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To investigate the time-dependent characteristics of jointed rock masses, a series of single-step true triaxial creep tests were conducted on natural jointed granite samples under different dip angles conditions. The results showed that the time-dependent failure of jointed granite behave as two different forms, namely, stress-controlled and stress-structure-controlled failure. When the dip angle of joint is less than 70° or greater than 77°, stress-controlled failure was the predominate mode. Otherwise, stress-structure-controlled failure occurred. There are similarities and differences in the evolution mechanisms of these two types of time-dependent failure. Similarly, the acoustic emission (AE) count rate demonstrated multiple peaks in both types of time-dependent failure processes. Differently, for stress-controlled failure, tensile microcracks and large-scale shear cracks develop alternately until the ultimate failure is reached, but for stress-structure-controlled failure, tensile cracks are dominant in the early stage and followed by plenty of shear cracks in the later stage. [ABSTRACT FROM AUTHOR]

Optimizing the microwave parameters adopted in microwave-assisted mechanical mining is of great significance to improving the efficiency of the deep resource development. The effects of varying the type of microwave antenna, microwave power, and working distance on the reflection coefficient and heating characteristics of Sishanling iron ore were studied. The optimized microwave operating parameters were proposed, and microwave irradiation and mechanical cutting tests were then carried out on the ore samples under true triaxial stress conditions to verify the feasibility of the optimized parameters. The results show that the ore's reflection coefficient increased at first and then decreased as the working distance (2–8 cm) was increased. The reflection coefficient increased as the microwave power was increased. Compared with other distances, the temperature and temperature gradient of the irradiated surfaces of samples were maximized when the working distance was 2 cm. The optimum microwave parameters of iron ore were: a microwave antenna format of horn antenna/standard waveguide antenna, a working distance of 2 cm. Under true triaxial stress conditions, samples suffered no macroscopic damage when irradiated for 300 s at a working distance of 8 cm. When the working distance was reduced to 2 cm, however, samples broke apart after just 10 s (failure mainly taking the form of intermittent spalling). Arcing readily occurred under true triaxial stress conditions when the working distance was 2 cm. This phenomenon occurred on the irradiated surface of the sample and the baffle of the antenna could also be ignited. Ways of optimizing the antenna were put forward and discussed. Highlights: The influence of microwave antenna type, microwave power and working distance on reflection coefficient and heating characteristics of Sishanling iron ore was studied. The influence of optimized working distance on microwave cracking effect of ore under true triaxial stress was studied. The optimization scheme of microwave antenna was proposed based on the arc phenomenon of ore. [ABSTRACT FROM AUTHOR]

The fracture of the surrounding rock in deep hard rock engineering is influenced by the stress path during excavation. Taking the stress path as the only variable, a true triaxial experimental system was used to study the loading–unloading stress path during excavation and the loading stress path with constant stress state after excavation, and combined with acoustic emission, 3D laser scanning and SEM to investigate the differential mechanisms of the two stress paths on the strength, energy storage and fracture of the rock. The results show that the strength, deformation and ultimate energy storage capacity of the rock under the loading−unloading stress path are greater than those under the loading stress path with constant stress state. Compared with the loading stress path with constant stress state, with the loading−unloading stress path, the rock fractures are more fragmented, the failure angles are slightly smaller, and the development of shear cracks is delayed, occurring closer to rock failure. This is due to the influence of the true triaxial stress state on rock fracture expansion. The changing direction and location of rock fracture expansion under the loading−unloading stress path inhibits crack penetration, which in turn delays the development of shear cracks. The effect of the stress path is influenced by the rate of loading and unloading; the greater the rate of loading and unloading of the rock is, the greater the strength and ultimate energy storage of the rock. Finally, the evolution characteristics of the tensile and shear cracks formed in the rock under the loading−unloading stress paths during excavation provide new understanding for the prevention and control of deep engineering hazards. [ABSTRACT FROM AUTHOR]

Shear failure along joints is one of the typical failure modes in deep underground rock masses, and indoor direct shear tests are commonly used to study the shear mechanical behavior of joints. However, deep underground rock masses are subjected to true triaxial stress conditions, and current indoor direct shear tests of joints only consider the shear stress and normal stress, without considering the influence of lateral stress. Therefore, in order to investigate the effect of lateral stress on the shear strength of jointed hard rocks, a series of true triaxial single-shear tests with different levels of lateral stress were conducted on intact hard rock, hard jointed rock (hard rock containing natural hard joints), and artificial jointed rock (hard rock containing open joints) using self-developed shear boxes and deformation measurement devices. The test results show that lateral stress can enhance the shear strength of intact granite and hard jointed granite but has little effect on that of artificial jointed granite. The shear strength of hard jointed granite is lower than that of intact granite and higher than that of artificial jointed granite. A simple linear function was used to describe the effect of lateral stress on the friction coefficients of these three types of specimens, and it was found that the enhancement effect of lateral stress on the friction coefficients of intact granite was greater than that on jointed granite. The failure mode of the specimens is jointly affected by normal stress, lateral stress, and joints. High normal stress and low lateral stress conditions often make the specimen prone to lateral peeling failure, and the presence of joints can reduce the lateral spalling failure of the specimen. [ABSTRACT FROM AUTHOR]

This study introduces a novel large three-dimensional physical model experimental system (PMES) for studying the geomechanical problems related to deep underground engineering. The apparatus is mainly composed of a counterforce frame, a hydraulic loading subsystem, a sample installation and transportation subsystem, an excavation subsystem, an integrated monitoring subsystem, and auxiliary facilities. During the design and development of the PMES apparatus, key technological issues were addressed. A high-stiffness frame structure with opening and closing function was developed. Uniform, gradient, and designed non-uniform stresses can be applied with high accuracy. An independent long-term load-holding unit and a surface dynamic disturbance unit were designed. The basic and innovative functions of the apparatus were verified through calibration experiments. A large physical model experiment was conducted to simulate the excavation of a deep-buried drainage tunnel in China's Jinping-II hydropower station. The experiment obtained the failure processes of the surrounding rock masses induced by high in-situ stress, specific engineering geological structures and excavation disturbance. The internal fracturing and strain evolution within the sample were successfully captured by the integrated monitoring subsystem. The results also validated the reliability of this novel apparatus. The development of this novel apparatus provides a new tool for the study of geomechanical problems in deep underground engineering. Highlights: A novel large three-dimensional physical model experimental system for studying the geomechanical problems related to deep underground engineering was established. The basic and innovative functions of the apparatus were verified through calibration experiments. A large physical model experiment was conducted to simulate the excavation of a deep tunnel in China's Jinping-II hydropower station. [ABSTRACT FROM AUTHOR]

An intense fault rockburst recently occurred in a deep tunnel excavated by tunnel boring machine in China, which was taken as a case to study the failure characteristics in detail. The temporal and spatial evolution of microseismic (MS) activity, the dynamic stress drop and the spatial distribution characteristics of rock fracture mechanism were analyzed. Stress-induced mechanism was studied using numerical simulation, and the development mechanism of the fault rockburst was revealed finally. The research results are as follows: (1) The development process of the fault rockburst is divided into six stages: calm stage, crack initiation stage, crack development stage, crack accumulation–slight fault dislocation stage, further crack accumulation stage and crack penetration–fault slip stage. (2) During the development of the fault rockburst, stress and energy accumulate in the deep of surrounding rock. When rockburst occurs, stress and energy are released in the shallow of the surrounding rock; at the same time, the dynamic stress drop increases significantly. MS energy is released intermittently during the development of fault rockburst. (3) Affected by the geological condition, the high in situ stress and the excavation unloading, horizontal shear slipping of the fault occurs, leading to fault rockburst. These research results can provide a reference for establishing the warning and mitigation methods of fault rockburst in deep TBM tunnels. [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]

Rockburst warning includes prediction of the position, intensity, and timing of potential rockburst. Rockburst time warning refers to the prediction of the moment at or time period during which a rockburst may occur. Due to the complex rockburst mechanism and many influencing factors, several key difficult-to-solve scientific problems currently remain in rockburst time warning research. In this article, microseismic (MS) monitoring is performed, and blasting is implemented as an iconic event to study the warning method for rockburst with blasting cycle as the unit of time. Focusing on this research goal, a deep learning method is applied to establish an MS information prediction model (MSIPM) and a rockburst time warning model (RBTWM) based on a long short-term memory network (LSTM). The MSIPM predicts the MS information for subsequent blasting cycles through the MS information time series of historical blasting cycles. The RBTWM predicts the potential rockburst intensity and which blasting cycle a rockburst may occur through the MS information time series obtained by fusing MS information from historical and subsequent blasting cycles. The developed method is applied in a railway tunnel excavated with the drilling and blasting method. The warning results of the test set demonstrate that the rockburst warning accuracies for the first, second, and third subsequent blasting cycles are approximately 74.6%, 71.2%, and 63.1%, respectively. In addition, further application and verification are carried out in the construction of another new railway tunnel. The rockburst warning accuracy for the first subsequent blasting cycles is approximately 80.0%. The application results show that the MSIPM and RBTWM provide warnings regarding the immediate rockburst time in blasting cycle units. The combination of MS monitoring and artificial intelligence represents a new idea for rockburst time warning. [ABSTRACT FROM AUTHOR]

Excavation of deep hard rock tunnels disturbs the initial stress imbalance, which can lead to rockburst, spalling and/or collapse. The disasters are related not only to the strength of the rock mass, but also to the excavation stress path. By analysing and summarizing a large number of engineering cases, the surrounding rock of a deep tunnel is divided into high-, medium- and low-risk fracture zones of disaster, and the excavation stress paths of the three risk fracture zones are generalized. Based on the true triaxial test system, the catastrophic failure mechanism of rock masses in high- and medium-risk fracture zones of hard rock tunnels in deep engineering under the excavation stress path is studied. It is revealed that the stress difference caused by the excavation stress path fundamentally determines the difference in the surrounding rock fracture degree. A method for evaluating the influence of the excavation stress path on the rock fracture anisotropy by using a three-dimensional stress difference coefficient is proposed. The results show that under the excavation stress path in the high-risk fracture zone, the energy stored in the rock is larger, the rock is more prone to failure, and the rock fracture is more violent. Under the excavation stress path of the medium-risk fracture zone, the rock is more prone to spalling failure. With the increase in the depth from the excavation boundary in the high-risk fracture zone, the stress difference decreases continuously, the rock bearing capacity increases, and the shear fracture component increases. The three-dimensional stress difference coefficient shows a linear positive correlation with the rock fracture anisotropy. The mechanism of the excavation stress path affecting rock burst disasters in deep hard rock tunnels is discussed from the perspective of energy storage and dissipation. This study also provides guidance for selecting a reasonable support scheme. Highlights: The excavation risk fracture zone of deep hard rock tunnel is divided, and its typical excavation stress path is summarized. The catastrophic failure mechanism of rock with different excavation stress paths under true triaxial stresses was investigated. A method for evaluating the influence of the excavation stress path on rock fracture anisotropy using a three-dimensional stress difference coefficient is proposed. The influence of the excavation stress path on field failure is discussed from the perspective of energy storage and dissipation. [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]

This paper presents a novel 3D printing technology for synthetic hard rock based on self-developed 3D printing equipment and a systematic 3D printing method. The 3D printing equipment adopts a wet-material extrusion deposition moulding (WEDM) process and consists of a frame structure, feeding subsystem and control module. Aiming at the characteristics of wet similar materials of rock, the feeding subsystem was designed based on pneumatic actuation-spiral control, which could realize the printing of multicomponent and multiparticle materials. A systematic 3D printing method considering similarity theory was established to achieve the printing of synthetic hard rock with different physical and mechanical properties. Using the aforementioned 3D printing equipment and method, the 3D printing of synthetic hard rock was successfully performed. The results show that the printability of similar materials of rock might be controlled to achieve a reasonable initial setting time, extrudability and certain self-supporting ability by adding a set retarder and water-retaining agent. The mechanical performance and failure patterns of the printed samples were conveniently controlled by adjusting the proportion of the components in the similar materials. The quantitative similarity evaluation of mechanical parameters between the printed samples and Jinping marble was performed based on similarity theory, and the optimal mix proportion for simulating Jinping marble was obtained. Compared with other 3D printing technologies for rock mechanics with WEDM, the surface microstructure and mechanical characteristics of the printed samples are more similar to those of natural rock. These research results can be used to expand the application of 3D printing technology in rock mechanics. Highlights: Sample-scale 3D printing equipment was independently developed based on WEDM process. A systematic 3D printing method for synthetic hard rock was established. The 3D printing of synthetic hard rock is successfully completed. [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]

In this work, an extremely intense fault-slip rockburst case, with a local magnitude of 2.3, observed in a deep tunnel in southwestern China was introduced as a case study. The rockburst caused a large economic loss and delayed the construction schedule for nearly 2 months. In-situ failure analysis, geological surveys, and microseismic (MS) monitoring were carried out to study the development and occurrence mechanisms of fault-slip rockburst. The fault-slip rockburst occurred in the hanging wall of a structure plane with dark green filling material and the rock mass slipped along the structure plane. It shows characteristic of intermittent occurrence, which results in great psychological pressure on workers. The dominant frequency of the rockburst MS event during the fault-slip rockburst is 13 Hz, which shows that the fault slip rockburst can be captured by the MS monitoring system. Extremely intense MS activity was recorded during the rockburst. The development and occurrence processes of the fault-slip rockburst were studied using this MS information. The stress evolution during the rockburst was studied by analyzing the dynamic stress drop. For the evolution mechanism of the fault-slip rockburst, many tensile fracture events occurred during the development stage, and a few shear fractures, corresponding to structure plane dislocation, occurred before the occurrence of the fault-slip rockburst. The presence of a large natural structure plane resulted in a higher stress and crack extension, and reduced the length of the penetration path, resulting in a stronger destructiveness. The results can be used for the further research on fault-slip rockbursts and planning appropriate warning and mitigation measures. Highlights: An extremely intense fault-slip rockburst with local magnitude of 2.3 in a deep tunnel was introduced. Occurrence characteristics of fault-slip rockburst were analyzed. The development and occurrence mechanisms of the fault-slip rockburst including the fracture type and evolution mechanism, were studied. [ABSTRACT FROM AUTHOR]

Radial chain rockbursts (RCRs) in deep-buried TBM tunnels have prolonged durations and can pose significant safety risks. Further research is needed to deepen our understanding of the mechanisms of recurrence of rockbursts under the influence of structure plane. In this study, the geological characteristics, occurrence characteristics, microseismic (MS) activity laws, and rock mass fracture mechanism of several RCRs influenced by weak band were summarized to reveal the mechanism of recurrence of RCRs. Research shows that under the influence of weak band, (1) RCRs tends to occur in the area where the dip angle is large, and the strike intersects with the tunnel direction at a large angle and forms a small angle with the maximum principal stress. The width of the weak band typically ranges from 0.01 to 1 m, which is positively correlated with the final depth of rockburst pit. The strength of the weak band is relatively "soft" compared to the surrounding bedrock. (2) The depth of each rockburst pit in RCRs gradually decreases, the time interval between adjacent rockbursts gradually increases, and the maximum thickness of rock block in each rockburst gradually increases. (3) The maximum MS energy of MS events and the maximum apparent stress gradually decreased in the development process of RCRs. The region of stress concentration gradually extended to deeper zone, and the distance difference between stress concentration region and sidewall during each rockburst occurrence stage gradually decreased. (4) The effect of weak band on RCRs attributes to the excavation induced local stress concentration near the weak band, and the interface between the weak band and surrounding rock controls the boundary of the rockburst body. The "chain effect" of RCRs mainly shows that the promotion effect of the previous rockburst on the subsequent rockburst, by promoting the stress and rock fracture transferring to the deep zone, and the proportion of shear fractures increases. • The geological conditions, occurrence characteristics and microseismic (MS) activity laws of radial chain rockbursts (RCRs) under the influence of weak band were summarized. • The mechanism of weak band acting on RCRs was obtained. • The mechanism of recurrence in RCRs under the influence of weak band was revealed, which is "chain effect". [ABSTRACT FROM AUTHOR]

Layered rocks usually show anisotropy in their deformation and failure characteristics. The anisotropy is mainly caused by the orientation of thin-layered structure (e.g., bedding or foliation). Thin-layered structures are likely to fail, thus jeopardizing the stability of underground excavations. Under different bedding or foliation orientations, the surrounding rock mass will represent different mechanical responses with changing depth. Due to the limitation imposed by testing machine performance, most studies focused on the mechanical properties of the layered rock under conventional loading conditions, which partially reveal the mechanical properties influenced by bedding (foliation) orientation of the layered rock subjected to multi-axial loading. To overcome these limitations, a series of true triaxial compression tests on a steeply-dipping thin-layered rock taken from a deep metal mine were conducted in two cases of foliation orientation in this research. The results show that the lateral stress differences (σ2–σ3) have significant effects on the strength and failure of this rock at different foliation orientations, and indicate that, in tunnel design, the angle between the tunnel axis and foliation strike should be as large as possible to maintain the stability of the tunnel to the greatest extent. The findings from this research show anisotropic rock behavior around underground excavations under various stress and foliation orientation states, and help designers to optimize tunnel orientation and prevent large anisotropic deformation. [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]

This paper introduces a test system for microwave-assisted dual-mode mechanical cutting of hard rock under true triaxial compression. The test system was developed by Northeastern University, China, to simulate the cutting and breaking of hard rock by a tunnel boring machine (TBM). The test system is mainly composed of a dual-mode mechanical cutting subsystem, a true triaxial loading subsystem and a high-power microwave presplitting subsystem. The system performs dual-mode cutting under true triaxial stress, applies a single sensor to monitor the three-dimensional force of the cutter head, and integrates the reduction in the vibration and resistance to torsion during the cutting process. Mechanical cutting tests of basalt under different amounts of microwave irradiation and true triaxial stress conditions were conducted in Chifeng, Inner Mongolia, China. The results showed that microwave radiation presplitting effectively improved the efficiency of mechanical rock breaking. However, after 180 s of microwave irradiation, true triaxial high stress inhibited the mechanical breaking of basalt in terms of the overall excavation depth. During the tests, each system performed stably. The test results verified the reliability of the test system. [ABSTRACT FROM AUTHOR]

Fault rockburst warning has always been challenging in deep tunnel engineering, and there is no quantitative warning method for deep tunnel boring machine (TBM) tunnels. This study clarified the geological characteristics and microseismic (MS) activity characteristics of fault rockbursts, and a quantitative warning method that integrates geological information and MS information was established. The results of this study show that fault rockbursts are prone to occur near faults without filling or with hard filling, where the faults run through the tunnel, and where the orientation of the fault is at a small angle to the direction of the maximum principal stress. Fault rockbursts exhibit significant precursor characteristics during the development process, that is, MS events are characterized by a significant distribution along faults, a cumulative apparent volume growth rate of adjacent MS events greater than 45%, and an overall rising trend in the ratio of cumulative apparent stress to cumulative dynamic stress drop (K value), which can be used to identify whether a potential rockburst will be a fault rockburst. To further predict fault rockbursts, seven MS parameters were comprehensively used, including the intraday and cumulative number of MS events, intraday and cumulative MS release energy, intraday and cumulative MS apparent volumes, and K value. Based on the CRITIC method for determining weight coefficients, a quantitative fault rockburst warning method was established and successfully applied to a deep TBM tunnel. This method achieves dynamic warning of type identification, intensity, and probabilities of fault rockbursts during the excavation process of deep TBM tunnels.HighlightsFault rockbursts are prone to occur near faults without filling or with hard filling, where the faults run through the tunnel, and that the fault orientation is at a small angle to the maximum principal stress.Fault rockbursts have significant precursor characteristics: MS events are characterized by a significant distribution along the faults, a cumulative apparent volume growth rate (CAVG) of adjacent MS events greater than 45%, and an overall rising trend in the ratio of cumulative apparent stress to cumulative dynamic stress drop (K value).The K value can be used to differentiate the intensities of rockbursts on the basis of characterizing the development process of fault rockburst.A quantitative fault rockburst warning method for deep TBM tunnels was established, which achieves dynamic warning of type identification, intensity, and probabilities of fault rockbursts during the excavation process of deep TBM tunnels.Fault rockbursts are prone to occur near faults without filling or with hard filling, where the faults run through the tunnel, and that the fault orientation is at a small angle to the maximum principal stress.Fault rockbursts have significant precursor characteristics: MS events are characterized by a significant distribution along the faults, a cumulative apparent volume growth rate (CAVG) of adjacent MS events greater than 45%, and an overall rising trend in the ratio of cumulative apparent stress to cumulative dynamic stress drop (K value).The K value can be used to differentiate the intensities of rockbursts on the basis of characterizing the development process of fault rockburst.A quantitative fault rockburst warning method for deep TBM tunnels was established, which achieves dynamic warning of type identification, intensity, and probabilities of fault rockbursts during the excavation process of deep TBM tunnels. [ABSTRACT FROM AUTHOR]

Excavation in deep weak rock engineering can significantly alter the stress state in the surrounding rock, often leading to varying degrees of floor heave. The excavation stress paths from the immediate floor to the interior floor were determined by simulating tunnel excavation and monitoring stress changes at fixed points on the floor. The characteristic stresses, failure modes, acoustic emission and deformation features of three types of clastic rock under excavation stress paths were experimentally investigated to reveal the floor heave mechanism. The study found that as the distance from the excavation boundary increased, the bearing capacity of the clastic rock gradually enhanced, and the risk of macroscopic failure decreased, transitioning from tensile failure to tensile–shear mixed failure. Simultaneously, the excavation-induced dilational deformation rapidly decreased. A novel method was proposed to evaluate the dilational deformation of surrounding rock induced by excavation based on the strain features of the samples under the different stress paths. The results demonstrated a rapid decrease in dilational deformation from the immediate floor to the interior floor for the three types of surrounding rock, with the maximum total dilation and the greatest risk of floor heave observed in contact cementation surrounding rock. Finally, the floor heave mechanisms of three types of clastic rock were summarized as follows: unloading dilation combined with macroscopic failure; slight rebound of the immediate floor coupled with multiple microcracks; notable rebound of the immediate floor accompanied by few microcracks. This study provides new insights into the mechanics disasters and selection of support measures for deep-buried tunnels. [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]

Hard rocks exhibit three-dimensional (3D) stress-dependent failure under true triaxial compression. The deformability and strength of hard rocks under true triaxial compression differ from those under traditional loading schemes of conventional triaxial compression tests. For the purpose of characterizing these distinctive features, including 3D stress-dependent brittleness and the failure process and 3D stress-induced anisotropy, a new model suited for hard rocks was proposed in this paper. In the new model, the 3D stress-dependent brittleness under true triaxial compression tests was reflected by a determination method of the internal variable considering the influences of both σ 2 and σ 3 . For the failure process, different evolutions in cohesion and the friction angle were applied to realize different failure processes under different 3D stresses, and their 3D stress dependency was identified by the 3D brittleness index defined in this paper. The 3D stress-induced anisotropy involved in the deformation and failure of hard rocks under true triaxial compression was described via the deformation modulus evolution. The formulation for the stress-induced stiffness matrix and the model framework is fully thermodynamically consistent. The model was implemented in the 3D elasto-plastic cellular automaton system, and good agreement was achieved between the numerical simulation and experimental results, indicating that the new model can be applied to describe the failure behaviours of hard rocks under 3D stress. [ABSTRACT FROM AUTHOR]

Microwave heating and grinding experiments were conducted on samples of Hongtoushan copper ore and Dandong gold ore with different particle sizes and forms. The differences between the heating effects observed were then investigated using the electromagnetic characteristics of the bulk and powdered ore samples. The results show that the bulk copper ore experiences a high shielding effect as the microwaves are strongly reflected. The dielectric constant and reflection from a powdered sample are significantly weaker. Microwaves of frequency 2.45 GHz produce an insignificant heating and grinding effect in a cylindrical sample of Hongtoushan copper ore; the heating and grinding effect using a particulate sample is significantly better. In contrast, the heating and grinding effect obtained using a cylindrical sample of Dandong gold ore is better than that obtained using a particulate sample. The reasonable use of microwave heating to process ore is also discussed. Hongtoushan copper ore is suitable for microwave treatment after fine crushing. Dandong gold ore is suitable for microwave treatment before fine crushing. In the latter case, the application of microwaves could be moved forward to the mining process stage before carrying out the other procedures further downstream. [ABSTRACT FROM AUTHOR]

Large deformations often occur during and after the construction of deep roadways, particularly in fractured hard rock. With persistent deformation, there may be mechanistically unexplained, repeated failure phenomena—they are difficult problems to analyse and address. One strategy is to analyse the failure characteristics of roadways and internal cracking characteristics of the surrounding rock which can be analysed with a digital borehole televiewer and three-dimensional laser scanning equipment. Here, we used this approach combined with research on geostress, joint information, and mechanical properties of rock to reveal the preliminary large deformation mechanism of deep fractured roadways. Based on a cracking-restraint method for engineering design, a control technique is proposed to prevent and control large deformations. The depth and degree of cracking are considered to design parameters and determine the appropriate installation time of the support system. To test this approach, the proposed control technique was applied to a roadway project more than 700 meters deep. The monitoring results demonstrated that this control strategy effectively restrained the cracking of surrounding rocks, improved the quality index of the rock mass, and reduced surface displacement. In addition, due to the improved deformation capacity of the bolt structure, an anti-deformation and energy-absorbing bolt can bear large deformations, preventing premature bolt failures in mining environments. [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]

Deep rock masses are in true triaxial stress (TTS) fields, and few studies have examined the brittleness evaluation of rock under TTS thus far. In particular, research has not been conducted on the brittleness evaluation index and the effect of TTS. Therefore, based on a summary and analysis of pre-existing brittleness indexes, this paper proposes a brittleness index of rock under TTS that considers both stress-induced anisotropy and energy conversion characteristics during Class I behavior and Class II behavior. Meanwhile, the studied brittleness index is analyzed and verified. The brittleness of rock was found to increase with increasing σ2 while σ3 is constant; when σ2 is constant, the rock brittleness decreases with the increase in σ3. When σ3 is high, the increase in the rock brittleness amplitude with increasing σ2 is large; conversely, when σ3 is low, the amplitude is small. For the same σ3 (σ3 = 5 MPa), the order of brittleness of the three rocks is basalt > granite > marble. Finally, this paper shows that the proposed brittleness index can be applied to conventional triaxial and uniaxial stresses. In addition, under different TTS, a brittleness threshold of σ2/σ3 = 5 was found for the rocks, and when σ2/σ3 < 5, the rock brittleness enters a stress sensitive zone. The research results in this study can provide a relevant basis and guidance for the study of the brittle fracture process and the brittleness of rock under TTS. [ABSTRACT FROM AUTHOR]

True triaxial short-term and long-term tests were conducted on three different types of hard rocks, namely, marble, basalt and granite. During true triaxial long-term creep tests, the crack evolution information was collected by using an acoustic emission system. The micro-structures before and after creep tests were analysed by scanning electron microscopy and polarising microscopy. The time-dependent propagation of cracks in hard rocks is affected by applied stress and the mineral composition. The high axial stress makes the cracks propagate with time development during creep stage. When the axial stress exceeds the damage stress, the AE signals intensity have increased significantly. The damage stress values determined through short-term and long-term tests are different, but the difference is small so that the former can be used efficiently to test the damage stress in terms of long-term strength analysis. Accumulation of extensile cracks induced by high applied axial stress eventually can cause creep failure of hard rocks; however, the time-effect seems to only increase the micro-cracking inside the rock. The ultimate macroscopic failure surface of the rock sample appears to only be affected by the difference between σ2 and σ3. The time-dependent dilatancy of Jinping marble and Baihetan basalt exceeded that of granite specimens. Under creep loading, calcite and pyroxene, which have lower hardness values, are more prone to crack time-dependent expansion than feldspar and quartz, which have higher hardness values. [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]

Rockbursts, stress-induced collapse, and spalling often occur in the excavation process of deep-buried hard rock tunnel, which affect the stability thereof. The early identification of these potential high-stress hazards during tunnel excavation can provide reliable basis for the selection of preservative and controlled measures. During the excavation of a deep-buried hard rock tunnel in southwest China (a railway tunnel with a maximum burial depth over 2000 m), rockbursts, stress-induced collapse, spalling, and cracking occurred frequently. In this study, the microseismicity of various high-stress hazards in the genesis process of the tunnel was analysed. The difference of microseismicity characteristics of these high-stress hazards was revealed. A matter-element extension mathematical model was established to quantify the microseismicity of the rock mass during tunnel excavation. Based on microseismic (MS) information, an identification method of potential high-stress hazards in deep-buried hard rock tunnels was established. The method was then applied to a real case, and reliability analysis thereof performed. Thirty test case samples were selected for reliability analysis and case verification. Through practical application, it is found that 90% samples were identified correctly. The results show that the identification method of potential high-stress hazards in the deep hard rock tunnel constructed in this study is both applicable and reliable, and can identify potential high-stress hazard types in the process of tunnel excavation by using MS information. [ABSTRACT FROM AUTHOR]

Time-delayed spalling is proposed to describe the time-dependent splitting failure near the tunnel periphery, which occurs in several days or several months after excavation. This kind of failure has been frequently encountered in a deep TBM tunnel, endangering workers' security and delaying the construction schedule. In this paper, two typical time-delayed spalling cases were systematically introduced, including geological and supporting conditions. Based on field observation, microseismic monitoring and laboratory creep tests, the characteristics and mechanism of time-delayed spalling were investigated. Results showed that time-delayed spalling is a stress-induced time-dependent failure in the vicinity of tunnel surface and appears as flake or plate-like splitting fracture subparallel to the free surface. Although the failure region where time-delayed spalling happened is intact, discontinuities are scattered in the peripheral region. Time-delayed spalling is sensitive to boundary conditions and timely support is an essential method for preventing time-dependent spalling. This research is helpful to the tunnel excavation under similar geological conditions and provides foundations for the prevention of time-delayed spalling. • This paper makes an all-around introduction to the evolution process and mechanism of time-delayed spalling. • The spatial-temporal characteristics of fracture activities of time-delayed spalling were investigated. • The fundamental drivers of time-delayed spalling and the effective prevention method were analysed. [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]

The occurrence of rockburst is related to the surrounding rock stress and geological rock mass strength. Rockburst occurs frequently during the construction at a deep tunnel in southwest China (a tunnel with a depth of approximately 2080 m, and a length of 13 km), leading to casualties, damage to construction equipment, and schedule delays. The application of the rockburst criteria based on the stress-strength/strength-stress ratio shows that they are not suitable for the construction stage. This paper suggests a simple and feasible rockburst intensity criterion based on the Geological Strength Index (GSI) for the construction stage. In this criterion, the strength of the rock mass exposed during excavation is estimated by the Hoek-Brown criterion and is used to reflect the geological conditions of the surrounding rock, considering the influences of the joint conditions and engineering disturbance. The maximum principal stress along the tunnel is obtained by the in situ stress inversion. The ratio of the rock mass strength to the maximum principal stress (σcm/σ1) is taken as the evaluation index. The criterion is as follows: σcm/σ1 > 1.2, none rockburst; 0.75 < σcm/σ1 ≤ 1.2, slight rockburst; 0.4 < σcm/σ1 ≤ 0.75, moderate rockburst; and σcm/σ1 ≤ 0.4, intense rockburst. The engineering applications show the reliability of this rockburst intensity criterion. The results of this paper provide a reference for rockburst intensity evaluation during the construction stage of deep tunnels. [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 failure process can be characterized by several distinct stress thresholds: the crack closure stress σcc, crack initiation stress σci, crack damage stress σcd and peak stress σp. Seventy-one true triaxial compression tests were conducted to investigate the crack damage stress σcd, which is the stress at the onset of unstable crack development and dilation. Two empirical models are developed to predict the crack damage stress σcd using the intermediate principal stress σ2 and the minimum principal stress σ3. In addition, the proposed models are validated by ten results derived from the German Continental Deep Drilling Program (KTB) deep hole amphibolite. A comparison with other failure criteria also indicates the accuracy and simplicity of the proposed models. The proposed composite models show that the crack damage stress σcd gradually increases with increasing intermediate principal stress σ2 and minimum principal stress σ3. The physical meanings and ranges of six material parameters are analyzed and explained. Both the influence of the tangent Young's modulus E and strain characteristics on the crack damage stress σcd and the inhomogeneity of the tested specimens are discussed in this paper. [ABSTRACT FROM AUTHOR]

In this work, the effect of mechanical damage on the thermal conductivity of granite was experimentally investigated. Granite samples were artificially damaged through cyclic loading and unloading tests, with the strain evolution and acoustic emission activity monitored simultaneously. P-Wave velocity of the damaged samples was tested to evaluate the damage effect. Thermal conductivity of the undamaged and damaged granite samples was tested using optical scanning method. The influence of mechanical damage on thermal conductivity was analyzed. Thermal conductivity difference in different testing directions was analyzed based on the experiment results in six scanning lines on the two surfaces of each disk sample. The results indicate that thermal conductivity of damaged samples reduced compared with that of the undamaged samples, while the thermal inhomogeneity factor increased. Thermal conductivity increased in water-saturated condition for both undamaged and damaged samples. The increased rate of damaged samples was bigger than that of the undamaged samples. The mechanical damage treatment caused the increase in the thermal conductivity difference in different testing directions. The difference decreased when samples were water saturated. Research results in this work can provide better knowledge to the evolution mechanism of thermal conductivity of engineering rock masses. [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]

Uncertainty in the locations where rockbursts may occur with respect to a tunnel cross-section greatly increases the difficulty in monitoring and providing warnings of rockbursts and mitigating their effects. In this paper, we analyze the characteristics of 40 rockbursts that occurred in a railway tunnel in China. The rockbursts are divided into three cases depending on their position with respect to the tunnel cross-section and the prevailing geological conditions. An in situ microseismic (MS) monitoring system was established to determine the likely position of rockbursts with respect to tunnel cross-sections. The spatiotemporal evolution of the MS events that occurred during the development of rockbursts belonging to the three cases was subsequently obtained. The mechanism responsible for the changing positions of rockbursts with respect to tunnel cross-sections was further elucidated by analyzing the fracture type of the MS events and conducting numerical simulations using three-dimensional distinct element code (3DEC). Finally, support schemes, based on the monitoring and simulation results, are suggested. [ABSTRACT FROM AUTHOR]

Based on the frequent occurrence of stress-induced hazards in deep rock tunnels, a physical model experimental study of the development process, failure mechanism, and onset conditions of deep hard rock spalling in horseshoe-shaped tunnels is carried out. The test process mainly includes model preparation, loading, excavation, and monitoring. A high-speed camera, distributed fiber optic sensing, acoustic emission (AE) system, and visual imaging correlation-2D (VIC-2D) techniques are integrated to monitor the mechanical response of the surrounding rocks. The test results indicate that the physical model test successfully simulates the spalling phenomena of the deep hard rock. The integrated monitoring system successfully captures the information on deformation, AE signals, and crack propagation in the instantaneous process of the surrounding rock spalling failure, which provides a sound basis for a more comprehensive understanding of the mechanism. Microcracks and dilation appear first on the sidewall surface; the cracks then continue to propagate and coalesce, forming thin rock flakes and spalling in a lamellar manner from shallow to deep. The quantitative relationships between the onset stresses, the critical strains, and the initial boundary stresses of the spalling failure are established. At the same depth from the excavation boundary, the radial strain of the sidewall is larger than that of the crown, and it decreases away from the excavation boundary to the interior. The increasing loading in vertical direction leads to the stress concentration in the shallow part at the sidewall and a sharp increase of deformation. There is a positive correlation between the spalling failure degree and the initial boundary stresses. [ABSTRACT FROM AUTHOR]

5.2 HT ht 13 HT ht Based on Eq. (1), the maximum sin I i SB I b i sb can be obtained when HT ht . 15 HT ht. [Extracted from the article]

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]

In a tunnel, instabilities in the surrounding rock mostly occur within the sidewalls and crown. After acquiring the rock mass structure, a combination of laboratory experiments, numerical simulations, and in situ monitoring data can permit a more reasonable stability analysis of the surrounding rock and engineering support design to ensure a safer engineering project. To overcome the shortcomings (e.g., inefficiency, high labor costs, and safety risks) of traditional methods for mapping the rock mass structures of the sidewalls and crowns of tunnels, this study proposes a safe, rapid, and efficient method that can acquire a 3D digital elevation model (DEM) of the sidewalls and crown of a tunnel and the corresponding rock mass structures by using digital photogrammetry (DP). The proposed method was then tested in an engineering tunnel. Error analysis of check points and discontinuity orientations showed that the errors were within a reasonable range. The method was further applied to traffic tunnel #1 of the China Jinping Underground Laboratory Phase II (CJPL-II), and the spatial coordinates and orientations of the joints were obtained. A 3D quasi-deterministic discrete model was subsequently established by converting the coordinates and orientations of the joints from a geological coordinate system to a local coordinate system in discrete element software. The quasi-deterministic model was then used to confirm that the joint persistence has an important influence on the stability of the surrounding rock of a tunnel and, thus, affects the support installation. Finally, the joint persistence value was determined by the size of the onsite unstable block. The results of this study provide a reference for the design, construction, and support of similar deep-buried jointed hard rock tunnels. [ABSTRACT FROM AUTHOR]

A new, high-efficiency technology for fracturing and breaking rocks is required. Due to various advantages including high efficiency, energy-saving, and having no secondary pollution, the technology of microwave-induced fracturing of hard rock has been considered as a potential method for rock fracturing and breaking. Aiming at the realisation of two engineering applications: microwave-assisted mechanical rock breaking and stress release from rock masses in deep underground engineering works to prevent geological disasters caused by high-stress concentrations such as rockbursts, a novel (open-type) microwave-induced fracturing apparatus (OMWFA) for fracturing hard rocks was developed. On this basis, the two modes of microwave-induced subsurface fracturing and microwave-induced borehole fracturing of hard rocks were proposed. Due to removal of the restraint of the microwave cavity, OMWFA can be used to fracture large-size rock samples and engineering-scale rock masses. Using the apparatus, the fracturing effects of the two fracturing modes on different dimensions of cuboidal basalt samples were investigated. By combining the microwave-induced fracturing apparatus with a press machine to explore the influence of unidirectional stress on the fracturing effect of microwave treatment on basalt. Moreover, field tests were carried out on rock masses encountered in underground engineering works at Baihetan Hydropower Station in Sichuan Province, China, and the fracturing effects were evaluated by applying a digital borehole televiewer and conducting acoustic wave testing. The results show that the apparatus had favourable fracturing effects on the subsurface and borehole samples of basalt. When no stress was applied, the cracks radially expanded from the approximate centre of the radiant surface and unidirectional stress promoted fracturing. The number and depth of cracks increased with prolonged microwave exposure. After microwave treatment, the P-wave velocity of the samples declined, and the longer the microwave exposure, the more significant the reduction in P-wave velocity was. The results of field test reveal that borehole fracturing can exhibit a favourable effect around boreholes. The sound velocity around the borehole and between the boreholes both declined to some extent. Microwave-induced hard rock fracturing offers guiding significance to those exploring and developing new rock breaking and tunnelling methods, and generally enhances construction safety in deep underground engineering works. [ABSTRACT FROM AUTHOR]

• Fully coupled thermo-hydro-mechanical model for hydraulic fracturing. • Fracture propagation in mixed tensile and shear failures for 3D modeling. • Thermally induced secondary fractures for the enhancement of hydraulic stimulation. • Anisotropic model for rock deformation, fluid-heat flow, and thermal expansion. Commercial development of enhanced geothermal systems in low-permeability rocks relies on fracturing treatments to create complex-fracture networks and an appropriate circulation strategy to maintain high flow rates at sufficiently high temperatures. However, it remains challenging to model complex-fracture propagation and heat energy extraction as a whole. This paper develops a fully coupled thermo-hydro-mechanical model to simulate reservoir stimulation and heat production in naturally fractured geothermal reservoirs. The proposed model is validated against a widely used model, TOUGH2, concerning heat sweep in a vertical fracture. This model is then applied to study multi-staged fracturing and geothermal extraction related to a doublet of horizontal wells. The hydro-geomechanical properties are chosen from the Soultz geothermal reservoir at a depth of approximately 3600 m. Numerical results demonstrate that: (1) mixed tensile and shear fracturing can constitute an important stimulation mechanism for naturally fractured geothermal reservoirs; (2) well interlinked, zigzag artificial fractures between injection and production wells readily lead to channeling flow; (3) keeping a segment of horizontal wells open and placing them further apart are beneficial to the formation of sufficiently diffuse flow pathways; (4) increasing well spacing tends to improve thermal performance; however, for the case of a one-stage opening, the improvement of heat sweep efficiency is not significant; and (5) an alternating circulation scheme could achieve superior thermal performance. This study establishes an effective modeling workflow for the design and optimization of naturally fractured geothermal reservoirs, and provides an integrated modeling framework for evaluating recoverable energy potential from geothermal reservoirs. [ABSTRACT FROM AUTHOR]

Microwave energy can be used to assist mechanical rock breakage for civil and mining engineering operations. To assess the industrial applicability of this technology, microwave heating of basalt specimens in a multi-mode cavity (a microwave chamber) at different power levels was followed by conventional mechanical strength and fragmentation effect tests in the laboratory. X-ray diffraction and scanning electron microscopy/energy-dispersive X-ray spectroscopy were used to determine the mineral composition and distribution of the basalt, to aid interpretation of crack propagation patterns and the associated strength reduction mechanism. These analyses demonstrated that cracks mainly occurred around olivine grains, primarily as intergranular cracks between olivine and plagioclase grains and intragranular cracks within olivine grains. Strength reduction during microwave fracturing of basalt is driven by heat from enstatite (a microwave-sensitive mineral) and volumetric expansion of olivine (a thermally expansive mineral). Uniaxial compressive, Brazilian tensile, and point load strengths all decreased with increasing microwave irradiation time at rates that were positively related to the power level. For a given power level, mechanical strength reduction can be estimated from linear relationships with irradiation time. On the other hand, a power function best described burst time (the irradiation time at which the specimen burst into fragments) vs. power level (for a given specimen size) and burst time vs. specimen size (for a given power level) relationships. Microwave-induced hard rock fracturing can be an integral part of new methods for rock breakage and tunnel excavation. [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]

The common practice in understanding some puzzling geotechnical and geomechanical issues by large-scale three-dimensional physical model tests is recently much improved by the introduction of 3D-printing technology which can realize the reconstruction of complex geological structures. However, during 3D printing process, the regular rock-like material suffers from some general problems such as short initial setting time, water segregation, decreasing fluidity induced by chemical reaction, and some other unclear influencing factors. To promote further development of physical model via 3D printing method and to fabricate large-scale high-precision 3D geological models, in this paper, the flow characteristics of rock-like materials were first investigated using a new fluidity testing apparatus. Based on the test results, a novel 3D-printing technology of geological material was formulated. Then, the technologies of fabricating the desired structural specimens, acquiring samples with mechanical properties and cracking behaviors similar to natural rock, printing large-scale complex geological models were formulated. The results show that the 3D-printing technology of geological materials had a principle: no losing fluidity during printing time. The parameters of print head diameter, line width, line span, and line slump were the four key factors affecting desired structural samples. From the enlightenment of printing small-scale sample, the printing methods of complex large-scale geological models including decreasing printing time, acquiring heterogeneous geological model with inner structures, desired density, and surrounding smooth surface were proposed. [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]

• Underground engineering instability is associated with rock damage evolution. • The progressive fracturing in intact and jointed marble is studied. • The irreversible strains and dissipated energy are computed as damage variables. • The influence of the intermediate principle stress and joint are quantified. Rock masses in underground engineering, such as mining and deep tunnelling engineering, are often repeatedly loaded and unloaded. Here, a series of true triaxial multi-level single cyclic loading tests (σ 1 > σ 2 > σ 3) were carried out on intact and jointed marble (50 × 50 × 100 mm3) to investigate the influence of the stress state and the absence or presence of a joint on the progressive damage process. The test results show that rock damage is directional and anisotropic. The irreversible strains in the σ 1 and σ 3 directions change linearly with cumulative damage, whereas the irreversible strains in the σ 2 direction decrease nonlinearly. The dissipated energy is appropriately linearly related to the equivalent irreversible strains. The damage variables based on the irreversible strains and dissipated energy are closely correlated, as determined by their similar fitted parameters. The stress state (σ 2 and σ 3), the absence or presence of a joint and the brittleness play a vital role in the rock damage evolution. More specifically, the parameter a has a logarithmic relationship with σ 3 and a linear relationship with σ 2. The relationship between the parameter a and the joint dip angle β can be fitted by a Gaussian distribution. In addition, the fitted parameter a decreases with increasing brittleness. [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]