A Displacement-Based Design for Axially Loaded Piles in Liquefiable Soils

Axially loaded piles in liquefiable soils can undergo severe settlements due to a shaking event. During shaking, the settlement is due to the pile’s shaft and tip capacity reduction arising from developed excess pore pressures (u_e) surrounding the pile. Post-shaking, additional pile settlement is caused by the surrounding soil settling due to reconsolidation and the associated development of drag load. A new displacement-based method is developed using a TzQzLiq analysis for designing axially loaded piles in liquefiable soils subject to seismic loading and liquefaction-induced downdrag. The new displacement-based design method offers several advancements to the state of practice forced-based design procedure by reasonably accounting for the mechanisms occurring on the pile during and after shaking. It accounts for the initial drag load on the pile, redistribution effects resulting in large u_e in the non-liquefied layers due to u_e migration from the liquefied layers, and the reduction in the pile’s shaft and tip capacity from u_e in the soil surrounding the pile. The new design procedure estimates the pile settlement and axial load distribution during the entire shaking event, that is, during shaking and reconsolidation. Design steps are provided to describe the procedure for obtaining design curves on the settlement and drag load on piles with varying pile lengths. The length of the piles is then selected based on serviceability criteria and the pile’s structural strength. An example design problem is presented to illustrate the application of the proposed design procedure in practice. The results are then compared to the Caltrans force-based design method, and important conclusions on pile design are discussed.