Negative skin friction on piles is dominant in

Concept:

Negative skin friction: It is usually a downward shear drag acting on a pile or pile group because of the downward movement of surrounding soil relative to the piles. Such a force can exist under one of the following conditions:

1. If a fill of clay soil is placed over a granular soil layer into which a pile is driven, the fill will exert a downward drag force on the pile due to consolidation.
2. If a fill of granular soil is placed over a layer of soft clay, it will induce the process of consolidation in the clay layer and thus exert a downward drag on the pile.
3. Lowering of the water table will increase the vertical effective stress on the soil at any depth, so it can induce negative skin friction on piles.

Now the axial capacity of a pile is the summation of upward reaction due to bearing at the base and net upward skin frictional resistance. As the negative skin friction (acting downward) lowers the net skin resistance, it, in turn, reduces the axial capacity of piles.

A schematic diagram of negative skin friction on piles is shown below:

∴ Negative skin friction on a pile under vertical compressive load acts downwards and reduce the load-carrying capacity.