The decisive mechanism through which lithological differences influence the mechanical properties and failure characteristics of anchored rock masses

To investigate the decisive mechanism through which lithological differences influence the pull-out mechanical properties and failure characteristics of anchored rock masses, pull-out tests were conducted using a self-developed true triaxial creep‒impact multifunctional test system for anchored rock. The mechanical properties, macro and micro failure modes, load transfer characteristics, and energy distribution laws of fully anchored rock with various lithologies, including coal, sandy mudstone, sandstone, and granite, were obtained. The results indicated that (1) the maximum pull-out load and interfacial stiffness level increased in a stepwise manner with increasing rock mass strength. Under unconfined conditions, soft rocks underwent sudden splitting and fragmentation, resulting in typical brittle failures, whereas hard rocks remained structurally intact and failed via tensile bolt ruptures. The confining pressure exerted a significant positive regulatory effect on the bearing capacity, interfacial stiffness, and ductility of the soft rocks. (2) The micromorphological features of the failed anchoring interfaces revealed the decisive influence of lithological differences on physical bonding. SEM characterization revealed that under unconfined conditions, the coal and mudstone interfaces were dominated by lamellar peeling and matrix tearing, respectively, which evolved into fine coal powder grinding–bonding and platy mineral stripping processes after confining pressure was applied. Moreover, the bolt-anchoring agent interface in the hard rocks was characterized by a unique thin-film peeling layer. (3) The axial force and interfacial shear stress monotonically decreased along the anchoring depth. In soft rocks under low confining pressure, the axial force and shear stress were highly concentrated in the shallow borehole region near the collar, whereas confining pressure and high rock hardness significantly improved the efficiency and uniformity of the load transfer process. (4) The storage and dissipation pathways of the external input energy were strictly controlled by lithological differences and the stress environment. During a soft rock failure, the work done by the interface served as the absolute dominant component of the external work (accounting for more than 60–70% of the total), and the confining pressure substantially increased the total energy dissipation level. Conversely, hard rock failures were dominated by the plastic deformation energy of the bolt (reaching 10–20 kJ); however, the high-strength rock mass and confining pressure restricted the effective deformation length of the bolt, leading to a slight reduction in the total energy absorption rate. Based on the experimental findings, a support design process was carried out for the deep high-stress top coal-retaining roadway in the Xinjulong Coal Mine. By implementing a combined support system consisting of “bolts, cables, mesh, steel strips, and steel beams” along with high prestress and a highly efficient surface protection scheme, the lateral constraint imposed on the shallow coal mass was enhanced. This approach successfully controlled the deformation of the surrounding rock after the excavation procedure, providing a valuable reference for implementing roadway support and control in complex geological environments.