Drainage and shear velocity dependent shear characteristics of granular materials: Rheological concept and debris flow mobility

This paper presents the shear characteristics of granular materials as a function of drainage and shear velocity using a ring shear apparatus, which was designed at Korea Institute of Geoscience and Mineral Resources (KIGAM). A series of ring shear tests were performed for the Jumunjin sands (SP) and gravels (GP) with mean grain diameter of 0.6 mm and 6 mm, respectively. The torque was measured for a given volume of sample (i.e., 2800 cm³) placed in ring shear box. The upper ring is fixed while the lower ring is rotatable under the unlimited shear deformation. The test results were compared under the various normal stresses, drainage and shear velocity conditions for a given volumetric concentration of solid (i.e., density dependent soil behavior). According to the preliminary test results, it is clear that the strain softening behavior is observed for the shear velocity higher than 0.1 mm/sec; however, the distinctive nature of strain hardening is clearly observed when the velocity is lower than 0.1 mm/sec, irrespective of drainage condition. In the flow diagram (i.e., the shear stress-shear velocity plot), the slope can be viscous resistance; and the yintercept value can be represented as the yield strength in soil rheology. Yield strengths (ty) are significantly influenced by sample density: as expected, the yield strength increases with increasing density. The yield strength obtained from the drained condition is larger than that from the undrained condition because of grain crushing effect.