Discrete-continuum analysis of shear banding in the direct shear test

The direct shear test is a widely used method for determining peak and critical state strength parameters for soil and for soil-manufactured material interfaces. The objective of this research was to examine the direct shear test by way of a discrete-continuum approach. Specifically, the effects of non-coaxiality between the principal stress and the principal strain increment direction and the deviation of zero extension direction from horizontal are examined. A new, mesh-free method of evaluating internal strains is proposed, and a new flow rule is also presented. The results from discrete simulations indicate that development of shear bands proceeds from the side boundaries toward the centre with both primary and secondary bands formed. A distinct shear band along the middle plane develops around peak state in a dense sample. Simulation data using the new flow rule and laboratory test data using a conventional flow rule are in good agreement, indicating that the effect of non-coaxiality is small at peak state. Deviation of the zero linear extension direction at peak state is less than 3°. It is concluded that boundary measurements made on the direct shear apparatus are suitable for determining strength and volume change behaviour of granular soils.