reducing pile NSF by using PVD's and surcharge

[AK92]

I'm designing some piles for a building with a soft compressible top layer that is currently undergoing consolidation and will be filled about 2m. Now the whole area is going to undergo ground improvement (PVD and surcharge) before the piles are done. Am I able to ignore negative skin friction in the pile design or perhaps even a percentage reduction since I think I would have already overconsolidated the soil during the ground improvement?

I'm thinking of ignoring both the positive and the negative skin friction to save on pile lengths, however from what I have read it only takes very little settlement to activate the negative skin friction, and I'm a bit concerned that I would be underdesigning the piles if I ignore the NSF, considering it is quite a significant component of the design due to the thickness of the soft clay layer (about 10m) and the fill on top of it.

 

[BigH]

If you preload and let it sit long enough to build out almost all consolidation settlement and the pile itself is not end-bearing on an unyielding surface, I would tend to not add on the negative skin friction. You might wish to refer to work by Fellenius. Poulos also wrote a nice paper you might be interested in. You can google the references below.

Fellenius, B. H., 1984. Negative skin friction and settlement of
piles. Second International Seminar, Pile Foundations, Nanyang
Technological Institute, Singapore, November 28 - 30, 12 p.
NEGATIVE SKIN FRICTION AND SETTLEMENT OF PILES
Dr. Bengt H. Fellenius, P. Eng.
University of Ottawa, Canada

Proceedings of the Institution of Civil Engineers
Geotechnical Engineering 161
February 2008 Issue GE1 Pages 19–27
by Harry G. Poulos Senior Principal, Coffey Geotechnics Pty Ltd, Lane Cove West, Australia
A practical design approach for piles with negative friction
H. G. Poulos, BE, DSc(Eng), FIEAust

 

[AK92]

@BigH

Thanks for the answer. Both papers that you gave references to were very informative. It is quite interesting that "all piles experience downdrag" due to pore pressures generated during pile installation. I think in this case, I would ignore both the NSF and PSF for the piles.