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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 is a disaster that is frequently encountered in tunnel engineering. Existing explanations of floor heave mechanisms have many limitations, and the most important of which is that the stress concentration, release and transfer phenomena and the influences of the in-situ stress lateral coefficient and tensile strength are ignored. Therefore, the combined finite-discrete element method (FDEM) is used to study the failure process of floor rock masses and propose a new mechanical mechanism of floor heave that can consider the influences of the in-situ stress lateral coefficient and tensile strength. The type I fracture energy corresponding to different tensile strengths is calibrated using a direct tensile simulation test. Then, the floor heave mechanism is investigated under different lateral coefficients and tensile strengths, and five different floor heave modes are proposed under different in-situ stress lateral coefficients and tensile strengths. The floor heave modes controlled by the various in-situ stress lateral coefficients and tensile strengths are different, but they can all be explained by the maximum concentrated tangential stress. [ABSTRACT FROM AUTHOR]

To investigate the effects of the oval hole on the strength and the failure mechanisms of rock materials, a series of uniaxial and biaxial compression numerical simulations were carried out using the combined finite-discrete element method (FDEM). Based on the numerical simulation results, the effects of dip angle (β), major–minor ratio (ω) and position of the oval hole on rock strength characteristics crack initiation and development process and final failure modes were analysed in detail. The numerical simulation results indicate that: (1) Under uniaxial compression, the ultimate failure modes of rock containing oval holes can be mainly classified into single inclined plane shear failure, X-shaped tensile-shear composite failure and Y-shaped tensile-shear composite failure. Under biaxial compression, the ultimate failure mode is single-inclined plane shear failure dominated by shear cracks. Whether it is uniaxial compression or biaxial compression, the crack initiation position of the main fracture surface is near the end of the major axis of the oval hole. (2) Under uniaxial compression, the axial stress–strain curve of rock containing the oval hole is smooth before and after the peak, but under biaxial compression, there is obvious fluctuation in the post-peak stage due to crack initiation and propagation. In both uniaxial and biaxial compression, the pre-peak elastic stage of the axial stress–strain curve of the rock containing the oval hole is significantly shortened and the peak stress is significantly reduced compared with the intact rock, which indicates that the bearing capacity of the rock is seriously reduced by the existence of oval hole. (3) When the position and major–minor axis ratio of the oval hole remain unchanged, the uniaxial compressive strength and biaxial compressive strength increase with the increase of the dip angle of the oval hole. When the position and dip angle of the oval hole remain unchanged, the uniaxial compressive strength and biaxial compressive strength decrease with the increase of the major–minor axis ratio of the oval hole. (4) At different dip angles, the brittleness index Bi of rock containing oval holes decreases with the increase of confining pressure, and the brittleness degree shows a gradual weakening trend. At the same time, when the dip angle increases, the decreasing amplitude of brittleness index Bi decreases with the increase of confining pressure. Three types of failure modes of rock with elliptical hole are systematically summarised, and the failure mechanisms of elliptical hole rock are revealed. The influence of different positions, major–minor axis ratio, and dip angle of rock containing oval holes on uniaxial compression and biaxial compression are studied. The relationship between brittleness index and failure mode of rock containing the oval hole is investigated. [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]

To better understand the rock fragmentation mechanism and optimize cutter design and selection for rock excavation by TBM, a set of three-dimensional indentation tests was conducted with different rock and cutter types. Acoustic emission (AE) monitoring technique was employed to capture the information of the rock damage evolution real-timely. It is found that the penetration by using the constant cross section (CCS) cutter tends to induce inconspicuous rock chips formation before the sudden occurrence of the macrocrack, but that by using the V-type cutter tends to induce gradual rock fragmentation accompanied by the multiple local rock chips formation and the sawtooth force-penetration curve. Meanwhile, rock fragmentation models for CCS and V-type cutters were compared, and the spatio-temporal evolution of AE events was quantitatively analyzed to reflect the rock damage zone development process. Results indicate that the V-type cutter has greater penetration ability and the CCS cutter can cause larger unit damage zone radius. Microscopic observation by using the scanning electron microscope (SEM) reveals that the fracture mechanism for the crushed zone of rock is mainly shear type and that for the major crack is tensile type. It does not matter with rock types or cutter types. [ABSTRACT FROM AUTHOR]

Rock has the characteristics of natural heterogeneity and discontinuity. Its failure phenomenon induced by external force involves complex processes, including the microcrack initiation, propagation, coalescence, and the macrocrack formation. In this study, the Weibull random distribution based on the rock microstructure characteristics is introduced into the combined finite-discrete element method (FDEM) to establish the heterogeneous rock model, and the mechanical response and damage evolution of rock samples in uniaxial compression test are simulated. The results show that FDEM simulation with loaded heterogeneous rock model can reflect the progressive development of rock damage, fracture, and acoustic emission (AE) activity in real rock well. Meanwhile, the statistical analysis indicates that the number and energy evolution of AE events with different fracture modes in the model are consistent with the macroscopic failure mode of rock. The change of b-value also agrees with the increasing trend of high-energy events in the loading process. This method provides a new tool for the analysis of rock damage and fracture evolution. [ABSTRACT FROM AUTHOR]

The excavation in layered rocks is an issue for a number of geoengineering applications; these kinds of rocks all exhibit transverse isotropic features due to the process of metamorphic differentiation. This paper focuses on providing two methods, i.e., the 3D printing technology and the combined finite-discrete element method, to simulate the anisotropic characteristics of layered rocks. The results showed that both the 3D-printed samples and the FDEM numerical models are considered as a good match, and both revealed that as the inclined angle increased, the UCS of the sample first decreased and then increased, showing a U-shaped pattern. The results of this paper served as a reference to the promotion of the 3D printing technology and the combined finite-discrete element method in the geotechnical engineering field and laboratory test research. [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]