Dominant mechanisms controlling sand liquefaction resistance under foreshock sequences

Abstract

Earthquake sequences frequently occur in seasonally frozen regions, where soils are simultaneously subjected to freeze–thaw cycles and repeated seismic loading, leading to complex liquefaction behavior. However, the mechanisms controlling sand liquefaction resistance under the influence of coupled freeze–thaw and foreshock sequences remain insufficiently understood. In this study, a series of undrained cyclic triaxial tests were conducted on saturated soil to investigate the effects of seismic loading sequences and freeze–thaw history on liquefaction resistance. Cyclic loads with varying stress ratios were applied to simulate foreshock sequences prior to a mainshock, and both unfrozen specimens and those subjected to one freeze–thaw cycle were examined. The evolution of excess pore water pressure (EPWP) and axial strain during foreshocks was analyzed, and liquefaction resistance was evaluated during the mainshock. The results indicate that the loading sequence of foreshocks significantly affects the cyclic response and liquefaction resistance of sand, with freeze–thaw cycles leading to greater EPWP generation and increased strain accumulation. When the maximum EPWP ratio during foreshocks does not exceed 0.8, liquefaction resistance exhibits a clear relationship with the accumulated EPWP based on the observed data trends. Conversely, in cases where large EPWP develops during foreshocks, the residual axial strain becomes the dominant factor influencing subsequent liquefaction resistance. These findings underscore the combined effects of seismic sequences and freeze–thaw processes on sand liquefaction resistance in seasonally frozen regions.