Catastrophic landslides have repeatedly occurred on gentle slopes in urban areas during past earthquakes in Japan, sometimes causing great loss of life and significant environmental damage. For gentle slopes less than 20º but greater than about 10º, the mechanism controlling the high mobility of the slide mass after failure appears to be the gradual loss in shear strength with progressive shear displacement. Laboratory shear tests indicate that this liquefaction phenomenon culminates in ultimate steady state strengths smaller than static (gravitational) driving shear stress at the landslide sites. Thus, the potential sliding mass may develop an accelerated motion under static conditions if the shear strength loss due to some transient disturbance (e.g., earthquake) was large enough to bring definitively the available shear resistance below the gravitational driving shear stress. For very gentle slopes of less than about 5º, the most likely mechanism governing the large lateral ground movements in liquefiable deposits is the void redistribution due to upward seepage associated with soil liquefaction. This process can cause the formation of a water film at the upper boundary of a liquefied deposit overlain by a lower permeability interlayer (e.g., silt or clayey material). The water film serves as a shear stress isolator, accommodating therefore the large observed lateral displacements for nearly horizontal ground conditions.





Back