Pile group 16 piles of 50 cm, efficiency

Given: 16 piles (4 × 4), $d = 0.50$ m, spacing $s = 1.0$ m, soft clay $c_u = 30$ kN/m², $\alpha = 0.6$, point bearing neglected. The length is printed as 90 m; 9 m is taken as the intended value (the efficiency does not depend on $L$, and for 90 m the capacities are 10 times larger).

Sum of individual piles

$$Q_u = \alpha c_u\pi dL = 0.6 \times 30 \times \pi \times 0.5 \times 9 = 254.5\ \text{kN},\quad 16Q_u = 4071.5\ \text{kN}$$

Block

$$\begin{aligned} B_g &= L_g = 3(1.0) + 0.5 = 3.5\ \text{m},\quad P_g = 14\ \text{m} \\ Q_{block} &= P_gLc_u = 14 \times 9 \times 30 = 3780\ \text{kN} \end{aligned}$$

Group capacity and efficiency

The smaller value governs: $Q_{g} = 3780$ kN.

$$\eta = \frac{3780}{4071.5} = 0.928\ (92.8\%)$$

Answer: ultimate load capacity of the group $= 3780$ kN and efficiency $= 0.93$ (block failure governs). For $L = 90$ m as printed: $Q_g = 37\,800$ kN, $\eta = 0.93$.