Discrete element simulation of triaxial tests is an important tool for exploring the deformation and failure mechanisms of geotechnical materials such as sands. A crucial aspect of this simulation is the accurate representation of lateral boundaries. Using coupled finite difference method (FDM)-discrete element method (DEM) approach, numerical simulations of consolidated-drained and consolidated-undrained triaxial tests were conducted under flexible lateral boundary conditions. These results were then compared with those of corresponding triaxial tests using rigid lateral boundaries. The results indicate that, compared to the rigid lateral boundary, the triaxial test using the FDM-DEM coupled flexible lateral boundary better captures both the macroscopic mechanical response and the microscopic particle kinematics of laboratory triaxial specimens. In the consolidated-drained triaxial tests, the strain softening and shear dilatancy of the specimen with the flexible lateral boundary are significantly weaker after reaching peak strength than those of the specimen with the rigid lateral boundary. In the consolidated-undrained triaxial tests, when the axial strain is large, the specimen with the flexible lateral boundary exhibits both a lower deviator stress and a smaller absolute value of negative excess pore pressure. Furthermore, in the consolidated-undrained triaxial tests, as the axial strain increases, the flexible lateral boundary provides weaker lateral constraint and support to the specimen compared to the rigid lateral boundary. Consequently, the stability of the force chains in the specimen with the flexible lateral boundary is lower, leading to more buckling events of force chains within the shear band. As a result, both the anisotropy and the deviator stress are reduced.

As common backfill materials, soil and rock mixtures (S-RMs) are widely used in high-fill slope engineering projects. The shear resistance of the interphase between the S-RM and bedrock is usually weak. To improve the stability of the slope, the bedrock can be excavated into a bench-like shape. However, the shear mechanical properties of benched interphases are complex and need to be clarified. The coupling of the finite difference method (FDM) and discrete element method (DEM) creates a powerful tool for simulating soil-rock contact. In this paper, a coupled FDM-DEM is proposed to simulate the benched interphase that considered the microstructure of an S-RM and demonstrated high computational efficiency. First, the method was validated with the results of laboratory tests. Then, the typical failure characteristics of the benched interphase were simulated and the impacts of the physical parameters of the S-RMs were discussed. According to the results, the macroscopic mechanical response of the benched interphase was closely related to the changes in the skeleton structure formed by the rock blocks and benched bedrock. Consequently, the rock block rotation, force chain distribution, crack distribution, shear stress-displacement response, and strength of the interphase underwent regular changes. Overall, the influence of the rock block proportion was more significant than the influences of the rock block shape and maximum rock block size. Therefore, to improve the stability of the high-fill slopes of S-RMs, the rock block proportion should first increase, and then the rock block shape irregularity and maximum rock block size should increase.