Seismic axial behaviour of pile groups in non-liquefiable and liquefiable soils

This paper investigates the axial load transfer during seismic events for pile groups installed in loose and dense sand layers. The analysis was conducted employing three-dimensional (3D) nonlinear finite element models (FEM) that were validated using the results from shaking table tests of a model superstructure supported by a pile group installed in saturated (liquefiable) and dry (non-liquefiable) soil profiles. The FEM was employed to explore the axial load transfer at the liquefiable and non-liquefiable tests. The results of the liquefiable test demonstrated that the excess pore water pressure and associated liquefaction during the shaking caused dramatic decrease in the shaft friction resistance. Nonetheless, positive shaft friction was observed through the loose sand layer until the soil becomes fully liquefied. In addition, the end bearing forces decreased dramatically, and the pile exhibited excessive settlement, which mobilized additional end bearing resistance. At the non-liquefiable test, the pile experienced compression and tension loading cycles, but the loads were less than its static capacity. The effects of ground motion intensity, pile diameter, and thickness of the loose sand layer on the load transfer mechanism were also evaluated. It was found that as the pile diameter increased, higher excess pore water pressure developed in the dense sand, which reduced the bearing pressure. Also, as expected, the reduction in shaft resistance was higher as the thickness of the loose (liquefiable) sand layer increased, especially for piles with a small diameter. For the non-liquefiable soil, the strain in the soil increased for larger pile diameters, and consequently, the bearing forces decreased but later increased as the pile settlement mobilized higher resistance.