DEM investigation of the microscopic mechanism of scale effect of sandy gravel material

Because of the limitations on laboratory space and testing apparatus, such as the load capacity, the mechanical properties of prototype rockfill materials are generally obtained from scaled-down samples. It has been generally accepted that the underestimation of the high rockfill dam deformation is primarily due to the scale effect of rockfill materials, i.e., there are differences between the mechanical properties of prototype materials and scaled-down samples. Recent experimental studies again demonstrate that the scale effect of rockfill materials consisting of sandy gravels and blasting rocks is different, and the underlying mechanism is still unclear. This study uses the discrete element method (DEM) to investigate the microscopic mechanism of the scale effect of sandy gravel material collected from Dashixia rockfill dam in China. The sandy gravel material composed of rounded gravel and pebbles is modeled as an assembly of spheres, and the rolling resistance at particle contacts considers the slight particle non-sphericity. The DEM input parameters are calibrated and verified by a series of single-particle crushing tests, angle of repose tests, and triaxial compression tests. The DEM simulations of triaxial compression tests are performed on samples with different particle crushing strengths and particle size distributions (PSD). Particle breakage weakens the shear strength and considerably lowers the deformation modulus of sandy gravel material. On the contrary, the widening of PSD has a significant effect on the force transmission structure, which is manifested as the increase in contact force and higher mobilization of frictional force at contacts, thus promoting the bulk resistance to deformation. The scale effect of sandy gravel material results from the competition between these two factors. As to the rounded gravel and pebbles studied here, the scale effect is dominated by the widening of PSD, which is confirmed by the increase in the deformation modulus and shear strength with an increase in maximum particle size and size span.