A numerical examination of the direct shear test

The paper presents results of two-dimensional simulations of the direct shear test using the discrete element method ( DEM). The heterogeneous distributions of stress and strain are illustrated and, taking advantage of the numerical technique, the evolution of the stress tensor and porosity changes in the shear zone are monitored. The results demonstrate that the dilation inside the shear zone is much greater than that deduced from boundary measurements. The stress ratio tau/sigma calculated from the boundary forces is about 10% greater than that calculated in the shear zone. It is also demonstrated that coaxiality of stress and strain rate exists at the critical state. Further simulations show that the influence of wall friction along the vertical faces of the specimen can be minimised by adopting the 'symmetrical arrangement' suggested by Jewell, and that the actual stress-strain-dilation behaviour within the shear zone is unaffected by the detailed boundary conditions. Comparison with previously published simulations results suggests that, in laboratory experiments, more reliable data, in the context of both stress ratio and dilation, would be obtained if the shear box aspect ratio was reduced below the value of 1:2 adopted in the traditional Casagrande shear box.