Interpretation of direct shear box testing of sands as quasi-simple shear

Interpretation of the direct shear box (DSB) test implicitly assumes a simple shear mode of deformation, but this assumption has not been fully verified, in particular in tests on sands. This paper describes a laboratory investigation, which was commissioned to find an optimum configuration of a DSB apparatus for measuring the strength and dilatancy characteristics in direct simple shear. The examination was made in a newly developed DSB apparatus using dry specimens of two standard sands: Toyoura sand and silver Leighton Buzzard sand. Boundary effects, such as the wall friction, the size of the opening between the two halves of the shear box and the constraint imposed by the loading platen, were each independently examined in pilot tests to determine their influence on the measurements of strength and angle of dilatancy. The optimum configuration was determined through careful examination of the boundary stresses and strains of rectangular specimens subjected to shearing under drained and constant-volume conditions, and also by comparing the results with those of comparative torsional simple shear and plane strain compression tests. On the basis of the test results obtained using an optimum configuration of the DSB device, the stress dilatancy of the sands was characterized by a non-associated rule based on a classical energy theory for the interrelationship between the angle of shearing resistance and the angle of dilatancy.