The microstructure of a soil-rock matrix (SRM) is a determinant of its macroscopic physical and mechanical attributes. The influence of the shape and volume fraction (VF) of rock blocks on the dynamic properties of the SRM has not been subjected to quantitative analysis. Consequently, a suitable construction technique was developed for the fabrication of small-scale triaxial specimens incorporating artificial rocks of various shapes. A series of homogeneous SRM specimens with differing rock VFs and shapes were fabricated. These specimens were then exposed to a long-term dynamic load consisting of 15,000 cycles at a frequency of 1 Hz. The principal findings are summarized as follows: The construction method proposed is capable of producing small artificial rocks with dimensions of 3 mm or 5 mm in arbitrary shapes while maintaining consistency with the prototypes. The method holds significant promise for application in geotechnical testing. Under long-term dynamic loading, the rock VF effectively elevates the threshold cyclic stress ratio of the SRM, diminishes the Pore Water Pressure within the mixture, enhances the dynamic stiffness, and mitigates the cumulative strain. SRMs composed of rock shapes with increased angularity and reduced block sizes exhibit higher dynamic stiffness and cumulative strain. The threshold cyclic stress ratio for an SRM with a 40 % rock VF is approximately 0.04, and the pore pressure increment in the SRM exhibits a gradual change, which contrasts with the test outcomes for pure clay. The exponential-hyperbolic model provided a satisfactory fit for the pore pressure data, while the hyperbolic model yielded good fitting results for the cumulative strain of the SRM with a low rock VF. These findings contribute to an enhanced comprehension of the dynamic properties of railway subgrades filled with SRM under cyclic train loading conditions.

Rock-encased-backfill (RB) structures are common in underground mining, for example in the cut-andfill and stoping methods. To understand the effects of cyclic excavation and blasting activities on the damage of these RB structures, a series of triaxial stepwise-increasing-amplitude cyclic loading experiments was conducted with cylindrical RB specimens (rock on outside, backfill on inside) with different volume fractions of rock (VF 1/4 0.48, 0.61, 0.73, and 0.84), confining pressures (0, 6, 9, and 12 MPa), and cyclic loading rates (200, 300, 400, and 500 N/s). The damage evolution and meso-crack formation during the cyclic tests were analyzed with results from stress-strain hysteresis loops, acoustic emission events, and post-failure X-ray 3D fracture morphology. The results showed significant differences between cyclic and monotonic loadings of RB specimens, particularly with regard to the generation of shear microcracks, the development of stress memory and strain hardening, and the contact forces and associated friction that develops along the rock-backfill interface. One important finding is that as a function of the number of cycles, the elastic strain increases linearly and the dissipated energy increases exponentially. Also, compared with monotonic loading, the cyclic strain hardening characteristics are more sensitive to rising confining pressures during the initial compaction stage. Another finding is that compared with monotonic loading, more shear microcracks are generated during every reloading stage, but these microcracks tend to be dispersed and lessen the likelihood of large shear fracture formation. The transition from elastic to plastic behavior varies depending on the parameters of each test (confinement, volume fraction, and cyclic rate), and an interesting finding was that the transformation to plastic behavior is significantly lower under the conditions of 0.73 rock volume fraction, 400 N/s cyclic loading rate, and 9 MPa confinement. All the findings have important practical implications on the ability of backfill to support underground excavations. (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/).