Noting that piles transfer loads from the superstructure by either end-bearing and/or by skin friction, they can be used at sites where the above load transfer mechanisms are possible in the subsoil. Construction of pile foundations require a judicious choice of piling system depending on soil conditions, loads coming from the superstructure and constraints of total and differential settlements besides special requirements. The installation of piles requires careful control on position, alignment, depth which involves skilled and experienced manpower. Pile load test (Section 9.6.7) is the most direct method to assess the above requirements and judging its performance. The details of testing are similar to the plate load test (Chapter 4) but instrumentation needs to be elaborate as described in standard text books such as Taylor (1964), Teng (1964), Terzaghi and Peck (1967) and Bowles (1996).
10.16.1
Construction Details
The construction of pile foundations involves two steps, namely construction of piles and the pile caps. The second step is a simple process similar to the construction of spread footings. Procedures and equipment required for installation of piles are described below.
Driven piles are installed by a pile driving device known as a pile hammer. The hammer may be suspended from the boom of a crane, supported on a large frame called a pile driver or carried on a barge for construction in water. In all cases, the hammer is guided between two parallel
steel members known as leads. The leads may be adjusted at various angles for driving vertical and batter piles.
Usually the information concerning the pile driving should be kept in an orderly form. It should include the details of the hammer and accessories. The behavior of the pile during the entire period of driving should be observed. It is time to stop driving a timber pile when the following phenomena are observed:
1. The pile vibrates and springs near the ground surface. 2. The pile hammer bounces.
3. The pile head shows distress under moderate driving.
Pile may be already damaged if the following behavior is noticed:
1. Penetration suddenly increases or becomes irregular, while the soil profile cannot account for it.
2. Pile suddenly changes direction.
Cast in situ piles are provided by drilling a bore hole with or without casing using drilling equipment and then placing reinforcement and concrete in the drilled bore hole (Section 9.2.2). 10.16.1.1 Alignment
Piles cannot be driven (or cast in situ) absolutely vertical and true to position. Even in ideal conditions the center of a pile head must be allowed to deviate a certain amount from the required location, and the pile at lower depth to vary from the required vertical or batter line. However, every precaution should be exercised to maintain the piles in position. The general procedure for determining the pile alignment and elevations is as follows:
1. Measure the elevation at top of piles immediately after driving of each and check the final elevations after the adjacent piles are driven or at the completion of all pile driving. If point- bearing piles are uplifted, they should be redriven.
2. Check the location of all piles after adjacent piles are driven or at the completion of all pile driving. In ordinary soil conditions an 8 cm tolerance is considered reasonable.
3. Inspect the pile shaft for verticality or required batter. In the case of heavily loaded piles measurement must be made by specially devised instruments, unless load test is made with piles having questionable verticality.
The above steps are equally applicable for cast-in-situ piles as well. 10.16.1.2 Defective Piles
A pile may be considered defective if: 1. It is damaged by driving.
2. It is driven out of position and/or is bent along its length.
To avoid damage to fresh concrete in a cast in place pile by the driving of adjacent piles, the pile should not be concreted until all piles within a certain radius are driven. The radius depends upon soil condition, length and size of pile and spacing.
A damaged or defective pile has to be withdrawn and replaced by another pile adjacent to it after checking the design with the new position of the replaced pile. Alternatively a new pile may be driven/cast while leaving the damaged pile as it is.
10.16.1.3 Effect of Pile Driving
Pile driving may introduce some of the following effects on the ground:
1. Subsidence. Vibration due to pile driving in loose sand may cause compaction of the sand. Consequently, the area may settle and adjacent structures may be affected. In saturated fine sand and silt, the shock may introduce large settlements.
2. Heave. Pile driving in clays and dense sand is commonly associated with surface heave and sometimes with lateral displacement. The heave of clay is followed by settlement immediately after driving. Piles uplifted by ground heave should be redriven. To avoid heave and lateral movement, pile driving should be started from the center of the ground and proceed outwards.
3. Compaction. Sand and gravel within a lateral distance of about three diameters of the pile and two diameters below the tip is largely compacted due to the displacement of pile. Consequently, a pile group in sand behaves as a rigid block of compacted soil.
4. Disturbance. Clayey soil surrounding the pile is greatly disturbed due to the displacement of pile. The disturbance may extend to a large lateral distance and the strength of clay is largely reduced. However, in ordinary cases it starts to regain its strength and in 30–50 days; 90% or more of its strength may be regained.