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• Conduct a series of triaxial tests on rectangular solid sandstone containing two non-coplanar fissures. • Analyze the effect of ligament angle on triaxial deformation parameters of fissured rectangular solid sandstone. • Evaluate triaxial damage threshold and peak strength behavior of rectangular solid sandstone containing two non-coplanar fissures. • Investigate the crack type and internal damage behavior of rectangular solid sandstone containing two non-coplanar fissures. Fissures are generally existing in engineering rock mass, and cracking pattern observed in specimens with multiple fissures is analogous to the pattern obtained in specimens with two fissures, whereas the cracking coalescence of real rock specimen under triaxial compression have rarely been investigated. Therefore, thirty fissured rectangular specimens containing two non-coplanar fissures with different ligament angles were prepared to perform triaxial compression in this work, and the stress-strain curves, strength and deformation parameters, ultimate failure modes and Acoustic Emission (AE) characteristic were investigated. The experimental results indicate that pre-existing fissures have a significant effect on the stress-strain curves, more stress drops were observed before and after the peak strength, whereas confining pressure cannot reduce stress-drop. Cohesion (C) of peak strength and damage threshold has different trend with ligament angle (β), and the C of the peak strength is larger than that of the damage threshold. Pre-existing fissures have more effect on damage threshold than peak strength; cracks are easier to initiate in fissured specimen. It is easier to coalesce when ligament angle is parallel to the maximum principal stress, and cracks are easier to expand in width direction than that in thickness direction. The coalescence of the two non-coplanar fissures is classified as five typical modes, and the CT result shows that crack distribution in depth direction is similar. Crack classification by AE parameter and k-mean method can reflect the cracking process in a certain extent. It is more drastic when shear crack initiates, and tensile crack propagates in a stable way. [ABSTRACT FROM AUTHOR]

Rock masses containing intermittent discontinues under dynamic loading have rarely been investigated, although step-path failure frequently occurs in field engineering. This work aims to explore the action mechanism of dynamic cyclic loading and discontinuities on step-path failure. Four treatments including the control (Intact sandstone), pre-flawed samples with assemblage of two 30° dip fissures (type-I), assemblage of 10° and 65° dip fissures (type-II) and assemblage of two 65° dip fissures (type-III) were replicated. Laboratory experiments consisted of conventional uniaxial compression tests (UCT) and multilevel constant-amplitude fatigue tests (MLFT). Real-time acoustic emission (AE) monitoring was conducted along with scanning electron microscopy (SEM) observation on the fractured surfaces. Results indicated that the mechanical properties, energy evolution, AE characteristics and microscopic features of sandstones were significantly influenced by pre-existing fissures. The weakened rates of peak strength ranged from 2.80% to 85.63%, and regardless of loading methods, always followed the sequence of type-I > type-II > type-III. We contribute the weakening effect to the reduction in the effective bearing area and the increase in the dip angle of the rock bridge. With the same pre-fissure morphology, the sandstone under MLFT has enhanced its deformation resistance compared to under UCT. In addition, the maximum AE count/energy during the whole cyclic stage were always significantly lower than those during the monotonic rising process. SEM images revealed that long-term fatigue loading lead to numerous micro pores and transgranular cracks. Ultimately, a simple constitutive model was proposed based on the statistical damage mechanics and geometric damage theory. [ABSTRACT FROM AUTHOR]

Highlights • The numerically simulated results of sandstone specimens containing two flaws were in agreement with the experiment. • The crack evolution process for pre-flawed specimen under triaxial compression was numerically investigated. • The internal crack patterns in pre-flawed specimens were revealed by X-ray CT scanning and PFC3D simulation. Abstract Flaws widely exist in engineering rock masses, which significantly influence the overall mechanical behaviors (strength, elastic modulus and failure, etc.) of rock masses. The study of crack coalescence behavior of rock specimen containing pre-existing flaws enhances the understanding of failure mechanism of rock. In this study, a series of PFC3D simulations on numerical specimens were carried out to investigate the crack initiation, propagation and coalescence process under conventional triaxial compression. First, numerical model for cylindrical sandstone specimen was constructed and a set of micro-parameters were calibrated by comparing with the experimental results of intact sandstone with different confining pressures. Then, two pre-existing open flaws which were the same as the tested sandstone were created by deleting balls. Triaxial compression simulations were conducted on pre-flawed specimens for a range of confining pressures (10–40 MPa). The numerically simulated results were compared with the experimental results, which showed that the simulations were in agreement with the laboratory tests. The processes of crack initiation, propagation and coalescence for pre-flawed specimens were investigated by program in PFC3D. The relationships among the cracking process, stress-strain curve and number of micro-crack were analyzed in detail. Finally, the internal crack patterns in pre-flawed specimens were revealed by analyzing the cross-sections from X-ray CT scanning and PFC3D. [ABSTRACT FROM AUTHOR]

• The mechanical parameters of sandstone specimens containing three pre-existing flaws at various inclination angles were investigated. • The evolution processes of three-dimensional crack of sandstone specimens containing three pre-existing flaws were explored. • The evolution of internal stress surrounding the pre-existing flaw during fracture process was discussed. Rock materials contain various scales of pre-existing flaws that greatly influence the stability of rock engineering. The study of strength and failure behavior of pre-flawed rock enhances the understanding the fracture mechanism of rock. In the present study, uniaxial compression tests were carried out on sandstone specimens containing three pre-existing flaws at various inclination angles. The experiment results indicated that the stress–strain curves, mechanical paprameters, acoustic emission and failure pattern of pre-flawed specimen were affected by the pre-existing flaws. To verify the test results, a PFC modelling was conducted on pre-flawed specimen using a set of calibrated parameters. The stress–strain curve, mechanical paprameters and failure mode of specimen containing three flaws were reproduced in the numerical simulation. Three types of crack, i.e., tensile crack, shear crack and mixed tensile-shear crack, were identified based on the laboratory micro observations and numerical displacement field. As the flaw angle increased from 0° to 90°, the tensile stress concentration gradually deteriorated and transferred from the middle to the tip of pre-existing flaw. The evolution of internal stress surrounding the pre-existing flaw during the crack initiation, propagation and coalescence process was further discussed. This experimental and numerical study aims to investigate the crack characteristic and internal stress evolution in fractured rock. [ABSTRACT FROM AUTHOR]

• Analyze the effect of fissure inclinations on the macro-and meso-mechanical properties of sandstone under uniaxial compression. • Comparing the energy evolution characteristics of sandstone containing different fissure inclinations under uniaxial compression. • Investigate the failure process of fractured sandstones under uniaxial compression by the evolution of microcracks. For the rocky slope with potential sliding hazard, there are a larger number of flaws such as fractures and joints, which greatly affect the stability and safety of the slope. Thus, it is important and relevant to the behavior of fractured rock from a macroscopic and mesoscopic perspective by experiments and numerical simulation. In this paper, the mechanical properties and crack evolution behavior of grey sandstone containing two parallel fissures under the uniaxial compression tests were evaluated by experimental and bond-based Peridynamics (BB-PD) simulation. The results show that with the increase of fissure dip angle, the peak strength, elastic modulus and Poisson's ratio of pre-cracked sandstone samples under uniaxial compression condition gradually increase. At the same time, the total energy, dissipated energy and elastic peak energy of fractured sandstone also increase with the increase of fissure dip angle. In order to validate the experimental findings, a set of calibration parameters were used to model the intact samples using BB-PD. The numerical simulation reproduced the stress–strain curves, mechanical parameters and failure modes of double-fractured sandstone. Based on the results of indoor experiments and numerical simulations, it is concluded that the failure mode of double-fractured sandstone has three types under uniaxial compression: tensile failure, shear failure and conjugate shear failure. As the fissure dip angle increases, the main control factor for failure of sample gradually changes from rock bridge failure to secondary crack coalescence, and the stress distribution of fractured sandstone gradually changes from high stress concentration in the rock bridge zone to uniform stress distribution as a whole. [ABSTRACT FROM AUTHOR]

• A new fluid-solid coupling model is improved for hydraulic fracturing modeling in shale reservoirs horizontal well. • The key factor affecting the fracture network in the vertical section of shale reservoirs is the interbedding fractures. • The secondary interbedding fractures generated by the primary through-bedding fracture can form a complex fracture network. • The optimal perforation spacing requires that the fractures extend to the maximum range and form an effective network. Hydraulic fracturing is one of the key technologies of shale gas extraction. Deeply comprehending the distribution characteristics of hydraulic fracture networks in shale reservoirs is significant for the shale gas fracturing design and production estimation. The efficient description of fluid–solid coupling is essential for modeling the hydraulic fracturing process. In this paper, based on the mechanical parameters from the laboratory test results, the fluid–solid coupling model in PFC2D is improved and employed to establish a DEM model for the hydraulic fracturing of shale reservoirs. The key factors affecting the hydraulic fracture propagation characteristic are investigated, furthermore, based on the geological parameters of Changning shale gas production areas, a multi-perforation fracturing numerical model of the shale reservoir horizontal well is established, and the distribution characteristics of the hydraulic fracture network in staged fracturing are analyzed and revealed. The results indicate that the main factor affecting the formation of a fracture network in the vertical section of the shale reservoirs is the coalescence of interbedding fractures. A large number of secondary interbedding fractures generated by the primary through-bedding fracture can form a complex fracture network, which is exactly the fracture morphology required for hydraulic fracturing in shale gas exploitation. According to the results of hydraulic fracture distribution characteristics under different perforation spacings, the optimal perforation spacing requires that the fractures extend to the maximum range under the premise of hydraulic fracture coalescence can form an effective network and guarantee low economic cost and construction technical difficulties. [ABSTRACT FROM AUTHOR]

• Analyze the effect of fissure inclination and confining pressure on mechanical behavior of fissured granite. • Investigate triaxial damage process of fissured granite by the evolution of microcracks. • Characterize three-dimensional crack characteristics by displacement field and X-ray CT images. In order to investigate the mechanical behavior and failure mechanism of fissured granite under triaxial compression, a three-dimensional grain-based model (GBM3D) was established in particle flow code (PFC) for simulating the triaxial compression of granite specimens with various fissure inclinations (α = 0°, 30°, 45°, 60° and 90°) under different confining pressures (0, 5, 10 and 20 MPa). First, a group of micro-parameters used in GBM3D was calibrated based on the experimental triaxial compression results of intact specimens, the simulation results of fissured specimens based on these parameters are in good agreement with the experimental results. Compared with intact granite, the fissured granite has smaller peak strength, crack damage threshold, elastic modulus and axial peak strain. Confining pressure can improve the mechanical properties of granite at the same fissure angle, and under the same confining pressure, the mechanical properties increase with the increase of fracture angle. With the increase of axial stress, the cracks first emerge from the crack apices and propagate toward the ends of the specimen along the direction of the principal stress, and the cracks penetrate each other and eventually lead to the failure of the specimens. The displacement field shows that the main cracks are all generated at the interfacial locations where large relative displacements occur between the particles. The three-dimensional spatial crack distribution characteristics of the damaged specimens are not only influenced by the confining pressure, but also related to the fissure angle. The confining pressure not only complicates the crack distribution characteristics, but also increases the total count of cracks. As the fissure angle increases, anti-tensile wing crack is more likely to appear in the specimen. [ABSTRACT FROM AUTHOR]

• Conduct the experiment and simulation on red sandstone containing a non-straight fissure. • Compare numerical and experimental results of red sandstone containing a non-straight fissure. • Analyze the mechanical parameters of red sandstone containing a non-straight fissure. • Explore the crack evolution behavior of red sandstone containing a single non-straight fissure. It is widely accepted that the strength, deformability, and failure behaviors of rock material are strongly influenced by widespread natural fissures. Research focusing on the crack initiation, propagation, and coalescence behavior in fissured rock material is important to ensure the reliability and predictability of rock engineering. However, the types and forms of pre-existing fissures are complex and variable; thus, mechanical experiments have rarely been conducted on rock samples containing non-straight prefabricated open fissures. In this research, the strength, deformability, and failure behaviors of sandstone samples containing a single non-straight fissure were systematically investigated by a series of uniaxial compression experiments and numerical simulations. First, the influence of the fissure angle (α) on the strength and deformation characteristics of red sandstone containing a single non-straight fissure was evaluated. Then, the ordinary state-based peridynamics model was used to investigate the fracture behavior of red sandstone specimens with respect to various α values. The results indicate that for α = 0°–15° and 75°–90°, tensile cracks first initiated from the convex points of the prefabricated fissure, and the crack initiation position transferred from the convex point to the tip of the prefabricated fissure for α = 30°–60°. In addition, for the α = 0°–90°, the cracks initiated in the form of tensile cracks, and the ultimate failure modes were mixed tensile and shear failure. Secondary cracks always appeared in the form of shear crack. Moreover, for α ≥ 75°, fewer cracks were initiated before the peak strength was reached; afterward, the crack initiations were more prolific, and more acoustic emission counts were observed. [ABSTRACT FROM AUTHOR]