Influence of linear coupling between volumetric and shear strains on instability and post-peak softening of sand in direct simple shear tests

Abstract

This paper focuses on the influence of volume change boundary condition on the instability and post-peak softening of sand. The laboratory program comprises an extensive series of Direct Simple Shear (DSS) tests with different degrees of linear coupling between volumetric and shear strains. It is observed that progressive loosening of sand associated with the volumetric expansion in a coupled strain path intensifies instability susceptibility, whereas volumetric contractive strains reduce vulnerability to instability and post-peak softening. Analysis of the experimental data using Hill’s second-order work criterion designates a family of state-dependent relational trends for instability lines, normalized peak shear strengths, and maximum pore-water pressure ratios depending on the degree of coupling between the volumetric and shear strain rates. It is found that a simple state-dependent constitutive model can predict relational trends in sand specimens suffering from instability and post-peak softening under different degrees of the linear coupling between the volumetric and shear strains.