Negative skin friction on 16-pile group

Given: 16 piles (taken as a 4 × 4 group), $d = 0.60$ m, through $H = 3.5$ m of unconsolidated (settling) soil, $q_u = 60$ kN/m² so $c_u = 30$ kN/m², $\gamma = 16$ kN/m³, $\alpha = 1$.

The printed spacing of "800 cm" is taken as 800 mm = 0.80 m; a spacing of 8 m is not reasonable for a 16-pile group.

The negative skin friction (NSF) on the group is the smaller of (1) the sum of the NSF on individual piles and (2) the NSF on the block of soil.

1. Individual piles

$$\begin{aligned} Q_{nsf} &= \alpha c_u\pi dH = 1 \times 30 \times \pi \times 0.6 \times 3.5 = 197.9\ \text{kN} \\ 16Q_{nsf} &= 3166.7\ \text{kN} \end{aligned}$$

2. Block

$$\begin{aligned} B_g &= 3(0.8) + 0.6 = 3.0\ \text{m},\quad P_g = 4(3.0) = 12\ \text{m} \\ Q_{ng} &= \gamma B_g^2H + P_gHc_u \\ &= 16 \times 3.0^2 \times 3.5 + 12 \times 3.5 \times 30 \\ &= 504.0 + 1260.0 = 1764.0\ \text{kN} \end{aligned}$$

Answer: negative skin friction on the group $\approx 1764$ kN (the block value governs). If the spacing were really 8 m, the piles would act separately and the NSF would be $16Q_{nsf} \approx 3167$ kN.