When the soil at or near the ground surface is unable in bearing the load of super-structure, deep foundations are required to transmit the load to deeper strata.
The most common types of deep foundations are pile, piers and caissons.
A deep foundation is generally much more expensive than a shallow foundation. It should adopted only when 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 earthquake.
5.Piles are required when the soil conditions are such that a wash out, 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 change in water content, piles are used to transfer the load below the active zone.
Classification according to material used:
- 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 hammer 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 The casing may or may not be withdrawn.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.
PILE LOAD TEST
The estimation of pile load capacity and settlement under a load is based on the results of field investigations, laboratory testing and the empirical and semi-empirical methods. These estimated values should then be confirmed by field pile load tests. Pile load tests, in practice, are normally executed in two alternative ways:
1. Test Pile Preliminary pile design is first carried out on the basis of site investigations, laboratory soil testing, and office study. Pile load tests are then carried out to refine and finalize the design. For these conditions, the test piles are generally tested to failure.
2. Test on a Working Pile In areas where previous experience is available, pile design is carried out based on the site investigations, laboratory soil testing, and office study. Pile load tests are then carried out on randomly selected actual piles to check the pile design capacities. In these situations, the piles are generally tested to two times the design capacity.
The equipment and test procedures for these two alternatives are essentially similar. The main difference is the level of final loading. Therefore, the details of tests presented below are applicable for both of the tests listed above. The pile load tests include axial compression, pullout, and lateral and dynamic loads.
An example of a typical axial compression load application arrangement