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Normal stress ( σ n ) under compression-shear conditions significantly affects crack propagation and failure modes of specimens containing non-persistent joints (NPJs). Crack propagation and failure modes of specimens vary for different σ n . In the present study, we adopt a strength-based local maximum stress criterion (SLMS) to describe tensile and shear cracks formed during shear and model crack propagation processes via the finite element method. Two classical cases are performed to validate the SLMS criterion for describing tensile and shear cracks under shear conditions. After that, crack initiation, propagation, and coalescence processes are modeled for specimens containing a pair of horizontal NPJs in coplanar and non-coplanar cases at different σ n . Variations of crack propagation and failure modes of specimens containing coplanar and non-coplanar NPJs are investigated for σ n ranging from low to high. Effects of rock bridge inclination angle (β ) on crack propagation and failure mode of specimens containing non-coplanar NPJs are investigated. Also, the effects of σ n on critical shear load ( τ sc ) for crack initiation in specimens containing coplanar and non-coplanar NPJs are examined. Results indicate that failure modes of specimens with coplanar NPJs transform from tensile to mixed tensile-shear, then to shear, and finally to pure shear as σ n increases. The increase of β promotes the appearance of tensile failure of specimens containing coplanar NPJs under high σ n while suppressing the appearance of shear failure. The modeling results provide a reference for understanding crack propagation and failure modes in rocks containing NPJs under compression-shear conditions. [ABSTRACT FROM AUTHOR]

The methane adsorption test on coal was conducted under three initial pressures of 0.50 MPa, 0.75 MPa, and 1.00 MPa using the coal bed adsorption simulation test system. The focus of this study is to analyze the evolutionary characteristics of CH4 pressure and coal seam temperature under three different initial pressures. The distribution law of methane gas pressure and coal seam temperature on the inner plane of a coal seam, as well as the coupling relationship between them, are studied by interpolating and plotting the collected data using MATLAB software. The results indicate that methane pressure decreases over time, while the coal seam temperature increases. There is no linear correlation between the decrease in pressure and the increase in temperature. When coal is adsorbed, the pressure and temperature in the central region are lower, while the temperature in the region with high pressure is higher. The distribution of pressure and temperature is synchronized. The higher the initial pressure, the higher the temperature of the coal seam, and the more methane is adsorbed by the coal. However, the strength of the coal's adsorption capacity cannot be simply measured by the magnitude of the pressure drop. The research content can be used as the basis for further investigation of the methane occurrence rule in coal seams and the evaluation of methane adsorption capacity of coal. [ABSTRACT FROM AUTHOR]

The information of fractures in rock mass is an essential indicator to evaluate the quality of rock mass. It is precise and efficient to obtain information of fractures by computer vision (CV)-based fracture detection technologies. However, rock masses in practical engineering always exhibit complex fractures with messy edges, shadows, and uneven rock surfaces. These factors negatively impact the effectiveness of fracture detection algorithms. To address such issues, this paper proposes a confidence score-based rock fracture detection algorithm that can effectively identify fractures in complex situations. The image with fractures is traversed to acquire candidate points at first. We proposed a method to evaluate the confidence scores of these candidate points according to the values of Hessian matrix eigenvalues, the degree of symmetry, and the gray-scale value. The candidate points are further filtered through threshold restriction and non-maximum suppression. Furthermore, the complete centerlines of the fractures are obtained by the connection of the candidate points and the connection of discontinuous centerlines. Finally, pseudo-fractures and noise are eliminated to get complete fractures. Experimental results show that the newly proposed algorithm can accurately identify fractures in complex situations. The algorithm avoids interference and noise, which achieves more precise results than other conventional fracture detection algorithms, is an effective method to identify fractures in rocky geological engineering and can also serve as a guide for other relevant fracture identification algorithm in rock mass. [ABSTRACT FROM AUTHOR]

Crack propagation of the rock specimen containing a central closed flaw at different end friction coefficients (µ) under uniaxial compression and biaxial unloading confining pressure has been modeled under different loading conditions in this study employing a strength-based localized maximum stress (SLMS) criterion. The contact problem at the ends of specimens is solved via an augmented Lagrangian method. The effects of µ on tensile and shear crack propagation paths and lateral deformation of specimen ends are evaluated. Based on the variation of contact state at specimen ends under different loading and end conditions, the evolution of kinetic and static friction distribution at the specimen ends during crack propagation and the influence of µ on the kinetic and static friction distribution is investigated. The results show that the contact state at the specimen ends varies with different µ and can be classified into three types: complete sliding (µ = 0), sliding-sticking mixed (0 < µ < 0.3), and complete sticking (µ ≥ 0.3). The contact state of the end surfaces varies dynamically as the crack propagates when 0 < µ < 0.3. The friction on the end surfaces is kinetic-static mixed when 0 < µ < 0.3, while it is complete static friction when µ ≥ 0.3. For 0 < µ < 0.3, the static friction section in the end surfaces gradually increases with the crack propagation, which is more significant under biaxial unloading confining pressure due to the propagation of shear cracks. [ABSTRACT FROM AUTHOR]

During the construction of the Hanjiang-to-Weihe River Diversion Project, frequent extremely intense rockbursts posed a serious threat to the safety of personnel and equipment. A microseismic (MS) monitoring system, which can effectively predict rockbursts, was established to monitor the sprouting and propagation of microcracks within the rock mass surrounding the tunnel in real time. First, the MS activity characteristics of the first excavated section (K33 + 870 ~ K37 + 011) and the second excavated section (K39 + 596 ~ K41 + 602) in the south Qinling section were compared under different surrounding rock classifications. Then, the time distribution characteristics of MS events from May 14 to June 14, 2020 were analyzed in detail, focusing on the MS event energy and spatial distribution of the first extremely intense rockburst. In addition, the effects of the daily excavation distance and excavation speed on the characteristics of extremely intense rockburst and MS activity were explored. Finally, the waveform and time–frequency characteristics of rockbursts with different intensities in the different surrounding rock classifications are discussed. According to the characteristics of the time–frequency, the extremely intense rockbursts in the classification I surrounding rock are divided mainly into low-frequency sustained type, low-frequency discontinuity type, and high-frequency discontinuity type, and the extremely intense rockbursts in the classification II surrounding rock are divided mainly into high-frequency discontinuity type and low-frequency discontinuity type. The study results are of great significance for improving the excavation efficiency and rockburst prediction accuracy in deep-buried tunnels with frequent rockbursts. [ABSTRACT FROM AUTHOR]

Water is a crucial factor that influences the mechanical behavior of rock and can induce failure. The aim of this study was to investigate the effects of water content and water distribution on the mechanical behavior of sandstone subjected to triaxial compression. Sandstone samples under different water states, including dry, unsaturated, saturated, and long-term saturated, were prepared. Some saturated samples were dried in air for different periods to prepare samples that were dry on the outside but remained wet on the inside. The triaxial compression tests were performed on the samples under four confining pressures: 5, 10, 15 and 25 MPa. The results indicated that the water state of a sandstone sample can be characterized by its water content, soaking time, water distribution, and long-term saturation. The different water states can affect remarkably the strength, deformation, and failure pattern of the sandstone samples subjected to triaxial compression. Several empirical equations were established to describe the evolution of the mechanical behavior of sandstone with increasing water content and soaking duration. The nonuniform distribution of water promotes the generation of tensile cracks, while increasing confining pressure promotes the development of shear cracks. The strength of sandstone samples with dry outside and wet inside are lower than that of samples with wet outside and dry inside. What is more, a long-term saturated state increases the strain-softening of the sandstone samples, thereby increasing their deformation and lowering their strength. Highlights: Influence of water state including water content and water distribution on sandstone is comprehensively investigated by triaxial experiments. A series of predictive empirical equations are established to describe the evolutions of the mechanical properties of sandstone with water content/soaking duration. The strengths of the sandstone samples in the dry outside but wet inside state are smaller than those of samples in the wet outside but dry inside state. A long-term saturated state increases the softening of the rock, thereby increasing the deformation and lowering the strength. Water especially nonuniformly distributed water promotes the propagation of tensile cracks near the surface of the rock. [ABSTRACT FROM AUTHOR]

Water is believed to be a significant factor affecting the short- and long-term strength of rocks. To further understand the effect of water on the mechanical behavior of rocks, we first performed a series of water absorption tests and uniaxial compression tests (different soak times) to guide a time-dependent creep test. Then, uniaxial creep tests under different water-stress sequence conditions and the traditional creep test were performed. Finally, the effect of water distribution on rock strength and failure patterns is discussed. The obtained results show that the water-stress sequences would result in different mechanical behaviors of rock, i.e., the specimens were more likely to fail under the condition of loading followed by soaking than under the condition of soaking followed by loading and loading after soaking. Furthermore, the strength of the specimen with a nonhomogeneous water distribution is greater than that of the homogeneous specimen if they have the same water content. Because the nonhomogeneous water distribution is dry inside the specimen, the homogeneous water distribution affects the entire specimen, leading to greater softening. According to the obtained results, the sequence of loading and soaking should be considered when predicting the stability of rock mass engineering. [ABSTRACT FROM AUTHOR]

Deep neural networks have achieved great success in microseismic event recognition tasks in rock engineering. However, a well-trained model on one project may not deliver satisfactory results on other new projects due to the different geological conditions, construction methods and monitoring configurations. In this study, the differences of microseismic monitoring in four monitoring project cases, including the geology conditions and monitoring settings, are compared to better understand the challenges of signal recognition by deep learning, and then the drawbacks and unavailability of current algorithms for microseismic monitoring scenarios are discussed. To address the challenges, a unified transfer learning strategy is proposed to perform knowledge transfer for microseismic recognition between multiple projects with varying difficulties, and it consists of a novel unsupervised two-level adversarial adaptation method and a semisupervised active learning mechanism; thus, it operates in a semisupervised fashion. The results experimentally demonstrate that a well-trained model cannot generalize well to other rock projects without performing transfer learning. This framework is evaluated on four real-world collections of monitoring recordings and compared with other benchmark methods. The results show that it outperforms all counterparts and is able to conduct knowledge transfer with varying difficulties. Moreover, the proposed active learning mechanism significantly reduces the manual labeling costs for both hard and easy tasks. The simulations suggest that we have preliminarily achieved deep model reuse in signal recognition of microseismic monitoring. Highlights: The transferability and vulnerabilities of the existing deep neural networks for microseismic event identification between different monitoring projects are discussed. A novel two-level adversarial domain adaptation method is proposed to perform transfer learning in unsupervised setting. A "performance sacrifice" method is proposed to improve algorithm design and explain models' performance. An active learning mechanism is implemented to greatly reduce manual labelling cost. A unified transfer learning strategy is proposed to perform knowledge transfer for microseismic event recognition of different underground construction projects with varying difficulties. [ABSTRACT FROM AUTHOR]

Frost heave damage of fractured rock mass is often involved in cold region engineering construction. In order to study the frost heave pressure caused by water-ice phase transition in saturated water crack affected by low temperature, which induce secondary crack initiation, formation, propagation, and penetration, a thermal-mechanical coupling model considering the heterogeneity of the rock is first established. Then, the equivalent thermal expansion coefficient method is used to solve the frost heave pressure value and verified with the theoretical model. Finally, the variation of frost heave damage crack growth in saturated water crack is analyzed. Results show that the frost heave deformation and failure process of fractured rock can be divided into linear elastic deformation stage, micro-crack initiation stage to secondary crack formation stage, and unstable fracture stage. The crack angle and frost heave pressure increased gradually with the increase of cracked ice width, and the cracked ice width is a significant factor on the growth of secondary cracks. When the cracked ice width is in the range of 0 to 1 mm, the secondary crack is an "I" shaped. When the cracked ice width is larger than 1 mm, the secondary crack shape is inclined to an oblique "I" shape. The secondary crack development under low temperature is less affected by the number of parallel cracked ice, and the secondary crack is mainly an inverted funnel shape. The results are of great significance to freeze-thaw damage and crack propagation of fractured rock mass at low temperature. [ABSTRACT FROM AUTHOR]

In order to solve the thermal damage caused by the long-distance high-temperature roadway mining stage in the late stage of the 20th middle section of the Daliuhang gold mine, fluent software and Ventsim software were used to simulate the tunnel cooling model. Firstly, the idea of sectional ventilation and cooling in long-distance high-temperature roadway is put forward, and the mathematical model of sectional uniform ventilation is established according to the theory of fluid mechanics. Secondly, the Fluent software is used to simulate and test the mathematical model. Thirdly, the Ventsim software is used to optimize the mine ventilation system, and the sectional ventilation cooling design is carried out. Finally, the cooling effect of roadway with conventional forced ventilation and sectional ventilation is simulated, and the simulation results are analyzed. The results show that: in the 220-m-long roadway in the late mining stage of the 20th middle section, the cooling effect of the roadway with sectional ventilation is about 15% higher than that of the conventional local ventilation. The cooling effect of the conventional forced ventilation near the working face of the 20th middle section is better than that of the sectional ventilation; when the return air distance exceeds 70m, the sectional ventilation cooling method is better than the conventional local ventilation cooling method. [ABSTRACT FROM AUTHOR]

The shear behavior of rock is markedly affected by the presence of water. To study the influence of water on the shear behaviors of rock, a series of direct shear tests were conducted on intact sandstone specimens with different water soaking durations (from dried to saturated) under three levels of normal stresses. Before direct shear tests were performed, water absorption experiments were conducted to guide the different levels of water saturation required for the direct shear tests. The influences of water on shear stress-shear strain responses and peak and residual shear strength envelopes were investigated. The results show that soaking duration significantly impairs the peak and residual shear strengths and other shear properties (e.g., cohesion, friction angle and shear stiffness). The reductions in fracture energy and friction coefficient and the chemical and corrosive deterioration are the main mechanisms for the decrease of intact sandstone's shear strength. A novel observation is that the failure patterns of specimens are sensitive to different water distributions. The results provided in this study reveal that the stability of rock engineering is affected by changes in water levels. [ABSTRACT FROM AUTHOR]

Microseismic technology has widely been used in many rock engineering applications to shield workers from engineering hazards and monitor underground construction. To avoid the heavy workloads imposed by the manual recognition of many microseismic signals, this study proposes a new end-to-end training network architecture to automatically identify microseismic events. A dataset including not only easily identifiable microseismic signals but also barely distinguishable nontypical data has been collected from a practical rock engineering project for training and testing the network model. The applicability of various networks for this task is discussed to select the best method for microseismic recognition. We modify the residual skip connections to make them more suitable for the signal classification task. Then, the novel depthwise spatial and channel attention (DSCA) module is proposed. This module can autonomously learn how to weight information with different levels of importance, similar to human attention, which greatly improves the network performance without incurring additional computational costs. Theoretically, it can be a useful tool to replace traditional denoising algorithms and model the interdependencies between the different channels of a multichannel signal. Furthermore, the DSCA module and the modified residual connections are combined with a traditional convolutional network to obtain a novel network architecture named ResSCA and the results of comparative experiments are presented. Finally, single- and multichannel models are constructed based on ResSCA, which achieved improved accuracy rates. Their advantages and drawbacks are analyzed. This study presents a modified network architecture suitable for identifying and classifying complex signals to enable intelligent microseismic monitoring, which is valuable for various rock engineering applications. [ABSTRACT FROM AUTHOR]

Thermal cracking is broadly observed in rock engineering. A finite element numerical model which considers the heterogeneity of rock materials and the damage evolution process was used to simulate the thermal cracking behavior of square rock samples heated from the central borehole. The thermal and mechanical behaviors of two cases, i.e., the case with large size but low heating rate and the case with small size but high heating rate were compared to study the crack initiation location in the models with different model sizes and heating rates. The simulated stress and temperature fields, as well as the failure pattern, were in good agreement with the experimental observations. The temperature and thermal stress distribution during the heating process in both cases indicated that high tensile stress was concentrated around the thermal gradient front, which resulted in the cracks initiating at the location with a certain distance away from the borehole. The results show that under the same heating rate, crack initiation location moves outwards with the increment of the model size then remains approximately at one location, which reflects the boundary-effect. Furthermore, the results indicate that the relative crack initiation locations in two cases are nearly the same if the ratio between the heating rate in two cases (Ṫx/Ṫy) nearly equal to the square of the inverse ratio of corresponding model side lengths ((ay/ax)2). This concept is named the scale-heating rate equivalence effect in this study. It is beneficial for studying thermal cracking of rock both numerically and experimentally since the model size can be significantly decreased. [ABSTRACT FROM AUTHOR]

A method that uses finite element analysis to determine the non-singular stress ( T-stress) at a crack tip is proposed in this study. T-stress includes two components: the T -stress parallel to the tangent of the crack at its tip and the T -stress perpendicular to this tangent. The effects of contact and friction on both the T - and T -stresses on the crack flanks are considered in the method. Because the method uses a single standard elastic finite element analysis derived directly from the equation of the stress fields around the crack tip and does not require any assumptions or simplification, it can be used to determine the T-stress for any given geometry and loading condition. Theoretical results are used to calibrate the results, which exhibited good agreement and to discuss the T-stress computational methodology. Furthermore, the T - and T -stresses in center-cracked Brazilian disc (CCBD) specimens subjected to diametrical or partially distributed compression were numerically computed, and the effects of contact and friction on the T - and T -stresses are discussed. [ABSTRACT FROM AUTHOR]

A discontinuous deformation and displacement (DDD) analysis method is proposed for modelling the rock failure process. This method combines the rock failure process analysis (RFPA) method (based on finite element method) and discontinuous deformation analysis (DDA) method. RFPA is used to simulate crack initiation, propagation and coalescence processes of rock during the small deformation state. The DDA method is used to simulate the movement of blocks created by the multiple cracks modelled by the RFPA. The newly developed DDD method is particularly suitable for modelling both crack propagation and block movement during the rock failure process because of the natural and convenient coupling of continuous and discontinuous deformation analyses. The proposed method has been used to simulate crack initiation, propagation and coalescence within a slope as well as the block movement during the landslide process. Numerical modelling results indicate that the proposed DDD method can automatically simulate crack propagation and block movement during the rock failure process without degrading accuracy. [ABSTRACT FROM AUTHOR]

The phenomenon of edge fractures is analyzed using a fracturing process approach. Such fractures often initiated from the free surface of layered materials, and often terminate at the interface that divides the fractured layer and the matrix layer, or continue to expand along the interface, or create a peel-crack. To understand better the pattern of fractures with different spacing and the fracture mechanism, a double-layer elastic model with the same material properties with a fractured overlying layer subjected to uniaxial tension is simulated firstly. The stress distribution between two adjacent fractures is periodically distributed as a function of the ratio of the fracture spacing to the thickness of the fractured layer (S/t ratio), and homogeneous and heterogeneous material properties are considered in the simulation. The simulation results show that both the stress distribution and the critical value of fracture spacing to layer thickness ratio are affected by material heterogeneity; the S/t ratio of heterogeneous material is much smaller and it is much easier to crack compared with the homogeneous materials. In particular, the fracture saturation mechanism is analyzed by stress state change. Then a numerical simulation is carried out to reveal the fracturing process from micro-fracture formation, propagation, coalescence, nucleation, fracture infilling, fracture saturation, termination, to interface delamination. A fitting curve of the relationship between strain and spacing to layer thickness radio is obtained. It is found that infilling fractures may grow near the bottom or the free surface of the fractured layer by coalescence of micro-fractures and flaws. We also find that fracture spacing in the case of interface delamination is greater than that without interface delamination. A study of stress transition between the two layers on fracture spacing in the fracture process is also carried out, with a focus on stress transfer mode. [ABSTRACT FROM AUTHOR]

The cracking mechanism of ice-saturated fissured rock is crucial when using the freezing method to construct shafts with water-rich soft rock layers. However, the dynamic cracking mechanism of ice-saturated rock with a flaw and the ice-filling mechanism still needs further investigation. In this study, the drop weight tests were conducted on sandstone with a single ice-saturated flaw to evaluate the influence of Flaw Dip Angles (FDA: 0°, 30°, 60°, and 90°) and flaw lengths (L: 8, 15, 30 mm) on the dynamic cracking process. Strain time history curves (STHC) and failure modes were analyzed, and PFC2D numerical simulations were used to discuss the cracking and ice-filling mechanisms. The results indicate the peak strain of the ice-filled sample is significantly lower than that of the intact specimen. The tensile and tensile–shear mixed modes are the domain failure patterns. The consumed energy of sandstone with horizontal or vertical flaws increases with the increase of L. For the inclined flaws, the energy increases first and then decreases. The crack initiation positions of the freezing samples are the same, but the propagation paths are different, and the wing and the splitting cracks control the failure. The filled ice can improve anti-deformation ability and crack initiation stress in the initial loading stage. Compared with unfilled flaw samples, the filled ice's support, tensile, and shear resistance lead to higher strength. [ABSTRACT FROM AUTHOR]