(1. State Key Laboratory of Intelligent Construction and Healthy Operation and Maintenance of Deep Underground Engineering，China University of Mining and Technology，Xuzhou，Jiangsu 221116，China；2. Yunlong Lake Laboratory of Deep Underground Science and Engineering，Xuzhou，Jiangsu 221116，China；3. School of Mechanics and Civil Engineering，China University of Mining and Technology，Xuzhou，Jiangsu 221116，China)
Abstract：In this work，direct shear tests，DEM simulation and theoretical analysis were conducted to reveal and characterize the multi-scale failure mechanism of soil-rock mixtures with lower rock block proportions. The experimental results show that the shear dilatancy of soil-rock mixture is determined jointly by soil density，volumetric block proportion(VBP) and normal stress condition. Significant nonlinear characteristics appears in the internal friction angle evolution of cohesionless soil-rock mixture(VBP<30%)，and the apparent cohesion induced by the rock block structure was generated from the strength analysis of Mohr-Coulomb criterion. Indicated by the DEM simulation results，the special mechanical pattern of soil-rock mixtures is formed by the block-in-matrix fabric on the micro and meso scale，such as the uneven distribution of contact force and the tortuosity of shear zone，which is distinguished from that of the homogenous sand matrix. Furthermore，based on the in-depth discussions on engineering geological characteristics and failure mechanism of soil-rock mixtures，a new formula was proposed to interpret the mechanism of the rock block structure. Through the stress transfer analysis，it was figured out that Lindquist's modified strength model is a simplified expression. Meanwhile，a novel strength criterion(equivalent roughness model) has been established，which presents the rationality in predicting the nonlinear strength evolution of soil-rock mixtures under medium to low VBP，in confrontations with the experimental results of cohesive and non-cohesive specimens.
