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Okabe (1977) reported response of instrumented piles in a wide pile group in subsiding soils and showed that the response of the perimeter piles was similar to that of single piles and that interior piles were unaffected by either drag force or shaft resistance. As the pile toes were in soft soil, the toe resistance was small for both perimeter and interior piles. Ordinarily, perimeter piles in a wide group will carry more load than interior piles. With time, downdrag on the perimeter piles will add force and soften the pile re-sponse, which means that some load is transferred to the interior piles. In the long-term, the interior piles may even have to carry larger load than the perimeter piles. Cooke et al. (1981) reported a case history of a wide group in overconsolidated soil and showed that the perimeter piles received larger load than the interior piles. The pile toe resistance was considerable and the observations indicated that the interior piles engaged the shaft resistance upward from the pile toe level. Hhigher up along the pile, the response was similar to that of the piles in the Okabe (1977) case, i.e., minimal shaft shear. Hansbo (1984) compared the response of two adjacent piled foundations subjected to very similar stress, 60 kPa and 66 kPa, respective-ly. One was conventionally designed with large safety factor and the other was designed considering the response to be that of raft and piles combined. The first had twice as many piles as the other. Observations over 13 years showed that the settlements of the two buildings were equal indicating that the ratio of load to ultimate bearing on a single pile is irrelevant to the response to load on a wide pile foundation. Russo and Viggiani (1995) confirmed the observation of the perimeter piles having a stiffer response as opposed to the interior piles, somewhat lessened by the presence of general subsidence.

Keywords:  pile group, force distribution, interior piles, perimeter piles, drag force, contact stress

B. H. Fellenius

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