Lateral Response of Three-Row Groups in Loose to Dense Sands at 3D and 5D Pile Spacing

Centrifuge tests were conducted on single and 3 × 3 pile groups at three-diameter (3D) and five-diameter (5D) spacings. In all of the tests, the piles were driven and laterally loaded in flight without stopping the centrifuge. The piles simulated 430 mm diameter by 13 m long pipe piles founded in medium loose ( Dr = 33%) and medium dense ( Dr = 55%) sands. Results of the tests showed that the ratio of lateral resistance of a group to a single pile, i.e. efficiency, was independent of soil density. The group efficiency at 3D spacing was 0.74, whereas at 5D spacing the group efficiency was 0.94. Drained triaxial compression tests were performed on the sand at multiple densities to obtain the necessary p - y strength data. Reese et al.'s p - y curves of 1974 resulted in excellent predictions of the prototype single-pile response for both relative densities. p - y multipliers were back-computed using the single-pile response and the measured total and individual row contributions within the group. It was found that the multipliers did not vary significantly at 5D spacing but did vary somewhat at 3D spacing. The latter was due to the variation in row distributions measured at the different soil densities. All tests were on free-headed piles.