Performance-based assessment of liquefaction-induced downdrag on piles

抄録

Earthquake-induced soil liquefaction can cause settlement around piles, which can translate to negative skin friction and the development of drag load and settlement of the piles. The overall phenomenon and the interaction of the individual mechanisms in time are still not fully understood, leading to either conservative or unconservative designs in practice. A series of centrifuge model tests were performed to assess liquefaction-induced downdrag and understand the interplay and effects of (i) pile embedment and pile-head load, (ii) excess pore pressure generation and dissipation; and (iii) reconsolidation and ground settlement on pile response during and post shaking. It was found that most of the pile settlements were co-seismic while full liquefaction (excess pore pressure ratio of 100%) is not a prerequisite for the development of significant drag loads. Furthermore, the patterns of pore water diffusion elucidated how they can either exacerbate or alleviate excess pore pressures in various soil layers. Informed by the results of the centrifuge testing program, follow on investigations focused on developing a validated TzQzLiq numerical model that essentially accounting for the change in the pile’s shaft and the tip capacity as free-field excess pore pressures develop/dissipate in soil. The numerical model was later leveraged to develop a simplified displacement-based design procedure for axially loaded piles. Ongoing advanced fully coupled nonlinear numerical models investigate the state-of-the-art in accounting for the aforementioned mechanisms and their potential for translation to practice. The paper presents an overview of these efforts.