Assessment of SSI effects on stiffness of single and grouped helical piles in dry sand from large shake table tests

Full-scale shake table testing was conducted to evaluate the seismic performance of single and grouped helical piles. Eight circular and one square helical piles with different properties including length, radius and number of helices, as well as one driven circular pile were installed in dry sand enclosed in a laminar soil shear box that was situated on the shaking table. Dynamic properties of sand bed and its natural frequencies as well as natural frequencies of single and grouped helical pile-soil systems were evaluated from the collected data during different shaking events. The effects of different pile configurations as well as successive shakings on the natural frequencies of the sand bed and pile-soil systems were also investigated. It was found that the soil’s disturbance increased as number of helices increased, which resulted in reduced pile-soil stiffness and natural frequency. Natural frequencies of single piles and pile groups decreased due to successive shakings owing to the degradation in the pile-soil stiffness and gap forming around pile shafts near the ground surface. The pile’s free length significantly reduced the stiffness of single and grouped helical piles and gap opening further increased the free length and hence resulted in additional reduction in stiffness and natural frequencies. These effects must be considered in seismic design of helical pile foundations. Furthermore, the seismic responses of single and grouped helical piles are greatly affected by the resonance condition, which causes large soil deformations and significant reduction in the stiffness of the pile-soil system. It was also demonstrated that the software DYNA6 could predict the single and grouped piles behaviour correctly by accounting for degradation of the soil’s stiffness and gap opening.