Search

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]

To investigate the failure process of fault rock burst in tunnels, a large-scale physical model test is conducted. Using the 3D printing technology of the wet-material extrusion deposition molding process, a 2 m × 2 m × 1.5 m model with a rigid closed fault is successfully printed. The mechanical similarities between the printed model and natural rock are quantitatively evaluated based on similarity theory. Based on the printed model, gradual excavation and multistage loading tests under true-triaxial stress conditions are conducted, and the failure process of the Jinping 11.28 rock burst is simulated. The test process is monitored with distributed fiber optical sensing, acoustic emission (AE), and video observation systems. During the excavation process, the surrounding rockmass is stable. The compressive strain concentration of the optical fibers near the fault is obvious. The AE events are mainly distributed along the fault and nucleated near the tunnel face. During the multistage loading process, the left sidewall primarily exhibits surface cracking accompanied by local spalling and slight rock burst, while several local intense rock bursts occur on the right sidewall, indicating the controlling effects of the fault on the failure modes. In the critical period of the intense rock bursts, the monitored stain of the optical fibers penetrating the fault changes from compressive to tensile suddenly, which can be related to the slip and dislocation of the fault. In addition, the AE events increase sharply and concentrate in the footwall of the fault. Failure mechanism analysis of AE events indicates that tensile failure is dominant during the excavation process, and shear failure increases significantly in the following multistage loading process. The research results can provide an important reference for better understanding of the fault rock bursts. [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]

The occurrence of various types of disasters in deep underground engineering is related not only to the strength of the rock mass but also to the crack propagation capacity (CPC) under corresponding stress conditions. The deep-seated rock mass is generally in the stress state of σ1 > σ2 > σ3, and the CPC under differential stress is very important to evaluate the brittle failure of rock. Therefore, based on the deformation characteristics of Baihetan (BHT) basalt under true triaxial compression (TTC), the evaluation index (IP) of the CPC is established, which considers the characteristics of stress-induced deformation anisotropy and the propagation angle of volumetric strain. The evaluation indexes of JP marble, BS granite and BHT basalt under TTC are analysed and compared. When the stress exceeds the crack initiation stress, the evaluation index of the CPC decreases with the increase of σ2, but increases with the increase of σ3, and the effect of σ3 on the CPC index is greater than that of σ2. Under TTC, the CPC of JP marble is the largest, followed by that of BS granite and BHT basalt. The analysis shows that the evaluation index (IP) proposed in this study is also suitable for conventional triaxial compression and uniaxial compression conditions. In addition, the test result shows that there is divisibility between the failure mode and IP under TTC. The relationship between IP and the failure mode of BHT basalt shows that splitting failure occurs when IP < 0.3, that shear failure occurs when IP > 0.4, and mixed failure occurs when IP is between these values. The evaluation index of the CPC proposed in this paper can provide a theoretical reference for evaluating the failure of surrounding rock near the opening boundary of deep underground rock engineering. [ABSTRACT FROM AUTHOR]

During underground excavation, rocks are generally subjected to weak dynamic disturbances caused by blasting, machine vibration, rockburst. To investigate the failure characteristics of hard rock subjected to weak dynamic disturbance, two types of tests, dynamic and static true triaxial tests, with different minimum principal stresses σ3 (0.5, 5, 10, 15, 20, 25, and 30 MPa) were conducted on granite specimens. In the former type of true triaxial test, a weak dynamic disturbance was included; while in the latter type of true triaxial test, only a static stress state was considered. Then, the strength and failure mode of the tested specimens were systemically investigated. The experimental results indicate that the rock strength from the dynamic true triaxial tests is lower than that from the static true triaxial tests, indicating a reduction effect of the weak dynamic disturbance on the rock strength. This reduction effect will gradually increase with σ3. In addition, the failure of the tested specimens from the dynamic true triaxial tests is much more serious than that of the tested specimens from the static true triaxial tests, which may be due to the promotion effect of the weak dynamic disturbance on the crack development. Specifically, when σ3 is low, the dynamic disturbance primarily promotes the development of subdominant cracks (vertical tensile cracks and short oblique tensile-shear cracks) within the tested specimens; when σ3 is high, the dynamic disturbance primarily promotes the development of predominant cracks (through oblique tensile-shear cracks). [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]

Rocks with a layered structure (bedding or foliation) usually exhibit different levels of anisotropy in terms of deformation, strength, and failure mode under multiaxial stress conditions. This anisotropy is influenced by the spatial relationship between the internal layered structure and the principal stresses; however, relatively few experimental studies have been conducted due to various limitations, resulting in insufficient knowledge of the failure mechanism of layered rocks. A series of true triaxial compression tests for a gneiss were carried out considering the orientation of schistosity. The results show that the orientation of schistosity has a significant impact on the strength and failure of this gneiss. More specifically, under the same intermediate principal stress σ2, the strength of specimens increases as the angle (ω) between the strike of schistosity and the direction of σ2 increases. The greater the angle ω, the more sensitive the strength varies with σ2. When σ2 is low, the schistosity plane acts as a weak plane to control the failure of the specimen, and the main failure plane is parallel to the schistosity. As σ2 increases, the weakening effect of schistosity decreases, and the influence of σ2 on the failure is enhanced. In this case, the failure of cutting through the rock matrix more commonly occurs, and the failure plane dips toward σ3 and strikes parallel to σ2. [ABSTRACT FROM AUTHOR]

To study the difference in the failures of laboratories #7 and #8 of the China Jinping Underground Laboratory Phase II (CJPL-II), different loading and unloading stress path tests were carried out under a true triaxial compression condition based on the measured in situ stress of CJPL-II. The strength, deformation and failure mechanisms of Jinping marble under different stress paths were thoroughly investigated. The results showed that under the same stress state as that at sample failure, the bearing capacity and deformation of the unloading stress path were greater than those of the loading stress path. The bearing capacity and deformation in the σ2 and σ3 directions of stress path II and stress path III were similar, and the others decreased from stress path IV to stress path V under unloading conditions. The failure mode results reflect the violence of the unloading failure. The failure features and mechanisms of different stress paths were analysed though the acoustic emission (AE) ring, cumulative energy and strain energy trend. This research may provide a reference for the excavation and support of deep underground works. [ABSTRACT FROM AUTHOR]

The rock structure and three-dimensional stress state play a vital role in the mechanical behaviour of rock masses. Here, a series of true triaxial compression tests (σ1 > σ2 > σ3) are conducted on jointed marble (50 × 50 × 100 mm3) containing a natural stiff joint, taken from the China Jinping Underground Laboratory (CJPL-II) project. The purposes of this study are to investigate the joint effect and estimate the stress dependency of jointed marble. The test results show that jointed marble can fail in four distinct forms, namely, splitting or shearing of intact marble, opening of the joint or sliding along the joint, and these failure modes are influenced by the joint configuration and the minimum and intermediate principal stresses. Generally, jointed marble has more brittle post-peak behaviour than intact marble. The linear Mogi-Coulomb failure criterion can be modified to describe the strength of the jointed marble under true triaxial compression. The jointed marble strength is more sensitive to the minimum principal stress than to the intermediate principal stress. A maximum decline of 25% in strength is observed, which corresponds to a joint dip angle of 60° at σ2 = 60 MPa and σ3 = 30 MPa. The link between the experimental results and in situ fracturing at CJPL-II is also demonstrated. [ABSTRACT FROM AUTHOR]

Energy principles, which can favorably explain the complete rock failure process, are one of the most reliable analysis approaches in rock mechanics and engineering. In this study, a strain energy approach under true triaxial compression (TTC) is proposed. On this basis, the energy evolution characteristics and variations of different failure behavior types (Class I, Class II and ductile failure) under TTC are analyzed. The variations of the strain energy characteristics of Beishan granite with σ2 and σ3 under TTC are studied. The results indicate that the total strain energy U and the elastic strain energy U e of Beishan granite increase with the increasing σ2 or σ3. The dissipated strain energy U d rapidly increases when the value of ε1/ε1peak is approximately 0.6–0.8. The influence of σ3 on the rock failure mode and energy evolution characteristics is greater than that of σ2. In highly brittle rocks, the tensile cracking of the rock microstructure is dominant, and the rock has a high strain energy storage capacity and a low strain energy dissipation capacity. The cumulative acoustic emission (AE) count rate curve shows the same trend as the total dissipated strain energy U d curve. The research results show that the proposed strain energy analysis method for TTC can explain the macroscopic failure behaviors, microscopic failure mechanism and AE characteristics of Beishan granite under TTC, thereby providing new ideas and methods for investigating the behaviors of deep underground hard rock. [ABSTRACT FROM AUTHOR]

The rockburst and spalling problems encountered in the Jinping Underground Laboratory are to be investigated using 3-D physical model testing. New similarity relationships for the brittleness, brittle–ductile transition and ductility characteristics of deep hard rock are first established based on the characteristics of the deep rock mass, thereby providing guidance for the development of analog materials with new characteristics that are appropriate for investigating deep rock engineering disasters via large-scale 3-D physical model tests. Achieving similarity simultaneously among the multiple physical–mechanical properties of deep hard rock constitutes the major challenge of research on analog materials. Consequently, a new method for designing analog materials by controlling the aggregate characteristics is proposed. The test results indicate that the aggregates of the analog materials can be controlled to achieve properties that are similar to those of the original rock. The newly developed analog material exhibits basic mechanical parameters and properties that are similar to those of Jinping marble, including high strength, high brittleness, and high unit weight in addition to brittle–ductile transition and ductility characteristics and deformation and failure modes. The new analog material successfully simulates the major physical–mechanical properties of Jinping marble and overcomes the limitations of preexisting methods, which develop analog materials via cementing agents, in which the achievement of similarity simultaneously among multiple properties is impossible. [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]

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]

In the process of underground engineering excavation, the adjustment of the stress field includes not only the magnitude of stress but also the direction of stress. The failure characteristics of Portuguese granite caused by interchange of the σ 2 and σ 1 directions are studied using a true triaxial testing system to investigate the effects of the interchange of principal stress directions on the failure characteristics of rock. After the interchange of the σ 2 and σ 1 directions, the bearing capacity of the rock increases, the anisotropic characteristics of deformation and fracture are interchanged, the macroscopic failure surface is deflected, and the brittleness of the rock is enhanced. When the rock failure degree increases upon the interchange of the σ 2 and σ 1 directions, the amplitude of the rock bearing capacity increases, the deformation difference decreases, and the number of deflection failure surfaces decreases. Based on the anisotropic characteristics of true three-dimensional stress-induced fractures, the reason for the change in failure mode induced by the interchange between the σ 2 and σ 1 directions is explained. By measuring acoustic emission parameters and performing scanning electron microscopy (SEM) observations, the tensile and shear fracture mechanism and fracture mode with interchange of the σ 2 and σ 1 directions are investigated. The effect of stress rotation on the rock strength and the reason for the change in the failure mode of deep hard surrounding rock excavated in underground engineering projects are discussed. [ABSTRACT FROM AUTHOR]

Microwave-assisted mechanical mining is a new mining method with great potential. A self-developed true triaxial system for microwave-induced fracturing and cutting of hard rocks was used to study the microwave cracking process, cracking mechanism, and its effects on subsequent cutting, grinding and mineral dissociation by taking Sishanling banded iron ore as the research object. The results show that compared with microwave irradiation alone, the failure mode and mechanism of ore samples were more complex under the combined action of true triaxial stress and microwaves. Large cracks that were parallel to the bedding plane appeared in the sample under microwave irradiation without triaxial stress. When true triaxial stress and microwave irradiation were used simultaneously, the surface of the sample was subject to spalling or spalling and cracking. Additionally, the correlation (parallel or vertical) between the direction of σ 1 and the direction of the bedding plane affected the fracture characteristics. Microwave irradiation could promote subsequent mechanical mining, grinding, and beneficiation of samples. Compared with no microwave treatment, under the action of stress and microwaves, the total amount of rock breakage of samples was increased by 55.1%, the powder content of the product after grinding of cut particles was increased by 21.9%, and the degrees of dissociation of cutting products (<0.1 mm and <0.2 mm) were increased by 11.6 and 12.7%, respectively. The influence of the strip width of magnetite on the microwave cracking effect of ore was estimated. At a given magnetite content, the microwave absorption was worse for wider strips, and the reflection performance was similar for strips of various widths. [ABSTRACT FROM AUTHOR]

Abstract: Stress-induced failures have typically been considered as engineering disasters in deep underground excavations. This paper highlights the importance of stress-driven rock failures as a means to estimate the in-situ rock stress at the depth. The method, which is based on the observation and documentation of a failure mode referred to here as “spalling” in deep tunnels or caverns, is put forward and presented to estimate the macro-scale rock stress. Using the proposed method, the orientation of the principal stresses in-situ can be deduced by the spalling intensity and associated stress-induced spalling veins in the rock mass surrounding the opening, and the magnitude of the principal stresses can be estimated by the assumed crack initiation stress required to generate spalling in the given rock by careful documentation of spalling failure characteristic in the field. A real case study at Baihetan powerhouse site has been carried out and indicates that the proposed method is an available way to enriching recognition of in-situ rock stress and a helpful supplement to traditional methods of stress measurements after strictly numerical verification. Analysis of the regional tectonic setting as well as measurements of in-situ stress supports the results by the suggested approach. [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]

For the in situ surrounding rocks of underground excavations, the radial stress corresponding to the minimum principal stress σ 3 varies with increasing distance away from the excavated boundary. To characterize the rock failure along the radial direction of an excavation, true triaxial compression tests with different σ 3 values (0.5, 5, 10, 15, 20, 25, and 30 MPa) were performed. The characteristic stresses (including the crack initiation stress σ i , crack damage stress σ d , and peak stress σ s) and failure mode of the tested specimen under different σ 3 conditions were systematically investigated. The experimental results indicate that with increasing σ 3 , the characteristic stresses increase monotonically and continuously. The increasing rate of the characteristic stresses with σ 3 ranging from 0 to 10 MPa is higher than that with σ 3 ranging from 10 to 30 MPa, indicating that low σ 3 impose a greater effect on the characteristic stresses than high σ 3. In addition, the failure mode of the tested specimen changes from tensile-shear failure mode I (with vertical tensile cracks and oblique tensile-shear cracks developing along the σ 2 direction) to tensile-shear failure mode II (with oblique tensile-shear cracks developing only along the σ 2 direction) to tensile-shear failure mode III (with oblique tensile-shear cracks along both σ 2 and σ 3 directions) as σ 3 increases. Then, the failure characteristic of the surrounding rocks of the underground excavation was characterized based on the obtained experimental results. When maximum radial stress σ r encountered near the excavated boundary is much smaller than the axis stress σ a , the surrounding rocks will exhibit a "Ternary failure characteristic", i.e., with an increase in the distance from the excavated boundary along the radial direction, the failure mode of the surrounding rocks after excavation changes from tensile/splitting failure mode to tensile-shear failure mode I and then to tensile-shear failure mode II. When the maximum σ r encountered near the excavated boundary approximates the original σ a , the surrounding rock masses will exhibit a "Quaternary failure characteristic". Specifically, the corresponding failure mode along the radial stress direction away from the excavated boundary changes from tensile failure mode to tensile-shear failure mode I then to tensile-shear failure mode II and finally to tensile-shear failure mode III. • True triaxial tests with different σ 3 are conducted. • Rock strength under different σ 3 condition are investigated • The influence of σ 3 on the rock failure mode is studied. • Rock failure near the boundary of underground excavations is characterzied. [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]

The failure modes of deep excavation absolutely relate to three-dimensional stresses. The effect of the intermediate principal stress (σ 2) on the failure of the granite surrounding an underground powerhouse was examined using true triaxial compression tests. As the initial major principal stress (σ 1 o) is parallel to the axis of the Shuangjiangkou Underground Powerhouse, for the cross-section the tangential stress around the excavation depends on the initial intermediate (σ 2 o) and minor (σ 3 o) principal stress components. Once the excavation is created, the radial stress becomes zero, and the tangential stress increases dramatically. The axial stress in the excavating direction remains almost unchanged. Slabbing will develop towards the excavation space that is parallel to the excavation axis. The stress σ 2 after excavation can help to improve the failure brittleness, crack initiation strength, crack damage strength, and peak strength of the rock when it falls within the range of stresses encountered in engineering practice. The ratio of the crack damage strength to crack initiation strength is approximately constant as σ 2 varies. More specifically, the crack damage strength is about three quarters of the value of the peak strength, and the crack initiation strength is about half the value of the peak strength. As σ 2 increases, the microcracks coalesce to form a major through-going crack. Meanwhile, the Poisson's ratio in the direction of strain ε 2 declines steadily and that in the direction of strain ε 3 increases moderately, so that the rock exhibits anisotropy. The fracture angle (the angle between the fracture plane and the plane normal to σ 1) is proportional to σ 2. The results of the true triaxial compression tests allow the brittle failure events in the Shuangjiangkou Underground Powerhouse to be adequately explained. It could help facilitate an optimization of the direction of the excavation axis and efficient supporting measures for deep underground powerhouse. • Intermediate principal stress σ 2 increases the Poisson's ratio ν 3 but decreases ν 2 • σ 2 increases crack initiation strength, crack damage strength, and brittleness • The σ ci (σ cd) value is about 50%(75%) of the value of the peak strength of the rock • For σ 1 parallel to tunnel axis the failure mode depends on σ 2 and σ 3 (normal to σ 1) • Increasing σ 2 could enhance the brittleness of rock, contrary to the σ 3 effect [ABSTRACT FROM AUTHOR]

In underground excavations, different failure features can be induced by complex geological stresses. Therefore, many researchers have investigated rock behaviour and failure mechanisms under high-stress conditions. This study investigated the different failure characteristics to determine the stress-induced cracking mechanisms. Accordingly, a series of tests were performed on a volcanic rock (rhyodacite) under one- to three-dimensional stress states. The results indicated that the samples exhibited different failure characteristics/modes under different stress states. Under 1-D and 2-D compression, the rock exhibited unstable brittle failure induced by tensile cracking (Mode I fracture). Under axisymmetric triaxial stress (σ 1 > σ 2 = σ 3 > 0), the rock exhibited stable failure, and as σ 3 increased, the failure mode transitioned from Mode I fracture to Mode II fracture to distributed cataclastic failure. In contrast, under a three-dimensional differential stress state (σ 1 > σ 2 > σ 3 > 0), the risk of unstable failure increased with increasing σ 2 , and the rock exhibited localized shear failure (Mode II fracture). Based on the acoustic emission (AE)-based cracking classification method, further studies were also conducted on the failure mechanism. Finally, based on the energy release and cracking propagation, the mechanism of the σ 2 effect on unstable failure was assessed. • Three stress-induced cracking mechanisms were revealed. • Tensile cracks were preferentially induced during the failure evolution. • The unstable failure can be induced by a higher (σ 2 -σ 3). [ABSTRACT FROM AUTHOR]

Strong unloading damaged zones at both slopes of the dam of a hydropower station in Southwest China have been attributed to the stress release. To study the deformation and failure mechanism of dacite (dam site material) under unloading conditions, three groups of true triaxial unloading tests were conducted on the rock sample with an initial minimum principal stress σ 3 0=5 MPa, and initial intermediate principal stresses σ 2 0= 10, 15, and 20 MPa. The experimental results indicate that the strain in the direction of σ 3 is the most sensitive to the unloading of σ 3 , whereas the three principal strains are further suppressed with increasing σ 2. Ductile deformation is a failure precursor of dacite under unloading conditions. Based on the analysis of the failure mode and acoustic emission monitoring data, it has been concluded that the unloading failure of dacite is caused by the initiation, propagation, and coalescence of microcracks, which form localized tension macrocracks. The observed anisotropic damage evolution is found to be determined by stiffness degradation and quantified the degree of fracture during the final unloading stage. The results indicate that damage is anisotropic under the true triaxial unloading conditions, and that the damage ω 3 is the most sensitive to the unloading stress σ 3. Therefore, the deformation or damage in the direction of σ 3 can be used as a guide in the design of engineering excavation plans. • The effects of unloading σ 3 on dacite is studied under true triaxial conditions. • The deformation in the direction of σ 3 is most sensitive to unloading. • The unloading failure of dacite is tensile cracks propagation in the direction of σ 1. • The evolution of damage is anisotropic under unloading conditions. [ABSTRACT FROM AUTHOR]