Mechanics of the instability of sand and non-plastic silt mixture using equivalent intergranular void ratio

Instability is a phenomenon which brings about abrupt reduction in the load carrying capacity of sand and silt mixtures undergoing deformation before the failure state. This study investigates the influence of adding a wide range of non-plastic silt on the strength of sand instability by a series of undrained triaxial tests. Several indices of static liquefaction and instability including different stress ratios in the plane of mean effective stress versus deviour stress, pˈ-q, have been evaluated using equivalent intergranular void ratio, es(eq), to predict sand and silt mixture instability behavior. The zone of instability has been investigated based on equivalent relative density at the initial condition before consolidation, Drin, in the pˈ-q plane with various consolidation pressures. Different values of Drin for sand and silt mixture are used and they can help to evaluate the static liquefaction and stress state of silty sand slopes which will be useful to determine the zone of liquefaction potential in the field. The results show that the better index is Drin for evaluating the instability behavior of sand and silt mixture. The slope of the instability line decreases as Drin increases. In addition, the occurrence of samples’ instability is definitely in the range of 25 to 40% of silt and sand mixture, and the Drin is more than 70%.