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At present, hydrofracture technology is mainly used for shale gas exploration and exploitation, and the effect of volume fracturing is tightly linked to the mechanical properties (including anisotropy and brittleness) of shale. Meanwhile, Sichuan Basin is also an important area for unconventional natural gas exploration and development in China. This work analyzed mineral composition and microstructure of outcrop shale matrix in Lower Silurian Longmaxi Formation through XRD test, slice observation, and scanning electron microscope (SEM). How anisotropy affects shale deformation, strength and failure characteristics at different bedding angles (0°, 30°, 45°, 60°, and 90°) under diverse confining pressures (0 MPa, 25 MPa, 50 MPa, 75 MPa, and 100 MPa) in conventional uniaxial/triaxial experiments were discussed. The acoustic emission (AE) evolution was introduced to reveal the cracks closure and propagation of Longmaxi Formation shale and qualitatively characterized the brittleness of the shale. What's more, based on the existing brittleness index, this study proposed an indicator BInew that considers the shape of the stress–strain curve before and after the peak stress to evaluate the brittleness of the Longmaxi Formation shale. The index BInew can better describe the changes of brittleness with confining pressure and bedding angle. Finally, this work elaborately explained the relationship between AE characteristics, brittleness, and failure modes of shale. AE characteristics can not only qualitatively characterize shale brittleness but also show the failure process and modes of shale samples. Change of failure modes rests with the difference in brittleness and also reflects the level of brittleness. The change of brittleness with failure modes can be expressed as shear along bedding plane (Sh-Al) > splitting through bedding plane (Sp-Th) > conjugate shear failure (Co-Sh) > shear along bedding plane (Sh-Al) > shear through bedding plane (Sh-Th). Highlights: Uniaxial/triaxial compression and acoustic emission (AE) monitoring of Longmaxi formation shale were carried out. Influence of anisotropy of shale on mechanical properties under different confining pressures was analyzed. Different brittleness indexes were used to evaluate the brittleness of shale, and a new brittleness index was established. Relationship between AE characteristics, brittleness, and failure modes of shale was discussed in detail. [ABSTRACT FROM AUTHOR]

Studying the frictional process, acoustic emission (AE) characteristics and permeability evolution of fractures under water injection is fundamental for proposing theories and methods to control the stability of geological structures. In this work, the shear-flow test was conducted on Longmaxi shale fractures with different specific roughness to analyze the frictional deformation and mechanical properties of shale fractures during the frictional slip process, and the influence of effective normal stress and surface morphology was investigated. AE monitoring technology was also utilized to reveal the AE evolution during the frictional slip process. Based on the AF/RA values, AE signals were subjected to cluster analysis using the Gaussian Mixture Model (GMM) to distinguish and quantify the crack types and proportions during frictional slip. Combining the development of cracks and considering the ratio of worn area, a theoretical analytical expression was established to reasonably describe the friction-damage evolution, classifying the friction-damage during the frictional slip process into tensile and shear damage, respectively. Meanwhile, the permeability evolution of Longmaxi shale fractures was also studied in detail. A theoretical model capable of describing and predicting the permeability evolution was proposed under the condition that the permeability evolution and damage process of Longmaxi shale fractures correspond to each other. Finally, the interaction between asperities on shale fractures with different surface morphologies during the frictional slip process was discussed, and the influence mechanism of surface morphology on permeability evolution was revealed. • There are differences in the post-peak frictional slip stages between Longmaxi shale sawcut and rough fractures. • AE evolution of Longmaxi shale fractures corresponds to the frictional slip stages. • The expression derived based on the development of cracks can describe the friction-damage evolution. • The theoretical model established based on the friction-damage evolution can predict the permeability evolution. • The interaction between asperities can explain the permeability evolution and the influence of surface morphology. [ABSTRACT FROM AUTHOR]

The frictional stability of rocks is fundamental in controlling the instability and activation of faults. Given the strong correlation between seismic events in the Sichuan Basin of China and hydraulic fracturing for shale gas extraction, we conducted a series of velocity-stepping friction experiments with Longmaxi shale from the Changning shale gas extraction block under fluid injection. The experimental results indicate that the frictional process of Longmaxi shale can be divided into four distinct stages and the effective normal stress can demonstrate the influence of injection pressure. Additionally, the phenomenon of a decreasing friction coefficient μ with increasing effective normal stress can be explained by the frictional adhesion theory and a competitive mechanism between the deformation of asperities and microscopic dilatant slip. As the injection pressure increases, the friction state of Longmaxi shale transitions from velocity-strengthening to velocity-weakening, which is beneficial for potential seismic slip. After friction, the fracture surface experiences wear, and the surface morphology of the left and right parts exhibits a symmetrical distribution. With increasing effective normal stress, the morphological evolution of fractures becomes more prominent, which is related to the variation in the effective contact area. Based on the experimental results, we propose that the uneven distribution of fluid pressure on the Longmaxi shale fracture may also contribute to the transformation of frictional stability. When studying the frictional properties of rocks under fluid injection, it is also necessary to focus on the evolution of fluid pressure distribution on the fracture surface during the frictional process. • The frictional process of Longmaxi shale under fluid injection can be divided into four stages. • Pressurized fluid can accelerate the transition of Longmaxi shale from the V-strengthening state to the V-weakening state. • The morphological evolution of Longmaxi shale fracture is related to the variation in the effective contact area. • Uneven distribution of fluid pressure is also a presumable factor changing the frictional stability of Longmaxi shale. [ABSTRACT FROM AUTHOR]

• Mechanical responses and failure characteristics of Longmaxi Formation shale were revealed. • Acoustic emission (AE) evolution of Longmaxi Formation shale during triaxial compression test was described. • Micro-mechanism of improving ductility by high confining pressure and real-time high temperature is different. For the safe and effective extraction of deep shale gas, it is essential to analyze the mechanical responses and failure characteristics of shale under real-time high temperature and high-stress coupling. Herein, triaxial compression experiments on Longmaxi Formation shale were carried out with various bedding angles under different confining pressures and real-time temperatures. The experimental results demonstrate that high confining pressure can transform the mechanical performance of shale from brittle to plasticity. Real-time high temperatures not only can aggravate this transition process but also cause discontinuous micro-rupture before shale failure. The acoustic emission (AE) evolution of shale under various confining pressures and real-time temperatures is generally similar and can correspond with the division of stress-strain stages. However, changes in confining pressure and real-time temperature have opposite effects on AE activity rate and associated parameters. In the deep environment, the real-time high temperature can also drive additional pores and microcracks, which change the microstructure of shale samples. Under high confining pressure, the impact of real-time high temperature on macroscopic failure modes diminishes, resulting in shale samples typically exhibiting the single shear failure mode. Furthermore, high confining pressure and real-time high temperature have different micro-mechanisms for improving shale ductility, as evidenced by their distinct effects on AE evolution. [ABSTRACT FROM AUTHOR]

To clarify the shear-slip process and characteristics of fault, we conducted triaxial shear tests driven by displacement on shale samples with prefabricated sawcut and natural fractures, and the acoustic emission (AE) technology was used to monitor the shear-slip process in real-time. According to the test results, the shear-slip process and mechanical characteristics of sawcut and natural fractures were analyzed. The test results indicate that the shear-slip process of fractures can be divided into four stages, improving the confining pressure will cause more serious plastic failure in the shear-slip process, and the increase of roughness will make the phenomenon of "stress drop" more obvious, which is closely related to the earthquake occurrence. Confining pressure and roughness all have implications for the stress thresholds of each stage. With the transition of shear-slip stages, the effects of confining pressure and roughness on the shear-slip of fractures are more drastic. The maximum static friction coefficient μ 0 increases with initial roughness increasing and is independent of confining pressure. Due to the strain-strengthening effect of fracture slip, the average value of dynamic friction coefficient μ d under simple shear deformation is greater than the maximum static friction coefficient μ 0. Based on AE response characteristics, microcracks development, and rupture scale during the shear-slip process were also explored. AE events are mainly distributed along the inclined fracture. The AE response in different shear-slip stages displays pronounced differences, and the AE evolution and macro mechanical characteristics of fractures show significant time-synchronization. The variation of peak frequency can reflect the rupture scale transformation and the influence of confining pressure and roughness. Additionally, the differences in the morphological evolution of sawcut and natural fractures are mainly reflected in the wear pattern, degree and the rock debris attach amount. Finally, we also discussed the micro-deformation mechanism of asperities during the shear-slip process. This work may provide a reference for the research of fault activation and slip in shale reservoirs. • Triaxial shear tests driven by displacement on shale samples with prefabricated sawcut and natural fractures were carried out. • Shear-slip process and acoustic emission (AE) evolution of sawcut and natural fractures in shale were analyzed. • Morphological evolution and characteristics of fracture surfaces before and after the test were revealed. • Micro-deformation mechanism of asperities during the shear-slip process was clarified. [ABSTRACT FROM AUTHOR]

• Triaxial compression coupled with permeability tests of limestone were carried out. • Mechanical properties of limestone under stress-seepage coupling were analyzed. • Effects of gas pressure and confining pressure on the mechanical properties, permeability and failure modes were studied. • Relationship between cracks propagation and permeability evolution of limestone were clarified. Limestone rock widely exists in various rock engineering, its mechanical behavior and permeability evolution have a great influence on the safety and stability of related engineering projects. To study the mechanical behavior, permeability and failure modes of limestone rock under stress-seepage coupling, triaxial compression coupled with permeability tests were carried out. Experimental results reveal that the failure of limestone still conforms to the Mohr-Coulomb criterion under stress-seepage coupling, and effective confining pressure σ 3 eff can indicate the effect of gas pressure P G and confining pressure σ 3 on the deformation parameters of limestone samples. Peak stress σ P and residual stress σ R increase with the enhancement of confining pressure and decrease with the ascent of gas pressure. Permeability evolution of limestone under different gas pressures and confining pressures is generally similar, and the trend of initial permeability k I , peak permeability k P and residual permeability k R decreasing with effective confining pressure σ 3 eff can be expressed by the exponential function. The increase of confining pressure and decrease of gas pressure will retard the acoustic emission (AE) response and inhibit the propagation of microcracks. More importantly, limestone samples always show composite failure modes dominated by tensile cracks under low confining pressure, while under high confining pressure, the limestone samples show shear failure modes. The difference is limestone samples are composed of tensile microcracks on the microscale under stress-seepage coupling, and the proportion of shear microcracks grows as gas pressure P G and confining pressure σ 3 increases. This research can provide a basic reference for relevant engineering applications. [ABSTRACT FROM AUTHOR]

This paper focuses on analyzing the mechanical properties, permeability and energy characteristics of limestone from coal measures strata of through-coal seam (TCS) tunnel in the context of the Taoziya tunnel as the engineering background. The improved MTS815 Flex test GT rock mechanics system and acoustic emission (AE) technology were utilized for the triaxial compression with permeability test of limestone from coal measures strata of TCS tunnel. The results indicate that gas pressure and confining pressure significantly affect the mechanical properties and permeability of limestone from coal measures strata of TCS tunnel. The gas pressure promotes the development and propagation of cracks, whereas increasing confining pressure strengthens the resistance of limestone samples to crack propagation. When limestone samples transition from the initial compaction stage to the unstable cracks propagation stage, the possibility of gas leakage during tunneling in the coal measures strata should be recognized, and the stability of surrounding rock should focus on the failure stage. In the process of progressive failure of limestone samples, AE events with larger amplitude are distributed near the main macroscopic fracture zone, and the distribution of AE events closely corresponds to the macroscopic failure mode of limestone samples. Exponential functions can be used to describe the trend of initial permeability k i , peak permeability k p and residual permeability k r decreasing with effective confining pressure σ e. The closure and propagation of microcracks are the fundamental reason for the corresponding relationship between the stress variation process, AE characteristics and permeability of limestone samples. Finally, the energy characteristics of limestone under triaxial compression were analyzed based on the stress-strain curves of limestone. This study may give fresh insights and necessary references to ensure the safe construction and development of TCS tunnels. • Mechanical properties of limestone from coal measures strata of TCS tunnel were studied. • Initiation and propagation of cracks in limestone samples were studied. • Relationship between cracks development and permeability of limestone was revealed. • Energy characteristics of limestone under triaxial compression were clarified. [ABSTRACT FROM AUTHOR]

• Roadways deformation with deep soft surrounding rock is serious and difficult to control. • The full-section layer-double arch synergy reinforcement technology was proposed. • The mechanism of layer-double arch synergy reinforcement technology was shown. • The deep soft rock roadway can be effectively controlled by the composite support scheme. In order to research the surrounding rock control technology of deep soft rock roadway, the air returning cross-cut of Xinhu Coal Mine in Bozhou City was taken as an example to study the failure characteristics, roadway deformation mechanism and surrounding rock repair technology. Before the surrounding rock of the roadway is repaired, the roof of the roadway sink seriously, the floor heave is obvious, and the surrounding rock strength is low, showing characteristics of soft rock. Therefore, The composite support method of " shotcreting + grouting anchor bolt + anchor bolt + grouting anchor cable + anchor cable " is proposed by the full-section layer-double arch synergy reinforcement technology. The technical parameters of the scheme are determined by theoretical analysis, engineering analogy and engineering practice. The numerical simulation is adopted to verify the rationality of the layer-double arch collaborative reinforcement technology. And, the on-site monitoring results show that the deep soft rock roadway can be effectively controlled by the composite support scheme, which can provide reference value for other similar engineering. [ABSTRACT FROM AUTHOR]

The behavior of fluid flow through rock fractures in the form of radial flow is a critical issue in various subsurface geological engineering projects. Previous work has considered to couple the radial seepage flow and the normal stress on the fracture surface, while ignored the influence of the fracture surface morphology on the seepage. Therefore, to quantitatively assess the fracture morphology influences on radial permeability, we conducted research adopting seepage testing and fracture surface scanning experiments and the tests results were present in this paper. The difference about fracture surface morphology of each sample can be quantitatively described by fractal dimension. Compared with natural fractures, artificial fractures had larger fractal dimension, rougher surface, and lower radial permeability. And the average radial seepage flow of natural fractures was about 1.57 times that of artificial fractures. In the same sample, the single hole permeability of different outlet holes was also different, which was related to the fracture surface morphology and its fractal dimension of the section line in this direction. The magnitude of fracture radial seepage flow increased with decreasing fractal dimension, and the change law of both followed a negative exponential function. A similar change tendency appeared in both artificial fracture samples and natural fracture samples. [ABSTRACT FROM AUTHOR]

Understanding the long-term creep mechanical behavior of granite is critical to evaluating the safety of the nuclear waste repository. It is unreasonable to extrapolate the creep test results of hard rock samples at high stress to lower stress. In this work, the uniaxial creep test of granite lasting for 1117 days is carried out at lower axial stress levels (60 and 87 MPa) to reveal its long-term time-dependent deformation characteristics. The test results show that under 60 MPa and 87 MPa, the creep deformation of granite accounts for only 21.7% and 36% of the total deformation, while that of salt rock is about 80%, making the creep of hard rock difficult to be detected. The steady creep rate of granite is 9.14 × 10−12 s−1, which is 1–2 orders of magnitude slower than that of salt rock under low stress, and 3–6 orders of magnitude slower than that under short-term granite creep test. At the same stress ratio of about 0.45, it takes 47.6 days for granite samples to enter the steady creep stage, which is much shorter than 213 days for salt rock samples. In the short-term (several hours or days) creep test under low stress level, the hard rock sample does not actually enter the steady creep stage. Based on the fractional derivative theory and damage mechanics, a novel nonlinear creep-damage constitutive model that can well describe the long-term brittle creep characteristics of granite is proposed. This research reveals the very long-term time-dependent deformation behavior of hard rock, which is conducive to the evaluation of long-term stability of nuclear waste repositories. [ABSTRACT FROM AUTHOR]

Previous prediction models have faced challenges in characterizing the three-dimensional (3D) surface subsidence of salt caverns with different shapes. In order to overcome these limitations, this study proposes a novel theoretical model based on stochastic medium theory. This model provides a theoretical formula for characterizing the 3D surface subsidence of salt caverns with different shapes (oblate spherical, spherical, and vertical ellipsoidal). By employing the Particle Swarm Optimization (PSO) algorithm to address the theoretical model, it can generate a three-dimensional settlement surface and subsequently compare it with the numerical results. The results demonstrate a strong consistency between the theoretical and numerical results. By conducting the parameter sensitivity analysis, it becomes evident that the shape of salt caverns holds paramount significance and should not be disregarded in the study of 3D surface subsidence. As the main influence angle increases and the burial depth decreases, the maximum settlement value of the salt cavern continues to increase while the maximum influence radius continues to decrease. The maximum settlement value and maximum influence radius of salt caverns gradually increase with the increase of the span. This suggests that the proposed theoretical model can effectively and intuitively describe the 3D surface subsidence characteristics of salt caverns. • A theoretical model of 3D surface subsidence has been proposed based on stochastic medium theory. • The proposed model can accurately predict 3D surface subsidence of salt caverns with different shapes. • The proposed model can effectively describe the 3D surface subsidence characteristics of salt caverns. [ABSTRACT FROM AUTHOR]

The mechanical model of the basic roof fracture structure is established on the basis of key block theory to study the roof breaking mechanism of gob-side entry retaining under roof cutting and pressure relief, and the analytical formula of roof support resistance is derived when the key block of the basic roof is stable. The influence of roof cutting angle and cutting height on roof support resistance is also analyzed. Determining the cutting seam parameters of the retained roadway roof is necessary to identify the support resistance of the roadway roof due to the correlation between the roof cutting parameters and the support resistance. Taking the II 632 haulage drift of the Hengyuan coal mine as the engineering background, FLAC3D numerical simulation is used in this paper to analyze the influence of different roof cutting angles and cutting heights on the surrounding rock structure evolution of retained roadways. Results show that the roof cutting angle and cutting height respond to the support resistance of the retained roadway roof, and the support resistance required by the roof increases with the roof cutting angle and cutting height. This condition ensures that the side roof of the gob can be cut off smoothly, and the support resistance required by the roof of retained roadways is within a reasonable range. Through theoretical and numerical simulation analysis, the reasonable roof cutting height of II 632 haulage drift is 8 m and the roof cutting angle is 15°. The theoretical analysis and numerical simulation results reveal that the required support resistance to maintain the stability of the roadway roof is 0.38 MPa. The supporting scheme of the roof of the II 632 haulage drift in the Hengyuan coal mine is then designed. Finally, the field industrial test is used for verification. The borehole imaging results show that the overall line of the retained roadway roof is small based on the description of field monitoring results. The deformation of the surrounding rock surface of the retained roadway is less than 100 mm, and the roadway is 40 m from the lagging working face. The deformation rate of surrounding rock decreases with the increase in distance from the working face. The integrity of the retained roadway roof is good, and the deformation of the surrounding rock is effectively controlled. [ABSTRACT FROM AUTHOR]

Hydraulic coupling often leads to progressive rock failure accidents. Mechanical tests were performed over a range of combined pore water pressure and confining pressure stress path conditions to study the progressive failure characteristics of sandstone under hydraulic-mechanical coupling and explore the crack initiation and pore water fracture mechanism. The closure stress and damage stress were determined by the axial deformation stiffness and volume deformation stiffness. The experimental results indicate that confining pressure is the main controlling factor in the crack propagation stage, and pore water pressure enhances crack evolution. With increasing effective confining pressure, the effective peak deviatoric stress strongly increases and the characteristic stress increases linearly. The initiation stress and damage stress decrease with increasing pore water pressure. The moduli in stages I, II, and III are similar to the law of the transverse and radial deformation ratio with notable differences in stage IV. The fracture trend angle was determined by the ratio of axial crack strain and radial crack strain. Compared with the experimental results, the internal cracks in the sandstone samples are mainly type-II cracks, and type-I cracks are also locally present. After stress damage, the cracks expand and extend at an angle close to the real fracture. [ABSTRACT FROM AUTHOR]