Pile Classification Based On Construction Method
On the basis of method of installation pile foundations can be divided into the following 6 types.
- Precast Driven Piles
- Driven Cast-in-situ Piles
- Bored Cast-in-situ Piles
- Bored Precast Piles
- Driven Steel Piles
- Driven Timber Piles
1. Precast Driven Piles
These are usually of RCC or pre-stressed concrete and generally small in size for ease in handling. The main advantage of this type of pile is that its quality, in terms of dimension, use of reinforcement and concrete, can be ensured as the piles are cast in a yard under controlled conditions. However care is needed while handling, transporting and driving the pile to avoid damages.
More to it, the limitation of length depending upon the capacity of the driving equipment is a disadvantage as these cannot be taken very deep except by joining. Generally, the depth over which these are used is restricted to 36 meter.
2. Driven Cast-in-Situ Piles
A steel casing pile with a shoe at the bottom is driven first to the required depth. The reinforcement cage for the pile is then lowered inside the casing and the pile is concreted. As the concreting of the pile proceeds upwards, the casing is withdrawn keeping a suitable overlapping length. When such piles are driven in soft soil and the tube is withdrawn while concreting, it affects resistance and changes the property of the soil and this also affects the capacity of individual piles. These are not suitable for use in soft soils, in greater depths or where keying with the rock is required.
3. Bored Cast-in-Situ Piles
In the bored cast-in-situ process, a larger diameter casing is used. A casing of 3 to 4 m in length is provided on top of the bore hole which is driven with the help of a bailor. Boring further below this casing is carried out by chiselling and the side walls are kept stable by circulating bentonite slurry inside the bore hole. The boring is continued up to the layer decided for founding the structure. After reaching the desired founding level, the chisel is removed, bore-hole flushed, reinforcement cage lowered into the hole, and held in position by tack welding it to the support bars at the top of the casing.
After this, concreting is carried out by using tremie, keeping its end always below the top level of rising concrete. The concreting is continued till a good quality concrete is seen at the top of the bore hole. After this, the tremie is removed and when the concrete has reached the top, the casing pipe on the top is also removed. The bentonite mix should be periodically checked for its specific gravity and changed as, due to constant use, it can get mixed with the soil and deteriorate in quality. This type of pile can be used even where the pile is keyed into the rock as chiselling in the rock can be carried out more easily. These piles serve as bearing-cum-friction piles. The diameters of such piles are generally more than 1.0m and can go up to 3.6m or more. They can be used singly or in group and are good replacements for well foundations required for bridge piers in rivers with clayey and mixed soils.
4. Bored Precast Piles
In this, as the name itself suggests, a hole is bored using a casing and a pre-cast pile is inserted into it. After securing it in position, the casing is withdrawn. A particular process used for bored pre-cast piles is the Benoto process which involves a steel tube being pushed into the soil, turned and reversed using compressed air. The tube is in the form of a casing and is driven for the entire depth after the soil is progressively grabbed from the tube. The process is continued till the tube reaches the pre-determined level. Then the pre-cast pile is lowered inside and held in position. The tube is lifted gradually after filling the annular gap between the pre-cast pile and the soil by grouting.
5. Driven Steel Piles
Steel piles can be circular or in other structural shapes. The circular ones are made in the form of either welded or seamless piles. Usually steel or cast iron piles used earlier for bridge structures are of longer diameter and screw type. These were used in past when loading was less. These piles are suitable for being driven through cohesive soil to reach up to the hard strata and to serve as bearing piles. They are not suitable where heavy scour is expected and for foundation for bridges when foundations are situated wide apart.
6. Driven Timber Piles
Timber piles have been extensively used in America. These have been used in India on the railways and highways, for temporary bridges. Timber piles are of hard wood, and used in natural form with thin end cut or suitably sized. They are used mostly as end-bearing piles in clusters. They are normally used in lengths of 12m and extended by splicing for use in deeper channels. The piles protruding above bed/low water level are suitably braced in cluster.