The problem involves evaluating the increase in stress Δp beneath a pile group when the group is subjected to a vertical load Qg . The computation of stresses depends on the type of soil through which the pile passes. The methods of computing the stresses are explained below:
Figure 15.31 Settlement of pile groups in clay soils
1. The soil in the first group given in Fig. 15.31 (a) is homogeneous clay. The load Qg is assumed to act on a fictitious footing at a depth 2/3L from the surface and distributed over the sectional area of the group. The load on the pile group acting at this level is assumed to spread out at a 2 Vert : 1 Horiz slope.
2. In the second group given in (b) of the figure, the pile passes through a very weak layer of depth L1 and the lower portion of length L2 is embedded in a strong layer. In this case, the load Q is assumed to act at a depth equal to 2/3 L2 below the surface of the strong layer and Q8 spreads at a 2 : 1 slope as before.
3. In the third case shown in (c) of the figure, the piles are point bearing piles. The load in this case is assumed to act at the level of the firm stratum and spreads out at a 2 : 1 slope.