Negative skin friction on pile 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 the pile.

Such settlement may be due to the weight of the superimposed as shown in Fig.1, 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 lead to large settlement or even failure in the underlying supporting soil.

**Computation of Negative Friction on Single Piles**

The magnitude of negative skin friction for a single pile in filled up soils may be taken as: [Fig.1 (a)]

**Cohesive Soils**

———- (1)

Where, Q_{ns }= Drag force per pile

A_{s}‘ = Shaft area of the pile subjected to Negative Skin Friction

s_{u}‘= Average s_{u} along the length of the pile subjected to Negative Skin Friction

= Adhesion factor in the region of Negative Skin Friction

**Cohesionless Soils**

———- (2)

Where,

A_{s}‘ = Shaft area of the pile subjected to Negative Skin Friction

K_{s} = Average coefficient of lateral earth pressure acting along the length L_{c}

K_{S }=1 (loose sand)

K_{S} = 2 (dense sand)

= Angle of friction between the pile and soil

L_{C} = Length of loose or soft soil layer underlying Negative Skin Friction

**Negative Skin Friction on Pile Groups**

When a group of piles passes through compressible filled up soil, the negative skin friction on the group may be found by any of the following methods [Fig.1 (b)].

———- (3)

———- (4)

The higher values obtained from these two should be used in design.

Where, A_{s}(g) = Perimeter Area of the pile group

A_{b}(g) = Area of the pile group at base

L_{C} = Length of soil under Negative Skin Friction

= Unit weight of soil within the pile group upto a depth of L_{c}

s_{u}‘(g) = Average unit cohesion of clay on the surface of group cylinder as a hole

**Remedial Measures for Negative Skin Friction**

To reduce the negative skin friction, the following measures should be taken:

- Provide small cross-sectional area of shaft as compared to the points
- Drive pile inside a casing with the space between pile and casing filled with a viscous material and withdraw the casing
- Coat the pile with bitumen