Negative skin friction on single pile and group

Single pile

The negative skin friction acts over the length $L_n$ of the pile that lies in the settling clay (down to the neutral point).

Unit negative skin friction: $f_n = \alpha c_u$ (total stress method, $\alpha \approx 1$ for soft clay), or $f_n = \beta\sigma'_v$ (effective stress method, Bjerrum and Burland, $\beta = 0.2$ to $0.25$ for clay).

$$Q_{nsf} = P \times L_n \times f_n = \pi D\,L_n\,c_u \quad (\alpha = 1)$$

where $P$ is the perimeter of the pile and $D$ the diameter.

Group of piles

A group in clay moves with the soil mass between the piles, so the drag is the lesser of:

1. The sum of the drag on all piles, $Q_{ng} = n \times Q_{nsf}$.
2. The weight of the fill carried by the group, together with the shear along the perimeter of the block of settling clay:

$$Q_{ng} = \gamma_f H_f B_g L_g + c_u\,P_g\,L_n$$

where $B_g \times L_g$ is the plan of the group, $P_g = 2(B_g + L_g)$ its perimeter, $H_f$ and $\gamma_f$ the thickness and unit weight of the fill (surcharge) that causes the settlement, $c_u$ the undrained cohesion of the clay and $L_n$ the thickness of the settling clay along the piles.

The calculated drag is added to the structural load when checking the pile capacity, since it acts in the same direction.