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For pile driving in certain soils like dense silt and weathered rock, the occurrence of false set phenomenon is not uncommon. During the driving process, negative pore water pressure is developed and the driven piles appear to have sufficient capacity during pile driving as the built-up of negative pore water pressure leads to an apparent temporary increase in driving resistance and strength.
However, some time after the pile driving, the dissipation of this negative pore water pressure would reduce the bearing strength in resisting the design loads. Sometimes, the presence of cracks along pile sections may bring about the problem of false set by the dampening effect of stress waves by these cracks.
To avoid the problem of false set, a certain percentage of constructed piles should be chosen to perform re-driving to check for the false set phenomenon
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.