Initiation of flow liquefaction in saturated sand with the presence of initial shear stress

Abstract

Water saturated loose sand, when sheared under undrained conditions, exhibits a peak strength at a small strain and then collapses rapidly with a very low residual strength. This flow-type failure, known as flow liquefaction or flow slides in soil mechanics, is a major concern in geotechnical applications involving dams, slopes and hydraulic-placed fills. A fundamental understanding of flow liquefaction behavior has been established through well-controlled laboratory experiments on sand specimens, mainly for the isotropic consolidation condition where no initial shear stress is present. The influence of initial shear stress on the initiation of flow failure, though being an issue critical to the safety of dams and slopes, is not yet well understood. This paper reports several interesting findings from an experimental study that aimed to address this issue. A number of undrained strain-controlled triaxial tests were conducted on two quartz sands, covering a range of confining stresses and void ratios and a range of initial shear stress levels. The initial shear stress was achieved by applying anisotropic consolidation and its level was characterized by an anisotropic consolidation ratio (ACR). One of notable findings is that the influence of initial shear stress is complicated, depending on the void ratio and confining stress prior to undrained shearing as well as on the ACR value. By normalizing the shear stress required to initiate flow failure with the initial shear stress applied, a fairly good correlation is established between this normalized shear stress ratio and the ACR value.