Piles Engineering

For direct circulation drilling and reverse circulation drilling, the major difference in drilling method is related to the direction of movement of drilling fluid. For direct circulation drilling, the drilling fluid is circulated from the drill stem and then flows up the annulus between the outside of the drill stem and borehole wall. The drilling fluid that carries the drill cuttings flows to the surface and the subsequent settlement pits. Pumps are employed to lift the cuttings free fluid back to the drill stem.

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In water bearing ground, some water head (about 1 m) above the existing ground water table is maintained to stabilize the bore during excavation below casings by pumping water to the pile bore.

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The PDA system consists of two strain transducers and two accelerometers attached to opposite sides of the pile to measure the strain and acceleration in the pile. The force is computed by multiplying the measured signals from a pair of strain transducers attached near the top of the pile by the pile area and modulus. The velocity measurement is obtained by integrating signals from a pair of accelerometers also attached near the top of the pile.

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High Strain Dynamic Testing consists of two main types, namely Dynamic Pile Monitoring and Dynamic Load Testing. Dynamic Pile Monitoring involves the use of PDA to perform real-time evaluation of Case Method pile capacity, hammer energy transfer, driving stresses and pile integrity for every blow count. On the other hand, Dynamic Load Testing is another technique that is evolved from wave equation analysis.

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In some design codes, it allows the simultaneous usage of shaft and base resistance of rock sockets to resist loading. Past experience showed that the shaft resistance could be mobilized in rock socket provided that the length of rock socket is less than three times the pile diameter.

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The possible methods to reduce downdrag on piles are mentioned below:-

(A) Coating piles with friction reducer such as bitumen.

(B) Predrill a hole firstly, followed by putting in the pile and subsequent filling of annulus with bentonite slurry.
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Coating driven piles with bitumen serves the following purposes:

(i) It acts as friction reducer and could effective reduce the effect of downdrag.

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It is a common practice in the construction of bored piles by adding a cement plug before concreting of bored piles takes place. One of the possible explanations is that the cement grout serves as a barrier and protects fresh concrete from being washed away by water inside pile bore.

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For normal soils, water in bentonite slurry penetrates into the sandy walls and leaves behind a layer of bentonite particles on the surface of the soils. The bentonite particles form the filter cake of low permeability on the excavated faces. The filter would be formed only when slurry pressure is greater than the pore water pressure in excavated soils. The filter cake serves as impermeable layer and allows the application of full hydrostatic pressure of bentonite slurry on the excavated surface of soils.

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The use of high density bentonite slurry could improve trench stability. It helps to retain cuttings and particles in suspension and reduce the loss of bentonite slurry into soils of high permeability such as sand.

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