Negative Skin Fraction on Single Pile and on Pile Group

Engineeering Projects

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.

negative skin friction

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

Q_{ns} = A_s's_u' \alpha ———- (1)

Where, Qns = Drag force per pile

As‘ = Shaft area of the pile subjected to Negative Skin Friction

su‘= Average su along the length of the pile subjected to Negative Skin Friction

\alpha = Adhesion factor in the region of Negative Skin Friction

Cohesionless Soils

Q_{ns} = K_s\sigma_v' tan\delta A_s'   ———- (2)

Where, \sigma_v' = \dfrac{\gamma L_c}{2}

As‘ = Shaft area of the pile subjected to Negative Skin Friction

Ks = Average coefficient of lateral earth pressure acting along the length Lc

KS =1 (loose sand)

KS = 2 (dense sand)

\delta = Angle of friction between the pile and soil (\delta = \dfrac{3}{4}\phi)

LC = 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)].

Q_{ns}(g) = n \times Q_{ns}(s)   ———- (3)

Q_{ns}(g) = A_s(g) \times s_u'(g) + \gamma \times A_b(g) \times L_c   ———- (4)

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

Where, As(g) = Perimeter Area of the pile group

Ab(g) = Area of the pile group at base

LC = Length of soil under Negative Skin Friction

\gamma = Unit weight of soil within the pile group upto a depth of Lc

su‘(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:

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

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