Multiscale Mohr–Coulomb Failure Criterion for Soil–Rock Mixtures Considering the Rotation Degree of Freedom of Rock Blocks

Soil–rock mixture (SRM) is a natural multiphase geological material composed of rock blocks and soil particles with different sizes, showcasing significant multiscale coupling mechanical responses. The shear strength of SRM is governed by the mechanical properties of rock blocks and soil particles at different scales. Thus, the SRM is decomposed into soil matrix and rock blocks based on the mechanical effects induced by the interaction between soil matrix and rock blocks. Based on the kinematic characteristics of soil particles and rock blocks at the microscale, a multiscale and hierarchical constitutive relation is established, which is combined with the conventional Mohr–Coulomb failure criterion (MCFC) to derive the multiscale MCFC of the SRM. Twelve groups of SRM samples with different soil matrix liquidity index and rock block content are prepared to conduct consolidated and undrained triaxial compression tests to determine the model parameters of the proposed multiscale MCFC to obtain its yield locus and to validate its effectiveness and accuracy. The results show that the multiscale MCFC can effectively reflect the influence of the coupled mechanism of soil particles and rock blocks on the shear strength of the SRM. The shape of the yield locus of the proposed multiscale MCFC is in good agreement with that of the conventional MCFC, which is an irregular hexagon and expands outward with a decrease in the soil matrix liquidity index and an increase in the rock block content.