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The pre-peak microstructure evolution of the material is crucial in determining the strength and failure scheme of the material, but is not captured by the existing shear strength parameters models which focus on post-peak stage information. Novel concepts of pseudo-cohesion and pseudo-internal friction angle that account for both the pre-peak and post-peak stage information were proposed in this study, based on the experimentally observed response of the rock mechanics parameters to an external load (the maximum principal stress). These two parameters were analytically derived as functions of maximum principal strain, with coefficients to be determined (i.e., by applying the Mohr–Coulomb criterion with these parameters to experimental data). The dependence of pseudo-cohesion and pseudo-internal friction angle on the axial strain and confining pressure was investigated. The evolution of the strength parameters was found to relate to the propagation of micro-crack developed in the loading process, as revealed by PFC2D simulations reproducing experiments. The results show that (1) analytical model characterizing the pseudo-cohesion and internal friction angle as the quadratic function of axial strain accurately reflects the mechanical response characteristics of the specimen; (2) before the residual stage, the pseudo-cohesion first increases and then decreases with increasing strain, while the pseudo-internal friction angle increases continuously; (3) for the sandstone specimens studied, the pseudo-cohesion at the peak stress increases by a gradually decreasing rate with increasing confining pressure, while the pseudo-internal friction angle at the peak stress decreases by a decreasing rate with the increasing confining pressure; (4) CPM well characterized the mechanical properties of rock under multiple confining pressures, as the total stress–strain curves and failure modes of numerical specimens reproduced the experimental results. The evolution of rock pseudo-shear strength parameters is closely related to the external load and the generation, propagation, and penetration process of rock cracks. The rock pseudo-shear strength parameters are gradually mobilized with the increasing load, and the propagating of cracks owing to the decrease of mobilizable maximum cohesion and the increase of mobilizable maximum internal friction angle. [ABSTRACT FROM AUTHOR]

The microcrack propagation-coalescence mechanism and stress–strain constitutive model of brittle rock is of great significance for prediction of mechanical properties. To establish a more applicable constitutive model of brittle rock, the crack model based on fracture mechanics was introduced. First, based on the crack slip model and maximum circumferential stress theory, a theoretical equation for crack propagation length was established. The microcrack propagation-coalescence mechanism of brittle rock material under loading was studied. On this basis, a macro-coalescence zone model was proposed. Through the relationship between the deformation characteristics of stress–strain curves at different crack growth stages and the behavior of microcracks, a complete stress–strain constitutive model of brittle rock based on the crack model was theoretically deduced. The constitutive model was verified by the results of a coupling hydro-mechanical test of granite carried out after different heat treatment temperatures. The advanced intelligent algorithm, genetic algorithm, was adopted to simulate experimental data. The research results showed that: (1) The derived stress–strain constitutive model of brittle rock is not only high in accuracy of stress–strain simulation, but also can simulate both the axial and circumferential stress–strain curves. Moreover, the dilatancy and crack initiation stresses can be obtained directly. (2) The parameters of the stress–strain constitutive model not only have clear physical meaning, but the mechanisms of rock mechanical properties, such as dilatancy, acoustic emission, and stress fluctuation, can be explained. [ABSTRACT FROM AUTHOR]

The Qingling-Bashan (QB) mountain region in southern Shaanxi mainly consists of strongly compressive zones from east to west, with tight folds and compressive fractures. There is a wide distribution of soft rocks of various types, such as phyllite and slate, accompanied by complex geological structures. Ironically, tunnel construction in these difficult grounds with complicated geological conditions embraces a high risk of extreme deformation due to various unpredictable reasons, which can frequently cause collapse and result in budget overruns during the construction period. Therefore, it is crucial to conduct effective countermeasures to eliminate or avoid such adverse impacts. This paper provides a case study on the “Yingfeng tunnel” (a tunnel constructed in soft rock consisting of a slate ground) based on a geological survey, indoor experiments and in situ monitoring. A successive rock mass deformation resulted in the tunnel lining seriously intruding into construction clearance and some other sections, even collapsing during the construction. The maximum displacement amount was 62.5 cm, while the maximum deformation speed reached as high as 34.18 mm/day. Additionally, to evaluate the construction impacts of tunnelling-induced geo-hazards, an investigation on extreme deformation was conducted. Considering the time-dependent features of the rock mass deformation, the constraint-convergence method was used to put forward applicable countermeasures in this paper. Finally, from the feedbacks of monitoring results, extreme deformation of the Yingfeng tunnel was effectively controlled. [ABSTRACT FROM AUTHOR]

From a global perspective, karst geological disaster has great influence on the tunnel construction. Once it is not handled properly, the karst problems in the tunnel area will cause great disaster, such as water inrush, mud gushing and collapse, etc. These karst disasters seriously affect the construction progress of the tunnel, resulting in equipment damage, casualties, and so on. The construction of Yichang-Wanzhou (YW) railway tunnel has created a miracle in the field of Chinese tunneling. The geology of tunnel area is quite complex, the karst disaster is remarkable, and the engineering construction is extremely difficult. In this paper, the karst characteristics, unfavorable geological disasters, and treatment methods of YW railway tunnel were analyzed systematically. The YW railway tunnel and Zhengzhou-Wanzhou (ZW) railway tunnel are both located in the Qinling-Daba (QB) mountainous area, and the regional geological conditions of them are very similar. Therefore, the research in this paper can be extended to the QB mountainous area, thus providing a reference for the construction of ZW railway tunnel. The investigation can also provide reference for the similar engineering. [ABSTRACT FROM AUTHOR]