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To investigate the asymmetric failure mechanism and failure mode of layered rock tunnels, a novel mixed-mode power exponent cohesive zone model (M-PECZM) is proposed in this paper, which can capture the crack propagation process under single-mode loading, tension-shear loading, and compression-shear loading conditions. The parameters in the formula can be obtained from pure tensile and shear tests, and the introduction of the post-peak characteristic parameter α makes the M-PECZM suitable for post-peak softening curves of different kinds of rocks. The proposed M-PECZM is embedded in FDEM to carry out uniaxial compression, direct tension, and triaxial compression simulations. By comparison, the numerical and experimental results show a good agreement, which verifies the reliability and effectiveness of the M-PECZM. Besides, the FDEM is adopted to simulate the excavation of gently inclined layered surrounding rock tunnel, and the influence of shear stress field and buried depth on the EDZ evolution process and convergence displacement is studied. Highlights: A novel mixed-mode power exponent cohesive zone model (M-PECZM) for FDEM is proposed. The introduction of the post-peak characteristic parameter α makes the M-PECZM suitable for various rocks. Comparisons with experiments verify the accuracy and completeness of the M-PECZM. The influence of shear stress field and buried depth on the EDZ evolution process and convergence displacement is analyzed particularly. The asymmetrical deformation mechanisms of the tunnel resulted from the coupling action between the layered soft rock and tectonic shear stress. [ABSTRACT FROM AUTHOR]

Floor heave disasters easily occur along tunnels in deeply buried, weak, anisotropic rock masses with high in situ stress. The previously presented floor heave mechanical models (buckling failure, shear dislocation failure, and slip line field theory) do not consider the processes of stress release, transfer, and concentration, and only the initial in situ stress state can be analyzed with these models, i.e., they are static mechanical models. Therefore, the combined finite–discrete element numerical method (FDEM) is employed here to study floor heave tunneling in anisotropic rock masses to propose a new floor heave mechanical mechanism based on progressive fracture and swelling processes and the influences of the in situ stress (lateral pressure coefficient) and the layer thickness are also investigated. The simulation results indicate that (1) the essential mechanical mechanism of floor heave is X-shaped conjugate shear failure accompanied by tensile failure caused by the concentrated maximum horizontal stress present after tunneling; the shallow rock fragments cannot bear the enormous concentrated horizontal stress, and thus, this stress is transferred deeper into the intact rock mass, further causing the shear and tensile fractures to propagate downward (a progressive evolution); (2) for anisotropic floor rock masses, in addition to the X-shaped conjugate shear fractures, shear fractures parallel to the bedding plane and tensile fractures perpendicular to the bedding plane form; (3) with the change in the in situ stress lateral pressure coefficient, the failure mode directly under the floor remains almost unchanged, but it will have a significant impact on the rock mass on both sides of the floor; and (4) the layer thickness has a weak influence on the failure mode of the floor rock mass. [ABSTRACT FROM AUTHOR]

The morphological characteristics of rock joints have a significant impact on the peak shear strength of a rock mass. In this study, three-dimensional scanning tests have been conducted on 39 joint specimens. Based on the scanning data, the morphological parameter C′ has been developed to describe the joint roughness. With an explicit physical meaning, this index is negatively correlated with the joint roughness and can reflect the anisotropy of surface morphology. Using a newly developed derivation method, the morphological parameter C′ of each joint from the earlier studies has been determined. The scanning test results in this study have been used to confirm the feasibility of this new derivation method. By applying the least square fitting method to the Grasselli’s (Shear strength of rock joints based on quantified surface description. Dissertation, Swiss Federal Institute of Technology, 2001) test results, a new peak shear strength model has been developed. Finally, a comparison of the peak shear strength of rock joint predicted by the new model and other classic models shows that the new model, characterized by simplicity, relatively higher precision, and a definite physical meaning, is feasible to predict the peak shear strength of rock joints. [ABSTRACT FROM AUTHOR]