Discrete-continuum Analysis of Shear Banding in the Direct Shear Test

The direct shear test is a widely used method of determining peak and critical state strength parameters for soil and for soil-manufactured material interfaces. However, stress conditions inside the box during shear are not well known. This paper presents a new micromechanics-based approach that modifies existing relationships between plane strain and direct shear friction angles taking into account the effects of non-coaxiality between the principal stress and the principal strain increment direction and the deviation of zero extension direction from horizontal. The results show that anisotropies of fabric and contact force rotate significantly after shearing starts, and reach their maximum at peak state. Strain localization is found to initiate from the side boundaries and extend slowly towards the middle of the box, and eventually develop into a distinct shear band along the middle plane around peak state. Simulation data are in good agreement with the proposed extended flow rule, with better agreement obtained when the effect of non-coaxiality is considered. Deviation of zero linear extension direction at peak from the horizontal is found to be less than 3 degrees and thus verifies the practice of measuring volume changes at the boundary in the direct shear device.