak to support the load transmitted by structure.
2.When the plan of the structure is irregular relative to its outline or load distribution. It would cause non-uniform settlement if a shallow foundation is constructed.
3.Pile foundations are required for the transfer of structural loads through water bodies to a firm strata.
4.Pile foundations are used to resist horizontal forces in addition to resist the vertical load in earth retaining structures and high structures that are subjected to horizontal forces due to wind and earthquakes.
5.Piles are required when the soil conditions are such that a washout, erosion, or scour of soil may occur underneath a shallow foundation.
6.In case of expansive soils, such as black cotton soil, which swell and shrink due to changes in water content, piles are used to transfer the load below the active zone.
Classification of piles:
- Steel Piles
- Concrete Piles
- Timber Piles etc.
Classification Based on mode of transfer of loads:
- End-bearing piles: These piles transmit the loads through their bottom tips.
- Friction piles: These piles transfer the load through skin friction between the embedded surface of the pile and the surrounding soil.
- Combined end bearing and friction piles: These piles transfer loads by a combination of end bearing at the bottom of the pile and friction along the surface of the pile shaft.
Classification based on method of installation
- Driven Piles: These piles are driven into the soil by applying blows of heavy hammering on their tops.
- Driven and Cast-in situ piles: These piles are formed by driving a casing with a closed bottom end into the soil. The casing is then filled with concrete. The casing may or may not be withdrawn.
- Bored and Cast in situ piles: These piles are constructed by excavating a hole into the ground and then filling it with concrete
- Screw piles: These piles are screwed into the soil.
- Jacked piles: These piles are jacked into the soil by applying a downward force with the help of a hydraulic jack.
Classification Based on displacement of soil
- Displacement Piles: All driven piles are displacement piles as the soil is displaced laterally when the pile is installed. The soil gets densified in the process.
- Replacement Piles: The Bored piles are called replacement piles as the soil equal to volume of piles is replaced.