Establishing an opening size criterion in direct shear test using DEM Simulation

Direct shear test has been widely used to examine the shear strength of geomaterials because of the simplicity of the testing method and apparatus. Three factors significantly affect the accuracy of the experimental results of direct shear tests, namely (1) the type of direct shear apparatus, (2) the specimen size (scale effect), and (3) the opening size between shear boxes. This study focused on the Threshold Line (TL), which is obtained based on experimental tests, as a guideline for setting the opening size between the shear boxes. The validity of the TL was examined using distinct element method (DEM) 3D simulations from the following four perspectives: the first and second perspectives investigated the influence of the mean particle size and particle size distribution for mean particle sizes larger than 0.8 mm. In the third perspective, the scale effect of the specimens for fixed and varying D:H ratios of the shear box to reduce the shear box size was examined. Lastly, in the fourth perspective, the validity of using the TL to determine the appropriate opening size for the samples with a mean particle size smaller than 0.8 mm was also examined based on the Threshold Point (TP). For each case, the results of the TPs obtained from the DEM simulations agreed well with those of the TL. These findings suggest that the TL is valid and the TL relational equation can be used for setting the opening size between the shear boxes in the direct shear test regardless of saturated and unsaturated soils.