Displacement-Based Design of Axially Loaded Piles for Seismic Loading and Liquefaction-Induced Downdrag

Abstract

Axially loaded piles in liquefiable soils can undergo severe settlements due to a shaking event. During shaking, the settlement is caused by the reduction of its shaft and tip capacity from the excess pore pressures generated around 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 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 AASHTO’s force-based design procedure by reasonably accounting for the mechanisms that occur on axially loaded piles during and post shaking. It accounts for the initial drag load on the pile, redistribution effects resulting in large excess pore pressures in the non-liquefied layers, and reduction in the pile’s shaft and tip capacity from excess pore pressures around the pile. The new design procedure estimates the pile settlement and axial load distribution during the entire shaking event, i.e., during shaking and reconsolidation. Design steps are provided describing 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. Finally, the new design procedure is applied on piles used in centrifuge model tests, and results are compared, followed by an example design problem that illustrates the applicability of the new method in practice.