Experimental and numerical study on the mechanism of liquefaction flow-slides in gently sloping ground

This study focuses on two major concepts related to the mechanism of liquefaction flow-slides in gently sloping ground, i.e., (a) ultimate steady state strength smaller than static driving shear stress, and (b) void redistribution and water film development in layered sand deposits. The results of a laboratory study based on undrained cyclic and monotonic triaxial tests conducted on a silty-sand characterizing the liquefiable deposit at the past liquefaction flow-slide sites throughout the Farmington Siding Landslide Complex (FSLC) in Davis County, Utah are used to investigate the minimum slope gradient that would generate flow-sliding in the light of concept (a). An infinite slope analysis using the postliquefaction undrained shear strength from laboratory testing indicates that a slope gradient much greater than the actual site conditions at the FSLC (characterized by a mild topography with an average slope gradient not greater than 1%) would be required to trigger flow failure. This outcome suggests that the flow-slide concept involving ultimate steady state strength smaller than the static driving shear stress is inconsistent with the mechanism of large lateral displacements associated with earthquake-induced ground liquefaction at the FSLC. The geologic setting at the FSLC strongly favors concept (b), i.e., water film development beneath low permeability interlayers within a liquefiable deposit, as the potential mechanism characterizing the past liquefaction flow-slides in the region. In this context, a one-dimensional void redistribution model for analyzing the generation and dissipation of water films in a liquefied deposit with a low permeability interlayer has been developed. A parametric study carried out using the developed numerical model revealed that a decrease in depth of the silt interlayer would result in thicker water films. The results of the simulation also indicate that in case of thicker silt interlayers, less water is trapped underneath the silt interlayer through time and that for lower permeability coefficients of the liquefiable deposit, larger ground displacements are expected due to larger times required for the water film to reach its maximum thickness and to dissipate.