4. Bearing capacity. The bearing capacity of clay layers underlying the piles must be evaluated.
The danger is that the piles will punch through the bearing layer and into underlying softer clay strata.
Similar to solutions for shallow foundations, charts have been developed for estimating the distribution of stress beneath deep foundations.
For example, Fig. 4.17 shows the pressure distributions for pile foundations for four different soil conditions.
For easy reference, Fig. 4.17 has been reproduced as Fig. 8.17, with an asterisk included that shows the zone of clay that would need to be evaluated
for primary consolidation settlement and secondary compression.
These charts are valid for relatively rigid pile caps, where the rigidity of the foundation system tends to restrict differential settlement across the pile cap.
In this case, the 2:1 approximation is ideal because with a relatively rigid
foundation system, it is desirable to calculate the average settlement of the pile cap. The four conditions in Fig. 8.17 are individually discussed below