Evaluation of Seismic Soil-Structure Interaction of Full-Scale Grouped Helical Piles in Dense Sand

A full-scale shake table testing program was performed with two helical pile groups supporting model structures to better understand the dynamic properties of a soil-pile-group-structure system. Each group had four piles embedded in dry, dense sand confined in a 6.7 m L x 3.0 m W x 4.6 m H laminar box. One group was comprised of 8.8-cm diameter and 3.66-m long helical piles, while the second group had 14-cm diameter and 4.27-m long helical piles. To investigate the influence of the pile-head fixity condition on group behavior, both pinned head and fixed head connections were implemented and tested. White noise excitation and two replicated strong earthquake motions were used to determine the natural frequency and observe the seismic behavior of the soil-pile-group-structure system, respectively. The experimental observations were analyzed to evaluate structural natural frequency and pile-group stiffness and damping. The experimental results were used to calibrate a numerical model that was then used to conduct a parametric study to gain a broader understanding of the seismic behavior of helical pile groups under varying conditions. The experimental and numerical results were compared and the effects of varying properties in a soil-pile-structure system, including their system seismic response, are discussed.