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Unloading processes are common in natural systems. Intense unloading activities can alter the frictional equilibrium of faults and induce their instabilities. Understanding the slip behavior of faults under stress unloading conditions is helpful in guiding engineering practices. We conducted a series of direct shear experiments under linear-unloading normal force conditions considering the influences of initial normal forces, initial shear forces, and normal unloading rates on planar and rough granite fractures. The experimental results showed that planar fracture exhibits sudden slip events during normal unloading, while rough fracture mostly displays stable sliding behavior. The planar fracture demonstrates an exponential increase in sliding distance and velocity at the end of each slip event. The rough fracture usually exhibits a quasi-static stage before rapid slip events. In addition, the accumulative sliding distance at the slip activation moment (at the first moment when sliding velocity is greater than 0.05 mm/s) for the planar fracture decreases with lower normal unloading rate, larger shear force and larger normal force, while its variation trend for rough fracture is opposite. These findings provide valuable insights into fault slip behavior under stress unloading, aiding in mitigating associated risks in engineering applications. [ABSTRACT FROM AUTHOR]

The safe and secure supply of strategic mineral resources is an important strategic need of China, and the extraction of mineral resources from the deep earth is an irreplaceable strategy for a long time in the future. To ensure the demand for mineral resources, the safe and efficient mining of high-stress hard rock is a major problem that must be solved in deep mining. Firstly, the technical challenges of deep high-stress hard rock mining and the transformative mining philosophy of "turning harm into benefit" was delineated. And a key engineering countermeasure to weaken the high-stress hard rock before mechanical rock breaking to achieve commercial mining was proposed. Next, the mechanism, characteristics and development status of contact and non-contact rock weakening methods were summarized, and the potentiality of these two types of rock weakening methods in the field of assisting mechanical breaking of high-stress hard rock was compared and analyzed. Finally, the safe and efficient continuous mining technology represented by mechanical rock breaking with thermal exfoliation damage weakening of deep high-stress hard rock was prospected. It provides useful guidance for the safe and secure supply of deep strategic mineral resources and the development of deep earth engineering. [ABSTRACT FROM AUTHOR]

The geometric properties of fracture surfaces significantly influence shear-seepage in rock fractures, introducing complexities to fracture modelling. The present study focuses on the hydro-mechanical behaviours of rough rock fractures during shear-seepage processes to reveal how dilatancy and fracture asperities affect these phenomena. To achieve this, an improved shear-flow model (SFM) is proposed with the incorporation of dilatancy effect and asperities. In particular, shear dilatancy is accounted for in both the elastic and plastic stages, in contrast to some existing models that only consider it in the elastic stage. Depending on the computation approaches for the peak dilatancy angle, three different versions of the SFM are derived based on Mohr-Coulomb, joint roughness coefficient-joint compressive strength (JRC-JCS), and Grasselli's theories. Notably, this is a new attempt that utilizes Grasselli's model in shear-seepage analysis. An advanced parameter optimization method is introduced to accurately determine model parameters, addressing the issue of local optima inherent in some conventional methods. Then, model performance is evaluated against existing experimental results. The findings demonstrate that the SFM effectively reproduces the shear-seepage characteristics of rock fracture across a wide range of stress levels. Further sensitivity analysis reveals how dilatancy and asperity affect hydraulic properties. The relation between hydro-mechanical properties (dilatancy displacement and hydraulic conductivity) and asperity parameters is analysed. Several profound understandings of the shear-seepage process are obtained by exploring the phenomenon under various conditions.

Photogrammetry, reconstructing three-dimensional (3D) models from overlapping two-dimensional (2D) photos, finds application in rock mechanics and rock engineering to extract geometrical details of reconstructed objects, for example rock fractures. Fracture properties are important for determining the mechanical stability, permeability, strength, and shear behavior of the rock mass. Photogrammetry can be used to reconstruct detailed 3D models of two separated rock fracture surfaces to characterize fracture roughness and physical aperture, which controls the fluid flow, hydromechanical and shear behavior of the rock mass. This research aimed to determine the optimal number of scale bars required to produce high-precision 3D models of a fracture surface. A workflow has been developed to define the physical aperture of a fracture using photogrammetry. Three blocks of Kuru granite (25 cm × 25 cm × 10 cm) with an artificially induced fracture, were investigated. For scaling 3D models, 321 markers were used as ground control points (GCPs) with predefined distances on each block. When the samples were well-matched in their original positions, the entire block was photographed. Coordinate data of the GCPs were extracted from the 3D model of the blocks. Each half was surveyed separately and georeferenced by GCPs and merged into the same coordinate system. Two fracture surfaces were extracted from the 3D models and the vertical distance between the two surfaces was digitally calculated as physical aperture. Accuracy assessment of the photogrammetric reconstruction showed a 20–30 μm digital control distance accuracy when compared to known distances defined between markers. To attain this accuracy, the study found that at least 200 scale bars were required. Furthermore, photogrammetry was employed to measure changes in aperture under normal stresses. The results obtained from this approach were found to be in good agreement with those obtained using linear variable displacement transducers (LVDTs), with differences ranging from 1 μm to 8 μm.

The infrared radiation temperature (IRT) variation concerning stress and crack evolution of rocks is a critical focus in rock mechanics domain and engineering disaster warning. In this paper, a methodology to extract the key IRT features related to stress and crack evolution of loaded rocks is proposed. Specifically, the wavelet denoising and reconstruction in thermal image sequence (WDRTIS) method is employed to eliminate temporal noise in thermal image sequences. Subsequently, the adaptive partition temperature drift correction (APTDC) method is introduced to alleviate temperature drift. On this basis, the spatial noise correction method based on threshold segmentation and adaptive median filtering (OTSU-AMF) is proposed to extract the key IRT features associated with microcracks of loaded rocks. Following temperature drift correction, IRT provides an estimation of the thermoelastic factor in rocks, typically around 5.29×10−5 MPa−1 for sandstones. Results reveal that the high-temperature concentrated region in cumulative thermal images of crack evolution (TICE) can elucidate the spatiotemporal evolution of localized damage. Additionally, heat dissipation of crack evolution (HDCE) acquired from TICE quantifies the progressive failure process of rocks. The proposed methodology enhances the reliability of IRT monitoring results and provides an innovative approach for conducting research in rock mechanics and monitoring engineering disasters.

Geometric morphological features significantly influence the shear mechanical properties of rock fractures, which are quantitatively assessed using roughness indicators. However, it remains uncertain whether fractures with identical roughness indicators exhibit similar shear mechanical behavior. This study employed three-dimensional scanning and the roughness index θ*max/(C + 1) to investigate the roughness of fracture profiles oriented differently on the fracture surface. The findings confirm that natural fracture profiles can have similar roughness levels but distinct morphologies. Consequently, a methodology was proposed to construct fractures with identical roughness but differing morphologies, based on 10 fracture profiles exhibiting varying roughness gradients. Numerical models of rough fracture samples were established, and direct shear tests were conducted using the discrete element method (DEM). The results reveal significant variability in shear strength, shear deformation, and macro-microscopic failure characteristics among fracture samples with identical roughness levels. As the θ*max/(C + 1) of fractures increases, more morphological asperities contribute to resisting external shear forces, although both the mean values and variability of shear mechanical indicators across fracture samples demonstrate an increasing trend. The distribution of fracture morphological features notably influences the characteristics of contact force distribution, as well as the scale and orientation of microcrack development within fracture samples. The contribution of different segments of fractures to resisting external shear forces correlates closely with their specific morphological feature distribution, particularly the location where θ*max/(C + 1)​ occurs. [ABSTRACT FROM AUTHOR]

A rapid and appropriate evaluation of the crack asymptote coefficients is essential for estimating the fracture growth in rock materials with inherent cracks and discontinuities. In the present study, the extended finite element method (XFEM), implemented with free coarse mesh, is used in conjunction with the finite element over‐deterministic (FEOD) method to determine the stress intensity factors (SIFs) and T‐stress for mixed‐mode I/II in‐plane loading of cracked rock specimens. Then, the generalized strain energy density (GSED) criterion is employed to estimate both fracture load and crack initiation angle for two types of rock pre‐cracked specimens. The predictions of the GSED criterion are then benchmarked against the experimental results, here obtained from three‐point bend tests on Neyriz marble. It is shown that such an approach to evaluate the fracture load dramatically reduces the computational cost and effort, meanwhile guaranteeing high accuracy and robustness. Highlights: XFEM is combined with FEOD to obtain stress fields around the crack tip.A new method for extracting test samples from rock cores is introduced.Various mixed‐mode fracture tests are conducted on rock samples.The results are satisfying even with unstructured coarse mesh. [ABSTRACT FROM AUTHOR]

The morphological characteristics of the fracture surface play a crucial role in determining its shear strength and deformation. Additionally, the fracture surface undergoes varying degrees of damage at different positions during the experimental process. This study aims to investigate the local damage mechanism of the fracture surface post-experiment, as it is essential for understanding the size effect and anisotropy of mechanical properties in rock masses. To achieve this, 3D scanning and 3D printing technologies were employed to create molds of rough fracture surfaces, and rock-like material mortar was used to cast samples of these rough fractures. The direct shear tests were successfully conducted on two rough rock fracture surfaces under varying normal pressures and shear directions. The analysis of the fracture surface's shear stress law indicates the presence of apparent anisotropy and varying damage modes in response to changes in normal pressure. Following the experiment, it was observed that the damage is localized on specific areas of the fracture surface, and its distribution characteristics are significantly influenced by the shear direction and normal pressure. Using 3D scanning technology, the JRC2D of various profiles and JRC3D of fracture surfaces are determined through the utilization of the geometric index WPA. Both local and overall roughness of the fracture surface were weakened after the shearing. Notably, the degree of local damage was found to be significantly related to the roughness feature at the corresponding position. [ABSTRACT FROM AUTHOR]

The mechanical and hydraulic behaviors of rock fractures are strongly dependent on the properties of the surface morphology. In this study, a sandstone fracture surface is taken as the research object. The morphological properties of the fracture surface characterized by parameters Z2s, Z2, and SRv are analyzed under the different sampling windows and profile lines. The results show that the large sampling window is more conducive to obtaining stable values of the morphology parameter Z2s. The coefficient of variation (CV) and the percentage of variation (PV) are proposed to determine the stable values of the morphology parameter Z2s, which is defined as a representative elementary surface (RES). It is confirmed that there is a RES with a size of 220 mm in the study region. The large sampling window generates a long correlation length of the morphology parameter Z2s. The differential sensitivities of the morphology parameters Z2 and SRv are compared under the different profile lines. It is found that the parameter Z2 can better describe the random variation properties of the surface morphology, while the parameter SRv is more suitable for describing the spatial structural variation properties of the surface morphology. The findings of this work are of great significance for investigating the size effect and anisotropy of rock fracture permeability. [ABSTRACT FROM AUTHOR]

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Aiming at the current groove blasting problems of additional damage, the feasibility of double base propellant for groove blasting was explored. Based on the propellant gas release behavior, the pressure change of the double base propellant in the closed hole was calculated. Combined with high-speed photography and digital image correlation (DIC) method, two groups of experiments were carried out with propellant loading density of 0.84 and 0.96 g/cm³ to investigate the dynmic destruction process of granite slabs under the action of propellant. The results show that the granite slabs in the two groups of experiments were cracked along the groove direction at 100 μs after ignition, and the cracks penetrated through the slabs at 200 μs; the specimen with a charge density of 0.96 g/cm³ had a larger separation speed between the upper and lower slabs after fracture, and the upper and lower slabs were cracked by the friction of the blocking rubber and the inertia of the specimen, and the cracks were in the vertical direction at 2 500 μs. The grooves around the blast hole provide space for the effect of the propellant gas, and the grooves can effectively guide the direction of crack propagation, no crushing zone formed around the hole wall. The quasi-static pressure generated by the combustion of double-base propellant is the main driving force for crack initiation and propagation. The experimental results have some implications for the use of double base propellant in controlled rock blasting projects. [ABSTRACT FROM AUTHOR]

Unloading processes are common in natural systems. Intense unloading activities can alter the frictional equilibrium of faults and induce their instabilities. Understanding the slip behavior of faults under stress unloading conditions is helpful in guiding engineering practices. We conducted a series of direct shear experiments under linear-unloading normal force conditions considering the influences of initial normal forces, initial shear forces, and normal unloading rates on planar and rough granite fractures. The experimental results showed that planar fracture exhibits sudden slip events during normal unloading, while rough fracture mostly displays stable sliding behavior. The planar fracture demonstrates an exponential increase in sliding distance and velocity at the end of each slip event. The rough fracture usually exhibits a quasi-static stage before rapid slip events. In addition, the accumulative sliding distance at the slip activation moment (at the first moment when sliding velocity is greater than 0.05 mm/s) for the planar fracture decreases with lower normal unloading rate, larger shear force and larger normal force, while its variation trend for rough fracture is opposite. These findings provide valuable insights into fault slip behavior under stress unloading, aiding in mitigating associated risks in engineering applications.

Identifying the real fracture of rock hidden in acoustic emission (AE) source clusters (AE-depicted microcrack zone) remains challenging and crucial. Here we revealed the AE energy (representing dissipated energy) distribution rule in the rock microcrack zone and proposed an AE-energy-based method for identifying the real fracture. (1) A set of fracture experiments were performed on granite using wedge-loading, and the fracture process was detected and recorded by AE. The microcrack zone associated with the energy dissipation was characterized by AE sources and energy distribution, utilizing our self-developed AE analysis program (RockAE). (2) The accumulated AE energy, an index representing energy dissipation, across the AE-depicted microcrack zone followed the normal distribution model (the mean and variance relate to the real fracture path and the microcrack zone width). This result implies that the nucleation and coalescence of massive cracks (i.e., real fracture generation process) are supposed to follow a normal distribution. (3) Then, we obtained the real fracture extension path by joining the peak positions of the AE energy normal distribution curve at different cross-sections of the microcrack zone. Consequently, we distinguished between the microcrack zone and the concealed real fracture within it. The deviation was validated as slight as 1–3 mm.

Rock fracture acoustic emission (AE) signals are commonly used non-destructive testing data in geological exploration, resource exploitation, and engineering fields. However, these signals are often accompanied by noise interference caused by environmental factors. In this study, we propose an enhanced model for denoising rock fracture AE signals, called simplified fully convolutional denoising autoencoder (SFCDAE). This model is based on the denoising autoencoder principle in the field of deep learning neural networks. The SFCDAE model consists of only seven layers, with minimal preprocessing of data input. By comparing denoising performance evaluation indicators, higher peak signal-to-noise ratio (PSNR) and lower root mean square error (RMSE) were achieved. On average, PSNR increased by 5.575% and RMSE decreased by 22.225%. Using simulated environmental noise to validate the model, it was found that the model has good robustness and can remove artefacts from sudden noise. The practical application value of the LSTM classification model was validated using data containing real experimental noise, resulting in a higher classification accuracy of 80.083%. These results indicate that the proposed model has better denoising performance compared to existing intelligent models and has certain practical value. [ABSTRACT FROM AUTHOR]

Crack initiation, growth, and coalescence in flawed rocks have been extensively studied for 2D (planar, penetrating) flaws under uniaxial/biaxial compression. However, little is known as to the mechanisms and processes of cracking from 3D flaws under true triaxial compression, where the intermediate principal stress ( σ 2 ) is distinguished from the major and minor principal stresses. In this work, we systematically investigate the effects of σ 2 on the 3D cracking behavior of rock specimens with preexisting flaws, through the use of mechanistic simulations of mixed-mode fracture in rocks. We explore how two characteristics of σ 2 , namely, (i) its orientation with respect to the flaw and (ii) its magnitude, affect two aspects of the cracking behavior, namely, (i) the cracking pattern and (ii) the peak stress. Results show that the orientation of σ 2 exerts more control over the cracking pattern than the flaw inclination angle. The peak stress becomes highest when σ 1 is parallel to the flaw, whereas it becomes lowest when σ 2 is parallel to the flaw. Also, the effects of σ 2 magnitude are more significant when σ 2 becomes more oblique to the flaw plane. On the basis of our observations, we propose mechanisms underlying the cracking behavior of 3D flawed rocks under true triaxial compression. Highlights: The effects of the intermediate principal stress ( σ 2 ) on the 3D cracking behavior of flawed rocks under true triaxial compression are systematically investigated. The orientation of σ 2 exerts more control over the cracking pattern than the flaw inclination angle. The effects of σ 2 magnitude become more significant when σ 2 are more oblique to the flaw plane. Mechanisms underlying the cracking behavior of 3D flawed rocks under true triaxial compression are proposed based on the observations made. [ABSTRACT FROM AUTHOR]

Physical models of dolomitic marbles from greek quarries with a pair of twin pre-existing cylindrical holes subjected to uniaxial compression are studied in the laboratory and then, they are simulated numerically with a Bonded Particles Model by employing the two-dimensional version of the Particle Flow Code and the Flat-Joint model. The micro-cracking, the fracturing process and the sequence of their appearance during the numerical tests of the hollow plates are in good agreement with the laboratory tested physical models. Each numerical fracture pattern is similar to the one of the respective physical models. Also, the regions of micro-cracking in the numerical models are similar to the regions of intense deformation observed from Digital Image Correlation analysis on the respective physical models. A comparison between the required applied axial stress for the primary fracture and sidewall spalling initiation of the physical and the numerical models is presented. Then, these stress values are compared to the ones of previously published hollow plates with a single cylindrical opening of the same diameter in compression of the same materials. [ABSTRACT FROM AUTHOR]

Abstract An accurate understanding of the heat transfer of water through rock fractures is essential for the extraction and utilization of thermal energy from high‐temperature rock masses. A systematic numerical simulation based on the double‐rough‐walled model has been presented to investigate the shear effect on convective heat transfer in rough rock fractures. On the basis of the modified successive random additions algorithm, four different self‐affine surfaces were generated and utilized to establish the 3D double‐rough‐walled fracture models. The fluid flow and heat transfer were simulated by directly solving the Navier–Stokes equation and energy conservation equation, respectively. The combined effects of shear and surface roughness on the heat transfer were investigated. The results show that the heat within rough‐walled fractures transfers preferentially along the main fluid flow channels, and the areas of fast and slow thermal transmission fit well with the high‐ and low‐flow regions, respectively. As shear advances, the heat transfer coefficient firstly increases, then decreases slightly and finally stabilizes within a certain range, in which stabilization occurs earlier in fracture with a larger joint roughness coefficient. The effect of surface roughness on heat transfer shows an opposite trend during shearing. When the shear displacement is small, the enhancement effect of surface roughness that provides larger heat transfer areas dominates the heat transfer. As shear displacement continues to increase, this enhancement effect will be gradually weakened until the decreasing effect that bumps on the rough‐walled surface hinder the fluid flow dominates the heat transfer.

Mining activities are key triggers for strong mine earthquakes and even rock bursts in coal mines. This study explores the impact of mining speed on the overlying strata's deformation and energy release through theoretical analysis, numerical simulation, and the digital speckle method. The temporal and spatial evolution characteristics of the impact energy during mining are simulated. The digital speckle method illustrates a positive correlation between rapid mining and increased fracture block degree of overburden rock and roof separation, confirming that accelerated mining speed extends the fracture distance of the stope. Furthermore, numerical simulations establish that both the energy associated with overlying rock breaking and the frequency of energy occurrence events are amplified during rapid mining, in contrast to slow mining. This observation corroborates that escalating mining speed augments the energy dispensed by the breaking of the upper rock. Consequently, this escalation induces a transformation in the energy levels of mine earthquakes, culminating in a heightened incidence of large-energy mine earthquakes. [ABSTRACT FROM AUTHOR]

The apparent permeability of a single rough fracture undergoes complex evolution in a non-Darcy flow regime, making description of the nonlinear flow challenging. The inertial permeability can be used to effectively solve this problem but is very sensitive to the geometric information and difficult to determine directly. Here, a model for predicting the inertial permeability is proposed by considering the geometric information of rough rock fractures. A massive training database of nonlinear flow in single rough fractures was built based on direct numerical simulations. The database consists of 1225 fractures and contains 12 geometric parameters, including 9 morphological and 3 aperture parameters. To predict the inertial permeability, four geometric parameters highly correlated with the inertial permeability were selected by correlation analysis. A robust prediction model was then established based on the support vector machine theory and the artificial bee colony algorithm. Forty-five fractures constructed from Barton's profiles were used to verify the model performance. The validation results show that the proposed method can accurately predict the inertial permeability based on the geometric information of rough fractures. Finally, the proposed prediction model was used to determine the critical Reynolds number. [ABSTRACT FROM AUTHOR]

This study focuses on the nonlinear flow characteristics of a single rough-walled fracture in a marble sample subject to various confining pressures (5.0–50.0 MPa) and deviatoric stresses (0–60.0 MPa). First, a series of seepage tests were conducted to investigate the impacts of fracture dilation and inertial effect on the nonlinear flow behaviors. Then, the competitive effects of fracture dilation and inertial effect on the transition of the nonlinear flow regime were theoretically evaluated. Finally, a novel seepage model incorporating fracture dilation was proposed based on the Goodman hyperbolic closure deformation relationship. This model was further validated through a comparison with both experimental observations and classical theory. The investigation reveals that variations in confining pressures and deviatoric stresses induce a substantial reduction in flow rate by factors of 5 and 2 magnitudes, correspondingly. Notably, the influence of the inertial effect diminishes as the fracture dilation effect becomes more pronounced, particularly evident under conditions of lower confining pressures. The proposed model considering the fracture dilation effect accurately predicts the relationship between flow rate and pressure conditions. Moreover, it demonstrates superior performance compared with the Forchheimer equation. This study can provide guidance for underground engineering practices when encountering high confining pressure, high deviatoric stress, and high water pressure. [ABSTRACT FROM AUTHOR]

Columnar jointed basalt (CJB) is a kind of jointed rock with a polygonal cylinder mosaic structure that has complex mechanical properties such as discontinuity and heterogeneity. The typical geological structure of the CJB is the intercolumnar joint plane and the implicit joint plane, which obviously affect the mechanical properties of the rock mass. Controlling the unloading relaxation of the CJB is a key problem during the construction of underground engineering. In this paper, in-situ acoustic wave and panoramic borehole camera measurements were carried out in the cavern of the Baihetan project to understand the failure mechanism of the collapse of the CJB. It was quite clear that the evolution of the excavation damage zone (EDZ) of the CJB depends on the time and spatial effects. The closer to the collapse zone, the greater the degree of relaxation failure of the columnar joint rock mass; the further away from the cavern perimeter, the more stable the surrounding rock. The correction between wave velocity and cracks in the rock mass was also discussed. This field test and theoretical analysis can provide a reference for studying the failure mechanism and control measures of CJB in underground caverns under high geostress. [ABSTRACT FROM AUTHOR]

An accurate understanding of the heat transfer of water through rock fractures is essential for the extraction and utilization of thermal energy from high‐temperature rock masses. A systematic numerical simulation based on the double‐rough‐walled model has been presented to investigate the shear effect on convective heat transfer in rough rock fractures. On the basis of the modified successive random additions algorithm, four different self‐affine surfaces were generated and utilized to establish the 3D double‐rough‐walled fracture models. The fluid flow and heat transfer were simulated by directly solving the Navier–Stokes equation and energy conservation equation, respectively. The combined effects of shear and surface roughness on the heat transfer were investigated. The results show that the heat within rough‐walled fractures transfers preferentially along the main fluid flow channels, and the areas of fast and slow thermal transmission fit well with the high‐ and low‐flow regions, respectively. As shear advances, the heat transfer coefficient firstly increases, then decreases slightly and finally stabilizes within a certain range, in which stabilization occurs earlier in fracture with a larger joint roughness coefficient. The effect of surface roughness on heat transfer shows an opposite trend during shearing. When the shear displacement is small, the enhancement effect of surface roughness that provides larger heat transfer areas dominates the heat transfer. As shear displacement continues to increase, this enhancement effect will be gradually weakened until the decreasing effect that bumps on the rough‐walled surface hinder the fluid flow dominates the heat transfer. [ABSTRACT FROM AUTHOR]

Anomalous solute migrations in fractured rocks are governed by geometric characteristics and flow regimes. Although existing inverse models can describe this behavior, the underlying physics for quantifying key transport coefficients remains largely unexplored. Here, we investigate the quantitative impacts of geometric heterogeneity and flow regimes on solute transport in rock fractures. We conduct numerical experiments to simulate water flow and conservative solute transport in 3D fractures with varying geometric features and Reynolds numbers. Our results show that the non‐Fickian transport is prevalent across the entire flow regime, with Darcy flows attributed to geometric heterogeneity and non‐Darcian flows influenced by additional eddy zones. We employ the mobile‐immobile (MIM) domain model and continuous time random walk (CTRW) model to inversely model simulated breakthrough curves. Inverse analyses demonstrate that both models effectively characterize anomalous transport behaviors. The fitted transport coefficients of the MIM model exhibit stronger quantitative relationships with aperture and roughness parameters, as well as Reynolds number, compared to the CTRW model. By incorporating parameterized transport coefficients, we propose physics‐ and statistics‐based models to directly predict anomalous transport behaviors under different flow regimes. These prediction models accurately reproduce solute transport processes of all simulated cases with acceptable errors. The feasibility of directly predicting solute transport under varying flow regimes using geometric information is thus validated. Our study not only supports the study of substance migration based on geometric structure features, but also serves as a foundation for investigating geological activities based on substance migration information. Plain Language Summary: The migration of substances within geological formations is governed by their geometric pore structure, while information about substance migration can reveal the geometric characteristics of these formations. In this context, these two aspects form an interdependent feedback system. Investigating quantitative relationships and models between the geometry of geological formations and substance migration is thus crucial for understanding various geological processes related to geometric structure changes and hydrogeochemical signals (i.e., chemical information carried by groundwater or surface water, which reflects various natural and anthropogenic geological processes) evolution. Here, we explore the impact of both geometric structure and hydrodynamic conditions on solute transport in rock fractures. We conduct extensive direct numerical simulations to determine the inherent quantitative relationships between key transport coefficients and geometric parameters under different flow regimes. Based on these relationships, we establish upscaling models for directly predicting solute transport processes using only geometric parameters within both physics‐ and statistics‐based frameworks. This research has significant implications for predicting solute migrations using measurable physical properties and offers potential applications for studying geological activities related to fractured rocks through analyzing released hydrogeochemical signals. Key Points: Non‐Fickian transport behaviors in 3D fractures under different Re were characterized within physics‐ and statistic‐based frameworksTransport coefficients within mobile‐immobile and continuous time random walk frameworks were systematically parameterized by geometric and hydrodynamic parametersDirectly predicting solute transport via geometric parameters under different Re is feasible in both physics‐ and statistic‐based frameworks [ABSTRACT FROM AUTHOR]

The article describes two single-mass models of a drill string and different methods of bottom-hole treatment: percussion and rotary percussion. The loads created by the piston are transferred to rocks via a progressively advancing bit in the first model and via a rotatable bit in the second model. The drag force applied to the bit is found from the nonlinear dependence on the penetration rate and kinematic parameters which govern the force impact on the rock and its loss of strength: initial blow velocity and rotational speed modulus of the bit. The optimal initial blow velocities are found at the preset blow frequency; they ensure elimination of short-term stick slips in penetration of the bit. For the mentioned cases of penetration, the processes of percussion and rotary percussion drilling in strong rocks are provided with the strict mathematical description. The numerical modeling results are presented. [ABSTRACT FROM AUTHOR]

Underground rocks in coal mining and oil exploration are usually subjected to in-situ stress and dynamic loading. In this paper, the tensile fracture behaviors of brittle rocks under coupled in-situ stress and dynamic loading are investigated numerically via the simulation of Brazilian disc (BD) tests of the modified split Hopkinson pressure bar (SHPB). To achieve this purpose, a cohesive zone model in the framework of the finite element method is used to model the cracking of the rock Brazilian disc under confining pressure. The pressure-dependent property of the rock is considered using the recently proposed bi-linear constitutive law. Dynamic Brazilian disc tests of the SHPB with two impact velocities of a striker are simulated for the rock. Comparisons between the simulated results and the reported experimental ones show a good agreement, demonstrating the accuracy and validity of the simulation models and the numerical approach. Due to the existence of holes in the rock in underground rock engineering practices, a pre-hole is inserted in the BD rock specimen in the modified SHPB tests to more realistically consider the rock fracture. The effects of the in-situ stress, the pre-hole size, and the pre-hole position on the dynamic fracture characteristics of the rock are numerically investigated using the modified SHPB test for BD rock specimen. [ABSTRACT FROM AUTHOR]

As two rough walls of a rock fracture approach each other under external compressive loading, the number of contact asperities increases. Some of the adjacent contact asperities may gradually come together to forming a new larger asperity, owing to the shared substrate. The geometrical and mechanical natures of the new asperity are determined by an equivalent transformation method, in which contact stiffness and force location remain consistent before and after the transformation. A theoretical contact model with the inclusion of asperity interaction and coalescence is developed, based on the famous Greenwood and Williamson theory. Fracture closure tests are conducted to validate the theoretical model, and comparisons between theoretical curves and experimental results demonstrate that the developed model can provide good predictions. The limitations of the developed model can mainly be attributed to the following aspects: (1) ignoring plastic deformation or damaged asperities caused by the large local stress at some contact spots, and (2) simplifying the three-dimensional geometrical description of contact asperities. Potential applications of the theoretical model are also preliminarily analyzed. [ABSTRACT FROM AUTHOR]

Underground space creation and energy extraction, which induce unloading on rock fractures, commonly occur in various rock engineering projects, and rock engineering projects are subjected to high temperatures with increasing depth. Fluid flow behavior of rock fractures is a critical issue in many subsurface rock engineering projects. Previous studies have extensively considered permeability evolution in rock fractures under loading phase, whereas changes in fracture permeability under unloading phase have not been fully understood. To examine the unloading-induced changes in fracture permeability under different temperatures, we performed water flow-through tests on fractured rock samples subjected to decreasing confining pressures and different temperatures. The experimental results show that the permeability of fracture increases with unloading of confining pressure but decreases with loading-unloading cycles. Temperature may affect fracture permeability when it is higher than a certain threshold. An empirical model of fracture hydraulic aperture including two material parameters of initial normal stiffness and maximum normal closure can well describe the permeability changes in rough rock fracture subjected to loading-unloading cycles and heating. A coupled thermo-mechanical model considering asperity damage is finally used to understand the influences of stress paths and temperatures on fracture permeability.

Abstract The frictional rupture mechanisms of rock discontinuities considering the dynamic load disturbance still remain unclear. This paper investigates the transitional behaviors of slip events happened on a planar granite fracture under cyclic normal stress with different oscillation amplitudes. The experimental results show that the activations of fast slips always correlate with unloading of normal stress. Besides, the intensive normal stress oscillation can weaken the shear strength which is recoverable when the normal stress return to constant. The rupture patterns are quantified by stress drop, slip length and slip velocity. With the effect of small oscillation amplitudes, the slip events show chaotic shapes, compared to the regular and predictable style under constant normal stress. When the amplitude is large enough, the big and small slip events emerge alternately, showing a compound slip style. Large amplitude of the cyclic normal stress also widens the interval differences of the slip events. This work provides experimental supports for a convincible link between the dynamic stress disturbance and the slip behavior of rock fractures.

Since its introduction, discontinuous deformation analysis (DDA) has been widely used in different areas of rock mechanics. By dividing large blocks into subblocks and introducing artificial joints, DDA can be applied to rock fracture simulation. However, parameter calibration, a fundamental issue in discontinuum methods, has not received enough attention in DDA. In this study, the parameter calibration of DDA for intact rock is carefully studied. To this end, a subblock DDA with Voronoi tessellation is presented first. Then, a modified contact constitutive law is introduced, in which the tensile and shear meso-strengths are modified to be independent of the bond lengths. This improvement can prevent the unjustified preferential failure of short edges. A method for imposing confining pressure is also introduced. Thereafter, sensitivity analysis is performed to investigate the influence of the calculated parameters and meso-parameters on the mechanical properties of modeled rock. Based on the sensitivity analysis, a unified calibration procedure is suggested for both cases with and without confining pressure. Finally, the calibration procedure is applied to two examples, including a biaxial compression test. The results show that the proposed Voronoi-based DDA can simulate rock fracture with and without confining pressure very well after careful parameter calibration.

Columnar jointed basalt (CJB) is a kind of jointed rock with a polygonal cylinder mosaic structure that has complex mechanical properties such as discontinuity and heterogeneity. The typical geological structure of the CJB is the intercolumnar joint plane and the implicit joint plane, which obviously affect the mechanical properties of the rock mass. Controlling the unloading relaxation of the CJB is a key problem during the construction of underground engineering. In this paper, in-situ acoustic wave and panoramic borehole camera measurements were carried out in the cavern of the Baihetan project to understand the failure mechanism of the collapse of the CJB. It was quite clear that the evolution of the excavation damage zone (EDZ) of the CJB depends on the time and spatial effects. The closer to the collapse zone, the greater the degree of relaxation failure of the columnar joint rock mass; the further away from the cavern perimeter, the more stable the surrounding rock. The correction between wave velocity and cracks in the rock mass was also discussed. This field test and theoretical analysis can provide a reference for studying the failure mechanism and control measures of CJB in underground caverns under high geostress.

Investigating deformation behavior of fractured reservoir bank slopes during impoundment plays an essential role in safety control of high arch dams. Because of changes in water levels during impoundment, the fractures are subjected to varying water pressure and cyclic wetting–drying conditions. In this numerical study, mechanical, physical, and chemical water–rock interactions in rock fractures are incorporated into discrete element modeling of fractured reservoir bank slopes, and effects of mechanical, physical, and chemical water–rock interactions on deformation of fractured reservoir bank slopes are investigated based on a case study of valley deformation at the Xiluodu Hydropower Station. The mechanical water–rock interaction is considered by the effective stress law, while the physical and chemical water–rock interactions are modeled by some empirical deterioration laws. The results showed that the modeled and monitored deformation behaviors of fractured reservoir bank slopes are in good agreement. The slip of shear zones due to water pressure elevation and mechanical parameter weakening is the main reason of impoundment-induced valley contraction. Discrete element method considering water–rock interactions can serve as a robust and reasonable tool to predict the deformation magnitude of fractured reservoir bank slope. Highlights: Mechanical, physical and chemical water–rock interactions in rock fractures are incorporated into discrete element modeling of fractured reservoir bank slopes. Four scenarios are designed to understand the role of mechanical, physical and chemical water–rock interactions on deformation of fractured reservoir bank slopes. The slip of shear zones due to water pressure elevation and mechanical parameter weakening is the main reason of impoundment-induced valley contraction. [ABSTRACT FROM AUTHOR]

In this paper a phase‐field formulation based on an extended F‐criterion (the normalized strain energy release rate criterion) is proposed to simulate tensile‐compressive‐shear rock fractures. By applying the F‐criterion, the phase‐field crack‐driving energy decomposition is determined by a direction search which maximizes the local fracture dissipation. In compressive‐shear states, the computation is supplemented by an explicitly expressed confinement‐dependent mode‐II fracture energy release rate, and the cracking angle is determined by both the fracture energy and strain states. The hybrid formulation and alternate minimization algorithm are adopted for the numerical examples in this paper. Fractures for rock and rock‐like specimens subjected to compression demonstrate the ability of the present model in capturing tensile‐compressive‐shear rock fracture behaviors. [ABSTRACT FROM AUTHOR]

Water inrush disasters in geotechnical engineering are mainly caused by seepage in fractures, and curtain grouting is the most common method to block water flow. To ensure the efficacy of the water blocking curtain, it is necessary to study the slurry's diffusion pattern. In this study, by means of laboratory experiments and theoretical deductions, the grout diffusion morphology in fractures with the slurry-rock stress coupling effect is revealed, and the corresponding theoretical model is established. First, based on the failure of water-blocking curtain in a hydropower station in Yunnan Province, a four-level four-factor orthogonal table was set up, grouting experiments were conducted using the self-developed fracture grouting device, and the relationship between the main influencing factors and fracture deformation and grout diffusion distance was revealed. Then, based on the Bingham fluid constitutive model, fracture deformation equation and grouting model with flowing water, a new fracture grouting model considering the coupling effect of slurry-rock stress was established. Finally, based on the calculations and experiments, the morphology of slurry diffusion is described, thereby proving the validity of the new model. [ABSTRACT FROM AUTHOR]

The stress background and rock fractures are essential factors affecting the stability of mines. In order to better understand the in situ stress background and rock fractures in the Guangdong Fankou Mine, we use ultrasonic borehole television scanning to measure rock fractures. The results indicate that rock fractures are intensively distributed at depths of −360 m to −450 m below the surface, suggesting the effect of intensive mining activities. The present maximum horizontal principal stress direction is NWW, which is consistent with the regional tectonic stress field direction. Systematic measurement of rock fractures is fundamental for further three-dimensional geological modeling and is significant for mining engineering.

The frictional rupture mechanisms of rock discontinuities considering the dynamic load disturbance still remain unclear. This paper investigates the transitional behaviors of slip events happened on a planar granite fracture under cyclic normal stress with different oscillation amplitudes. The experimental results show that the activations of fast slips always correlate with unloading of normal stress. Besides, the intensive normal stress oscillation can weaken the shear strength which is recoverable when the normal stress return to constant. The rupture patterns are quantified by stress drop, slip length and slip velocity. With the effect of small oscillation amplitudes, the slip events show chaotic shapes, compared to the regular and predictable style under constant normal stress. When the amplitude is large enough, the big and small slip events emerge alternately, showing a compound slip style. Large amplitude of the cyclic normal stress also widens the interval differences of the slip events. This work provides experimental supports for a convincible link between the dynamic stress disturbance and the slip behavior of rock fractures. Highlights: Fast slip events always happen at the unloading stage under cyclic normal stress conditions. The cyclic normal stress weakens shear stress and the slip styles are guided by the oscillation amplitudes. Compound rupture sequences can be observed when the amplitude of cyclic normal stress is large. [ABSTRACT FROM AUTHOR]

Piezoelectric excitation of quartz mineral phase in granite using high-frequency and high-voltage alternating current (HF-HV-AC) is a potential new weakening pretreatment in comminution of rock. The present study addresses this topic numerically by quantifying the weakening effect on the compressive strength of granite. For this end, a numerical method based on a damage-viscoplasticity model for granite failure under piezoelectric actuation is developed. The rock material is modelled as heterogeneous and isotropic. However, the piezoelectric properties of quartz are anisotropic. The governing global piezoelectro-mechanical problem is solved in a staggered manner explicitly in time. Numerical simulations predict that the weakening effect on compressive strength of granite is 10% with the excitation frequency and voltage of 274.4 kHz and 150 kV of the pretreatment. As the weakening effect takes place at a natural frequency of the numerical rock sample, the quartz content has only a slight effect on the frequency at which maximum weakening occurs. Moreover, the weakening effect depends strongly on the orientation of the quartz crystals. In a more practical application of simulating low-rate compression of a sphere-shaped rock sample, a weakening effect of 8% after the HF-HV-AC pretreatment was predicted. Highlights: Substantial damage can be induced on granite by piezoelectric excitation by high-voltage and high-frequency alternating current. This pretreatment by piezoelectricity of quartz can be used to weaken granite samples before mechanical comminution. The frequency of the excitation needs to match one of the natural frequencies of the rock sample. A weakening effect of 10% on the compressive strength can be achieved at frequency of 274 kHz and voltage of 150 kV. Experimental validation of the theoretical/numerical predictions is needed. [ABSTRACT FROM AUTHOR]

With the rapid development of underground engineering, the problem of sudden surge water has become one of the main problems that restrict engineering safety. Grouting method is commonly used to seal the fracture water flow. However, grouting in flowing water streams still faces many challenges. In this study, a rough fracture model is established based on the Barton curve, and the effects of fracture roughness, water flow rate, fracture inclination, and grouting pressure are considered, and orthogonal tests are used to study the diffusion pattern of slurry under the four factors. According to the observed phenomena, there are four main diffusion patterns of the slurry: (1) full slurry coverage with no water passing through; (2) little coverage with watercourses and cavity areas along both sides of the fracture; (3) partial slurry coverage with no obvious watercourses but some cavity areas; and (4) less coverage with larger watercourses and cavity areas. The reduction rate of flow rate at the outlet before and after grouting was used to express the sealing effect. The results show that the degree of influence on the slurry sealing effect from strong to weak is fracture roughness, water flow velocity, grouting pressure, and fracture angle in order. A method to characterize the sealing effect by slurry coverage is proposed based on the final distribution of residual slurry. Considering the mutual influence between different factors, the sealing effect of two factors is analyzed in order to provide reference for engineering practice. [ABSTRACT FROM AUTHOR]

The mining stress formed in the surrounding rock after coal seam mining is the root cause of the damage and fracture of the stope surrounding rock. In order to study the evolution characteristics of mining stress and the fracture process of overlying strata on the mining face, a numerical model was established to simulate the evolution law of stress field and fracture field in the mining process of the working face by taking the 40101 mining face of Gaoliang Coal Mine as an example. The results show that the relationship between rock collapse and failure can be judged by using Mohr-Coulomb criterion to analyze the variation law of the difference between maximum and minimum principal stresses. The correctness of the time-varying evolution of fracture field and stress field in working face mining can be verified by comparing the numerical simulation results with the field measured results.

Abstract Fractures are widely present in geomaterials of civil engineering and deep underground engineering. Given that geomaterials are usually brittle, the fractures can significantly affect the evaluation of underground engineering construction safety and the early warning of rock failure. However, the crack initiation and propagation in brittle materials under composite loading remain unknown so far. In this study, a three‐dimensional internal laser‐engraved cracking technique was applied to produce internal cracks without causing damage to the surfaces. The uniaxial compression tests were performed on a brittle material with internal cracks to investigate the propagation of these internal cracks at different dip angles under compression and shear. The test results show that the wing crack propagation mainly occurs in the specimen with an inclined internal crack, which is a mixed‐Mode I–II–III fracture; in contrast, Mode I fracture is present in the specimen with a vertical internal crack. The fractography characteristics of Mode III fracture display a lance‐like pattern. The fracture mechanism in the brittle material under compression is that the internal wing cracks propagate to the ends of the whole sample and cause the final failure. The initial deflection angle of the wing crack is determined by the participation ratio of stress intensity factors KII to KI at the tip of the internal crack.

In the process of rock excavation, cyclic loading may occur before or after the peak value. In order to obtain the fracture damage evolution characteristics of rock under the pre-peak and the post-peak cyclic loading condition, the fracture tests of three-point bending granite beams were conducted by using the digital image correlation method and the acoustic emission technology. Based on the secant modulus, the acoustic emission energy and effective crack length, the rock fracture damage indexes DE, DAE, and DL were constructed respectively, and the rock fracture damage process was quantitatively analyzed. The results show that the damage index of the rock sample has an obvious rate effect. At the same number of cycles, as the loading rate increases, the damage index decreases. For the pre-peak cyclic loading condition, the fracture process of rock from elastic stage to elastic-plastic stage can be well reflected by DE. Under the condition of post-peak cyclic loading, DL can better represent the fracture damage process of rock from step up to continuous failure.

For various rock engineering, injection of fluids into rock fractures through boreholes is quite common. It is of great significance to investigate the characteristics of radial flow (RF) in rock fractures for these activities. In this study, macroscopic and mesoscopic characteristics of RF in rough rock fractures were investigated and compared with those of unidirectional flow (UF) by theoretical analysis, tests and simulations. An equation for nonlinear RF was derived for rock fractures according to conservation law of mass and Izbash's law. Four scanned rough rock fracture models were established and used to experimentally investigate the macroscopic flow characteristics in both UF and RF. Numerical simulations were performed to clarify the mesoscopic differences in fluid pressure distributions and the flowlines of RF and UF in rock fractures. The parameters of hydraulic aperture and equivalent width for RF were obtained and correlated to those for UF. A method to calculate fracture roughness coefficient of fractures for RF related to the flow direction was proposed. The characteristic parameters, i.e., critical Reynolds numbers for the flow transition from linear to nonlinear flow, effective hydraulic apertures and non-Darcy coefficients, were obtained for the UF and RF based on the test results. It was indicated that the fracture roughness plays a critical role in the macroscopic and mesoscopic characteristics of both RF and UF. According to the test results, the macroscopic characteristic parameters for RF are related to those for UF, and the nonlinearity of RF was stronger than that of UF at a specified flow rate, which was consistent with the mesoscopic characteristics observed in the simulation that the distribution of water pressure, flow velocity and the streamlines in RF were more non-uniform than that in UF. The study results were useful to describe the RF characteristic in rock fractures with the characteristic parameters for UF, which have been investigated extensively in literature. Highlights: A nonlinear flow equation for radial flow in rock fractures was derived to describe the relationship between the hydraulic head and flow rate. The differences and relations between radial and unidirectional flow were studied from macroscopic and mesoscopic aspects. The parameters of hydraulic aperture and equivalent width for radial flow were obtained and correlated to those for unidirectional flow. The effect of fracture roughness on radial and unidirectional flow was related to the flow direction and was incorporated in the Forchheimer equation. [ABSTRACT FROM AUTHOR]

Acoustic emission (AE), a phenomenon of elastic waves released by localized fracture generation, has been extensively utilized as an effective tool for monitoring rock failure processes in many rock mechanics related fields. Within the framework of the combined finite-discrete element method (FDEM), we develop a new AE simulation technique based on moment tensor theory considering the clustering effect of microcracks. The technique first integrates forces around the AE source to obtain the moment tensor, and then estimates the AE magnitude associated with the acquired moment tensor. In addition to quantifying the seismic source mechanisms of the modeled AE events, the technique can also distinguish fracture types based on moment tensor decomposition approaches when an AE event contains multiple microcracks. The effectiveness of the newly developed approach for capturing the distribution of AE event magnitude is firstly verified by establishing a heterogeneous rock model under uniaxial compressive load. Then, we perform four typical tests to validate the effectiveness of the proposed approach for distinguishing the source mechanism of microcracks, and further revise the traditional criterion to better accommodate the discrimination of the full spectrum of AE source types. Furthermore, the fractures generated in the heterogeneous model demonstrate the capability of the moment tensor decomposition approach in distinguishing macro-fracture types on laboratory scales. As an exemplar application, we also establish a numerical model to analyze the failure mechanism in a bridge region of two pre-existing flaws in a rock specimen through laboratory-scale uniaxial compression tests. The work may provide a new means to analyze fracturing and failure in rocks and the associated seismic behaviors. Highlights: Moment tensor-based AE simulation approach is realized in FDEM for the first time. A clustering algorithm to integrate spatially and temporally connected microcracks is implemented. The effectiveness of the proposed approach for capturing AE event magnitude is verified. The proposed approach's capability to distinguish macroscopic fracture types is validated. The traditional criterion is modified to better discriminate full-spectrum AE source types. [ABSTRACT FROM AUTHOR]