For the high-fill slope with soil-rock mixtures (SRMs), the bedrock under the SRMs can be excavated into many benches to improve the mechanical properties of bedrock-SRMs interphase and the stability of the slope. However, this improvement effect by bench size is still unclear. The continuous-discrete coupled method is a powerful tool for analyzing the interaction between soil and structure, soil and rock, etc. Firstly, this paper proposes a fine discrete element modeling method for rock blocks and develops a continuous-discrete coupled method for the benched bedrock-SRMs interphase. Then, a series of numerical direct shear tests for the benched bedrock-SRMs interphases with different bench sizes are conducted. The effect of bench size on the shear mechanical properties of the interphase is systematically investigated in terms of rock block rotation, contact force chain distribution, crack distribution, shear stress-displacement curve, and shear strength. The numerical results demonstrate that the bench size has a considerable impact on the strength and deformation properties of interphase. Raising the height or height-width ratio of the benched bedrock can enhance the interaction and skeletons between the benched bedrock and SRMs, thereby improving the strength and deformation properties of the interphase. Compared to increasing the bench height, increasing the height-width ratio has a more significant effect. Finally, a shear strength prediction method for the benched bedrock-SRMs interphase is proposed based on the Mohr-Coulomb strength criterion, which is practical in the design and stability evaluation of high-fill slope with SRMs. A reverse reconstruction method for building the refined SRMs model is proposed.A coupled FDM-DEM is proposed to simulate the benched bedrock-SRMs interphase.The impacts of bench size on mechanical properties of the interphase are discussed.A shear strength prediction method for the interphase is proposed.

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.