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.

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This paper discusses methods for realistic simulation of slope failure owing to heavy precipitation in a centrifuge. A rain simulator was developed to provide precipitation that satisfies the conditions in a centrifuge so that the impact pressure on the ground surface in the centrifuge (pm) was reduced to the same level in the prototype (pp). Pneumatic spray nozzles, producing fine droplets with a mean diameter of 20 µm, yield large precipitation intensity of 1500 mm/h on the model at 50g. Accordingly, the model precipitation intensity (rm) was provided by n times the corresponding prototype precipitation intensity (rp). Several sets of centrifuge tests were conducted to observe the progress of failure in the shallow section of the slope. Heavy precipitation induces an increase in the saturation of the soil from the surface. Flow failure was observed repeatedly in the shallow section through progress of the wetting front. In addition, experiments using different viscosity used in the liquid for precipitations were carried out to compare the types of failure. Larger failure by precipitation of the viscous liquid was observed than that of water even when equivalent permeability conditions are maintained. This paper also introduces a method of measuring shear deformation in the shallow section by bending strains, allowing monitoring of the progress of failure.

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