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To investigate the statistical laws of acoustic emission energy (AEE) avalanche dynamics of sandstone under varying fracture lengths and dip angles, as well as to determine the relationship between acoustic emission (AE) parameters and damage variables, we studied the mechanical properties and AE characteristics of sandstone with a single fracture subjected to uniaxial compression with the aid of the Shimadzu AG-IS test system and the PCI-2 AE system. The AEE characteristics of fractured sandstone under load were analyzed based on the statistical method of avalanche dynamics, with emphasis on AEE distribution, aftershock sequence, and waiting time distribution. The Weibull distribution function that incorporates a correction coefficient β was employed to optimize the Weibull parameters based on the strain equivalent hypothesis theory, which led to the establishment of a statistical damage constitutive model for fractured rock. The results indicated that the peak strength of rock samples initially decreases and then increases with increasing fracture dip angle, and decreases with increasing fracture length. The AEE of the rock sample followed a power-law distribution, with a power-law index ranging from 1.62 to 1.81, which was independent of fracture length and dip angle. The aftershock sequence also adhered to a power-law distribution, whereas the waiting time exhibited a multi-power-law distribution. The proposed statistical damage constitutive model aligned more closely with the experimental curves compared to previous models that normalizes AEE. [ABSTRACT FROM AUTHOR]

To investigate the changes in the strength and deformation of the blast load-damaged sandstone roof plate under cyclic loading and unloading conditions at different confining pressures, a triaxial loading device was used to carry out graded cyclic unloading tests on specimens with different degrees of damage, and the test results were summarized. The effects of blast-load-induced damage, confining pressure and loading stage on the strength, cohesion, internal friction angle, residual strain and volumetric strain were analyzed. (1) Compared with that of the undamaged specimen at a confining pressure of 0 MPa, the peak stress reductions in the vibration-damaged and blast-damaged specimens were 4.93% and 9.04%, respectively. The peak stress, cohesion and internal friction angle characteristics of the undamaged specimens under triaxial cyclic loading and unloading and conventional triaxial loading were greater than those of the vibration-damaged specimens. In addition, the corresponding parameters of the vibration-damaged specimens were greater than those of the blast-damaged specimens; thus, the damage induced by the blast load reduced the ability of the rock body to resist external loads. (2) The peak stresses of the specimens with different degrees of damage increased linearly with increasing confining pressure, and the presence of confining pressure restricted the deformation of the rock body and enhanced the ability of the specimens to resist external loads. (3) Under cyclic loading and unloading, the first cycle generated the largest residual stress variable. When the confining pressures were 0, 5, 10, 15 and 20 MPa, the levels of plastic deformation of Grade I cyclic loading and unloading specimens (the amounts of total deformation and peak strain of specimens under different numbers of loading and unloading cycles) were 4.19, 4.30, 4.75, 5.11 and 5.43%, respectively. An increase in the confining pressure enhanced the deformation and failure resistance characteristics of the specimens. (4) Compared with those under conventional triaxial loading, specimens under cyclic loading and unloading failed faster after reaching the ultimate volumetric strain, and specimens damaged by blasting were more likely to fail. During deep coal mining, the bearing capacity and deformation resistance of rock masses damaged by blasting after repeated disturbance loads were greatly reduced. Therefore, the safety and stability of the surrounding rock should be strictly monitored when applying blasting pressure relief technology to avoid safety accidents. [ABSTRACT FROM AUTHOR]

To study the influence of the spatial distribution and structure of multi-scale cracks on the mechanical behavior of rocks, triaxial compression tests and cyclic triaxial complete loading and unloading tests were conducted on sandstone, with real-time wave velocity monitoring and CT scan testing. The quantitative classification criteria for multi-scale cracks on sandstone were established, and the constraint effect of confining pressure was analyzed. The crack with a length less than 0.1 mm is considered a small-scale crack, 0.1–1 mm is a medium-scale crack, and larger than 1 mm is a large-scale crack. As the confining pressure increases, the spatial fractal dimension of large-scale cracks decreases, while that of medium-scale cracks increases, and that of small-scale cracks remains stable. The respective nonlinear models of the aspect ratio were established with the length and density of multi-scale cracks. The results indicate significant differences in the effects of cracks of different scales on rock damage. The distribution density of medium-scale cracks in the failed specimen is higher, which is the main reason to produce damage. The small-scale cracks mainly originate from relatively uniform initial cracks in rocks, mainly distributed in medium-density and low-density areas. The results of this research provide important insights into how to quantitatively evaluate the damage of rocks. [ABSTRACT FROM AUTHOR]

To investigate the mechanical and energy evolution characteristics of fractured rock under true triaxial stresses, true triaxial strength compression experiments on fractured sandstone were conducted with varying crack lengths and widths. The results indicate that under true triaxial stresses, the peak stress of the rock exhibits a gradual decline with an increase in crack length and width. Meanwhile, crack initiation stress and crack damage stress of fractured sandstone also demonstrate a declining trend overall, and the influence of crack length on the characteristic stress (crack initiation stress and crack damage stress) of sandstone is more pronounced than that of crack width. According to the energy analysis results, the total strain energy of fractured sandstone gradually decreases with an increase in crack length and width. The results offer a theoretical foundation for the strength assessment and stability management of fractured rock materials during deep coal mining operations. [ABSTRACT FROM AUTHOR]

In order to investigate the influence of shear on contact characteristics and fluid flow evolution of rough rock fractures, a series of shear-flow tests were carried out by numerical experiments. Firstly, a sandstone specimen with a rough fracture was made in the laboratory, and the numerical model of the fracture was reconstructed in FLAC3D software. Experiments were conducted to investigate the depth of penetration of the fracture under different normal stress (1, 3, and 5 MPa) and shear displacement (2, 4, 6, 8, and 10 mm). By establishing the relationship between the shear direction and the apparent inclination of the fracture asperity, the effects on the asperity contact characteristics and seepage properties are explored. The results of the study indicated that the larger the normal stress the larger the surface contact section and the smaller the aperture, showing the opposite trend with increasing shear displacement. Positive apparent inclination distribution can effectively predict the location of shear damage during the shear process. Negative apparent dip implies that those regions increase permeability after shear, and decrease with increasing crack opening. Fracture seepage shows obvious nonlinear characteristics, where the nonlinear coefficients are 5–6 orders of magnitude larger than the linear coefficients. The critical Reynolds number Rec is used to distinguish the linear and nonlinear categories of fluid flow, and the calculated results show that the range of Rec is between 0.0081 and 0.11.3, and the Rec increases with shear displacement and decreases with normal stress. [ABSTRACT FROM AUTHOR]

Fracture effectiveness plays a key role in gas productivity of ultra‐deep tight sandstone reservoirs, Kuqa depression, Tarim Basin. Based on cores, thin sections, well logging, well testing and production data, the study evaluated fracture effectiveness and illustrated its impacts on gas productivity. High‐angle and vertical shear fractures are the most important types. Distribution of effective fractures shows great heterogeneous. Fracture effectiveness is influenced by tectonism, diagenesis and in‐situ stress. Earlier fractures or fractures in close to gypsum rock are easier to be filled. Completely filled fractures can be reopened under late tectonism. Dissolution improves local fracture effectiveness. Minerals spanning fracture surfaces protect fracture effectiveness from late compression. Fractures filled with calcite can be activated by acidification. Effective fractures parallel to maximum horizontal principal compressive stress direction show larger aperture. Overpressure can decrease the effective normal stress to maintain fracture effectiveness. With exploitation, decline in pore pressure reduces fracture effectiveness. Linear density, aperture, and strike of effective fractures influence gas productivity. Effective fractures greatly enhance matrix permeability. Therefore, more abundant and larger aperture fractures are always corresponded to higher productivity. However, effective fractures also facilitate late water invasion, especially, both mutually parallel. Intense water invasion leads to rapidly declines in productivity. [ABSTRACT FROM AUTHOR]

Cracked rock masses in cold regions are subjected to freeze–thaw cycles over extended periods, resulting in freeze–thaw deformation. The combined effects of freeze–thaw cycling and the depth of cracks significantly influence the stability and durability of underground rock engineering in these regions. In some cold regions with minimal annual rainfall, rock masses are unable to absorb external water during freeze–thaw cycles. As freeze–thaw deformation progresses, the rock transitions naturally from a saturated state to an unsaturated state. To investigate the deformation damage mechanisms and evolution patterns of saturated red sandstone with initial non-penetrating cracks of varying depths (20 mm, 30 mm, 40 mm) under freeze–thaw cycling conditions without external water replenishment and with naturally varying saturation levels, relevant freeze–thaw cycle experiments and strain monitoring were conducted. The results indicate that cracked red sandstone experiences residual strain in each freeze–thaw cycle, which gradually accumulates, leading to irreversible freeze–thaw damage deformation. The cumulative residual strain of the rock specimen after 45 freeze–thaw cycles was 40.69 times greater than the residual strain from the first cycle. Additionally, the freeze–thaw strain characteristic values exhibited a clear correlation with crack depth. These findings provide experimental methods and data references for analyzing the deformation and failure mechanisms of cracked rock induced by freeze–thaw damage in cold regions. [ABSTRACT FROM AUTHOR]

The onset of brittle failure in rocks includes dilatancy and strain localization. To better understand this nucleation process, we analyze the evolution of the local three‐dimensional strain tensor using X‐ray tomograms acquired during triaxial compression experiments on granite and sandstone. The onset of the localization of the compaction, dilation, and shear strain occurs when ∼65% of the rock volume experiences dilation. Tracking the locations of the high strains throughout loading suggests that the deformation that occurs early in loading influences the location of the system‐sized fracture network that produces macroscopic failure. This influence is larger in the sandstone experiments than the granite experiments, likely due to the microstructure of the sandstone. These results have important implications for detecting precursors to catastrophic failure. Plain Language Summary: We investigate the fundamental processes that lead to brittle failure in rocks. We deform two common types of crustal rocks, granite and sandstone, under upper crustal stress conditions. As the stress applied to the rock increases, the rock tends to expand (dilate) more than compact, particularly as it approaches catastrophic, macroscopic failure. A larger portion of the rock undergoes dilation when the strain field starts to localize, indicating that accelerating dilation is a precursor to macroscopic failure. We observe different localization patterns in the rocks: in sandstone, strain localization progresses monotonically with increasing stress, whereas phases of delocalization can occur in the granite. Two competing models describe the development of the system‐sized fracture network that produces macroscopic failure: the network develops from (a) the coalescence of fractures that form early in loading, or from (b) the propagation of a process zone of interacting fractures through relatively intact rock. We find that the high strain events persist at the same location throughout the experiments more than expected by chance, particularly in experiments on sandstone. The results provide perhaps the most robust experimental confirmation yet that the fracture network that causes macroscopic failure evolves from the deformation that occurs earlier in loading. Key Points: X‐ray tomography quantifies the evolving spatial evolution of high strain events during brittle failureAt the onset of strain localization, on average 65% of the volume of the rock cores undergo dilationLocalized zones of high strain events persist in space from the onset of loading [ABSTRACT FROM AUTHOR]

To investigate the deformation and failure characteristics of loaded sandstone under the influence of fracture dip angles and characterizing the propagation patterns of wing cracks in fractured rocks, the stress-strain characteristics and failure modes of sandstone samples with different fracture dip angles were analyzed through uniaxial compression tests. A wing crack propagation model in fractured rocks was established based on relevant theories of fracture mechanics. Further, a rock strength calculation method that considers the influence of fracture dip angles was proposed by combining the M-C strength criterion and the Box-Lucas1 function and validated by experimental data. Results show a close match between theoretical and experimental results. As the distance between the wing crack initiation position and the fracture end increases, the fracture dip angle increases while the wing crack initiation angle decreases first, then increases, and finally shows a single descending trend. When the fracture dip angle increases, the longitudinal difference between the peak strength curves first rises and then drops. The conclusions obtained in this study provide theoretical basis and technical support for the stability assessment and engineering application of fractured rocks. [ABSTRACT FROM AUTHOR]

Water plays a crucial role in underground engineering, significantly influencing the short-term and long-term strength and deformation behavior of rocks. Understanding the mechanics of rock deformation under water–rock coupling is vital for assessing the stability of underground structures. In this study, short-term strength compression and creep experiments were conducted on sandstone samples with varying water contents. Critical water contents (0%, 0.8%, 1.6%, 2.4%, and 3.3%) were determined through water absorption experiments, with corresponding tests performed under confining pressures of 0 MPa, 6 MPa, 9 MPa, and 12 MPa. Results revealed an exponential decrease in uniaxial compressive strength and elastic modulus with increasing water content, while confining pressure influenced radial deformation, leading to a linear increase in compressive strength. Water content also significantly impacted micro-cohesion and internal friction angle, with cohesion being more sensitive to water. The creep experiment demonstrated a deformation trend similar to compressive strength, with analysis revealing three stages of creep deformation: viscoelastic, plastic, and nonlinear acceleration. Critical strengths entering different stages were identified, with their ratio to compressive strength stabilizing between 0.44–0.59 and 0.59–0.74. A strain parameter εac, related to steady-state and accelerated creep failure time, showed little correlation with water content but exhibited a linear relationship with confining pressure. Short-term and long-term failure modes of sandstone were analyzed, indicating a transition from "X-shaped" conjugate shear failure to single oblique shear failure with changing confining pressure and water content. Mitigation measures, such as groundwater isolation and the application of high confining pressure, were identified as effective means to mitigate long-term rock deformation's adverse effects on structural integrity in onsite engineering projects, ensuring structural stability throughout construction. The study's findings and data are crucial for informing the long-term stability of engineering structures. Highlights: Water content significantly affects the short-term and long-term strength and deformation behavior of rocks. The creep behavior of rocks is influenced by water content, confining pressure, and shear stress. The water content and confining pressure affect the formation and evolution of cracks in rocks during creep. Different failure modes are observed in rocks, depending on water content and confining pressure. [ABSTRACT FROM AUTHOR]

With the development of large-scale tunnel construction and green energy mining toward the depths of the Earth, the study of the mechanical properties and the deformation, and fracture characteristics of the surrounding rock mass under high confining pressures and high temperatures has become an urgent requirement. Therefore, we compare and study the effects of different high temperatures and confining pressures on the mechanical behavior and fracture characteristics of rock samples under loading and unloading conditions. In this paper, uniaxial and triaxial compression tests of sandstone at high temperature, and the stress–strain curves of red sandstone under different temperatures and confining pressures, are obtained. On this basis, the characteristics of energy conversion in the process of rock deformation and failure are studied. The research results indicate that the high-temperature effect weakens the brittle characteristics and enhances the plasticity of red sandstone, and there is a certain lag in the time of rock failure relative to the stress reaching peak strength. The high confining pressure weakens the brittleness and enhances the plastic properties of red sandstone, enhancing the overall resistance of the rock to deformation and enhancing its ability to store elastic deformation energy. According to the law of energy evolution, a new strength criterion is established to describe the three-dimensional spatial critical state surface of macrofailure caused by rock fracture coalescence. The study of the mechanical properties and failure criteria of rocks under high temperatures and pressures from the perspective of energy not only has important value for deep resource extraction but also has important theoretical significance for deep underground engineering construction. [ABSTRACT FROM AUTHOR]

Roof fall in underground coal mines is a time-dependent process involving various behaviors such as intact and failed rock, strain-softening, and strength degradation. Current laboratory tests have observed these behaviors, but no comprehensive model represents all of these critical behaviors. To investigate the time-dependent manners of intact and failed sandstone rocks under unconfined conditions using unconfined relaxation testing in underground coal mines in Iran. Unconfined compression and relaxation tests were conducted on sandstone specimens to study the time-dependent behavior of intact and failed sandstone rocks. Two tests were conducted, with one group initiating relaxation examinations in the pre-failure region and the other in the post-failure region. The results show that intact and failed rock specimens' relaxation behaviors differ. The stress relaxation curves in the pre-failure area exhibit a smooth stress relaxation behavior. In contrast, those in the post-failure region exhibit stepped behavior with sudden and significant stress drops. The viscous parameters defined at distinct failure conditions did not show significant differences, and their average represents the time-dependent properties of failed rock. The developed model, which combines the Burgers and strain-softening models, can accurately describe the time-dependent strain-softening behavior of rocks and simulate laboratory tests. This study demonstrates the feasibility of using unconfined relaxation testing to determine the time-dependent properties of intact and failed rocks under unconfined conditions. The developed model provides a comprehensive representation of the time-dependent behavior of rocks, including strain-softening, strength degradation, and viscous behaviors. [ABSTRACT FROM AUTHOR]

This study proposes a new algorithm to enhance the accuracy of measuring internal deformation, identify and quantify damage in rocks using digital volume correlation (DVC). This algorithm integrates the multimesh refinement technology into a mechanically regularized global DVC algorithm that incorporates nodal force equilibrium. The accuracy of different DVC algorithms was assessed through scanning experiments on particles with different sizes and digital virtual fracture models. The internal deformation and damage patterns of red sandstone were quantitatively analyzed using in-situ CT scanning during uniaxial compression experiments. The results indicate that the introduction of the mechanical regularization term reduced the measurement uncertainty of the global DVC method by 1-2 orders of magnitude. For the red sandstone specimens, the new algorithm was able to identify sub-voxel microcracks with an aperture of 0.15 voxels, while reducing the computational cost by 85.15%. The damage quantification analysis revealed that newborn cracks with a maximum aperture of 0.32 voxels occurred in the specimen when the axial stress reached 50.58% of its peak value, and these microcracks evolved into voxel-scale fractures when the stress increased to 84.27% of the peak value. The new algorithm not only improves the computational efficiency, but also demonstrates a strong capability in identifying sub-voxel scale newborn cracks, providing a novel method for quantifying internal deformation and damage in rocks. [ABSTRACT FROM AUTHOR]

[Objective] The hydraulic coupling behavior of rocks is closely related to the safety and stability of various rock engineering projects, including tunnel excavation, dam construction, oil and gas extraction, and rock slope stability analysis. The failure of rock masses and the evolution of permeability are part of a gradual dynamic process that is interrelated. On the one hand, the external load applied to the rock mass alters its microphysical, macrophysical, and mechanical properties, thereby affecting its permeability. On the other hand, changes in permeability can alter the distribution of permeability pressure, affect the effective stress within the rock mass, and consequently influence its stability. Additionally, the rock mass contains numerous fractures and fissures resulting from diagenetic processes, tectonic movements, and excavation disturbances, which complicates the assessment of the stability and reliability of the rock mass. Therefore, fully quantifying the mechanical properties and permeability evolution of prefabricated fractured rocks under hydraulic coupling is essential for minimizing damage in rock engineering projects. Despite recent advances in rock mechanics testing technology, measuring the microstructural changes in rocks under hydraulic mechanical loads during laboratory tests remains a challenge. [Methods] The rock failure process is closely associated with acoustic emission (AE), which is defined as transient elastic waves generated by the rapid release of energy within a material. Owing to its high sensitivity to crack initiation, propagation, and coalescence in loaded rocks, AE monitoring is a powerful tool for studying brittle rock failure. This technology has been widely applied in rock mechanics research and engineering applications. This article employs AE technology to conduct triaxial hydraulic coupling tests on intact sandstone and prefabricated fractured sandstone samples. The primary objective is to explore the micro-mechanisms of permeability evolution under hydraulic coupling loads through a series of laboratory tests. The strength and deformation characteristics, macroscopic failure modes, and permeability evolution patterns of prefabricated fractured sandstone under hydraulic coupling were examined. Additionally, the spatiotemporal evolution of AE events and the statistical distribution of fracture types during the failure process of prefabricated fractured sandstone were analyzed using AE technology. [Results] The experimental results indicate the following: 1) Antiwing cracks occur when the inclination angles of prefabricated cracks are 45°, 75°, and 90°, whereas specimens with other prefabricated crack inclination angles predominantly exhibit wing crack failures. 2) During the spatiotemporal evolution of AE events, the compression events resulting from the closure of cracks and pores during the initial compression stage represent the primary microscopic mechanism for the decrease in permeability. 3) The increase in permeability is mainly attributed to tensile and shear microcracks associated with volume expansion, whereas the decrease in permeability is caused by shear microcracks associated with volume compression and compression microcracks. [Conclusions] Under triaxial hydraulic coupling, the strength and deformation characteristics, macroscopic failure modes, and permeability of prefabricated fractured sandstone specimens are influenced by the inclination angle of the prefabricated fractures. The decrease in permeability mainly results from compression events that lead to fracture closure and pore collapse, whereas the increase in permeability is attributed to the continuous accumulation of tensile and shear events, which is also influenced by the inclination angle of pre-existing fractures. [ABSTRACT FROM AUTHOR]

Tunnel stability is influenced by the rheological properties of the surrounding rock. This study, based on the Ganshen high-speed railway tunnel project, examines the rheological characteristics of siltstone and sandstone through laboratory tests and theoretical analysis. Rheological curves and parameters are derived, revealing the time-dependent deformation mechanisms of the surrounding rocks. A numerical simulation model is created using these parameters to analyze deformation and stress characteristics based on different rock levels and inverted arch closure distances. Results indicate that sandstone follows the Cvisc model, with the Maxwell elastic modulus increasing under higher loads while the viscous coefficient decreases. The vault displacement is mainly affected by the surrounding rock strength; lower strength leads to greater displacement, which also increases with the closure distance of the inverted arch. These findings are crucial for determining the optimal closure distance of inverted arches in sandstone conditions. [ABSTRACT FROM AUTHOR]

Engineering rock mass often involves bedding planes and U-shaped defects. Under the combined influence of bedding planes and defects, the fracture mode of engineering rock mass is more complicated. To better understand the failure law of U-shaped defects in rock mass with bedding planes and better control or predict the crack propagation morphology, three-point bending tests were conducted on semicircular disk specimens with U-shaped notches under three different notch radii. The load variation law, different stages of the load–displacement curve, and fracture characteristics of specimens under five bedding angles were analyzed. Then, numerical models of semicircular three-point bending tests were established in particle flow code 2D (PFC2D). The influence of notch radius and bedding angle on the failure path of semicircular specimens was simulated and analyzed, and the three failure modes were verified again. In addition, the stress at the notch tip of the model was monitored by the measurement circle in the discrete-element method, and the fracture toughness of the model specimen was calculated, which provides a new calculation idea for fracture toughness. The research results help us more comprehensively understand the mechanical behavior and failure characteristics of layered rock mass with notch under external load. [ABSTRACT FROM AUTHOR]

Borehole breakout is an important geological phenomenon that provides a significant reference for the estimation of the direction and magnitude of in situ stress. To better understand the water content effect on borehole breakouts, a series of experiments on red sandstone was conducted. The standard cylindrical samples and cubic samples containing a circular hole were loaded under uniaxial compression in four water states including dry, unsaturated, short-term saturated, and long-term saturated. Then, the water effect was discussed in terms of rock strength and deformation, initiation stress of borehole breakout, failure evolution process, geometries of breakout, fragments, and acoustic emission (AE) signals. In the water-bearing state, rock properties significantly deteriorate with the decline of strength, the enhancement of plastic deformation, and the restrain of the brittle failure mode. For cubic samples, the axial stress required for borehole breakout increases under the short-term saturated condition but decreases after the long-term immersion. In addition, obvious extrusion deformation of borehole walls toward the borehole space is observed in saturated samples, resulting in vague V-shaped void notches. Before the overall failure, more obvious AE signal fluctuation occurs in the water-bearing state and AE locations reveal a larger fracture concentration region. Two mechanisms are proposed to explain the water content effects on the borehole breakout initiation stress, involving strength weakening effect and stress redistribution mechanism. The shrinkage of borehole breakout geometries is discussed further on the influence on the in situ stress estimation. Highlights: The influence of water content on sandstone properties and borehole breakout is comprehensively investigated. Short-term immersion results in a slight rise of borehole breakout initiation stress, while it declines again after long-term immersion. Double mechanisms are proposed to explain the water content effect on borehole breakout initiation. Water induces the shrink of both breakout depth and angular span, which may further affect the in situ stress estimation. [ABSTRACT FROM AUTHOR]

To investigate the influence of joints on the rockburst process in the surrounding rock of deep tunnels, a free-face true-triaxial test was conducted on the rockburst of the jointed surrounding rock of a tunnel. The rockburst ejection failure process (i.e., failure mode, rock strength, and deformation characteristics), rockburst debris characteristics, and acoustic emission evolution characteristics of red sandstone specimens under different joint conditions were investigated. The experimental results indicate that the failure modes of red sandstone specimens with a joint condition perpendicular to the tunnel direction generally exhibit 'internal shear and external splitting', which can suppress the occurrence of rockbursts, whereas the failure mode of red sandstone specimens with a joint condition parallel to the tunnel direction is mainly shear fracture, which can also promote the occurrence of rockbursts. The variation trend of the peak strength and the joint angle of the specimen exhibits a 'U' type shape, and the debris pertaining to the joint specimen's rockburst exhibits fractal characteristics. With the increase of the joint inclination angle, the fragment's ejection kinetic energy exhibits a trend of decreasing first and subsequently increasing. There are significant differences in the total number pertaining to impacts of acoustic emissions during rockburst under different joint conditions. The distribution range pertaining to the dominant frequency signal of the acoustic emission signal at the time of rockburst is mainly a low-frequency signal, and the intermediate-frequency, high-frequency, and ultra-high-frequency signals also exist, thus indicating that the rockburst damage exhibits strong complexity. Finally, the strain rockburst mechanism of jointed rock is significantly different from that of jointless intact rock. The rockburst mechanism is mainly divided into two typologies: shear fracture and slab buckling-shear fracture. The research results contribute to a deeper understanding of the underlying mechanisms of rockburst disasters in jointed rock formations. Highlights: Successfully performed a free-face true-triaxial rockburst test in the jointed surrounding rock of a tunnel. The presence of different joints under true-triaxial conditions significantly alters the mechanism of strain-induced rockburst. Joints perpendicular to the tunnel direction reduce the occurrence of rockbursts, whereas joints parallel to the tunnel direction promote the occurrence of rockbursts. With the increase of the joint inclination angle, the fragments' ejection kinetic energy exhibits a trend of decreasing first and subsequently increasing, where the joint inclination angle of 30° is the transition's inflection point. [ABSTRACT FROM AUTHOR]

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Natural fractures are widely developed and distributed in tight sandstone reservoirs in the western Sichuan Basin, China, influenced by complex tectonic movements and diagenetic processes. These natural fractures serve as the main flow channels and important storage spaces in such reservoirs. Understanding their development characteristics and controlling factors determines the positioning and production efficiency of tight gas wells. This paper first explores the types and characteristics of natural fractures in the second member of the Upper Triassic Xujiahe Formation (T3x2) in the western Sichuan Basin through outcrop investigations, core observations, image log interpretations, and thin section analyses. We study the main influencing factors controlling the development and distribution of tectonic shear fractures by combining the characterization of fracture attributes. The results show that the tight sandstone reservoirs in the T3x2 mainly contain tectonic fractures, diagenetic fractures, and overpressure fractures, among which shear fractures in tectonic fractures are the main types. There are four sets of tectonic shear fractures in the study area, oriented in the E-W, N-S, NE-SW, and NW-SE directions. The dip angles of fractures range from 20° to 80°, with an average dip angle of 53°, predominantly consisting of high-angle fractures. The lengths of fractures are mainly within 100 cm, with a mean value of 47.53 cm, and 76.2% of fractures have lengths less than 50 cm. Fracture apertures range from 2.51 to 163.19 μM, with an average of 30.54 μM. The proportion of effective fractures in tectonic shear fractures reaches 85.7%. The development of tectonic shear fractures is primarily influenced by lithology, rock mechanics stratigraphy, diagenetic processes and facies, and faults. Lithology stands as the fundamental factor influencing the degree of fracture development in tight reservoirs. In rocks with the same composition, as the grain size decreases, the degree of fracture development increases. Simultaneously, with the rise in muddy content, fracture development diminishes in siltstone, argillaceous siltstone, silty mudstone, and mudstone. The formation and distribution of fractures are governed by rock mechanics stratigraphy, primarily manifested as a decrease in fracture density with an increase in rock mechanics layer thickness within a certain range. Diagenetic processes affect the abundance of tectonic shear fractures by influencing the mechanical properties of rocks. The stronger the compaction and cementation, the poorer the physical properties, and the higher the brittleness of the rocks, resulting in a greater abundance of tectonic shear fractures. Rocks with strong compaction and robust cementation facies exhibit high brittleness, rendering them more susceptible to fracturing under identical stress conditions. Fractures are highly prevalent in both the hanging wall and footwall of faults. As the distance from the fault increases, the linear density of fractures significantly decreases, owing to the uneven distribution of stress perturbation caused by fault activity. [ABSTRACT FROM AUTHOR]

Rock material, such as sandstone, frequently shows a wide range of fracture characteristics and mechanical responses due to variations in weathering grades and the existence of natural fractures. These natural fractures in sandstone are regularly cemented by different minerals or cementation removed by weathering and/or stained with iron, which can affect the stress-induced fracture pattern and rock strength. These effects, along with fracture orientation and location on mechanical behavior and failure modes resulting from quasi-static loading, are poorly understood. The focus of this paper is to assess the effects of cemented and stained natural fractures on stress-induced sandstone failure mechanism and mechanical behavior, with a focus on natural fracture angle and location. To investigate sandstone mechanical behavior and failure mechanism, six unconfined compressive tests were conducted on specimens with pre-existing natural fractures at different angles (∅) to estimate unconfined compressive strength (UCS) and failure modes. ANSYS was also utilized for finite-element modeling and simulation to analyze comprehensive 3D fractured models. The results revealed that UCS of sandstone decreased with increasing fracture angles of 0°, 20°, 30°, 45°, 50°, to 60°, respectively. Sandstone with a 90° fracture angle exhibits higher UCS than samples with a 0° fracture angle. The findings indicated that sandstone with 0° and 20° fracture angles had multiple fracturing, while specimens with 90° fracture angles provided axial splitting. On the other hand, the shear failure in sandstone specimens occurred as the fracture angle superseded 50°. The simulation results demonstrate that fracture location affects only the UCS of sandstone. [ABSTRACT FROM AUTHOR]

The deformation, strength, and failure characteristics of layered composite sandstones (LCSs) are critical issues in a stability evaluation in underground engineering. These issues were examined in this study using true triaxial experiments, with the main focus being on the influences of mechanical contrast (MC) between the layers in a three-dimensional (3D) restricted stress state. The overall strength of LCSs is mainly controlled by the strength of the weaker sandstone when MC exists between the layers. Failure under true triaxial stress results in σ2-oriented fractures coexisting with σ3-oriented fractures within LCSs. The overall failure of an LCS test specimen exhibits the σ3-transverse symmetric fracture mode, i.e., σ3-transverse symmetric tension near the σ3-loading surface and σ3-transverse symmetric shear in the middle of the specimen. A new strength criterion is proposed to predict the true triaxial strength of LCSs effectively. The study results provide insights into the cracking morphologies of LCSs with mechanical property changes in individual layers under true triaxial stress. [ABSTRACT FROM AUTHOR]

The quantification microstructural parameters has gained significant attention to estimate the strength of rock materials. Some researchers have qualitatively explained the influence of microstructural parameters on rock mechanical properties and many developed empirical relations between various microstructural indices and engineering properties of different rock types. A comprehensive review on the relation between microstructure and strength of rock materials has been carried out in this study based on the available literature and the tests performed on granite and sandstone from Malanjkhand and Kothagudem respectively. It has been observed that the correlation of rock strength with mineral content, textural parameters and micro-crack patterns does not signifies any perceptible correlation or any definite trend. This is attributed to the fact that rock strength is a manifestation of cumulative responses of a number of microstructural parameters and thus, correlating rock strength with any single parameter would overlook the impact of others. The study also discusses the challenges and potential benefits of both quantitative and qualitative analysis of microstructure in rock mechanics, providing a way forward for future research in the field. It is ascertained that microstructural studies are time consuming approach and involve significant limitations when quantifying the parameters from the microscopical images and relating them individually with strength of the rocks. However, they might be an important aspect in comprehending the variation in strength and deformation of rock materials and in this respect the implication of modern technological setups would be efficient over the conventional methods. [ABSTRACT FROM AUTHOR]

Flaw length has a significant effect on the cracking behaviour of fractured rock. PFC2D was used to simulate the uniaxial compression of red sandstone samples with secondary flaw lengths L2 of 0 mm, 5 mm, 10 mm, 15 and 20 mm under different primary flaw angles α(α = 0°, 15°, 30°, 45°, 60°, 75°, and 90°). Based on the simulation results, the effects of the secondary flaw length on the mechanical parameters, crack initiation location and crack evolution process of the fractured rock were analysed. Then, a method for judging the initiation location of the first cracks was proposed. Finally, the characteristics of the particle displacement field at preexisting flaw tips during first and secondary crack initiation were analysed. The results of this study are helpful for more deeply understanding the cracking behaviour of fractured rock. [ABSTRACT FROM AUTHOR]

The study of rock burst tendency of rock masses with different sizes plays a key role in the prevention of rock burst. Through theoretical analysis, it is proposed that uniaxial compressive strength and deformation modulus ratio are the key mechanical parameters affecting rock burst occurrence. In order to find out the size effect of uniaxial compressive strength and deformation modulus ratio, theoretical analysis and uniaxial compression experiment are carried out on rock samples with different heights, different cross-sectional areas and different volumes. The results show that the smaller the uniaxial compressive strength is, the larger the deformation modulus ratio is, and the more likely rock burst are to occur. On the contrary, rock burst is still not easy to generate. The uniaxial compressive strength of rock samples with different heights, different cross-sectional areas and different volumes increases with the increase of rock sample size. The deformation modulus ratio of rock samples with different heights and different volumes shows an upward trend on the whole, while that of rock samples with different cross-sectional areas shows a downward trend on the whole. The fracture forms of rock are analyzed using the energy conversion law in the process of deformation and failure for three kinds of rock with different shapes and sizes. [ABSTRACT FROM AUTHOR]

Bedrock temperature contains effective information about changes in the crustal stress. A new method of detecting crustal stress change by bedrock temperature (DSCT) has been proposed. Understanding the stress-induced temperature response characteristics of loaded rocks is fundamental for applying DSCT. In this study, temperature observation experiments of different rocks and water-saturated sandstones subjected to tiered cyclic loading were conducted to investigate the temperature–stress relationship throughout the whole rock deformation and failure process. Through experiments, some valuable results are obtained: (i) temperature changes synchronously with stress and has a very strong linear correlation with it; (ii) the magnitude of temperature response to stress (TRS) is approximately 1 mK MPa−1, ranging from strong to weak are sandstone, marble, diorite and basalt, respectively. The differences in TRS of various rocks are determined by their major rock-forming minerals, textures and structures; (iii) the evolution of TRS experiences three stages: the TRS rises rapidly in the compression stage, slowly in the acoustic emission quiet period and a significant increase in TRS before rock failure is observed on marble, sandstone and basalt, consistent with the abnormal bedrock temperature rise preceding earthquakes; (iv) the TRS of water-saturated sandstones is higher compared to the dry ones, and the abnormal sharp increase in TRS before rock failure is also more significant in the former. These findings mentioned above promote the understanding of thermal anomalies preceding earthquakes. [ABSTRACT FROM AUTHOR]

Geological carbon sequestration in jointed reservoirs will require the use of fracture network for the flow of CO2 plumes. However, acidic solution formed at the interface between brine and CO2 can cause chemical erosion of the local rock mass, especially in rocks with high carbonate content. The use of the water alternating gas technique for injection stimulation can exacerbate this issue, as the water–CO2 interface occurs in areas near the injection point. As a result, acidic flow can impact the surrounding rock mass, particularly around the main flow paths where fracture network conductivity is much higher than matrix permeability. To investigate the impact of acidic flow on fracture conductivity, we conducted an experiment on a fractured sandstone sample that was exposed to CO2-saturated water. Our findings revealed a nearly ten-fold increase in post-experimental water-relative permeability, and restriction of flow within established flow channels, which consist one third of the fracture surface. In conclusion, our study sheds light on the dynamic behavior of fractured sandstone under the influence of CO2–H2O flow, revealing significant changes in transmissivity and fracture geometry. These findings contribute to a better understanding of the hydraulic performance of fractures in the context of geological carbon sequestration. [ABSTRACT FROM AUTHOR]

Rock mass is one of the most important load-bearing media in geotechnical engineering. It has been continually vulnerable to geological tectonic movements, natural calamities, and human excavation activities. Its inherent weak surfaces such as primary pores, joints, and fissures have resulted in varying damage degrees. In mining operations, the damaged rock mass has a variety of negative impacts on the stability of its overlying structures and is frequently disturbed by the load. To study the damage law of rock mass under cyclic loading, in this paper, an acoustic emission (AE) device was employed to monitor the rock under the action of two types of cyclic loads: the variable upper and lower pre-loads, and the fixed upper and lower pre-loads. The damage of the loaded rock was split into three stages in this research, based on the features of the AE signals of the rock under uniaxial load, and the damage evolution of the loaded rock was analyzed in distinct stages. The AE signals of the rock under cyclic loading were mainly emitted in the first loading stage. When the stress did not exceed the maximum stress value in the stress history of the loaded rock, few new AE event was generated in the loaded rock. After the low-frequency cyclic static load, the AE signals varied with the load-bearing stress of the rock during the whole process from initial loading to failure, which was consistent with the characteristics of the AE signals of the loaded rock. The research results can be adapted to rock mass in open-pit mines stability analysis and risk prediction while providing some references for the early warning and danger relief of rock masses in engineering. [ABSTRACT FROM AUTHOR]

To investigate the deformation and failure taw of deep sandstone, the relationship between plastic zone development, crack propagation and acoustic emission (ABE) characteristics is established. Tang deep sandstone samples as the research object, the axal static compression ABE test is charred out, the evolution law of plastic zone and maternal crack of rock samples is analyzed by numerical simulation. The results show that the dissipation of maternal energy density is relatively stable in the compaction and e!mastic stage,gradually increases in the hp!mastic stage, and all the energy is released immediately before allure. There is a strong core!nation between the hp!mastic zone and crack ABE characteristic.. The ranging count and energy Increase first and then decrease in the compaction stage. Before the elastic stage, the plastic zone and micro-cracks appear from both sides of the rock scamp! In the e!mastic stage, the ranging count and energy continue to increase,the Increase of the latter is greater than that of the former,the hp!mastic zone develops diagonally inward, and the number of cracks increases. In the hp!mastic stage, the ringing count continues to increase, the energy surges, and the plastic zone develops outwards. In the allure stage, both the ringing count and energy surge,and the simulation test shows a shape shear crack, which is consistent iii the actual allure crack. [ABSTRACT FROM AUTHOR]

Fissured rocks deteriorate with increasing water content, and the mechanical behavior is significantly influenced by the filling materials within their fissures. Understanding the effects of tailings water on the mechanical properties and failure modes of rocks under different filling conditions is crucial for assessing the stability of tailings ponds. In this study, uniaxial compression tests were conducted on single-fissured sandstone filled with gypsum, cement, and epoxy resin at various immersion heights, and acoustic emission signals were monitored. The results indicate that the mechanical properties of sandstone deteriorate significantly upon immersion, but the rate of deterioration decreases with increasing immersion height. The use of stronger and more cohesive filling materials can improve the mechanical properties of fissured sandstone, but there remains a gap compared to intact samples. Differences in physical properties and uneven stress distribution between immersed and dry portions lead to the formation of complex crack networks in partially immersed samples. The strong bonding between epoxy resin and sandstone results in local stress exceeding the sandstone's bearing limit, leading to increased fragmentation. The acoustic emission activity generally exhibits a pattern of gradual increase, quiescence, and then activation. As the immersion height increases, the number of acoustic emission events and energy release decrease. The average frequency and rise angle analysis reveals that tensile cracks dominate the failure process. Near failure, the b-value drops sharply and exhibits intense fluctuations, accompanied by the emergence of numerous high-frequency signals. These phenomena provide a basis for predicting rock instability and failure. [ABSTRACT FROM AUTHOR]

In order to study the hydration absorption behaviors and characteristics of sandstone in Mogao Grottoes in China, the pressure-less hydration absorption experiment on the outcrop porous sandstone of Mogao Grottoes was carried out by using the self-developed real-time monitoring experimental system. The hydration absorption was measured and the curve of hydration absorption with time was drawn. At the same time, the digital image correlation method (DIC) was used to measure the full-field deformation, and the speckle pattern of the sample was analyzed using Match ID, and the displacement field and strain field of the sandstone sample at different hydration absorption moments were computed. Moreover, the sparse area and dense area of sandstone are used as regions of interest (ROI) for DIC analysis. According to the test results, it is concluded that the hydration absorption of sandstone increases rapidly in the initial stage, and gradually tends to be stable with the change of time. This corresponds well with the deformation characteristics of sandstone analyzed using DIC. In the initial stage, the deformation of sandstone increases rapidly. With the change in time, the deformation of sandstone samples gradually slows down. When the hydration adsorption reaches saturation, the sandstone continues to deform for a period of time before stopping hydration absorption. The results of the mercury injection test and the XRD test show that the porosity of the sparse area is larger than that of the dense area and the particle content of the dense area is lower. When the sandstone is saturated with water, the liquid is immersed in the pores between the solid particles, which makes the sparse area more prone to stress concentration, and the deformation in the sparse area is larger. Therefore, when analyzing the hydration absorption deformation of sandstone, the porosity should be considered. [ABSTRACT FROM AUTHOR]

In engineering practice, layered rock masses often display obvious anisotropy while deforming and failing, and the failure mode directly impacts the engineering construction stability. In this study, the fracture failure load, fracture toughness, crack deflection angle, and failure mode of a layered rock mass under different fracture modes were analyzed by utilizing improved asymmetric semi-circular disc specimens. According to the constitutive model of transversely isotropic materials, the maximum tensile stress (MTS), maximum energy release rate (MERR), and maximum strain energy density (MSED) calculation formulas were modified, and the calculation formulas of the three prediction criteria under anisotropic materials were derived. The calculation results were compared with the experimental results. The results show that the fracture toughness and crack deflection angle were significantly affected by the weak bedding plane. As a result of applying the MTS criterion, the results are closer to the experimental results, providing a solid foundation for engineering deformation, failure, and fracture analyses. [ABSTRACT FROM AUTHOR]

To explore the causes of mine tremors in coal mines with sandstone roofs, a three-point bending loading experiment was designed for composite sandstone layers, and the fracture and interlayer shear slip characteristics of the composite sandstone layers were studied using optical measurement and acoustic emission techniques. The results show that the bending of the rock layers led to interlayer sliding deformation, while the fracturing greatly promoted interlayer sliding. The maximum interlayer slip accelerations during bending deformation and fracturing were 0.6 mm/s2 and 3.8 mm/s2, respectively. During the fracturing of the rock layers, the proportion of acoustic emission shear fracture events increased with the continuous occurrence of long-lasting and high-amplitude acoustic emission events. The mechanism of mine tremors in thick sandstone roofs is as follows: the increase in the area of the goaf causes rock bending deformation and fracturing, accompanied by interlayer shear slip, fracturing of the sandstone layer, and friction dislocation at the cementation surface of the adjacent sandstone layers, which jointly cause vibration of the roof. [ABSTRACT FROM AUTHOR]

Through a series of static mechanical tests and (SHPB) dynamic impact tests, the static and dynamic mechanical parameters of rock as represented by yellow sandstone are determined, and the Holmquist-Johnson-Cook model parameters of the rock are calibrated using the test data and theoretical calculations. The feasibility of a numerical model is verified, and numerical analysis of the SHPB impact process under different radial pressure pre-loading is carried out on the basis of good verification. The results show that with increasing impact load, the degree of rock breakage increases, as does the dynamic tensile strength. With the application of increasing pre-static pressure, the dynamic tensile strength of the rock decreases gradually, and the maximum radial cumulative strain increases continuously under a given impact pressure, indicating that micro-cracks in the rock develop initially and then expand under the influence of pre-static pressure; the rock is more easily broken, and its weakening degree increases. Under coupled dynamic and static loading, the energy utilization rate of rock in the Brazilian splitting process is jointly affected by axial compression ratio and impact load. Too large a pressure ratio will reduce the strain-rate sensitivity of rock, resulting in low energy utilization rate, while too low an axial compression ratio will make the dynamic tensile strength of rock relatively high, which is not conducive to tensile failure. Therefore, on the premise of clear fracture form requirements, a suitable combination of axial compression ratio and impact velocity can improve the rock crushing effect and energy utilization rate. [ABSTRACT FROM AUTHOR]

To investigate the characteristics of crack propagation and microscopic structure under water–rock coupling effect in deep surrounding rock, a triaxial mechanical experiment was conducted on sandstone strata in the Shandong Wanfu Coal Mine under the action of pressurized water absorption. The experimental results showed that the mass increment of the rock sample was exponentially related to the confining pressure, and the mathematical relationship curve of the mechanical strength variation of the rock sample under different confining pressures was obtained. Based on this, the strength softening formula of the rock sample with the absorption time under different confining pressures was obtained, and the softening process was divided into three stages: the relatively stable stage, the rapid softening stage and the deceleration softening stage. The T2 spectrum curve of the rock sample was normalized, and the pore distribution and strength softening curve under different confining pressures were constructed. The speed of water molecules entering pores in different softening stages was analyzed, and it was found that in the relatively stable stage, the speed was micro-pore > meso-pore > macro-pore, in the rapid softening stage, the speed was meso-pore > macro-pore > micro-pore, and in the deceleration softening stage, the speed was micro-pore ≈ meso-pore ≈ macro-pore. According to the law of microscopic pore structure changes under water–rock coupling, a pore structure model of sandstone under different confining pressures was constructed. Based on the crack propagation model and crack propagation rate formula, it was concluded that the increase in crack strain propagation rate was the result of both water saturation and permeation water pressure, and a distribution map of the crack strain propagation rate was established. By comparing the crack strain propagation rate, it was found that under the same characteristic stress, the axial strain propagation rate of the crack was greater than the radial strain propagation rate, and the radial strain propagation rate was greater than the volume strain propagation rate. Quantitative relationships between crack propagation in sandstone and water saturation under different stress conditions were obtained. [ABSTRACT FROM AUTHOR]

Creep and acoustic emission tests were conducted under uniaxial compression on sandstone samples with no (i.e. intact), one and two prefabricated fractures collected from the Xiaolangdi Reservoir area using a RLJW-2000 rock microcomputer-controlled rheology servotest system and PCI-2 acoustic emission testing equipment, respectively. The change laws of the acoustic emission amplitudes, peak frequencies, acoustic emission events and b values of samples with different fracture numbers (i.e. number of fractures) were systematically analysed. The differences between the prediction made by acoustic emission b values and cumulative ringing count for the precursor time of creep failure in a rock mass were studied comparatively, thereby identifying the effects of fracture number on the creep and acoustic emission characteristics of sandstone. According to the study results: (1) With an increase in the prefabricated fracture number, the acoustic emission signal counts of samples increased within the amplitude intervals of 60–80 and 80–100 dB. The signal counts were especially high in the high-amplitude interval of 80–100 dB but they decreased within the low-amplitude interval of 45–60 dB. (2) Under each stress level, the total acoustic emission event counts generated by the single- and double-fracture samples were 1.90 and 2.36 times that generated by the intact sample, respectively. Under the last stress level, the event counts generated by the single- and double-fracture samples were 4.64 and 6.19 times the event count generated by the intact sample, respectively. A higher prefabricated fracture number implied higher total acoustic emission event counts generated by samples and higher acoustic emission event counts generated under the last stress level. (3) Under the same stress level, the frequency band concentration of acoustic emission signals of samples became more pronounced as the prefabricated fracture number increased. Under the last stress level, as the prefabricated fracture number increased, the peak frequencies of the acoustic emission signals of samples were concentrated in a greater number of frequency ranges (four, seven and eight frequency ranges, respectively). (4) With an increase in the prefabricated fracture number, the average acoustic emission b values of samples presented an overall declining trend, with an increased Δ b range. (5) The plunge rates of acoustic emission b values of the single- and double-fracture samples before accelerated creep failure increased by 8.0 and 11.2%, respectively, compared with the intact sample. (6) The acoustic emission b value curves and cumulative ringing count curves for samples with different fracture numbers predicted creep failure in the rock mass earlier than the strain curves. For all samples, the precursor time points of creep failure in the rock mass predicted by acoustic emission b value curves were all earlier than those predicted by corresponding cumulative ringing count curves. As the prefabricated fracture number increased, the precursor time points of creep failure in the rock mass predicted by acoustic emission b value curves occurred even earlier. Thematic collection: This article is part of the Engineering Geology and Hydrogeology of the Anthropocene collection available at: https://www.lyellcollection.org/topic/collections/engineering-geology-and-hydrogeology-of-the-anthropocene [ABSTRACT FROM AUTHOR]

The instability of hard and brittle rock often leads to disastrous consequences in underground engineering. Under various surrounding rock pressure conditions, in situ stress induces corresponding deformation and damage to the floor post-mining. Therefore, it is crucial to examine the effects of mining under different confining pressures on rock disturbance, damage characteristics, and their distribution. Consequently, triaxial loading experiments under varying intermediate principal stress conditions were conducted on red sandstone specimens, using an acoustic emission monitoring system to track energy changes during rock damage and failure. This approach aids in studying crack generation, propagation, and fracture damage evolution. The results indicate that rock deformation results in axial compression and dilatancy, aligned with the direction of minimum and intermediate principal stresses. Ductility in rock failure becomes more pronounced with increased stress, primarily manifesting as shear failure. Internal cracks in the specimen lead to stress concentration and marked plastic deformation under compression, yet do not result in macroscopic surface cracks. The fracture angle θ of specimens post-failure generally exceeds 45° and varies with stress changes; at consistent burial depths, the angle of the sandstone failure surface increases with intermediate principal stress. This paper preliminarily establishes the informational linkage between rock failure and energy release, analyzing the rock samples over time and space. This research offers insights for analyzing and mitigating sudden rock instability. [ABSTRACT FROM AUTHOR]

An understanding of the rheological characteristics of surrounding rock is key to predicting the large deformations of tunnels in soft rock in cold regions, particularly due to freeze–thaw (F–T) damage of the surrounding rock. Therefore, in this study, a series of F–T cycle and triaxial creep tests of surrounding rocks were conducted on the basis of typical tunnel projects in cold regions, and the parameters of the test data were identified by the fractional-order creep damage constitutive equation. Then, a constitutive model was embedded into the Fast Lagrangian Analysis of Continua in 3 Dimensions software to analyze the coupled creep deformations of the surrounding rock and supporting structure in response to F–T cycles while excavating the Ningchan tunnel. The calculated results accurately reflected the creep deformation characteristics of the surrounding monitoring points after tunnel excavation under different numbers of F–T cycles. Finally, a tunnel operation model was derived and used to analyze the creep deformation characteristics of a tunnel during operation in a cold area. The calculation results showed that with increasing time, the lining structure and surrounding rock at different monitoring points in the tunnel underwent various degrees of creep deformation. Additionally, the creep deterioration effect of the surrounding rock was more severe after the F–T cycles, which endangered traffic safety. [ABSTRACT FROM AUTHOR]

This study investigates the impact of intermediate (s2) and minimum (s3) principal stress unloading on damage behavior and the confining pressure influence on crack initiation stress (sci) in true triaxial stress conditions, utilizing large-scale cubic samples. Two distinct true triaxial tests were executed, examining the effects of confining stress (s2 and s3) unloading on porous sandstone damage and the correlation between confining stress and sci. Acoustic emission (AE) parameters, signal characteristics, and wave velocity variations were utilized to elucidate cracking mechanisms and damage development in the samples. Unloading tests reveal consistent velocities in three orthogonal directions (V11, V22, and V33) during the initial two unloading stages. In the subsequent three stages, confining stress unloading leads to a decrease in wave velocity in the corresponding direction, while velocities in the other two directions remain nearly constant. Notably, s2 unloading generates higher amplitude AE signals compared to s3 unloading, with over 70% of the micro-cracks categorized as tensile. In the incremental loading tests, sci is found to be contingent on confining pressure, with s2 playing a crucial role. During s1 loading, V33 decreases, indicating additional crack formation; conversely, s3 loading results in V33 increase, signifying the continuous closure of existing cracks. Limitations of the experiments are summarized and prospects in this domain are outlined. [ABSTRACT FROM AUTHOR]

To investigate the dynamic response of fractured rock under cyclic loading and unloading, a WHY-300/10 microcomputer-controlled electro-hydraulic servo universal testing machine was used to conduct uniaxial cyclic loading and unloading tests. Simultaneously, acoustic emission (AE) and a CCD high-speed camera were employed to monitor the fracturing characteristics of sandstone. The mechanical properties, energy evolution, AE characteristics, and deformation of 45° sandstone were analyzed. The results indicate that as the load cycle level increases, both the elastic modulus and deformation modulus exhibit a "parabolic" increase, with a rapid rise initially and a slower rate of increase later. The damping ratio generally shows a decreasing trend but tends to rise near the peak load. The total energy, elastic energy, dissipated energy, damping energy, and damage energy all follow exponential function increases with the load level. The b-value fluctuates significantly during the stable crack propagation phase, unstable crack propagation phase, and peak phase. When the FR (Felicity ratio > 1), the rock is relatively stable; when the FR (Felicity ratio < 1), the rock gradually extends towards an unstable state. The Felicity ratio can be used as a predictive tool for the precursors of rock failure. Shear fractures dominate during the compaction and peak phases, while tensile fractures dominate during the crack propagation phase, ultimately leading to a failure characterized by tensile fracture. High-speed camera observations revealed that deformation first occurs at the tips of the prefabricated cracks and gradually spreads and deflects toward the ends of the sandstone. This study provides theoretical support for exploring the mechanical behavior and mechanisms of fractured rock under cyclic loading and unloading, and it has significant practical implications. [ABSTRACT FROM AUTHOR]

Influence of compression on the cyclic mechanical properties of rock leaves a significant scope for research to judiciously design surface and subsurface structures in the Himalaya. The paper aims at analysis of the cyclic strength and mechanical properties of Disang sandstone from Indian Himalaya under complete and incomplete uniaxial cyclic loading tests. Two series of cyclic loading tests were performed. First series comprises of complete cyclic loading test under stress-controlled mode to determine the fatigue limit of the rock. Second series comprises of incomplete cyclic loading tests under amplitude and cycle controlled mode performed below the determined fatigue limit of the rock. The key findings from the study indicate that under incomplete cyclic loading, with increase in both amplitude and number of cycles, static compressive strength of the rock decreases. Cyclic elastic properties decrease with increase in peak stress under complete cyclic loading, whereas the incomplete test shows a progressive decline in its elastic properties during initial cycles followed by an asymptotic behavior with increasing number of cycles. Progressive evolution of irreversible axial strain and hysteresis energy under complete cyclic loading shows primary, secondary, and tertiary stages of deformation behavior whereas in incomplete cyclic loading, only primary and secondary stages are visible. Three non-linear strain–based empirical models were proposed to characterize all the three stages of cyclic deformation and hysteresis behavior of rock. [ABSTRACT FROM AUTHOR]

To explore the effect of hob temperature on the rock-breaking characteristics of full-section tunnel boring machines (TBMs) in sandstone strata, high-temperature furnace heating experiments of sandstone and physical and mechanical experiments at room temperature and high temperatures were conducted to obtain the mechanical properties of sandstone at different temperatures. The mechanical properties at different temperatures were calibrated using PFC3D to obtain micro-mechanical and thermodynamic parameters and establish a rock-breaking model. The orthogonal experiments were used to establish the simulation experiments of rock breaking under different temperatures, confining pressure conditions, knife tip distances, and penetration degrees. The results show that the hob tip force is gradually increasing with an increase in the confining pressure. When below 600 °C, there is little temperature transfer from particle to particle as the temperature increases. At this time, with the two sides of the rock slag flaking, the hob knife tip force is the first to reduce. After 600 °C, with the expansion of the rock extrusion hob, the temperature rises on both sides; at this time, the hob tip force also increased. The hob tip force is minimal at a tip distance of 70 mm and an S/P of 14. As the surrounding pressure increases, the rock-breaking efficiency of the hobber decreases. The highest rock-breaking efficiency is achieved at 25 °C and 600 °C. The rock-breaking efficiency is highest when the knife tip distance is designed to be 70 mm, and when the S/P is 14. The three-dimensional constitutive analysis of rock-breaking particles showed that the increment caused by the hob temperature is mainly distributed in the normal force direction in the surrounding rock without any confining pressure, and the increment caused by the hob temperature exposed to the confining pressure occurs in all directions. [ABSTRACT FROM AUTHOR]

The impact of high temperature environments on the physical and mechanical properties of rocks is a significant factor to consider. The investigation into the impact of elevated temperatures on the physical and mechanical characteristics of rocks holds great importance in the advancement and exploitation of deep- seated mineral reserves, as well as in ensuring the safety and stability of subterranean engineering projects. This study utilizes the state-of-the-art GCTS Mechanical Loading Test System to conduct uniaxial and triaxial compression tests on sandstone after thermal treatment from 25°C to 650°C. In addition, XRD, SEM and nuclear magnetic resonance experiments were carried out on the sandstone after thermal treatment. The aim of the experiments is to provide a quantitative characterization of mechanical properties and behaviors of the rock samples. The results show that the mass, density, and wave velocity of sandstone decrease with increasing temperature, while volume and porosity increase. The mass, volume, and rate of density change of sandstone exhibit a significant increase when subjected to temperatures above 500°C. The uniaxial compressive strength and elastic modulus exhibit an initial increase followed by a subsequent decrease as the temperature rises, with 300°C serving as the critical turning point. The axial peak strain and Poisson's ratio increase with increasing temperature. The cohesion decreases with increasing temperature, while the internal friction angle increases. Additionally, it is observed that the rate of change for both properties exhibits an increase beyond the temperature threshold of 400°C. [ABSTRACT FROM AUTHOR]

Featured Application: This article may be applied to the excavation and support of coal mine roadways. In order to study the impact of fracture thickness on the dynamic mechanical properties of rock specimens, impact compression tests on prefabricated sandstone specimens with different fracture thicknesses and intact specimens were carried out by using a split Hopkinson pressure bar (SHPB) test device, and the dynamic mechanical parameters of the fractured sandstone specimens were obtained. The results showed that the dynamic stress–strain curves of the prefabricated fractured sandstone specimens are similar to those of the intact sandstone specimens, which can be divided into three stages: elasticity, plasticity, and failure. With the increase in the thickness of prefabricated cracks, the dynamic compressive strength of the sandstone specimens decreases in a quadratic function, the dynamic strain decreases in a power exponential function, and the dynamic elastic modulus decreases linearly. Attempts were made to quantitatively analyze the crushing degree of sandstone specimens. The average particle size of the crushed specimens was negatively correlated with the thickness of prefabricated cracks, and the fractal dimension was linearly negatively correlated with the dynamic compressive strength. The 1.5–2.5 mm prefabricated fractured sandstone specimens produced airfoil cracks and secondary cracks; the 3–3.5 mm prefabricated fractured specimens produced airfoil cracks, coplanar secondary cracks, and secondary oblique cracks; and the complete specimens were subjected to axial failure to produce axial cracks. [ABSTRACT FROM AUTHOR]

In backfill mining, surrounding rock damaged by impact loading (e.g. blasting) is immersed in backfill water after backfilling, and the instability of the impact-damaged and water-soaked (IDWS) surrounding rock can occur under blasting loads. To study the dynamic mechanical properties and the size effect of rock under such working conditions, triaxial dynamic compression tests were conducted on four groups (four length-to-diameter ratios) of first impact-damaged and then water-soaked sandstone specimens using an improved split Hopkinson pressure bar device. The test results show that, at a similar strain rate (approximately 10 s−1), the peak strength and elastic modulus of IDWS specimens are lower than those of the intact specimens, whereas the peak strain is higher than that of the intact specimens. Both the peak strength and peak strain of the IDWS specimens increase with strain rate, exhibiting a strong strain rate dependency. The peak strength, peak strain, and elastic modulus all display a size effect. This size effect varies over different strain rate ranges. When the strain rate is approximately 10 s−1, the peak strength decreases as the length-to-diameter ratio increases. In contrast, at a strain rate of approximately 30 s−1, the peak strength increases with an increase in the length-to-diameter ratio. There is a linear relationship between the incident energy and absorbed energy of the intact and IDWS specimens. Under the same incident energy, the IDWS specimens show higher absorbed energy than the intact specimens. This relationship is independent of the length-to-diameter ratio of rock specimens. [ABSTRACT FROM AUTHOR]

Increasing the diameter of the drillhole can facilitate drillhole breakage using soundless chemical demolition agents, but it is prone to cause drillhole blowout, resulting in crushing failure. This paper conducted a blowhole prevention test on a large borehole using the internal insertion cooling pipe method (ICBPM) to test the expansion pressure of cooling pipes with different diameters. During this test, a fracture occurred in a hole with a 75 mm inner diameter in the rectangular sandstone specimens with high strength. It was found that utilizing the ICBPM can effectively hinder the development of blowholes. Expansion and blowhole prevention are optimized with a 0.14 mass ratio of the cooling water to demolition agent and a maximum expansion stress of 49.0 MPa. The guiding effect of the minimum resistance line is significant. In repeated tests, all fissures are distributed in a Y-shape on the free surface where the minimum resistance line is located. The acoustic emission signals from statically fractured hard rock increase abruptly before damage, and the development of rock expansion and fracturing can be obtained through strain monitoring. These results suggest that the ICBPM can reduce the expansion time with a strong crushing effect, satisfying the need to process more crushing projects. [ABSTRACT FROM AUTHOR]

Safety is the primary guarantee for achieving coal mine production. The degree of water in the overlying rock of the coal seam has a significant impact on the mechanical strength of the rock, which can lead to subsidence, roof collapse, and rock burst in the mining area. Therefore, it is important to study the law of waterrich damage and deterioration of coal seam overlying rock, as well as the characteristics of bursting liability to prevent and control mining disasters. This paper aims to investigate the change in compressive strength, elastic modulus, and bursting liability of coal measure strata under different water content and their relationship with lithology, particle size, pore characteristics, and mineral characteristics. We carried out uniaxial compression tests on drilled and prepared samples of coal measure strata in the east of Ordos City. We also monitored the acoustic emission characteristics of the rock failure process under different water content. The results demonstrate that, in the dry state, the compressive strength of siltstone >fine sandstone >medium sandstone >coarse sandstone. As the water content increases, the rock transitions from brittle to plastic, and the compressive strength, elastic modulus, and bursting liability gradually decrease. The stress-strain curve changes from a single peak to bimodal and multimodal, and the active degree of acoustic emission events decreases, especially the low-value ringing count. Calcareous cementation, clay minerals, and chlorite exhibit strong softening properties when subjected to water, while the grain skeleton (comprising quartz, feldspar, mica) and siliceous cementation displays weak softening properties in such conditions. The most significant is the mechanical strength deterioration of siltstone containing a clay matrix. The research findings can provide reference and theoretical support for safe mining practices in coal mines. [ABSTRACT FROM AUTHOR]

The deformation of rocks well below their ultimate strengths is frequently described through the constant stiffness moduli of the linear elasticity theory. In addition, the isotropic material approximation is usually used as the basic approach. Particularly in sedimentary rocks, local inhomogeneities, anisotropy, and irreversible processes, mainly related to the gradual breakage of grain joints, can affect their deformation behavior from the beginning of the loading. Consequently, the stiffness moduli measured under triaxial conditions are effective parameters, which may depend on the loading path in the stress space. Therefore, a complex experimental study of the deformation response of compact sedimentary rock (Brenna sandstone) along various paths in triaxial stress space was prepared to understand this dependence. In addition, a detailed analysis of the rock composition and structure of this compact sandstone was carried out. This paper presents an initial experimental study that is based on a loading method using alternative stress paths that correspond to different modes of the monotonic increase in differential stress from an initial isotropic compression state. In the experiment, the dependence of stiffness moduli on the loading path was found. Differences in rock deformation for conventional triaxial compression and extension can be attributed to a slight rock anisotropy originating from the stratification. The different behavior of deformation and related stiffness moduli along so-called reduced triaxial paths and conventional paths indicates that irreversible processes must be taken into account. Highlights: The study brings new experimental data on the deformation of compact sedimentary rock in specific triaxial stress conditions. The measured effective stiffness moduli of compact sandstone depends on the loading path in the triaxial stress state. In general, the deformation response of the sandstone is nonlinear from the start of loading. The mechanical properties of the sandstone show anisotropy as a consequence of stratification. In standard triaxial tests, failure criterion combining stress and strain conditions can be used for the investigated sandstone. [ABSTRACT FROM AUTHOR]

Tight gas sandstone reservoirs play an important role in the field of exploration and development of unconventional oil/gas resources. However, these reservoirs typically exhibit low-porosity and poor-permeability, with pore structures of strong heterogeneity comprising connected and isolated pores. The traditional rock physics models (such as the Gassmann model) are established under the assumption of fully connected pores, which cannot reasonably describe the complexity in tight reservoirs. In this regard, this work proposes a rock physics modeling approach aimed at tight sandstone reservoirs based on a reformulated Xu–White model. Specifically, the differential effective medium model and Xu–White model are combined to analyze the influences of isolated pores on the dry rock skeleton. To characterize the general effects due to the different pore types on rock elastic moduli, the volume content of connected pores is defined as a pore-connectivity parameter. The simulated annealing algorithm is applied to invert the parameter, which is treated as a weighting coefficient for correcting the wet rock elastic moduli, thereby improving the precision of rock physics modeling. The proposed approach is tested and verified by using the log data of the work area of Sichuan Basin, West China. The result shows that, compared with the conventional method, the P- and S-wave velocities predicted by the proposed method are consistent with the log data. In addition to the inversion results of the conventional petrophysical parameters, the inverted pore-connectivity parameter can be a good auxiliary attribute that assists locating potential targets for tight gas sandstone reservoirs. [ABSTRACT FROM AUTHOR]

To reveal the failure mechanism of fractured sandstone under the influence of the infilling material strength, uniaxial compression tests, and PFC3D numerical simulation analyses were conducted on fractured sandstone with different infilling material strengths, and the influence of the infilling material strength on the basic mechanical properties of the fracture‐filled sandstone was studied. The results showed that, with increasing infilling material strength, the peak strength and elastic modulus of the specimens increased nonlinearly and eventually stabilized. The failure pattern of the specimens changed from shear to tensile. When the infilling material strength was low, a wing crack or anti‐wing crack was generated; however, when the infilling material strength was high, a vertical tensile crack was generated. From the perspective of fracture mechanics, the filling of sandstone cracks can reduce or even suppress the stress concentration effect at the prefabricated crack tip and improve the bearing capacity of the fractured sandstone. Highlights: The basic mechanical properties of the fracture‐filled sandstone specimens were obtained.PFC3D numerical simulation analyses were conducted on fractured sandstone.The influence mechanism of filling materials on the fractured rock was revealed. [ABSTRACT FROM AUTHOR]