Search

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]

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 large deformation mechanisms and predictions of soft rock tunnels have always been important but difficult to solve problems in the field of geotechnical engineering. A combined finite–discrete element numerical simulation method (FDEM) was used to study large deformation mechanism, classification and prediction. The deformations or failure mechanisms of tunnel surrounding rock were revealed, and the failure modes and displacement value prediction of surrounding rock with different strength-stress ratios were also investigated. The following conclusions were obtained: (1) Critical hysteresis damping can be adopted to simulate the progressive large deformation process of soft rock tunnels, which can obtain the final deformation of unsupported tunnels and avoid a dynamic response. (2) Under concentrated tangential stress, the surrounding rock undergoes X-shaped conjugate shear fracture, and this type of fracture network continues to propagate toward the depth of the surrounding rock until the model reaches a stable state; the reduction in tunnel cross-section is mainly caused by the macroscopic movement and volume expansion of rock fragments, and the latter is due to the generation of a large number of macroscopic voids. (3) As the strength-stress ratio decreases, the deformation or failure modes of the surrounding rock can be divided into four categories: elastic–plastic deformation, closed fracturing, shear dilation and broken expansion. (4) Finally, a prediction equation for isotropic and homogeneous soft rock unsupported tunnel deformation with general size driven by hydrostatic in situ stress is obtained, which indicates that with an increasing strength-stress ratio, the surrounding rock displacement decreases as an exponential function, with a correlation coefficient of R 2 = 0.997. In addition, the robustness and reliability of the prediction equation is verified. [ABSTRACT FROM AUTHOR]

The failure mechanism and failure mode of anisotropic rock masses are very different from those of isotropic rock masses. For unsupported tunnels, the failure modes of anisotropic rock masses can be summarized as composite failure, V-shaped notch failure and spalling of bedding planes. Different failure modes are controlled by the mechanical parameters of the rock masses, the environmental occurrences and the excavation conditions. The combined finite-discrete element method (FDEM) was employed to investigate the failure mechanism and failure process of layered rock masses and the influence of different factors on the failure mode. The study results indicate that (1) composite failure is the basic failure mode of the layered rock masses and that it is caused by the combined action of shear-slip fractures F1 parallel to the bedding plane, tensile fractures F2 perpendicular to the bedding plane, and conjugate shear fractures F3; the rotation movement of slab-like rock fragments induced by fractures F1 and F3 into the tunnel causes significant deformation of the surrounding rock masses; (2) on the basis of composite failure, when fractures F1 disappear and fractures F2 obliquely intersect with the bedding plane, V-shaped notch failure occurs; furthermore, spalling of the bedding plane occurs when the propagation of fractures F3 is blocked at the bedding plane; and (3) with increasing rock mass strength, lateral pressure coefficient of in situ stress and layer thickness, or decreasing tunnel span, the surrounding rock masses change from composite failure to V-shaped notch failure and then to spalling of the bedding plane. However, the effect of the elastic modulus on the failure mode of anisotropic rock masses is weak, and the influence of the dip angle of the rock layer on the failure mode is complicated. [ABSTRACT FROM AUTHOR]

• A new phenomenological tensile failure criterion with an anisotropic coefficient was proposed. • Direct tensile test of the Kangding slate was carried out to verify the accuracy of the new criterion. • The tensile strength data of eight typical layered rocks were collected and compared with the other six typical tensile strength criteria to verify the universality and superiority of the new criterion. • Based on the sensitivity analysis of tensile rate, the loading rate of 0.01 m/s was recommended for the samples with various foliation angles. • Layer thickness has little effect on tensile strength, but a significant effect on tensile failure modes. The tensile strength is a key parameter for the design of many geotechnical engineerings such as tunnel support, slope support, and petroleum drilling. The tensile strength in layered rocks, however, is anisotropic due to the existence of bedding planes. In this paper, to study the anisotropic tensile behaviors, a series of direct tensile tests were conducted using Kangding slate with five different foliation angles (β = 0°, 30°, 45°, 60°, and 90°). Based on the N-Z criterion, a new tensile failure criterion with the anisotropic coefficient was proposed. The tensile strength data of nine typical layered rocks were collected to evaluate the prediction ability of seven anisotropic tensile failure criteria including the new criterion. Embedding the new criterion into FDEM, the loading rate-dependent and layer thickness-dependent tensile mechanism of layered rock under direct tensile test were simulated. The experimental and numerical simulation results showed that the foliation angle significantly affected the tensile strength and failure modes of the slate. With the increase of foliation angle (β), the tensile strength of nine typical layered rocks, including Kangding slate, presented a non-linear increase trend. Compared with the other six typical anisotropic tensile failure criteria, the new criterion has the best prediction capability on the tensile strength of rocks with four different anisotropic degrees, and therefore the new criterion was recommended. The new criterion was embedded in FDEM to carry out direct tensile simulation of slate with different β. The numerical simulation results were in good agreement with the experimental results, further verifying the accuracy of the new criterion. The loading rate sensitivity analysis indicated that a tensile rate of 0.01 m/s was recommended to assure quasi-static loading of the specimen. Through the sensitivity analysis of layer thickness, it is concluded that layer thickness has very little influence on tensile strength, but has a very significant influence on the final failure modes of the specimens. [ABSTRACT FROM AUTHOR]