Modeling of scale effects on the micromechanics of granular media under direct shear condition

Shear strength parameters of granular soils measured in direct shear tests are largely affected by the testing device scale. In this paper, results from a DEM (Discrete Element Modeling) study on the direct shear behavior of granular materials as affected by the test scale are presented. The study focuses on exploring the effects of specimen length scale and height scale with respect to the median particle size on the bulk material shear strength and shear banding behavior. Simulation data show that the maximum shear strength measured at the model boundaries increases with decreasing specimen length scale and increasing specimen height scale. Micromechanics-based analysis indicates that the local/global fabric change and failure is the major mechanism for the specimen size scale effect. Global failure along the primary shear band prevails when the specimen length scale and aspect ratio are small, while progressive failure becomes more likely when the specimen length scale and aspect ratio become larger. Recommendations are given on the selection of optimum test device scale for minimizing the boundary effects on test results. © 2009 American Institute of Physics.