Instability and liquefaction flow slide of granular soils: the role of initial shear stress

Abstract

Loose granular soil, when subjected to shearing, may collapse rapidly to large strains with a very low residual strength. This flow-type failure, known as flow slide or flow liquefaction, is a major concern in geotechnical applications involving slopes, dams and embankments. A fundamental understanding of the flow liquefaction phenomenon has been established through extensive laboratory experiments on isotropically consolidated sand samples over the past decades. In real situations, however, the element of soil in a slope or dam is not under the isotropic consolidation, but is subjected to a static, driving shear stress prior to external loading. What role played by this static shear stress in the initiation of flow liquefaction is an issue of importance but is not yet fully understood. This paper presents new data from a specifically designed experimental program along with analysis in a sound theoretical context. A marked finding of the study is that the gradient of the flow liquefaction line—which indicates the onset of flow slide in the stress space—is almost uniquely related to the initial state parameter defined in the critical state theory, regardless of the presence or absence of initial shear stress. Based on the characteristics of the observed behavior, an alternative definition for the factor of safety against flow failure is put forward, which takes proper account of the key factors involved and thus is more rational in certain aspects than the conventional one used in engineering practice.