Settlement of pile group in sand

The settlement of a pile group in sand is greater than that of a single pile carrying the same load per pile, as a larger volume of soil is stressed. The main methods are:

1. Vesic's method (empirical):

$$S_g = S\sqrt{\frac{B_g}{D}}$$

where $S$ is the settlement of a single pile at the same working load, $B_g$ is the width of the group and $D$ the pile diameter.

2. Meyerhof's SPT method:

$$S_g\ (\text{mm}) = \frac{2q\sqrt{B_g}\,I}{\bar N},\qquad I = 1 - \frac{L}{8B_g} \ge 0.5$$

where $q$ is the net pressure on the group (kN/m²), $B_g$ is in metres, $L$ is the length of pile embedded in the bearing stratum and $\bar N$ the average SPT value within a depth $B_g$ below the tip (for CPT: $S_g = \dfrac{qB_gI}{2q_c}$).

3. Equivalent raft method: the pile load is assumed to act on an imaginary raft of the group size at a depth $\tfrac{2}{3}L$ from the top (for friction piles) with a 2:1 load spread, and the settlement of the raft is computed from elasticity or from SPT/plate test data.

4. Elastic (interaction) methods (Poulos): settlement from interaction factors between piles.