Fault fracture zones, characterized by high weathering, low strength, and a high degree of fragmentation, are common adverse geological phenomena encountered in tunneling projects. This paper performed a series of large-scale triaxial compression tests on the cohesive soil-rock mixture (SRM) samples with dimensions of 500 mm x 1000 mm to investigate the influence of rock content P-BV (20, 40, and 60% by volume), rock orientation angle alpha, and confining pressure on their macro-mechanical properties. Furthermore, a triaxial numerical model, which takes into account P-BV and alpha, was constructed by means of PFC3D to investigate the evolution of the mechanical properties of the cohesive SRM. The results indicated that (1) the influence of the alpha is significant at high confining pressures. For the sample with an alpha of 0 degrees, shear failure was inhibited, and the rock blocks tended to break more easily, while the samples with an alpha of 30 degrees and 60 degrees exhibited fewer fragmentations. (2) P-BV significantly affected the shear behaviors of the cohesive SRM. The peak deviatoric stress of the sample with an alpha of 0 degrees was minimized at lower P-BV (60%). Based on these findings, an equation correlating shear strength and P-BV was proposed under consistent alpha and matrix strength conditions. This equation effectively predicts the shear strength of the cohesive SRM with different P-BV values.

Soil-rock mixtures in fault fracture zones are composed of rock blocks with high strength and fault mud with low strength. In this paper, in order to study the mechanical properties of the soil-rock mixture with non-cohesive matrix, a large-scale laboratory triaxial compression test with a specimen size of 500 mmx1000 mm is conducted, combined with numerical simulation analyses based on the two-dimensional particle flow software PFC2D. The macroscopic mechanical response and mesoscopic fracture mechanism of soil-rock mixtures with varying rock block proportions, block orientation angles and matrix strengths are studied. The results indicate the following: (1) When the proportion is less than 30%, the shear characteristics of the mixture are similar to those of its non-cohesive matrix. When the proportion is in the range of 30-70%, the internal friction angle and cohesion increase rapidly, and the softening characteristics of the mixture become more apparent. When the proportion exceeds 70%, the aforementioned effect slows. (2) The strength of the mixture is positively correlated with its matrix strength, and the influence of the matrix strength on the loading curve of the mixture is related to the block proportion. (3) When the block orientation angle is 0 degrees, the cohesion and internal friction angle are slightly greater than those at an angle of 90 degrees. Based on the above, for the soil-rock mixture with non-cohesive matrix, a strength prediction model based on the block proportion is given when the block orientation angle and matrix strength are consistent.