Seismic Response of Driven and Helical Piles in Non-liquefiable and Liquefiable Soils
Abstract
The thesis investigates the nonlinear soil-pile-structure interaction through three-dimensional nonlinear finite element models (FEM) employing the OpenSees platform. The FEMs were validated with the results of large-scale shaking table tests of model pile groups-superstructure systems in dry and saturated sand and large-scale field tests on single piles installed in cohesive soil. The numerical models correctly predicted the different pile deformation modes that were exhibited in the experiments. The results illustrated that the inertial interaction contributed to the bending moments at the pile top, while the kinematic interaction contributed to the bending moment at the layers interface. In addition, the excess pore water pressure and associated liquefaction during the shaking caused a dramatic decrease in the pile shaft friction resistance. Nonetheless, positive shaft friction was observed through the loose sand layer until the soil was fully liquefied. The larger pile diameters caused higher excess pore water pressure in the dense sand, which reduced the bearing pressure.
The lateral displacements of the helical piles (HP) and adjacent soil decreased compared to the driven piles. Meanwhile, HPs demonstrated excellent performance in maintaining their capacity during and after liquefaction and controlling the post-liquefaction settlement. The response of the HP groups in the saturated test was dominated by the rocking behaviour, while the flexural behaviour dominated the response in the dry tests. Moreover, the FEMs were employed to examine the effect of model scale on seismic response of prototype HPs. A set of scaling factors was suggested for extrapolating the soil and HP responses from 1g shake table model tests.
The seismic performance of bridge-helical pile foundation was explored based on the seismic fragility analysis. The results revealed that the HPs were the most fragile component in the non-liquefiable and liquefiable tests. The liquefiable soil decreased the seismic demand on the column lateral deformation and increase the demand dispersion. The reinforced concrete pier exhibited a large drift response in the non-liquefiable soil. Finally, the dynamic response of driven and HPs was explored in cohesive soil. The HP could replace the driven piles with shorter lengths, and it reduced the required width of the improved soil.