Performance of Single and Grouped Helical Piles under Strong Earthquake Shaking

Full-scale shaking 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 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 frequency, as well as the 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 characteristics of the sand bed and pile-soil systems were also investigated. In addition, responses of single and grouped helical piles were computed analytically using the software DYNA6, which agreed with measured responses. These analyses accounted for degradation of soil stiffness and gap opening. Furthermore, effects of the earthquake characteristics (i.e., intensity and frequency content) on seismic performance of single and grouped helical piles were evaluated from the measured responses. The performance characteristics of helical pile groups were discussed in terms of the interaction between piles within a group and the contributions of vertical and lateral stiffness of individual piles to the rocking stiffness and the overall capacity of the pile group. The effect of pile head connection to the pile cap (1-bolt connection versus 2-bolt connection) was evaluated and compared. In addition, behaviour of a single pile and a pile within a group were compared in terms of their normalized responses. A three-dimensional nonlinear dynamic finite element model was constructed employing the software ABAQUS to simulate the shake table testing. The numerical model was validated with the experimental results, which was then used to perform an extensive parametric study. The parametric study explored the effect of the level of a single helix, diameter of a single helix, number of helices, spacing between helices as well as the effect of the pile diameter. Results revealed the superior performance of helical piles under strong ground shakings due to the significant contribution of the helices to the rocking resistance of the pile group