Static liquefaction of sand-fines mixtures with the presence of initial shear stress

Abstract

Loose sand, when sheared under undrained conditions, will exhibit a peak strength at a small strain and then collapse rapidly with a very low residual strength. This flow type of behavior, known as static liquefaction or flow liquefaction in soil mechanics, can cause devastating effects on geotechnical structures involving dams, slopes and hydraulic-placed fills. In the past, much effort has been made to understand the mechanism underlying this behavior through well-controlled laboratory tests on clean sands, mainly for the isotropic consolidation condition with initial shear stress. However, most natural sands encountered in situ contain some quantity of fines, and it is widely accepted that the mechanical behavior of silty sands can be significantly affected by the fines. Besides, 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. The aim of this research was set out to investigate the static liquefaction of sand-fines mixtures with the presence of initial shear stress. To this end, a number of undrained strain-controlled triaxial tests were conducted on manufactured mixed soils, covering a range of confining stresses and void ratios and a range of initial shear stress levels. Fines with different particle shape but similar size were added into clean sand. The initial shear stress was achieved by applying anisotropic consolidation and its level was characterized by an anisotropic consolidation ratio (ACR). The addition of fines to sand significantly affected the behavior of soils while both the fines content and particle shape were found to have prominent influence. The presence of initial shear stress didn’t change the critical state of soils but affected the triggering condition of liquefaction. Regarding the strain softening behavior of soils, a modified collapsibility index was used to characterize the degree of strain softening of anisotropically consolidated soils. In order to provide a convenient method to predict of undrained behavior of soils, the void ratio –mean stress space was divided into three zones corresponding to three types of undrained soils behavior. Based on the two boundaries of undrained soils behavior zones, a new parameter called state index was proposed to predict the undrained soils behavior and estimate the collapsibility index of soils. The correlation between initial state and undrained shear strength of soils was established by using the liquefaction surface in three-dimensional void ratio –shear strength–initial mean effective stress space. The parameters of liquefaction surface were dependent on materials and correlated to the value of ACR. A modified liquefaction surface was proposed by using the parameter called index of void ratio so that the undrained shear strength of Toyoura sand based soils with same ACR could be estimated by using one single modified liquefaction surface. In order to illustrate the application of liquefaction surface, liquefaction evaluation was performed based on the 1983 Nerlerk berm failure. Discussion and comparison on different liquefaction evaluation methods suggested that the liquefaction surface based evaluation method could appropriately predict the factor of safety of slopes.