Contribution by W. J. Neely
Equations 1 and 2 in the paper by Roohnavaz (2010) imply that the phenomenon of negative skin friction will reduce the geotechnical capacity of the piles. The drag load (termed NSF in the paper) induced by negative skin friction does not affect the bearing capacity of a pile since geotechnical failure requires that the pile move downward relative to the settling soil and negative skin friction cannot be sustained when this happens. The drag load, or negative skin friction load, must be considered, but not with respect to geotechnical capacity. Drag load influences only the structural strength of the pile section at the location of the maximum load in the pile shaft, which occurs at the neutral plane.
The source of the negative skin friction is the compression of the soft Bangkok clay in response to deep pumping. Equation 2 includes the sum of the negative skin friction load from the soft clay and the specified working load (SWL) multiplied by a factor of safety plus positive shaft resistance in the soft clay. The author states that this approach was necessary to allow an assessment of pile load adequacy based on short-term information from pile testing. In the case of static loading tests, it is not necessary to include the negative skin friction load (NSF) since in a loading test the pile moves downward relative to the soft clay, thereby eliminating the drag load and creating positive resistance, which contributes to the geotechnical capacity. The practical usefulness of Equation 2 is questionable, given the fact that it includes both negative and positive shaft resistances from the same layer of soft Bangkok clay.
Author's reply
Discussion comments regarding the theoretical definitions of ‘drag load' and ‘geotechnical failure' are noted.
Equation 2 is to provide an assessment of the adequacy of geotechnical capacity of the pile in the long term using the data from the static load test (SLT). The long-term pile capacity as given in Equation 1 is (SWL + NSF) factor of safety (FoS). However, at SLT pile movement significantly exceeds that of any ground movement as the pile is taken to ‘geotechnical failure'. In the long term, in response to deep pumping, the soft clay compresses and the positive contribution in the soft clay at the time of the SLT is imposed as negative skin friction, as the ground movement exceeds that of the pile.
Therefore, to verify the long-term conditions from the SLT, the positive contribution of soft clay is taken away from the SLT value at failure (Q) and that capacity is required to resist the NSF added as a load to SWL. Hence, in the long term
At the time of the SLT
It is noted that the FoS practitioners apply to the SWL and NSF respectively may not be the same.
