Rock Crack Research Articles

A coupled thermo-mechanical model for investigating cracking and failure of composite interbedded rock

Composite interbedded rock is a special geological phenomenon that occurs when layers of rock (beds) of a given lithology lie between or alternate with other layers of a different lithology. The thermal and mechanical properties of the composite interbedded rocks exhibit strong anisotropy. Studying the thermomechanical coupling properties of natural composite interbedded rocks using existing experimental techniques is challenging. A coupled thermo-mechanical composite interbedded rock model was developed based on Fourier's heat-conduction law and Newton's second law. The thermodynamic responses and biaxial mechanical properties of composite interbedded rocks with different interlayer angles under the coupling of initial stress and thermal loading were studied for the first time. The results indicate that the thermal stresses within the granite interlayers are higher than those within the sandstone interlayers. Throughout the thermal loading process, thermal cracking occurred primarily within the (stiffer) granite interlayers. The number of thermal cracks generated during the cooling process far exceeds that during heating; at 400 °C, the number of thermal cracks during cooling is more than 35 times that during heating. When the interlayer dip angle approaches 90° and is aligned with the major principal stress direction, the phenomenon of the “double peaks” and layered failure becomes more pronounced. Within the range of variables considered, the relative impact of the three variables on peak stress in composite interbedded rock is as follows, from greatest to least: interlayer angle, thermal shock temperature, and model size. The results and developed modeling methods will contribute to the management and optimization of future engineering geology projects.

Simulation of cracking processes of the rectangular cavern surrounding rock based on a continuum–discontinuum method

In a self-developed continuum–discontinuum method that considers the internal cracking of quadrilateral elements, the computational domain is divided into quadrilateral elements with high accuracy. After cracking, the cracks propagate along the boundaries of the elements and the diagonal lines of the quadrilateral elements. Deformation, cracking, and contact of rocks can be simulated using this method. For Brazilian disc rock specimens, the rapid increase of tensile crack segments corresponds to a rapid decrease of the load. Compared with the original method, crack propagation is smoother in the improved method. This is because the propagation paths of cracks are fewer in the original method, and the propagation of cracks is hindered. For uniaxial compressive rock specimens, when two shear cracks intersect, one crack may dominate, forming a main crack, and its direction is consistent with the theoretical direction obtained using the Mohr − Coulomb criterion. For the cavern surrounding rock models, some long and narrow shear cracks appear in the surrounding rock, and their shapes are like logarithmic spirals. An interval exists between the shear cracks, and the more to the interior of the surrounding rock, the larger the interval between cracks. The higher the hydrostatic pressure, the faster the shear crack propagates and the easier it is to form a shear slip line. With an increase of cavern size, the depth and range of the shear cracks increase. With a decrease of the internal friction angle, the interval between the cracks increases, and the propagation direction of the cracks become more internal.

Numerical simulation of blasting behavior of rock mass with cavity under high in-situ stress

With the shift of coal seam mining to the deep, the in-situ stress of coal and rock mass increases gradually. High ground stress can limit the generation of rock cracks caused by blasting, and blasting usually shows different crushing states than low stress conditions. In order to study the blasting expansion rule of rock mass with cavity under high ground stress and the rock mass fracture state under different side stress coefficients. In this paper, the effective range of blasting and the stress distribution under blasting load are analyzed theoretically. The RHT (Riedel-Hiermaier-Thoma) model is used to numerically simulate the blasting process of rock mass with cavity under different ground stress, and the influence of ground stress and lateral pressure coefficient on the crack growth of rock mass is studied. The results show that when there is no ground stress, the damage cracks in rock mass are more concentrated in the horizontal direction and the fracture development tends to the direction where the holes are located, which confirms the guiding effect and stress concentration effect of the holes in rock mass, which helps to promote the crack penetration between the hole and the hole. The length difference of horizontal and vertical damage cracks in rock mass increases with the increase of horizontal and vertical stress difference. Under the same lateral stress coefficient, the larger the horizontal and vertical stress difference is, the stronger the inhibition effect on crack formation is. For blasting of rock mass with high ground stress, the crack formation length between gun holes decreases with the increase of stress level, and the crack extends preferentially in the direction of higher stress. Therefore, the placement of gun holes along the direction of greater stress and the shortening of hole spacing are conducive to the penetration of cracks between gun holes and empty holes. The research can provide reference for rock breaking behavior of deep rock mass blasting.

Simulation of Rock Crack Propagation and Failure Behavior Based on a Mixed Failure Model with SPH

Simulation of Rock Crack Propagation and Failure Behavior Based on a Mixed Failure Model with SPH

Macroscopic experimental study and microscopic phenomenon analysis of damage self-healing in salt rock

Salt rock is characterized by a low porosity, low permeability and self-healing ability, and it is one of the most common materials used for underground energy storage and construction in underground nuclear waste repositories. Studying the healing of macroscopic cracks in salt rock under different conditions is important for ensuring the stability of the rock surrounding a salt cavern. To evaluate the healing ability and effect of cracks in salt rock at the macroscale, two tests were designed to directly assess the healing ability of salt rock by analyzing the tensile strength and permeability recovery of healed crack surface. Furthermore, the microstructure of the healed macroscopic fractures in salt rock was observed using scanning electron microscopy (SEM), and the microscopic mechanisms of salt rock damage healing were analyzed. (1) The results show that the presence of water is an important condition for the healing of cracks in salt rocks. Fully fractured salt rock cracks can heal at a low pressure of 160 kPa, and the uniaxial tensile strength of the healed specimens can reach a maximum of 121.1 kPa. It was found via computed tomography (CT) that a healed crack was still a weak surface in the whole specimen. (2) Increases in temperature, environmental humidity, and normal stress all promote the self-healing of salt rock fractures. Based on the experimental data, a model describing the evolution of salt rock damage healing variables with respect to temperature, humidity, and stress was established. It was also observed that the change in salt rock damage healing variables over time during the healing process can be represented by a first-order exponential decay function. (3) Using SEM, detailed micro healing experiments were conducted on NaCl single crystal and impurity-containing salt rock specimens. The healing characteristics of intracrystalline cracks and intercrystalline cracks were studied. It was observed that salt rock damage healing manifests at the microscopic level as the filling of microcracks and crack segmentation caused by grain growth. Additionally, the presence of a certain amount of impurities was found to promote the growth of internal structures that heal cracks within salt rock. These findings are of great significance for evaluating the stability and tightness of the surrounding rock of salt cavern reservoirs.

Rock crack initiation triggered by energy digestion

The critical value of rock failure is determined by irreversible deformation (inelastic deformation, damage, and other internal dissipation) processes and external conditions before rock failure. Nevertheless, a thorough explanation of the mechanism causing cracks in rock material has not yet been provided. The strain energy theory is applied in this work to assess the initiation of rock cracks and investigate the relationship between energy digestion and rock strength. Firstly, the uniaxial compression test was conducted on sandstone samples under quasi-static loading conditions and the results of energy evolution, non-linear cumulative digestion, and stored ultimate energy were obtained. Then, a novel algorithm for assessing the initiation of rock cracks has been put forth. The concept of energy digestion index (EDI), which is the ratio of digested energy over the external loading energy, has been developed to characterize the energy absorption capacity of rock material. The result shows a relationship between the maximum growth rate of energy digestion and the increasing rate of variable elasticity modulus and crack initiation. The mechanical characteristics and peak strength of the rock material are negatively correlated with the EDI. By monitoring the digested energy status, an evaluation of the residual strength is introduced based on the relationships, which will initiate further research into in-situ monitoring and failure prediction.

An adaptive phase field approach to 3D internal crack growth in rocks

Modeling the 3D internal crack under compression entails complex fracture mechanics (mode I-II-III fracture), resulting in substantial computational costs and challenges in characterizing fracture morphology characterization for Phase Field Method (PFM) simulations. In the present paper, we developed a 3D adaptive crack propagation model integrated into the PFM to study the internal cracks in rocks. The proposed locally refined solution was implemented within the isogeometric analysis (IGA) framework, utilizing PHT-splines and employing the phase field variable as an error indicator to accurately capture non-smooth crack surfaces. The simulation begins with a coarser mesh, and the spatial discretization is adaptively refined in real-time as the computation progresses and the phase variable threshold is reached. By comparing our results with previous numerical tests, we validated the effectiveness of our model in simulating crack growth. Notably, a smaller length scale parameter led to a higher peak force in notched square plates under tension, and our adaptive PFM successfully reduced errors caused by length scale parameters. We further validated our model by comparing it with uniaxial compression experiments on 3D-printed resin samples with internal penny-shaped cracks, achieving good agreement with the experimental data. Using the 3D adaptive PFM, we predicted changes in crack morphology and fracturing mechanisms by varying the inclination angles of internal penny-shaped cracks. Our numerical results showed that as the inclination angle increased, wing cracks initiated closer to the end tips of the original crack, and their length decreased due to the wings wrapping effect. The corresponding fracturing mechanisms transition from mode I-III fracture (0°) to mode I-II-III fracture (30°, 45°, 60°), and eventually to mode I fracture (90°) as inclination angles increase. Comparing 2D and 3D models, we found that the growth of wing cracks in 3D was significantly influenced by the intermediate principal stress. We conclude that the extended 3D adaptive PFM-based model accurately predicted complex mechanical behavior and crack growth patterns in rock.

Can laser irradiation improve the strength of weak rock mass?

Controllable rock cracking technology is crucial for the exploration and exploitation of deep underground resources. Many existing studies have been dedicated to the laser-assisted rock-weakening technology. It has been proved that laser irradiation can improve drilling and blasting efficiency when combined with mechanical rock fracturing methods, which are irrelevant for borehole stabilization. To improve the latter, this study used laser ablation for borehole reinforcement. The high-power laser was applied to typical rock samples (sandstone, mudstone and coal) in both dry and saturated conditions. Multi-technique observations and measurements were used to fully understand the peculiar modifications of the specimens under laser treatment, i.e. mechanical loading, acoustic emission (AE) monitoring, digital image correlation (DIC) strain field evaluation, infrared thermography (IRT) monitoring and X-ray computed tomography (CT) scanning. The results showed that, in addition to the effects already demonstrated, laser irradiation can improve the strength of the soft rock, especially in the saturated state. The process involved a complicated phase change including melting and evaporation of the matrix under high-temperature and high-pressure to form a glassy high strength silicate material. This process is similar to the reaction between molten lava and water, or the impact of an asteroid on the earth. Inspired by the results, a conceptual path for a new borehole stabilization technology using laser ablation was outlined.

Modelling a foreland fold-and-thrust petroleum system: a Permian example in the NW Sichuan Basin, Southwest China

ABSTRACT Foreland thrust belts are globally significant areas for oil and gas exploration. These belts undergo unique tectonic activity, resulting in distinct variations in the accumulation of oil and gas. Studying the genetic evolution of deep oil and gas in these thrust belts is a crucial means of revealing the patterns of deep oil and gas accumulation. This study utilized a novel approach that combines reservoir geochemistry and basin simulation to determine the migration process of oil and gas in the foreland thrust fault belt of northwestern Sichuan Province. To achieve this goal, we established a thermal evolution model of lower Palaeozoic source rocks through organic chemical analysis. The balanced profile technique was used to determine the paleotectonic evolution pattern of the critical period in this area. Additionally, the mixed flow migration algorithm was employed to simulate the evolution of the Palaeozoic petroleum system. Through basin petroleum system modelling (BPSM), it was discovered that the bitumen found in the thrust nappe belt originated from the lower Cambrian strata. Furthermore, the gas reservoir in the hidden fault belt primarily migrated from the lower Cambrian strata and was also partially mixed with thermogenic gas from Permian source rocks. After establishing the genetic models of various tectonic belts in the study area, it became clear that there are notable differences in hydrocarbon accumulation within these belts. The BPSM results revealed that the oil present in the thrust nappe belt was generated in Lower Cambrian source rocks through early Indosinian hydrocarbon expulsion and continuous charging. The gas found in the thrust nappe belt was produced through the process of oil cracking in the deep reservoir located in the hidden front belt during the late Indosinian. The depth of the thrust nappe belt has helped to preserve hydrocarbon reservoirs. The natural gas present in the hidden fault belt primarily originates from lower Cambrian source rocks, which are combined with gas generated by oil cracking in lower Permian source rocks after the Yanshanian. Therefore, hydrocarbons accumulate deeply in the footwall of the thrust fault belt. The BPSM offers a fresh approach to hydrocarbon genetic models of thrust fault zones in the western Sichuan Basin, and it is beneficial for accurately evaluating the potential for deep hydrocarbon resources in comparable geological settings.

Effect of crack density on crack initiation and damage for high confining pressure using wave velocities

The study of progressive failure of reservoir rocks is very important for the stability analysis of ultra-deep boreholes. The progressive failure can be characterized by the crack initiation stress (CI), crack damage stress (CD) and peak stress (Peak), which is controlled by the evolution of cracks in rock and is significantly affected by confining pressure. To explore the effect of high confining pressure and initial crack on the progressive failure, the initial crack density of 121 sedimentary rock samples was characterized by acoustic waves, and the progressive failure under different confining pressures was analyzed. Experiments show that the ratios of CI to Peak (CI/Peak) and CD to Peak (CD/Peak) increase significantly with confining pressure, which is agreement with the existing conclusion from low confining pressure (0–52 MPa), but those decrease for high confining pressure (68–85 MPa) above critical point of sandstone. The CI/Peak and CD/Peak decrease with increase of initial crack density, which is sensitive to confining pressure but independent of rock type and grain size. The CD of sandstone is pertinent to the friction coefficient of cracks. These results laid a foundation for analyzing the evolution mechanism of wellbore damage and predicting the timing of wellbore instability in ultra-deep wells.

Influence of pre-existing crack on rockburst characteristics in coal-rock combination: A laboratory investigation

The existing cracks in the roof rock are critical to stability of the coal-rock combination structure during mining activities, which is vital for safe exploitation in coal mines. However, the physical mechanisms by which pre-existing crack in the roof rock induces rockburst in coal mines are still poorly understood. In this study, to investigate the rockburst characteristics in an underground mine, triaxial compressive experiments on coal-rock combinations containing a pre-existing crack in the roof rock were carried out. Both mechanical and failure behaviours of composite specimen containing a pre-existing crack were analysed. The testing results showed that a pre-existing crack in roof significantly affects mechanical properties of combinations. The average peak stress and average elastic modulus of specimen exhibit an M-shaped change trend when dip angle is in a range of 0°–90°. The average peak strain increases before decreasing with increase in dip angle. In addition, the roof rock is more likely to fail when it contains a pre-existing crack. Both the initial failure location and failure pattern of combinations are highly related to angle of pre-existing crack. A novel rockburst model considering in-situ stress and pre-existing crack was proposed to determine the physical mechanism of rockburst. The findings are useful to understand characteristics and physical mechanism of rockburst in mine engineering.

Quantitative Analysis and 3D Visualization of Crack Behavior in 3D-Printed Rock-Like Specimens with Single Flaw Using In-Situ Micro-CT Imaging

3D printing technology allows for precise control of preparing complex geometries and internal defects in printed rock analogs, while in-situ Micro-CT imaging enables real-time observation of crack behavior. The combination of these technologies offers a new research approach for studying rock crack behavior. In this study, 3D-printed rock-like specimens containing a pre-existing flaw were prepared using a gypsum powder-based 3D printer. An advanced in-situ Micro-CT system equipped with a loading device was used to quantitatively and visually investigate the crack behavior in 3D-printed specimens under uniaxial compression testing. 2D CT images obtained from in-situ compression testing at different deformations could be used to reconstruct a 3D model and visually identify the crack patterns of the extracted cracks in 3D-printed specimens. The initiation angle of cracks, volume of the pre-existing flaw, volume of newly formed cracks, and damage value with respect to strains were analyzed to quantitatively investigate crack behavior. The results indicated that within the 3D-printed specimens, tensile cracks were first initiated near the internal flaw, followed by the occurrence of shear cracks or tensile-shear mixed cracks at the flaw tips. Additionally, there was a negative linear correlation between the initiation angle of newly formed cracks and the initial flaw angle. For flaw angles in the range of 0° ≤ α ≤ 45°, a higher number of newly formed cracks were observed in the 3D-printed specimens, and the rates of increase in crack volume and damage values with strain were faster. However, for flaw angles in the range of 45° < α ≤ 90°, the results showed the opposite trend. Furthermore, through comparison with the crack behavior of natural rocks containing a single flaw, it was found that the failure modes and crack behavior of the 3D-printed specimens exhibit certain similarities with natural rocks.

Quasistatic progressive crack propagation behavior of crystalline rocks based on SCB tests

To explore the influence of mineral grains on the mechanical strength and crack propagation behavior of rocks, this study investigates the quasi-static crack propagation characteristics in four types of granular structured rocks (three varieties of granite and one type of diabase rock) through a semi-circular bend three-point bending test. The implementation of a servo-controlled crack-opening displacement mechanism facilitated the realization of quasistatic crack propagation. The entire experimental procedure was documented on a camera. Subsequently, the quasistatic propagation and development of cracks in crystalline rocks were examined using digital image correlation, whereas the distribution of mineral grains and their relationship with the pattern of crack evolution were investigated under optical microscopy. The paths of the cracks exhibited noticeable deviation, which were further influenced by the mineral grains. Specifically, cracks tended to circumvent harder minerals while gravitating towards softer ones, highlighting the significant role of mineralogical composition in determining the fracture behavior of granite.

Study on the characteristics of granite rock impact crack based on grain-based model

The type and heterogeneity of minerals control the initiation, aggregation, and expansion of rock blasting cracks and significantly affect the rock failure process. Granite was used to investigate the law of rock cracking under impact loading at the scale of mineral particles, and a two-dimensional grain-based model (GBM) was developed using PFC based on CT) scanning and laboratory mechanical tests. The GBM’s reliability was validated using laboratory uniaxial compression tests and numerical simulations. The specimen failure modes and evolution of microcracks during rock impact were analyzed, as well as the impact cracks’ characteristics under various impact velocities. The findings show that the GBM can simulate the microfracture behavior of several types of mineral fractures during rock impact. The evolution of granite cracks under impact loading can be divided into three stages: rapid growth, decreased growth, and gradual stabilization. The impact failure cracks of the samples were primarily tensile and intragranular. Granite’s tensile and shear fractures and intergranular and intergranular cracks of granite exhibit various trends with varying impact velocities. The GBM is viable for researching the dynamics of crystalline rocks and is a powerful tool for exploring the dynamic characteristics of rocks at the mineral particle level.

Analysis of precursor response characteristics and time-frequency vibration parameters of rock failure: a laboratory experimental method

ABSTRACT To explore the vibration characteristics of rock failure precursors, a vibration optical force combined monitoring method was adopted. In this study, quasi-sandstone and granite samples were selected for uniaxial loading experiments to monitor the surface crack propagation and vibration. The variations in acceleration, displacement, energy, and waveform characteristics of quasi-sandstone and granite were studied from the perspective of the time domain. Additionally, a warning parameter SKE (Skewness, Kurtosis and Energy) based on waveform characteristics and energy was proposed. From the perspective of time-frequency analysis, a warning parameter CFEFD (Center Frequency Energy and Frequency Difference) is proposed by comprehensively utilising short-time Fourier analysis and statistical methods. The prediction effectiveness of both parameters was evaluated. It was observed that the vibration acceleration, energy, and displacement of rocks can all reflect the propagation of rock cracks. Prior to reaching the peak stress, the vibration acceleration and energy of rocks undergo sudden changes. The development of rock fractures releases energy, leading to high-energy frequency diffusion. Before reaching the peak stress, the range and energy level of high-energy frequency diffusion show significant improvement. Finally, it was found that both the SKE and CFEFD warning indicators can provide a certain degree of warning effect.

Study on overburden failure characteristics and displacement rule under the influence of deep faults

Deep faults significantly impact the structural stabilities and deformation behaviors of their overburden rocks, which are key factors in underground engineering and geological hazard research. Considering the problem of deep-fault-overburdened breaking during mining of super-thick coal seams and taking the Yaoqiao Coal Mine as the research object, the mining fracture evolution characteristics and overburden displacement law of the non-structured and fault-bearing overburden corresponding to fully mechanized caving mining are compared and analyzed using numerical simulations and physical similarity simulations. The results of this study show the following: 1) The fracture height of the overlying rock presents a specific change law with advancement of the working face; the initial rapid increase to a maximum height of 74 m is achieved when the working face advances to 90 m; with the development of the plastic zone indicating past yield, the fracture height decreases to 54 m and becomes stable, and the final caving angle of the fracture stabilizes at 70°. 2) In coal mining under normal fault conditions, when the working face advances from the upper to lower walls, the roof forms a masonry beam structure that slows down fault activation and crack development. When moving away from the fault, the overburden movements and water-conducting cracks are fewer, and the crack height is lower than that without faults. When approaching the fault, the influence of the faults in the fracture zone increases, and the height of fracture development reaches the maximum value after crossing the fault, highlighting the significant influence of the fault on fracture development. 3) Through a similarity simulation test, it is shown that the overburden caving zone is further compacted by the overburden rock and that the roof collapses in a large range, resulting in rapid upward development of the overburden rock cracks and separation of the central overburden rock cracks that are gradually compacted and closed. These findings are expected to have important theoretical and practical significance for deep underground engineering design, geological disaster prevention, and fault activity monitoring.

Developmental Characteristics and Genesis of Ground Fissures in Wangjiacun, Emei Plateau, Yuncheng Basin, China

The Yuncheng Basin is part of the Fenwei Graben System, which has developed ground fissure hazards that have caused serious damage to farmland, houses, and roads and have brought about huge economic losses. Located in Wanrong County on the Emei Plateau in the northwestern part of the Yuncheng Basin in China, the Wangjiacun ground fissure is a typical and special ground fissure developed in loess areas, and its formation is closely related to tectonic joints and the collapsibility of loess. In order to reveal the formation and genesis of the Wangjiacun ground fissure, the geological background, developmental characteristics, and genesis pattern of the Wangjiacun ground fissures were studied in detail. A total of three ground fissures have developed in this area: a linear fissure (f1) is distributed in an NNE-SSW direction, with a total length of 334 m; a circular fissure (f2) is located near the pool, with a total length of 720 m; f2-1, a linear fissure near f2, has a fissure length of 110 m and an NE orientation. This study shows that tectonic joints in loess areas are the main controlling factors of the linear fissure (f1); differential subsidence in the pool caused by collapsible loess is the main source of motivation for the formation of the circular fissures (f2, f2-1), and tensile stresses produced by the edges of subsidence funnels lead to the cracking of shallow rock and soil bodies to form ground fissures (f2, f2-1). This study enriches the theory of ground fissure genesis and is of great significance for disaster prevention and the mitigation of ground fissures in loess areas.

Numerical simulation of rock blasting under different in-situ stresses and joint conditions.

High primary rock stress can limit the generation of rock cracks caused by blasting, and blasting usually shows different rock breaking states under different primary rock stress conditions. There are a large number of naturally formed joints in rock mass, due to the limitations of laboratory tests, a numerical model of jointed rock mass was established using LS-DYNA software to investigate the evolution of blasting damage under various in-situ stresses and open joints. In this simulation, using the Lagrange-Euler (ALE) procedure and the equation of state (JWL) that defines explosive materials, the study considered different joint thicknesses (2cm, 4cm, and 6cm), joint angles (0°, 30°, 60°, and 90°), and in-situ stress conditions (lateral stress coefficients of 0.5, 1, and 2, with vertical in-situ stresses of 10MPa and 20MPa), through stress analysis and damage area comparison, the relationship between damage crack propagation and horizontal and vertical stress difference is explored. The research aimed to understand the mechanisms underlying crack initiation and propagation. The results show that: (1) The presence of joints exerts a barrier effect on the expansion and penetration of cracks. When explosion stress waves reach the joint surface, their propagation is impeded, leading to the diffusion of wing cracks at the joint ends. When the lateral stress coefficient and joint angle are the same, an increase in initial in-situ stress results in a reduction in the area of the blasting damage zone. (2) Under the same initial in-situ stress conditions, the area of the blasting damage zone initially increases and then decreases with an increasing joint angle. However, it remains larger than that without a joint, and there exists an optimal angle that maximizes the damage area. In the simulated conditions, the area of damage cracks is greatest when the joint angle is 60° dip angle. (3) The presence of initial in-situ stress has a certain impact on the initiation and expansion of blasting cracks. The degree and nature of this influence are not solely related to the lateral stress coefficient but also depend on the joint's angle and thickness. When in-situ stress is present, the initial in-situ stress field's pressure is not conducive to the initiation and propagation of blasting cracks. However, the existence of a joint has a noticeable guiding and promoting effect on crack propagation, and the pattern of crack propagation is influenced by both joint and in-situ stress conditions.

Failure precursors recognition method for loading coal and rock using the fracture texture features of infrared thermal images

Early warning of the catastrophic failure of rocks is a challenging rock mechanics problem. This article proposed a failure precursor recognition method based on infrared thermal image texture features for coal and rock. Based on spatiotemporal background noise correction for infrared thermal images, a new thermal image parameter of loading coal and rock, Contrast Texture Feature Values (CTFV), is proposed to extract subtle changes in the thermal image caused by crack evolution based on the Gray Level Co-occurrence Matrix (GLCM). The cumulative Crack Texture Thermal Image (CTTI) is reconstructed using CTFV, which can accurately reflect the spatial evolution process of loading coal and rock cracks. The CTFV remains at 0 in the early stage of loading and gradually increases with stress increase at the unstable crack propagation stage, which can serve as a reference for precursor warning of coal and rock failure. For shale, sandstone, and limestone, the precursor of CTFV at grayscale levels of 7, 8, and 9, can be classified into high failure risk, medium failure risk, and low failure risk, respectively. For coal samples, the CTFV is only applicable for the critical warning of high failure risk when the grayscale level is 7. Then, an adaptive identification method for failure precursors based on the sliding window probability density estimation method is proposed. The research results can enhance the reliability of IR monitoring technology for rock failure and instability early warning and can provide support for the prevention and control of rock engineering and geological disasters.

Wmic-GMTS and Wmic-GMERR criteria for micron-scale crack propagation in red-bed soft rocks under hydraulic action

Micron-scale crack propagation in red-bed soft rocks under hydraulic action is a common cause of engineering disasters due to damage to the hard rock–soft rock–water interface. Previous studies have not provided a theoretical analysis of the length, inclination angle, and propagation angle of micron-scale cracks, nor have they established appropriate criteria to describe the crack propagation process. The propagation mechanism of micron-scale cracks in red-bed soft rocks under hydraulic action is not yet fully understood, which makes it challenging to prevent engineering disasters in these types of rocks. To address this issue, we have used the existing generalized maximum tangential stress (GMTS) and generalized maximum energy release rate (GMERR) criteria as the basis and introduced parameters related to micron-scale crack propagation and water action. The GMTS and GMERR criteria for micron-scale crack propagation in red-bed soft rocks under hydraulic action (abbreviated as the Wmic-GMTS and Wmic-GMERR criteria, respectively) were established to evaluate micron-scale crack propagation in red-bed soft rocks under hydraulic action. The influence of the parameters was also described. The process of micron-scale crack propagation under hydraulic action was monitored using uniaxial compression tests (UCTs) based on digital image correlation (DIC) technology. The study analyzed the length, propagation and inclination angles, and mechanical parameters of micron-scale crack propagation to confirm the reliability of the established criteria. The findings suggest that the Wmic-GMTS and Wmic-GMERR criteria are effective in describing the micron-scale crack propagation in red-bed soft rocks under hydraulic action. This study discusses the mechanism of micron-scale crack propagation and its effect on engineering disasters under hydraulic action. It covers topics such as the internal-external weakening of nano-scale particles, lateral propagation of micron-scale cracks, weakening of the mechanical properties of millimeter-scale soft rocks, and resulting interface damage at the engineering scale. The study provides a theoretical basis for the mechanism of disasters in red-bed soft-rock engineering under hydraulic action.