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Most rockbursts are associated with jointed rock masses as the energy stored inside the rock is prone to abruptly release at such discontinuities. Moreover, remote blasting at working faces can trigger the brittle failure of jointed rocks at opening boundaries. In this work, a testing system combining true-triaxial compression with dynamic disturbance is employed to investigate the failure modes of cuboid jointed granite specimens. A three-dimensional tunneling-induced stress path is created in cuboid jointed granite specimens by first applying static loads to simulate the stress state encountered around an open boundary. Then, a dynamic disturbance (load) is continuously applied to the granite specimen to simulate the effect of the vibrations experienced due to remote blasting. A dynamic disturbance in the σ2 direction is found to reduce the peak strength of the specimen (σp) by approximately 5% (compared to static stress-induced failure) and promote shearing failure at the fracture surface. The major fracture surfaces form a 'V'-shape, along with several nearby minor cracks. However, a dynamic disturbance in the σ3 direction can reduce the peak strength by 10% and accelerate the evolution of tensile failure resulting in subvertical tensile fracture. Due to the natural stiffness of the joint, the Poisson's ratio (υ3) in the ɛ3 direction is larger than that in ɛ2 direction, highlighting the anisotropy of the deformation. The proportion of the total stored energy that is released in the jointed rock is greater than that released in intact hard rock. More strain energy is dissipated at the joint fracture. The joint provides a location at which energy can be released, which considerably increases the severity of potential rockbursts. The energy-storage limit of granite with different dip angles is also explored based on complete stress–strain curves. Post-peak granite specimens with joints dipping at 60° are found to be the most susceptible to external dynamic disturbance but release less kinetic energy. Specimens with joints that dip at 80° incorporate the maximum amount of stored energy and are not prone to rockburst when subjected to dynamic disturbance. Once a rockburst does occur in 80°-dipping jointed granite, a strong tremor will appear, however. Highlights: Disturbance in σ2 direction decreases strength by 5% and expedites shearing failure. Disturbance in σ3 direction reduces strength by 10% and accelerates tensile failure. Poisson's ratio (ν3) in ε3-direction presents more than that (ν2) in ε2-direction. The joint dipping of granite significantly affects energy components of rockbursts. [ABSTRACT FROM AUTHOR]

Foliated rocks are often encountered in underground engineering, and the spatial orientation relationship between its foliation and in situ stress controls the stability of the surrounding rock. This is closely related to the anisotropy of the mechanical properties of foliated rocks. The anisotropic mechanical properties of thin foliated rock under the influences of foliation and stress can be obtained by laboratory testing; however, there have been few experimental studies on foliated rocks under true triaxial compression at present. Foliated rocks are in a three-dimensional unequal stress state in deep excavation engineering; thus, to evaluate the stability of surrounding rock and scientifically guide the support design, a systematic true triaxial test considering the loading orientations (β, ω) of schistosity for a foliated gneiss was conducted. The results show that the strength and failure of the gneiss are greatly affected by inherent structure and stress conditions. More specifically, the larger the ω and σ2, the greater is the strength, and the failure mode tends to be controlled by the differential stress. Finally, a new empirical true triaxial anisotropic failure criterion was proposed according to the variations of strength with loading angle and stress conditions. This criterion can reflect the tendency and sensitivity of the change in the strength to σ2 at different ω and β, and can satisfy the need to model degradation from the true triaxial stress state to the conventional triaxial stress state. This criterion provides a new approach to characterize the strength of anisotropic rocks and improve the design of engineering works in practice. Highlights: The strength and failure of the foliated gneiss are affected by the loading angle of schistosity structure and differential stress. When the differential stress is larger, the larger the angles ω and β, the more does the failure mode tend to be controlled by stress induction. The strength of the foliated gneiss has a strengthening effect on ω, and the larger the β, the weaker is the strengthening effect of ω. A new empirical true triaxial anisotropic failure criterion is proposed according to the variations of strength with ω, β, and stress conditions. [ABSTRACT FROM AUTHOR]

Microwave-induced hard rock fracturing technology has potential application prospects in tunnel boring machine (TBM) hard tunnel excavation. It is of great significance to research the hard rock cracking induced by rotary microwave applicator irradiation to promote the application of microwave–TBM coupling. Based on a rotary open microwave irradiation hard rock fracturing true triaxial test system independently developed by Northeastern University, Chifeng basalt is taken as the research object. The crack propagation characteristics of hard rock irradiated by a microwave applicator at different rotation speeds under the conditions of no stress and true triaxial stress are studied, and the fracturing mechanism of hard rock irradiated by rotary microwave applicator is revealed. The results show that the temperature in the same area of hard rock rises in a "step shape" under rotary microwave applicator irradiation. The faster the microwave applicator rotation speed is, the shorter the intermittent irradiation time of the same area of hard rock, the quicker the temperature rise is, and the earlier the hard rock cracking time is first detected. The degree of crack propagation and the decrease in compressional wave velocity are positively correlated with the microwave applicator rotation speed. In the process of rotary microwave applicator irradiation, the cracks in the irradiated hard rock surface develop and expand from the high principal strain region at the boundary to the central region of the irradiated surface. Under the condition of no stress, tensile failure in hard rock is induced by rotary microwave applicator irradiation. Under the condition of true triaxial stress, tensile failure induced by rotary microwave applicator irradiation mainly occurs in hard rock, although shear failure occurs at the same time. This makes the crack development in the irradiated hard rock surface denser and the crack network more complex. This research provides a new perspective for microwave–TBM coupled tunneling in hard rock tunnel engineering. Highlights: A rotary open microwave irradiation hard rock fracturing system is developed independently. The influence of microwave applicator rotation speed on cracking effect of hard rock under a true triaxial stress condition is studied. The fracturing mechanism of hard rock irradiated by rotary microwave applicator is revealed. This study provides a new perspective for microwave–TBM coupled rock breaking in hard rock engineering. [ABSTRACT FROM AUTHOR]

Because true triaxial compression testing is generally more complicated, time-consuming, and expensive than conventional triaxial compression testing, further research is needed to reduce the cost and time of true triaxial compression tests. In this study, the true triaxial apparatus developed by Northeastern University is used, which can change the principal stress in one direction independently. Multi-stage true triaxial compression tests are carried out on Jinping marble, and the effectiveness of the new test method is evaluated. The strength parameters obtained from the multi-stage tests and single-stage tests are compared. It is suggested that constant slope unloading should be carried out before the peak strength to ensure reliable and effective experimental results. The multi-stage true triaxial compression test exhibits good repeatability and can obtain reliable strength parameters in a short time for reference in engineering practice. Under different multi-stage loading conditions, Jinping marble presents different strength, deformation and failure characteristics, and the macrofailure mechanical properties of a specimen are affected by the propagation of microcracks within that specimen. [ABSTRACT FROM AUTHOR]

Shear failure is a typical failure mode of rock masses in underground engineering. Conducting shear tests of rocks in the laboratory is an effective research method. In this paper, a shear box is presented that can be used for the direct shear test of hard rocks under three-dimensional stress conditions with a true triaxial compression testing apparatus. The new normal linear variable differential transformer (LVDT) sensor fixture is used to make the obtained normal deformation data more accurate. The lateral deformation measurement adopts a U-shaped sensor design, which solves the problem of the lateral deformation measurement. Under different lateral stresses, a series of direct shear tests were conducted on the sandstone specimens, and the test results verify the reliability and practicability of the novel laboratory shear box. With the increase of lateral stress, the peak shear strength and peak normal displacement of the sandstone specimens also increase, and the lateral deformation is restrained. The lateral splitting block can be effectively reduced by applying lateral stress. [ABSTRACT FROM AUTHOR]

To study the deformation and failure mechanism of hard rocks, true triaxial compression tests were conducted on four types of hard rocks to obtain the complete stress–strain curves and failure modes. Under true triaxial compression conditions, the shapes of the complete stress–strain curves can be divided into three types: elastic–brittle (EB), elastic–plastic–brittle (EPB), and elastic–plastic–ductile (EPD) types. According to the different post-peak deformation behaviours, the stress–strain curves of EPB type can be subdivided into three sub-categories: post-peak instantaneous brittle (EPB-I), post-peak multi-stage brittle (EPB-M), and post-peak delayed brittle (EPB-D). The stress–strain curves change from EPD to EPB-D, EPB-M, EPB-I, and finally EB with increasing differential stress (σ2 – σ3). The deformation characteristics are dependent on intermediate principal stress σ2, minimum principal stress σ3, mineral compositions, and mineral textures of rock samples. An increase in σ3 leads to an increased ductility, while an increase in σ2 leads to an increased brittleness. Moreover, rocks with regular mineral textures and low mineral hardness are more prone to ductility. When the deformation curve is transformed from EPD to EPB and then to EB, the tensile crack gradually predominates, and the macroscopic failure angle gradually becomes steeper. [ABSTRACT FROM AUTHOR]

In view of a previous study of the intermediate principal stress effect at a limited σ2 range, a series of true triaxial tests, covering a full range of intermediate principal stresses that vary from the generalized triaxial compression stress state (σ2 = σ3) to the generalized triaxial tensile stress state (σ1 = σ2), was carried out on sandstone and granite samples. The experimental results revealed that the deformation, failure strength and failure mode have a significant dependence on the stress state. As an effect of the intermediate principal stress on crack evolution, the deformation difference known as stress-induced deformation anisotropy occurred and should be considered when developing the mechanical model. Moreover, a post-peak deformation with a step-shaped stress drop is observed and illustrates that there will be a multi-stage bearing capacity after the rock failure. The peak strength is non-symmetrical with the increasing σ2 and is closely related to the Lode angle. Based on the final fracture surface and SEM analysis under true triaxial compression, three failure modes and failure zones, including tension failure, shear failure and mixed failure, are delineated and discussed. Combining the failure mode and the strength under true triaxial compression, it is found that the strength variation exhibited a close relationship to the failure mechanism. [ABSTRACT FROM AUTHOR]

The purpose of this ISRM Suggested Method is to introduce a guideline on determining deformation and failure characteristics of rocks subjected to true triaxial compression on different stress path. The true triaxial testing apparatus was reviewed by means of the function and engineering application. Some key techniques, such as stress and strain measurements, and reduction of end effect between specimen and metal platens, preventing metal platens interference, were stated and suggested in detail. Methodology of specimen processing, specimen shape, and testing procedure are characterized. There is an explanation of the experimental data processing on stress–strain curves, strength, and fracture mode. [ABSTRACT FROM AUTHOR]

The study of rock fracturing is a fundamental research topic in rock mechanics and engineering. In this paper, based on complete stress–strain curves obtained under TTC and the corresponding AE and strain energy results, the mechanical properties and fracturing process of Beishan granite are studied. A representative test result (σ 2 = 75 MPa, σ 3 = 20 MPa) is selected to analyse and illustrate the three-dimensional fracturing process of the Beishan granite. The results show that the variations in the characteristics of the brittle fracture behaviour, AE and strain energy of Beishan granite change with σ 2 or σ 3 following certain relationships, and the mechanisms driving these various relationships are very different. The AE and strain energy characteristics of Beishan granite are also quite different among the five fracturing evolution stages. The variations in the increments of AE count, AE event and total elastic strain energy per unit time (ΔS c / Δt , ΔS e / Δt and ΔU e / Δt) are studied at different times and under different stresses during rock failure. The rock fracturing evolution can be illustrated through the variation in these parameters, providing deep insight. This research provides a perspective for studying deep underground fracture-forming processes, such as earthquakes, tunnel sudden failure, spalling, splitting and rockburst. [ABSTRACT FROM AUTHOR]

To investigate the deformation and failure mechanism of rock influenced by initial stress and unloading rates, this study presents minimum principal stress unloading tests of Jinping marble under true triaxial compression conditions under various initial stresses and unloading rates based on the in situ stresses measured in the China Jinping Underground Laboratory Phase II (CJPL-II). The effects of initial stresses and unloading rates on the bearing capacity and the deformation and failure characteristics of Jinping marble were analysed. The relationship between the strains in the three principal stress directions and the unloading extent of σ 3 is defined using parameters such as the strain-unloading extent ratio and the unloading strain rate. With an increase in the initial σ 1 , the increase in strain-unloading extent ratio and unloading strain rate changed from linear to exponential earlier, and sample failure occurred with a smaller unloading extent. Meanwhile, the number of acoustic emission rings and the amount of cumulative energy increased, and the sample failure changed from tensile failure to a mixture of tensile and shear failure that showed more transgranular cracks in the microscopic images. With an increase in the initial σ 2 or σ 3 , the overall trend of the failure characteristics was similar to that with an increase in the initial σ 1 , but the sample failed with a larger unloading extent, and the increase in strain-unloading extent ratio and unloading strain rate changed from linear to exponential later. The influence of unloading strain rate shows that as unloading strain rate increased, the sample failed with a larger unloading extent and changed from a mixture of tensile and shear to tensile failure, and more transgranular cracks were observed in the microscopic images. • σ 3 unloading tests with different unloading rates and initial stress levels under true triaxial stresses were carried out. • The bearing capacity, deformation process, and failure mode of Jinping marble under σ 3 unloading condition were examined. • The relationships between strains along principal stress directions and unloading extent of σ 3 (Δσ 3) were established. • Failure mechanisms of Jinping marble were identified based on AE data and SEM pictures of fractures. [ABSTRACT FROM AUTHOR]

High stress induced by excavation disturbance in deep hard rock engineering can cause rockburst accidents. Microwave-induced borehole fracturing of hard rock is a promising technique for preventing rockbursts, the principle of which is to reduce energy and stress concentration levels in rock mass through microwave heating. The characteristics and mechanism of microwave-induced borehole fracturing of hard rock under different true triaxial stresses were experimentally investigated. After microwave-induced borehole fracturing under different σ 1 and σ 2 conditions, a complex crack network dominated by tensile cracks was generated in the rock specimen, which is consistent with the results obtained by AF and RA. According to the P-wave velocity before and after the test, the degree of thermal fracture is positively linearly correlated with σ 1 and σ 2 as a whole, and has good consistency with the length and number of thermal cracks. The results show that the technique will have better applicability and higher fracturing efficiency in high-stress areas. The thermal fracture process of basalt specimens under different σ 1 and σ 2 conditions is similar, which can be divided into a silent period, quiet period, dense period, and persistent period. In addition, thermal fracture exerts a significant threshold temperature effect, and fracture rate increases rapidly when the temperature reaches 150–192 °C (average about 172 °C). The relationship between thermal fracture and stress provides guidance for parameter design. • Microwave-induced borehole fracturing of hard rock is a promising technique for preventing rockbursts. • Microwave-induced borehole fracturing tests are conducted under different true triaxial stress. • The characteristics and mechanism of microwave-induced borehole fracturing under different σ 1 and σ 2 conditions are investigated. • Microwave-induced borehole fracturing process is investigated by AE technique. [ABSTRACT FROM AUTHOR]

• The stress-strain curves and mesoscopic voids of shale were obtained their evolution exhibited staged characteristics. • The relationship between the damage variable and the axial strain follows the quadratic function. • The mesoscopic mechanism of the macroscopic response of shale was analysed. • A damage constitutive model of shale under uniaxial compression conditions was proposed. With the continuous development of shale gas engineering in China, an accurate and adequate understanding of shale damage mechanism has become a critical scientific problem addressed by rock mechanics. So far, many researchers have studied the fracturing process and mechanism of shale in multi-scales. The mesoscopic structural characteristics of shale during loading can be obtained using the in-situ X-ray computed tomography (CT) technology. Some scholars have observed the structure evolution of shale under in-situ uniaxial compression conditions, but the quantitative investigation of damage evolution and constitutive model of shale still requires further studies. Given this, the in-situ micro-CT device was adopted to study the macroscopic mechanical response and the mesoscopic structural evolution of Longmaxi shale under uniaxial compression conditions. The evolution law of the damage variable with the axial strain was analysed according to the characterisation of the damage variable based on the CT image grey variance. In combination with the macroscopic stress-strain curve shape, evolution stage of the mesoscopic void, evolution of the damage variable, mesoscopic mechanism of the macroscopic mechanical response was investigated, and a damage constitutive model of shale under the uniaxial compression test was proposed. The conclusion of this study can provide a foundation for further research into the multi-scale failure mechanism of shale and exert theoretical guidance significance on shale gas fracturing engineering practice. [ABSTRACT FROM AUTHOR]

Abstract: The influence of the intermediate principal stress on rock strength has been studied comprehensively by previous researchers. However, the reason why rock strength firstly increases and subsequently decreases with the increase of intermediate principal stress is still unclear. In this paper, the mechanism of the intermediate principal stress effect on rock failure behaviour is revealed through a numerical method using the EPCA3D system (Elasto-Plastic Cellular Automaton). In this study, both homogeneous and heterogeneous rocks are considered. The heterogeneity of a rock specimen is modelled by introducing Weibull''s statistical distribution. Two criteria, i.e. the Drucker–Prager and Mohr–Coulomb models, are used to determine whether a meso-scopic element in the rock specimen is in a failure state or not during the polyaxial stress loading process. The EPCA3D simulation reproduces the typical phenomenon of the intermediate principal stress effect that occurs in some rock experiments. By studying the EPCA3D simulated acoustic emission and complete stress–strain curves illustrating failure initiation, propagation and coalescence in the failure process of rocks, the essence of the intermediate principal stress effect is tracked. It is concluded that the heterogeneous stress distribution induced by the natural heterogeneity of rocks and the effect of the loading platen are two of the reasons producing the intermediate stress effect. Studies indicate that a moderate intermediate principal stress delays the onset of local failure, which in turn leads to an increase in the rock strength. However, once the intermediate principal stress reaches a certain value, local failure will be formed through the application of the intermediate principal stress. It is the number of failed elements in the pre-peak region that determines whether the rock strength decreases or not. The extent of rock strength reduction when the intermediate principal stress reaches a certain value is lessened with the increase in the minimum principal stress. [Copyright &y& Elsevier]

The conventional (σ 1 > σ 2 = σ 3) multi-step creep test and true triaxial (σ 1 > σ 2 > σ 3) multi-step creep test of Jinping marble with rectangular shape were conducted. In this way, the effects of σ 1 , σ 2 , σ 3 and time on creep behaviour were compared and analysed. Unlike conventional creep tests, Jinping marble showed differential creep deformation and failure characteristics in σ 2 and σ 3 -directions under true triaxial stress. The crack time-dependent evolution was-directional, and the crack tip growth was nearly parallel to the σ 1 -direction with high stress and opened along the σ 2 and σ 3 -directions with low stress. Moreover, when σ 2 > σ 3 , the crack growth in the σ 2 -direction was inhibited, and the crack evolution mainly occurred along the σ 3 -direction. This difference eventually led to the macroscopic failure plane of Jinping marble generally acting parallel to the σ 2 -direction and along the σ 3 -direction. The larger the value of σ 2 , the steeper the macroscopic failure surface, and the failure mode of the rock samples changed from shear to near-splitting failure: however, increasing σ 2 can only cause closure of initial cracks opening along the σ 2 -direction, but did not change the number of initial cracks opening along the σ 3 -direction. The experimental data also showed that the differential creep deformation in the σ 2 and σ 3 -directions caused by true triaxial stress did not change with time and σ 1 , but was only affected by σ 2 and σ 3. The differential creep deformation in σ 2 and σ 3 -directions can be described by the formula DI = a (1-exp(b ((σ 2 -σ 3)/σ 3))). [ABSTRACT FROM AUTHOR]

Rock fracture process is associated with the degradation and mobilization of the mechanical and strength properties of rock. A number of true triaxial cyclic loading and unloading tests were carried out on three types of hard rocks (i.e., granite, marble and sandstone) to investigate the evolution of the rock properties as a function of accumulated damage. In addition to stress and strain measurements, acoustic emission (AE) signals were monitored and used to associate different stages of crack development with different stages of the mechanical and strength parameter evolution. Young's modulus and five strain increment ratios were calculated from the measured increments of stresses and strains in each cycle. It was found that hard rocks exhibited profound deformation anisotropy and directional dilation. A general trend of decreasing volumetric strain reversal stress and peak stress with increasing plastic strains was observed for all the tested specimens. The evolution of the strength properties of rock under true triaxial compression was investigated using a linear 3D hard rock failure criterion and was found to be generally consistent with the cohesion weakening and frictional strengthening concept. The influence of the intermediate principal stress on the evolution of the rock properties was also discussed. [ABSTRACT FROM AUTHOR]

The post-peak behaviour of rocks plays an important role in studying the stability of surrounding rocks. A series of true triaxial compression tests in high-low stress states were carried out on 58 Jinping marble specimens. The characteristics of complete stress–strain curves, including the post-peak stage, the corresponding meso- and micro-failure characteristics with scanning electron microscopy (SEM), and the whole progressive failure processes with acoustic emission (AE), were analysed. With either decreasing σ 3 or increasing σ 2 , the shapes of the post-peak curves change from Class I to Transition and then to Class II. Additionally, the changes have obvious stress boundaries. As σ 2 increases at low-constant σ 3 or σ 3 decreases at constant σ 2 , the failure mode changes from tensile–shear mixed failure or shear failure to tensile failure, and these changes correspond with the changes in the post-peak curves. Micro-failure of marble changes from intergranular failure at Class I to transgranular failure at Class II. The AE characteristics of the three typical post-peak progressive failure processes are significantly different, matching the brittle failure behaviour from weak to strong. This study enhances the knowledge of in situ failure mechanisms and the stability of deep-buried tunnels and caverns. • The complete post-peak deformation and failure results for 58 Jinping marble specimens under various true triaxial stresses were obtained • σ 2 induced a change in the post-peak deformation curve shapes from Class I to Class II, especially at low σ 3 • σ 2 induced multiple parallel fractures, and the failure mode changed from mixed to tensile failure at low σ 3 • The in-depth study of rock post-peak behaviour is conducted by linking the 3D stress states (σ 2 , σ 3), shapes of the post-peak curves, meso- and micro-failure, and AE characteristics • The impact of the post-peak deformation and failure characteristics of marble on engineering failure mechanisms is explored [ABSTRACT FROM AUTHOR]