Abstract

A flowslide overriding liquefied substrate can vastly enhance its disaster after failure initiation, due to rapid velocity and long-runout distance during landslides mobilized into flows. It is crucial to provide improved understanding to the mechanism of these catastrophic flowslides for hazard mitigation and risk assessment. This study focuses on the Saleshan landslide of Gansu in China, which is a typically catastrophic flowslide overrode a liquefied sand substrate. Geomorphologic and topographic maps along with analysis of seismic signals confirm its dynamic features and mobilized behaviors. ERT surveying detected abundant groundwater in the landslide, which is fundamental to its rapid long-runout distance. Particle size distributions and triaxial shear behaviors affirmed more readily liquefied behavior of superficial loess and underlying alluvial sand than red soil sandwiched them. We also examined the liquefaction susceptibility of the alluvial sand under loading impact at undrained and drained conditions. The alluvial sand is readily liquefied in the undrained condition while it is difficult at drained condition due to rapid water pore pressure dissipation. The results showed that the landslide experienced a sudden transformation from slide on the steep slope where it originated to flow on a nearly flat terrace with abundant groundwater that it overrode. This transformation can be attributed to the liquefied alluvial sand substrate enhancing the whole landslide body mobility. Along with recent, similar findings from landslides worldwide, substrate liquefaction may present a widespread, significant increase in landslide hazard and consequent mobility and our study reveals conditions necessary for this phenomenon to occur.