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 fu lly verified, in particular in tests on sands. This paper describes a laboratory investigation, which was commissioned to find an optimum co nfiguration of a DSB apparatus for measuring the strength and dilatanc y characteristics in direct simple shear. The examination was made in a newly developed DSB apparatus using dry specimens of two standard sa nds: Toyoura sand and silver Leighton Buzzard sand. Boundary effects, such as the wall friction, the size of the opening between the two hal ves of the shear box and the constraint imposed by the loading platen, were each independently examined in pilot tests to determine their in fluence on the measurements of strength and angle of dilatancy. The op timum configuration was determined through careful examination of the boundary stresses and strains of rectangular specimens subjected to sh earing under drained and constant-volume conditions, and also by compa ring the results with those of comparative torsional simple shear and plane strain compression tests. On the basis of the test results obtai ned using an optimum configuration of the DSB device, the stress dilat ancy 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.