Influence of Low Permeability Capping Layers on Liquefaction Induced Failure in Stratified Ground

Abstract

Large-scale permanent ground deformations due to an earthquake are generally termed as flow liquefaction (FL). Flow failures are often observed in gentle slopes where minimum undrained shear strength remains less than static shear stress even after ground motion ceases and the induced deformations continue until a balance is achieved. Such failures are caused by significant strength loss in saturated granular soils due to the seismic induced excess pore water pressure (PWP). As the soil strength is reduced, plastic deformations can occur even due to static shear stresses such as building loads or driving forces on sloping ground. While such deformations are rare in level grounds with saturated sandy deposits, the triggering of such events depends on the ability of soil in retardation of dissipation of excess PWP which controls the effective stress in the layer. The dissipation of PWP is influenced by the permeability of interlayers in the layered sand deposit. As in natural conditions, sand deposits are highly heterogeneous and stratified, they often tend to contain low permeability capping layers sandwiched between them. This low permeability tends to inhibit the dissipation of PWP after the onset of ground shaking, which leads to further instability of overlying layer. This study aims to investigate the effect of such capping layers on the PWP dissipation and deformation of saturated sand deposits under dynamic loading conditions. To evaluate the buildup and dissipation of PWP in a simple way, multiple one-dimensional (1D) model tests were conducted by imparting dynamic loading to a soil column with different capping layer conditions. Multiple parametric studies were conducted with varying relative density of sandwiching layer, capping layer thickness, and capping layer material.