Concrete piles can be divided into following two categories :
- precast concrete piles, and
- cast in-situ concrete piles.
These piles are usually cast near. the site of work in specially prepared casting yard with adequate supervision and control to produce good quality concrete. The casting yard should be a leveled firm area with proper drainage and located as close to the site as possible so that expenses on transporting are limited. The form work of steel or timber should be of the required specifications and should be properly cleaned and oiled before placing the reinforcement cage. As far as possible, longitudinal reinforcement shall be in one length. In case this is not possible, overlaps shall be staggered and preferably joints butt welded. Necessary stirrups shall be provided and they shall be closely spaced near the top and bottom of the pile to avoid damage due to high impact stresses. The concreting of each pile has to be in one continuous operation and thoroughly compacted with vibrator. The exposed face must be trowel finished to provide a dense even surface. Side shuttering can be removed after a day and piles cured by wet gunny bags for a period of ten days. The piles should be carefully examined to see whether there are any defects, before they are taken to site for driving.
These piles are reinforced not only to carry the load on the foundation but also to withstand the stresses produced in lifting the piles and carrying them to the place of installation. In precast concrete piles, generally the reinforcement required to withstand the stresses during handling and driving are more than that required to take the load on the foundation. Piles can be lifted by hooks or clamps at a single point or at two points (Figure 4.16). Hooks can be embedded at the time of casting or proper markings made so that slinging is done correctly. The reinforcement has to be suitably designed according to the proposed mode of lifting. The high stresses produced while lifting and driving, necessitates proper structural design of the pile to take the bending and shear stresses. Once the pile is driven into the ground, much of the steel becomes redundant as the stresses are mainly compressive. Piles can also be manufactured in a factory but transportation of long
piles to the site could pose problems.
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| Lifting of Precast Piles |
If it has been decided to adopt precast piles for the foundation, the length of piles has to be assessed fairly correctly, as cutting the piles or extending them cannot be done easily. They also require large casting yards and heavy equipment for handling and driving. However, in situations where soil is such that driving is easy or large number of piles of predetermined length are to be provided or where reinforcement is required from considerations of lateral pressure or tensile steel is required to resist uplift, precast piles are advantageous. The quality of concrete in precast piles is better as they are cast above ground under controlled conditions and hence, such piles are sometimes preferred in aggressive soil (e.g. sulphates) conditions.
Precast concrete piles for small loads and short lengths can be square in cross-section with chambered comers, while for longer length and heavier loads they are generally of octagonal or circular section. Sometimes hollow sections are also used which are filled with concrete after driving. The tips are pointed to facilitate driving. As the reinforcement in the pile is mainly to resist the handling stresses, this objective can also be achieved by prestressing. It can be either pretensioned or post tensioned. Prestressed concrete piles are not widely used.
Pile Driving
Piles are commonly driven by means of a pile driver, basic elements of which are a frame and a hammer. The frame or trestle is generally of steel and has a pair of vertical guides, known as leaders within which the hammer is held. Mobile units are also available and are convenient when a large number of piles are to be driven.
The hammer could be a simple drop hammer or more efficient steam, air, diesel or hydraulic hammers. If the fall of hammer is due to gravity alone, it is known as single acting. If pressure is applied by steam etc., it is known as double acting.
Drop hammers weigh from 1000 to 5000 kg and fall through a distance of about 1 metre. As a rough rule, the weight of a drop or single acting hammer could be the same as the weight of the pile. For heavy piles, this would not be possible but for proper driving the hammer should weigh not less than a third of the weight of the pile. A cast steel helmet is placed over the top of the concrete pile with a resilient dolly on top to prevent the pile head from shattering under the impact of the hammer blow.
The resistance to pile driving is expressed in terms of number of blows per inch of penetration. Resistance of 6 to 8 blows per inch are specified for concrete piles. If piles have to be driven through dense layers, jetting around the pile is resorted to in order to loosen the soil and ease penetration. If piling is planned to be done by. jetting, it is preferable to insert an M.S. pipe of 50 to 75 mm in diameter at the centre of the pile while casting.
In saturated plastic clays, displacement of soil on account of driving of pile may cause heaving of adjacent area, and in such cases piles can be placed in holes made by angering.
Cast in-situ Concrete Piles
There are two types of concrete cast in-situ piles - driven or bored.
Driven Cast in-situ Piles
In this type of piles, a heavy sectioned metal tube, with a detachable metal shoe at the bottom, is driven into the ground by a drop hammer or any other type of hammer upto the required depth. Thereafter reinforcement is placed, if required, and concrete is filled into the tube and the tube simultaneously withdrawn leaving the shoe at the bottom. In another version, a thin steel shell is driven with the help of a mandrel, which is then withdrawn, reinforcement placed, if required, and concreting done, the shell being left permanently in the ground.
Raymond concrete pile company have developed a thin steel shell pile, known as Raymond Pile. The casing pipes consist of either uniformly tapered or step tapered light corrugated steel tubes. An internal mandrel is used to drive the casing, the mandrel is then withdrawn and the shell filled with concrete.
In Franki piles, a concrete plug is formed at the bottom of the steel casing and by repeated hammering the pipe is thrust downwards into the ground. When the bearing stratum is reached, additional concrete is poured and the plug hammered out of the tube to form a bulb end. Then the reinforcement cage is lowered into fie tube, concreted and the casing withdrawn.
Vibro pile uses a detachable steel or cast iron shoe with a steel tube casing. With the help of a hammer, the tube with the shoe on is driven to the required depth, reinforcement cage lowered. concreting done and the tube withdrawn leaving the shoe at the bottom of the pile. Simplex piles are also similar to this sort of arrangement.
Bored Cart in-situ Piles
In this system, a hole is bored into the ground, reinforcement (if required) lowered and concreting done. Boring is generally carried out by rotary or percussion type drilling rigs. Kelly mounted hydraulically operated grabs are also used.
If the soil is such that the walls of the bore would cave in. casing tubes have to be used which is subsequently extracted while concreting. Sometimes the casing is left in the ground and the concreting done within the casing. Such piles are known as cased piles. Another method to keep the sides of the hole in position is by pumping bentonite slurry into the borehole as the soil is removed. Bentonite is a clay of the monunorillonite group. Its slurry forms a membrane along the walls of the bore hole and also am hydrostatically to retain the stability of the sides of the hole. After the required depth of the bore is reached a high slump concrete is placed by means of a tremie.
For piles of small diameter and depths upto 10 m, the minimum cement content of the concrete should be 350 kg/m3 while for larger diameter and deeper piles it should be 400 kg/m3. Slump of concrete shall range between 100 to 180 mm depending on the manner of concreting.
Where drilling mud is used before concreting, the bottom of the hole shall be flushed with fresh bentonite slurry. Throughout the boring operation it has to be ensured that the drilling mud suspension is of the required consistency.
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