Skin friction of drilled piers (Piles)
Skin friction of drilled piers (Piles)
(OP)
Why there is difference in upward and downward skin friction of drilled piers (piles)? If someone can explain theoretically.
Jaleel Ur Rehman
Geotechnical Engineer
Osaimi Engineering Consulting Office, KSA





RE: Skin friction of drilled piers (Piles)
Assumption: A single friction pile's uplift capacity is lower than its compression capacity.
For compression loading, all of the soil surrounding the pile is more or less supported by underlying soil. Therefore, the total surface area of the pile is mobilized to resist movement by friction.
For uplift, only a portion of the pile's surface area is available to resist movement. The soil closest to the surface has nothing above it to directly resist upward movement. The soil in contact with the pile tends to move upward with the pile.
In summary, my guess is that for compression loading there is both more useful surface area and a higher coefficient of friction than there is for uplift loading.
I'm sure this is too simplistic an explanation; I would not be surprised if these observations point in the right direction.
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Skin friction of drilled piers (Piles)
Poulos and Davis' Pile Foundation Analysis and Design suggests for granular soils a reduction of 1/3 can be assumed - but best to do a pile load test. They also say that the published reports have quite a bit of scatter and in some cases in conflicting.
RE: Skin friction of drilled piers (Piles)
RE: Skin friction of drilled piers (Piles)
This article by England provides interesting hypotheses based on bidirectional load tests.
Basically, he concludes that there is no real significant difference from tensile and compressive capacity and observed differences may be due to locked stresses and interpretation of load tests in relation to definition of ultimate failure. Interesting and heretical views, summarized just before chapter 4, page 13 of the linked PDF.
http://www.loadtest.com/INT_media/680-693%20Englan...
www.mccoy.it
RE: Skin friction of drilled piers (Piles)
RE: Skin friction of drilled piers (Piles)
If 'the proof is in the pudding' then we should eat that pudding and judge by ourselves. Of course, no way I wouldn't follow what the relevant codes suggest. the Italian building code, which follows the European code, has a safety factor for shaft lateral friction in tension which is 9% larger than lateral friction in compression.
2 quick non-heretical references, the sources I use most:
www.mccoy.it