When the soil at or near the ground surface is not capable of supporting a structure, deep foundations are required to transfer the loads to deeper strata. Deep foundations are, therefore, used when soil surface is unsuitable for shallow foundation. The most common types of deep foundations are pile, pier and also caisson foundation. The mechanism of transfer of the soil is essentially the same in all types of deep foundations.
A deep foundation is generally much more expensive than a shallow foundation. It should be adopted only when a shallow foundation is not feasible. In certain situations, a fully compensated floating raft may be more economical than a deep foundation.
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 filling it with concrete. A pier is a vertical column of relatively larger cross section than a pile.
Necessity of Pile Foundation
Pile foundations are used in the following conditions:
- When the strata is at / just below the ground surface is highly compressible and very weak to support the load transmitted by the structure.
- When the plan of the structure is irregular relative to its outline and load distribution. It would cause non-uniform settlement if a shallow foundation is therefore constructed. A pile foundation is required to reduce differential settlement.
- Pile foundation are required for the transmission of structural loads thus through deep water to a firm stratum.
- Pile foundations are used to resists horizontal forces in addition to support the vertical loads in earth-retaining structures and tall structures that are subjected to horizontal forces due to wind and earthquake.
- Piles are required when the soil conditions are such that a wash out, erosion or scour of soil may occur from underneath a shallow foundation.
- Piles are used for the foundations of some structures, such as transmission towers, off-shore platforms; which are subjected to uplift.
- In case of expansive soils, such as black cotton soil (swells or shrink as the water content changes), piles are used to transfer the load below the active zone.
Classification of Piles
Piles can be classified according to followings –
- the material used
- mode of transfer of load
- method of construction
- the use
- displacement of soil
1. Classification according to material used
There are four types of piles according to material used :
- Steel Piles
- Concrete Piles
- Timber Piles
- Composite Piles
2. Classification based on mode of transfer of loads
Based on the mode of transfer of loads, the piles can be thus classified into 3 categories :
- End-bearing Piles
- Friction Piles
- Combined end-bearing and friction Piles
3. Classification based on method of installation
Based on method of installation / construction the piles may be classified into the following 5 categories :
- Driven Piles
- Driven and Cast in-situ Piles
- Bored and Cast in-situ Piles
- Screw Piles
- Jacked Piles
4. Classification based on use
Piles can be classified into the following 6 categories, depending upon their use :
- Load bearing Piles
- Compaction Piles
- Tension Piles
- Sheet Piles
- Fender Piles
- Anchor Piles
5. Classification based on displacement of soil
Based on the volume of the soil displaced during installation, the piles can be classified into 2 categories :
- Displacement Piles
- Non-displacement Piles
Piles are driven into the ground by means of hammers or similarly by using vibratory driver. Such piles are thus called driven piles. In some special cases, piles are installed by jetting or partial augering.
Following methods are commonly used –
1. Hammer Driving
- Drop Hammer
- Single acting Hammer
- Double-acting Hammer
- Diesel Hammer
2. Vibratory Pile Driver
3. Jetting Techniques
4. Partial Augering Method
Construction of bored piles
1. Drilling Holes
Bored piles are constructed after making a hole in the ground and however filling it with concrete. Following methods are used for drilling the hole.
- Hang Auger
- Mechanical Auger
- Boring Rig
- Belling Bucket
Before placing of concrete, the bored hole is bailed dry of water. Any loose or softened soil is cleaned out and the bottom of the hole is rammed. A layer of dry concrete is placed and rammed if the bottom of the hole is wet. Then the concrete with a readily workable mix, not leaner than 300 kg cement/m3 of concrete, is poured into a hopper placed at the mouth of the hole. A retarder is added to the concrete if there there is a risk of the concrete setting before the casting is lifted out. However, the quality of concreting done under water is not good. This method should be avoided as far as possible.