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.