Liquefaction-induced uplift responses of deeply buried structures under the main-aftershock

Deeply buried underground structures in liquefiable sand layers possess the potential for liquefaction-induced uplift under main-aftershock sequences. However, understanding of this risk is limited because prior investigations have largely been confined to either shallow burial conditions or analyses of single seismic events. To address this gap, this study employs a three-dimensional finite difference model incorporating fluid-solid coupling to systematically investigate the influence of burial depth (ranging from 10 m to 40 m) on the displacement and internal forces of structures subjected to the main-aftershock. The effects of multiple aftershocks and soil relative density are also examined. The research results validate the known phenomena that multiple low-intensity aftershocks and an increase in relative density can enhance the liquefaction resistance of sandy soils and confirm that the degree of soil liquefaction beneath structures is a key factor controlling their uplift behaviours. In addition, the key findings include: (1) The soil-structure interaction increases with the depth of the structure, the influence range of the structure on the surrounding sand and liquefaction resistance also increase accordingly; (2) For a given set of soil and seismic parameters, a critical burial depth exists at which the structure experiences negligible net uplift or settlement after the seismic sequence; (3) The structural internal forces significantly increase after a strong mainshock and may remain at levels close to or exceeding the design values, posing a risk of structural damage. These findings can have direct implications for engineering practice: The identified critical burial depth can provide a quantitative guideline for optimising the design of underground structures in liquefiable sites, while the sustained elevation of structural internal forces suggests a potential long-term risk of creep failure that should be addressed in design codes and safety evaluations. Consequently, this study demonstrates that a rigorous seismic design should account for the full effects of main-aftershock sequences and cannot simply assume that greater burial depth invariably leads to improved safety.