(1. Hunan Provincial Key Laboratory of Geotechnical Engineering Stability Control and Health Monitoring，Hunan University of Science and Technology，Xiangtan，Hunan 411201，China；2. Fujian Research Center for Tunneling and Urban
Underground Space Engineering，Huaqiao University，Xiamen，Fujian 361021，China；3. China Railway
No.5 Engineering Group Co.，Ltd.，Changsha，Hunan 410114，China；4. College of Civil
Engineering，Hunan University，Changsha，Hunan 410082，China)
Abstract：The geographical distribution of discontinuous graded soil-rock mixtures is extremely widespread，and they are commonly used as backfill materials in subgrade and slope engineering. The discontinuous grading effect exhibited by their shear mechanical properties is a critical factor in the control of compaction quality. First，a discontinuity index is proposed，which can effectively quantify the discontinuous grading characteristics of soil-rock mixtures. Then，large-scale triaxial standard specimens of soil-rock mixtures with different discontinuity indices were prepared，and triaxial shear tests were conducted on the standard specimens using the DJSZ–150 large-scale triaxial testing apparatus. The influence of and confining pressure on the strength and deformation characteristics of soil-rock mixtures was analyzed. Subsequently，the intrinsic relationships between the parameters of the composite power-exponential model and the deviatoric stress strength， ，and were investigated，leading to the proposal of the CPE model that considers the effects of varying interrupted grain fractions and stress states. Finally，the influence of changes in the discontinuous particle size fractions on the mesoscopic structural characteristics of soil-rock mixtures was explored，elucidating the discontinuous gradation effect on the shear mechanical properties of soil-rock mixtures. The results indicate that under triaxial compression，discontinuous graded soil-rock mixtures generally exhibit strain hardening characteristics，the deviatoric stress strength increases with the rise in ，while the internal friction angle initially increases and then decreases with the increase in . In contrast，the cohesion exhibits an opposite trend；The parameters k，b，and n all exhibit a linear relationship with the . Both k and n show a linear relationship with the ，while b follows a quadratic function relationship with . The improved CPE model not only accurately predicts the strength and deformation characteristics of soil-rock mixtures，but also effectively reflects the influence of discontinuous grading features and stress state on the deformational mechanical behavior of soil-rock mixtures. Different discontinuous particle groups constitute distinct mesostructural characteristics of soil-rock mixtures. Under axial compression loading，an increase in the content of the largest particle group enhances the overall deformational mechanical properties of the soil-rock mixture. However，excessive amounts of large particles can increase the voids between coarse particles，leading to a rise in deviatoric stress strength followed by a decrease.
