Surrounding rocks at different locations are generally subjected to different stress paths during the process of deep hard rock excavation. In this study, to reveal the mechanical parameters of deep surrounding rock under different stress paths, a new cyclic loading and unloading test method for controlled true triaxial loading and unloading and principal stress direction interchange was proposed, and the evolution of mechanical parameters of Shuangjiangkou granite under different stress paths was studied, including the deformation modulus, elastic deformation increment ratios, fracture degree, cohesion and internal friction angle. Additionally, stress path coefficient was defined to characterize different stress paths, and the functional relationships among the stress path coefficient, rock fracture degree difference coefficient, cohesion and internal friction angle were obtained. The results show that during the true triaxial cyclic loading and unloading process, the deformation modulus and cohesion gradually decrease, while the internal friction angle gradually increases with increasing equivalent crack strain. The stress path coefficient is exponentially related to the rock fracture degree difference coefficient. As the stress path coefficient increases, the degrees of cohesion weakening and internal friction angle strengthening decrease linearly. During cyclic loading and unloading under true triaxial principal stress direction interchange, the direction of crack development changes, and the deformation modulus increases, while the cohesion and internal friction angle decrease slightly, indicating that the principal stress direction interchange has a strengthening effect on the surrounding rocks. Finally, the influences of the principal stress interchange direction on the stabilities of deep engineering excavation projects are discussed. (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/).

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/).