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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.
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.
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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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.
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.
Bentonite slurry is one of the most common excavation fluid used in constructing diaphragm wall. Bentonite clay (in powder form) and water are combined in a colloidal mixer and clay particles bond to each other and set to form a gel when left to stand for a period of time. When the bentonite is set in motion, it reverts back to the fluid state rapidly.
The size of tremie pipe is about 300mm with sections having flange couplings fitted with gasket to prevent water leakage. The tremie pipe should be closed initially to prevent water from entering the pipe. It should be designed with sufficient thickness and weight so that it would not be buoyant when empty inside water.
