Soil-pile-structure kinematic and inertial interaction observed in geotechnical centrifuge experiments

This paper reports the results of a series of centrifuge tests examining dynamic responses of single and 3×3 grouped piles embedded in sand and supporting SDOF and 2DOF structures. A total of 7 model tests were conducted with the centrifugal acceleration of 40 g. Each model was subjected to 12 sinusoidal waves with constant acceleration amplitude and varying frequencies. The results of the tests indicate that pile-head motion is dominated by two sequential frequencies: a lower frequency (fSSI) where pile-head motion is substantially maximized and a higher one (fpSSI) where the response is minimized with respect to free field surface motion. These results confirm recent published numerical results on single piles supporting SDOF structures and generalize their findings to grouped piles supporting SDOF and 2DOF structures. The results show also strong mobilized kinematic interaction effect generating significant pile bending when the ground is excited at its resonant frequency. On the other hand, structural vibrations tend to impose large bending moments as the excitation frequency approaches the natural frequency of the coupled soil-pile-structure system. Distribution of pile bending moments in the group is found to be a function of the pile position and the excitation frequency. In contrast to inner piles having the greatest kinematic bending moments, outer piles have a more pronounced inertial ones.