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Negative Skin Friction

A negative skin friction is a phenomenon that arises from a settlement of soil in the vicinity of a pile. The soil deforming around the pile tends to pull the pile downwards thus reducing its bearing capacity for a given pile settlement. 

The input parameters for assessing the influence of negative skin friction is the settlement of ground surface w and a depth of influence zone of this deformation h. For a uniformly distributed load around the pile the value of w should be measured in the distance equal to three times the pile diameter from its outer face. The value then represents the depth influenced by the ground surface settlement and below which the soil is assumed incompressible with no deformation. 

Computation of negative skin friction is carried out first while determining the limit shear forces transmitted by the pile skin Tlim. The solution procedure assumes that the soil settlement decreases linearly with depth from the value of w on the ground surface up to 0 at a depth of h. The specific value of the soil settlement is therefore assumed for each level below the ground surface till the depth of h. The forces developed in springs of pile segments due to their deformation are determined and then subtracted from Tlim to reduce the bearing capacity of the pile skin.  

From the presented theory it is evident that for large settlement w or large depth h the values of Tlim may drop down to zero. In extreme cases the negative skin friction may completely eliminate the skin bearing capacity so that the pile is then supported only by the elastic subsoil below the pile heel.