Soil-rock mixtures are composed of a complex heterogeneous medium, and its mechanical properties and mechanism of failure are intermediate between those of soil and rock, which are difficult to determine. To consider the influence of different particle groups on soil-rock mixture's shear strengths, based on the mesomotion properties of the particles of different particle groups when the soil-rock mixture is deformed, it is classified into two-phase composites, matrix and rock mass. In this paper, based on the representative volume element model of soil-rock mixtures and the Eshelby-Mori-Tanaka equivalent contained mean stress principle, a model of shear constitutive of the accumulation considering the mesoscopic characteristics of the rock is established, the influence of different factors on the shear strength of the accumulation is investigated, and the mesoscopic strengthening mechanism of the rock on the shear strength of the accumulation is discussed. The results show that there is a positive correlation between the rock content, the surface roughness of the rock, the stress concentration coefficient, coefficient of average shear displacement, and the accumulation's shear strength. When the accumulation is deformed, it stores or releases additional energy than the pure soil material, so it shows an increase in deformation resistance and shear strength on a macroscopic scale.

The shear behavior of backfill-rock composites is crucial for mine safety and the management of surface subsidence. For exposing the shear failure mechanism of backfill-rock composites, we conducted shear tests on backfill-rock composites under three constant normal loads, compared with the unfilled rock. To investigate the macro- and meso-failure characteristics of the samples in the shear tests, the cracking behavior of samples was recorded by a high-speed camera and acoustic emission monitoring. In parallel with the experimental test, the numerical models of backfill-rock composites and unfilled rock were established using the discrete element method to analyze the continuous-discontinuous shearing process. Based on the damage mechanics and statistics, a novel shear constitutive model was proposed to describe mechanical behavior. The results show that backfill-rock composites had a special bimodal phenomenon of shearing load-deformation curve, i.e. the first shearing peak corresponded to rock break and the second shearing peak induced by the broken of aeolian sand-cement/fly ash paste backfill. Moreover, the shearing characteristic curves of the backfill-rock composites could be roughly divided into four stages, i.e. the shear failure of the specimens experienced: stage I: stress concentration; stage II: crack propagation; stage III: crack coalescence; stage IV: shearing friction. The numerical simulation shows that the existence of aeolian sand-cement/fly ash paste backfill inevitably altered the coalescence type and failure mode of the specimens and had a strengthening effect on the shear strength of backfillrock composites. Based on damage mechanics and statistics, a shear constitutive model was proposed to describe the shear fracture characteristics of specimens, especially the bimodal phenomenon. Finally, the micro- and meso-mechanisms of shear failure were discussed by combining the micro-test and numerical results. The research can advance the better understanding of the shear behavior of backfill-rock composites and contribute to the safety of mining engineering. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).