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Note on Negative Skin Friction



 Negative Skin Friction
Negative skin friction is the downward drag that increases load on the pile, therefore generally increase the pile settlement but does not change the pile capacity. It adds the structural load of pile. It occurs when the surrounding soil settles relatively more than the pile, causing drag force that increases the load on the pile and resulting in the reduction of carrying capacity of pile. Such settlement is due to the weight of superimposed fill, to ground water lowering or as a result of disturbance of clay caused by pile driving. The additional load due to negative skin friction may be so large as to cause overstressing of the pile material or may be lead to large settlement or even failure in the underlying supporting soil.
Settlement computations should be performed if necessary to determine the amount of settlement the soil surrounding the piles is expected to undergo after the piles are installed. The amount of relative settlement between soils and pile that is necessary to fully mobilize negative skin friction is approximately 0.5 inches. At that movement the maximum value of negative skin friction is equal to the soil adhesion or friction resistance. The negative skin friction can not exceed these values because slip of the soil along the pile occurs at this value. It is particularly important in the design of friction piles to determine the depth below which the pile will be unaffected by negative skin friction. Only below that depth can positive skin friction forces provide support to resist vertical loads.


The net ultimate load carrying capacity of the pile is given by the equation
Qu’ = Qu - Qnsf
Qnsf = Negative skin friction
Qu’ = Net ultimate load
Qu= Load carrying capacity of pile
An approximate estimate of the force can be made by empirical formulae such as following
Fnsf= force due to negative skin friction
Fnsf  = (perimeter*soil depth)*Cu [for clays]
Fnsf= 0.5(perimeter*(soil depth)2*γ Ktan𝛿) [for sands]
Cu= undrained shear strength
γ= unit weight of soil
K= coefficient of earth pressure
𝛿 = angle of internal friction.
Where it is anticipated that negative skin friction would impose undesirable, large downward drag on a pile, it can be eliminated by providing a protective sleeve or a coating for the section which is surrounded by the settling soil.
The remedial measures for negative skin friction are:
·         Coating the pile surface by bitumen.
·         Driving first the casing and then fill the space between the casing and the pile. Viscous material is used and casing is withdrawn later.
·          Use of belled pile.
·         Use preloading to accelerate the consolidation of soil before pile drilling.

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