Abstract: Coarsegrained soils are extensively used as roadbed embankment material for highway construction in the mountainous areas. To obtain the strength and deformation parameters of coarsegrained soils from highly weathered basalts, a series of laboratory largescale direct shear tests were conducted. Based on the particle discrete element software PFC2D, a twodimensional direct shear numerical model was established with soil particles modeled as disks. In the model, the particles size distribution and the parallel bonding contact model (PBM) effects were considered. The direct shear tests were simulated under different vertical pressures. Results indicate that: Under a 1.9×107 N/m of normal and shear stiffness, a magnitude of 8.0×105 N/m for the bond normal and shear stiffness, and 0.39 for the bonding radius multiplier factor, the resulting shear strength indices are 98.33kPa for the cohesion and 27.02° for the internal friction angle, respectively. The shear stress and shear displacement have a good agreement in simulated and tested results. During initial testing, the simulated and tested vertical displacementshear displacements were different. From parameter calibration, it is noted that the friction coefficient among disks increases when the vertical pressure increases from 0.15 to 0.60. Due to the rotation and the stress concentration of the contact pressure, irregular rotational movements of the disks near the shear band were observed.

Key words: Coarse Grained Soils, PFC2D, Largescale Direct Shear, Numerical Simulations