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A large amount of slates is distributed along Kangding No. 2 tunnel of Sichuan–Tibet Railway (China). The bedding of the Kangding slate is rich, showing apparent anisotropy of strength. For underground tunnels excavated in the anisotropic rock mass, the rock material is vulnerable to damage in tension because of its low tensile strength. Therefore, the tensile strength and fracture modes are important for the support design of layered surrounding-rock tunnel. In this study, first, a series of direct tensile tests were carried out on slate samples with five kinds of bedding dip angles. Second, the effect of bedding dip angle on the slate's tensile strength, failure modes, and acoustic emission characteristics was analyzed. Third, based on the Nova–Zaninetti criterion and the experiment results, a new phenomenological anisotropic criterion was proposed by introducing anisotropic coefficient, which could describe the nonlinear variation trend of tensile strength with foliation angles. Compared with four typical tensile strength criteria, the accuracy and universality of the new criterion were verified. Finally, the new criterion was implemented into the combined finite-discrete element method (FDEM) to conduct direct tensile simulation of slate with different foliation angles. The results indicated that the direct tensile strength followed an S-shaped increasing trend with the variation of foliation angles. The direct tensile failure modes of slate samples with different foliation angles could be divided into layer activation failure, mixed failure, and rock matrix failure. The acoustic emission process of slate with different bedding inclination under direct tension could be roughly divided into the following three stages: quiet period—stepped saltatory increase—drastic increase. The fitting effect of the new criterion was the highest among Kangding slate and other six typical layered rocks, which showed that the new criterion was suitable for predicting its Brazilian strength and direct tensile strength. At the same time, the numerical simulation results were in good agreement with the experimental results, which further verified the accuracy of the new criterion. Highlights: The acoustic emission characteristic parameters helped to reveal the direct tensile failure characteristics of layered rocks. A new phenomenological anisotropic tensile failure criterion was proposed. The direct tensile test data and the Brazilian splitting test data were used to study the accuracy and universality of the new anisotropic tensile failure criterion. FDEM was used to study the accuracy of the new anisotropic tensile failure criterion. [ABSTRACT FROM AUTHOR]

In this study, a two-dimensional Voronoi element based – explicit numerical manifold method (VE-ENMM) is developed as a new approach to investigate the acoustic emission characteristics during the failure process of intact rocks. The micro-granular structures of the intact rocks are approximated by the random Voronoi polygons and the interactions among rock grains are interpreted using a modified interface contact model. A new acoustic emission (AE) monitoring technique considering the clustering effect of micro-cracks is developed based on the explicit time integration scheme of numerical manifold method. With the newly developed approach, a series of numerical simulations are carried out. Firstly, numerical uniaxial compressive and Brazilian tests are conducted to validate the new approach on simulating micro-/macro-mechanical behaviors of granite samples. After that, the acoustic event properties, such as time evolution, spatial clustering and frequency-magnitude distribution of AE events during the failure process under uniaxial compression are studied in details. Simulation results show that the development of acoustic emissions is in good consistence with the damage evolution process of the numerical model, which may help to reveal the micro-fracturing mechanism of intact rocks in terms of seismic energy dissipation. To further illustrate the capability of the developed approach in monitoring the failure process as well as failure characteristics during TBM tunnel excavation, an additional numerical simulation is conducted and the fracturing behaviors as well as the related AE properties of the surrounding rock mass during TBM tunnelling are investigated. [ABSTRACT FROM AUTHOR]