Recently, I've some High Strain Dynamic Pile Test (HSDPT) results for pile driving monitoring on a test pile. I notice that there is a obvious trend of tensile stress increase after jointing the first extention pile, where the starter pile is 11m into the ground.
At the end of driving, the pile is about 17m long with 12m starter pile and 12 extension pile. There is about 3m toe penetration into hard layer (SPT-N > 50). The general overburden has SPT-N of 5 for first top 4m and 15 for the subsequent 10m. The subsoil is mainly Silty Clay with some gravel at the lower stratum.
The tester's explaination on this phenomenon can be contributed by the following:
a. The welded joint tends to open up during the upward reflection wave passing back to the upper extention pile, which allows the upper extension pile to move more than the lower starter pile, therefore tensile stress is recorded. b. The contrast in shaft friction allow the upper portion of the pile segments, which have lower friction, move a little bit more than the lower pile segments, which have more restraints for movement from the higher shaft friction, during the impact wave reflecting upward.
Is there any other reasons for the occurrence of high tensile stress on pile in this ground condition?
Yes the return wave from the pile toe is a compression wave in this case. When a pile is just driven with very little skin friction and the pile toe rest on rock, the compression wave from the hammer blow travels to the toe and reflected as double the compression wave, when the wave returns back to the pile head, as little skin friction along the pile shaft reduces the energy of the wave and also no protection exist at the pile head and the compression wave(Double) reflects back to the pile as tension.
Pile monitoring should not be used just to measure what happen to the pile but to be used as a tool to solve and eliminate any piling problems. The tester should be able to advise the piling contractor during monitoring to limit the stresses experience in the pile by giving instruction to the piler to change the hammer drop(for the case of Hydraulic Hammer) but at the same time achieve the capacity required in the pile driving.
I have experience in a pile monitoring for a 500mm diameter 100mm thick prestressed concrete pile using a 7 ton Hydraulic Hammer facing the problem of broken pile during piling. Most other tester could not solve the problem.