Negative Skin Friction?

[GalileoG]

I haven't done much pile design and I am sure this question is an easy one for you geotechnical experts. The geotechnical report that I am reading is suggesting that negative skin friction -10 kPa from top of pile to -20 kPa to the bottom of the engineering fill (5 meters below grade.) What is negative skin friction, how is engineering fill causing it and how do I account for it? Actually, any free online resources for pile design? Would appreciate it. Thanks.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[apsix]

Settlement of the fill is anticipated to cause negative skin friction dragging on the pile. This needs to be added to the design load, unless a slip layer is used.

 

[BigH]

google Fellenius and access many of his papers. there was a good chapter in the Timber Pile Foundation Manual (old hand way of handling it - Louisiana, I belive). If you find SlideRuleEra's web site - you can download the manual from it. You can also google it.

 

[ishvaaag]

So you integrate the negative skin friction along the length for your given values and add the load atop along the biggest column load (conservative approach for vertical downwards force on the pile). Think if following your code you need a safety factor (or how the combination must to be made). In walls we factor the earth push, so being some kind of earth push maybe we should use a factor, except we see the estimate as overly conservative. The fact of the values being somewhat unreliable (like earth push) also support the use of a safety factor.

You can also apply the load where it is applied but this only will have some effect when using some nice FEM or specialized pile design software.

 

[Ltwine]

Just a simple fact here.

A positive skin friction goes against the direction of gravity, while negative skin friction means the soil sinks/drags the pile, like throwing a stone into muddy swamp. I heard piles were nowhere to be found, could be a joke though. But, you need to consult with your geotech to determine the proper method to overcome this phenomenon.

 

[SirAl]

Negative skin friction is usually associated with settling soil relative to a pile that extends through and to depths well below the settling zone. The settling material imposes a downward force on the pile shaft. There are a number of semi-empirical formulas that can be considered to estimate the value. Most foundation design textbooks would carry a blurb on this. Even well compacted fill will settle a small amount after placement. And that does not necessarily account for possible consolidation settlement of the soil below the fill subject to the load of the fill. Fellenius, as BigH has referenced, is as good a source of information in this regard. It seems to me his web site has several papers and references available.

 

[vulcanhammer]

Some information can also be found in the FHWA's driven pile manual:

http://www.vulcanhammer.info/soils/piles/dpf.pdf

http://www.pz27.net