Shear and viscous characteristics of gravels in ring shear tests

The shear and viscous characteristics of a material are strongly dependent on the material’s properties, such as cementation, density and grain size distribution, as well as the testing conditions, such as drainage and shearing speed. More complex shear behaviors can occur when the tested materials have diverse grain sizes. The shear and viscous characteristics of gravels are examined in terms of drainage and shear velocity under the constant normal stress using a ring shear apparatus, in which the materials used can be sheared under a large deformation. In this study, the materials used are commercial aquarium gravels with mean diameter of 6 mm. Test results show that the materials typically exhibited strain-hardening behavior when subjected to low shear velocity (i.e., ≤ 0.01 mm/sec) and strain-softening behavior when subjected to high shear velocity (i.e., > 0.1 mm/sec) in the ring shear torque measuring system. As expected, higher shear velocities correspond to higher shear stresses, regardless of the drainage condition. For a given shear velocity (V) ranging from 0.01 to 1 mm/sec, a gradual decrease in shear stress is observed in the drained condition; however, a relatively constant shear stress is observed in undrained shear stress. It is clearly exhibited that the vertical displacement is large in the drained condition but relatively unchanged in the undrained condition. The grain crushing is significant for both drainage conditions when the materials experienced a large deformation. There are two regions: (a) non-dominant grain crushing and (b) dominant grain crushing (with a large amount of sand and fine contents) as a function of shear velocity. For both drainage conditions, a large shear resistance occurred when the shear velocity reaches 1 mm/sec. Through the contraction-particle rearrangement-grain crushing process during shear, when V ≥ 1 mm/sec, it appears that the shear stress is easily reached to the residual state of shear. The amounts of fine contents from grain crushing are much larger in the drained condition than in the undrained condition. In the extreme case that grains are highly crushed, it may result in a high mobilization of materials in natural hazards.