Some Observations on the State of Stress in the Direct Simple Shear Test Using 3D Discrete Element Analysis

Assessment of the mobilized friction angle in a soil specimen tested in a given shear apparatus requires adequate information to establish the stress state at the instance of interest. In the most commonly used version of the direct simple shear (DSS) apparatus, where a cylindrical soil specimen is confined by wire-reinforced membrane, only the normal and shear stresses on the horizontal plane are measured. The knowledge of these stresses alone does not provide adequate information to construct the Mohr circle defining the state of stress. In this context, drained and constant volume (i.e., equivalent to undrained) discrete element simulations of a cylindrical DSS specimen were performed and the results are presented with emphasis on the DSS mobilized friction angle during shearing. It was found that planes of maximum stress obliquity rotate with the development of shear strain. This finding implies that it is not possible to calculate the friction angle accurately from typical DSS laboratory tests with unknown normal stress on the vertical plane. However, it seems that at large shear strains, the horizontal plane becomes a plane of maximum stress obliquity, and the friction angle calculated using the stress state on the horizontal plane is a good approximation to the mobilized friction angle at such shear strain levels.