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The Double-Yield (D-Y) model is a widely used method for predicting the mechanical properties of caved rock. However, determining the numerical simulation parameters for this model typically requires extensive trial and error. In this paper, a new approach for parameter inversion of the D-Y model is proposed and applied to a real case study involving repeated mining in multiple coal seams. The proposed framework combines FLAC3D, a finite element numerical simulation software, with a new computational tool developed using MATLAB® App Designer. By applying this framework to the case study, the accuracy and efficiency of parameter determination in the mining area are improved. In addition, a 3D finite element model of multiple coal seams is generated based on the geological information of the case study, and the stress evolution is analyzed. The simulation results are then validated through field measurements, providing valuable reference information for safe and efficient production. Overall, this improved method for parameter inversion of the D-Y model, along with the combined numerical simulation and field measurement approach, enhances the understanding of stress evolution in multiple coal seams and contributes to safer and more efficient mining operations. Highlights: An in-depth analysis was conducted to examine how variations in Double-Yield parameters impact the numerical simulation curve. A breakthrough approach for parameter inversion of the Double-Yield (D-Y) model is introduced, and the computational tool is developed. A 3D finite element model is crafted to analyse the dynamic stress evolution during repeated mining of multiple coal seams. The accuracy of simulation outcomes is bolstered by validation against field measurements of mining-induced stress. [ABSTRACT FROM AUTHOR]

High-intensity longwall top coal caving (LTCC) mining of super-thick coal seam in high stress conditions often leads to significant movement and failure of overburden. The competent roof above the super-thick coal seam is easy to form large unstable structures, and the breaking and movement of the roof strata often induce high weighting strength and large dynamic disasters which severely affect the mine safety. The key to control the behavior of competent roof is to understand the fracturing law of the roof strata. In this paper, a case study based on microseismic (MS) monitoring was performed at LW8309 working face in Tongxin coal mine, where super-thick coal seam under competent sandstone roof in high stress conditions was mined by LTCC method. The spatial distribution characteristics of MS events were studied and four impact zones in the horizontal direction and 'four-zones' in the vertical direction were identified. Multifractal analysis of MS events was carried out to study the multifractal characteristics of the fracturing of competent overburden. In addition, a large and small periodic law of fracturing of competent roof was revealed from the temporal distribution of MS events and on-site monitoring of hydraulic support pressure. A key stratum (KS) structural model was then proposed to explain the strong behaviors of competent strata during high-intensity mining. The research results are of great significance for safe and efficient mining of super-thick coal seam under competent roof in high stress conditions. Highlights: The impact zones in front of the mining face and four vertical zones were identified and the fracture characteristics of the competent overburden in the impact zones and the vertical zones were revealed. Multifractal characteristics of the fracture of the competent overburden were obtained. A large and small periodic law of fracture of competent overburden was revealed from the temporal distribution of MS events and on-site monitoring of hydraulic support pressure. A key stratum (KS) structural model was proposed to explain the behavior of the fracture of competent overburden. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is a type of coal mine dynamic disaster with complex causes and serious consequences. In China, the number and scale of outburst disasters are among the highest in the world. It is an inevitable stage for outburst that the mechanical instability of the combination of intact coal and tectonic coal (CICTC) in strata. The dip angle of the interface has an important influence on the instability of CICTC-containing gas. To further understand the instability process of CICTC, the uncoordinated deformation of CICTC was defined. According to this, three strain conditions were delimited. The variations of stress under three strain conditions were analyzed, the expressions of additional interfacial stress were established. Then, based on the Mohr–Coulomb criterion, the strength characteristics and failure forms were analyzed. It was found that the dip angle deflects the principal stress in the interface, and the influences of additional stresses on the value of principal stress are opposite. These two reasons make the principal stress state and instability process of the combination complicated. Finally, combined with actual parameters, the instability processes of CICTC under different stress paths and buried depths were studied. It can be found that the failures of CICTC under different stress paths were concentrated appearing in the tectonic coal body, intact coal interface, and tectonic coal interface. With the increase of buried depth, the initial maximum principal stress increases and the reduction from additional stresses to the strength of intact coal interface enhances, which makes the combination damage easily and the failure form change. The research results in this paper can provide a theoretical basis to prevent and control the outburst of the combination containing gas considering the dip angle of the interface. Highlights: The uncoordinated deformation of combination considering the dip angle was first defined. The expressions of additional interfacial stress of combination considering the dip angle were established The influences of different stress paths and buried depths for the instability of combination were discussed quantificationally. [ABSTRACT FROM AUTHOR]

A theoretical basis for evaluating safety and stability of underground engineering is to study changes in rock strength and failure. In the process of coal mining, the failure mechanical properties in coal present a complex change principle that, under various in situ stresses and in a hydrological environment, results in material failure because of its unstable state being driven by energy. Therefore, by establishing a relationship between energy change and deformation process of coal failure, it would contribute to reflecting the essential characteristics relating to coal strength and failure. In order to explore the mechanics and energy evolution characteristics of water-bearing coal under the triaxial compression process, triaxial compression mechanical experiments of coal under different water contents and confining pressures have been conducted in this paper, whose effects relating to different confining pressures, including water content on changes in total strain energy, elastic strain energy, and the dissipation energy of coal have been analysed. As coal is a porous medium containing a pore and fissure structure, the volume change law relating to coal during the deformation and failure process is characterised by porosity, consequently, a damage constitutive model for coal was established from the perspective of energy dissipation based on statistical damage theory. It is believed that this model could better reflect the deformation behaviour of coal under triaxial compression, in which the relationship between damage evolution characteristics and energy dissipation changes has been reflected in the model. Further, the methods of determining the model's parameters have been proposed to verify the rationality of the established model. Finally, the influence of confining pressure and water content on the brittleness in coal has been discussed from the position of energy evolution. Highlights: Triaxial compression experiments were conducted on coal specimens to investigate the effects of water content and confining pressure on energy evolution. Based on statistical damage theory, the constitutive model that combined energy dissipation and coal failure was established. The effects of water content and confining pressure on the coal brittleness were discussed from the aspect of energy evolution. [ABSTRACT FROM AUTHOR]

The damage caused by coal fine retention to the permeability of hydraulic propped fractures is one of the key factors restricting coalbed methane (CBM) production. In this paper, permeability experiments of propped fractures under different coal fine retention conditions are carried out, and the retention coefficient of the coal fines is introduced. In addition, the permeability model of propped fractures is established to study the influence of constant rate flow and discontinuous flow on the coal fine retention, along with the influence of coal fine retention coefficient on the permeability of propped fracture in the single-phase flow drainage stage. The study results show that, with the increase of the coal fine flow rate, the retention of the coal fines decreases exponentially, in turn resulting in a logarithmic increase of the porosity and a logarithmic increase of the permeability in the propped fracture. The larger the coal fine particle size is, the likelier it is to remain, and likelier it is to reduce the porosity of the propped fracture, thereby resulting in a decrease in the propped fracture permeability. It is also shown that the retention coefficient of coal fines decreases exponentially with the increase of the flow rate, and increases with the increase of the coal fine particle size. In addition, the retention coefficient ratios of the two flow conditions increase linearly with the increase of the flow rate. The slower the flow rate is, the likelier the coal fines will be deposited in the constant rate flow; and the larger the flow rate is, the likelier the coal fines will be deposited in the discontinuous flow. Highlights: Retention coefficient and permeability model of propped fractures were established. Discontinuous flow of permeability experiments with coal fines were carried out. Coal fine flow rate changed retention exponentially and porosity logarithmically. Large coal fine particle size resulted in badly retention and damage of permeability It is worth noting that we added a Acknowledgements Section after the Conclusion of the article that I have labled. [ABSTRACT FROM AUTHOR]

Thin Spray-on Liner (TSL) is a new class of skin support which can be applied to the roof or sidewalls of an excavation for enhancing load-deformability behaviour. In this study, fibreglass-reinforced TSL (FR-TSL) was coated around the surface of samples made of rock-like materials and coal to compare the compressive and tensile behaviours with those of unlined samples. It was found that 5-mm FR-TSL could enhance the tensile strength of hydrostone by four times and change the failure mode from tensile to shear. A distinct "slight strain-softening zone" was observed in the stress–strain curves of cubical concrete and coal samples when coated with FR-TSL. The dissipated energy density and its rate were also higher in the post-yield regime, indicating effective confinement by the liner. The paper describes the experimental procedures and results in terms of deformability characteristics, elastic strain energy, and dissipated energy with and without the liner. The efficiency of liner performance is also quantified for its potential use in field applications. Highlights: Load-deformation behaviour of rock-like material and coal by coating fibreglass reinforced TSL (FR-TSL) is studied experimentally. The load-carrying capabilities of FR-TSL reinforced hydrostone beam samples increases, and failure mode changes from tensile (without liner) to shear. FR-TSL coated coal, and concrete blocks exhibit higher peak strength and slight strain-softening after peak till the liner tears. The rate of energy dissipation in lined samples is higher as compared to unlined concrete and coal samples. [ABSTRACT FROM AUTHOR]

The assessment of the strength of the coal pillars is essential for the safe extraction of the coal seam. All the pillar strength formulae used worldwide are developed for the flat coal pillars. Therefore, their adoption in evaluating the strength of the inclined coal pillars may endanger the workings of the inclined coal seams. The strength of an inclined coal pillar should be estimated by considering the inclination of the coal seam and its associated behaviour, because the shearing effect along the true dip aggravates the instability of the inclined coal pillars. In this paper, generalised analytical solutions have been developed to estimate the strength of the coal pillars which can be applied for both the inclined and flat coal pillars. The mathematical models are derived to obtain the confining stress in the coal pillar and the corresponding peak stress at the time of its failure using a rock mass failure criterion. The mathematical expressions are also developed for the stress distribution over the pillar considering the increase of the shearing effect with the dip of the coal seam. The Mohr–Coulomb criterion is considered for the shear characteristics at the interfaces of pillar–floor and pillar–roof. The asymmetrical stress distribution and failure along the dip-rise and the strike directions of the inclined coal pillars are addressed in this study. The concept of the confined core and three-dimensional stress distribution over the coal pillar are used to derive the strength formulae for the square, rectangular, and very long pillars. The performance of the derived strength formulae is assessed by the stable and failed cases of the flat and the inclined coal pillars. It is observed that all the inclined and flat coal pillars cases are correctly predicted by the derived generalised strength formulae. According to the derived strength formulae, the strength of the inclined coal pillar decreases with the increase of the inclination of the coal pillar. This paper also describes the variation of the strength of the inclined coal pillars with respect to the coal seam inclinations and the frictional properties of the contact planes for the different width-to-height ratios. [ABSTRACT FROM AUTHOR]

To solve the difficult problems of gas drainage in coal seams with a high gas content and low permeability, a novel technology based on a series of mechanically over-excavated cavities along drainage boreholes in coal seams to increase permeability was proposed. In this paper, the fluid–solid coupled model of the mechanically over-excavated borehole and the gas drainage in soft coal seams was analyzed, and COMSOL Multiphysics numerical simulation software was used for validation. The influence of the parameters for the mechanically over-excavated borehole (i.e., cavity diameter, cavity length, and interspace between the cavities and cavity number) and the reservoir properties of coal seams (i.e., initial permeability, initial porosity and elastic modulus) on gas drainage were investigated. The results show that the gas drainage volume increases with cavity volume and has little effect when the interspace between the cavities exceeds 0.5 m. This indicates that mechanically over-excavated boreholes can significantly increase the coal seam permeability and gas drainage volume. Based on numerical analyses, a single-wing embedded mechanically over-excavated borehole drill bit was developed. The effectiveness of the drill bit was tested in situ at the Licun Coal Mine of the Shanxi Lu'an Group. The results indicate that the effective gas drainage volume of a mechanically over-excavated borehole is 147% higher than that of an ordinary borehole and is not much different from that of a hydraulic flushing borehole. However, mechanically over-excavated borehole technology has great advantages with respect to operational efficiency and cost. Highlights: The influence of the parameters for the mechanically over-excavated borehole (i.e., cavity diameter, cavity length, interspace between the cavities and cavity number) on gas drainage was studied. The influence of reservoir properties of coal seams (initial permeability, initial porosity and elastic modulus) on gas drainage was studied. Combining the actual coal seam drilling conditions of the Licun Coal Mine of the Shanxi Lu'an Group and considering the economic investment, based on numerical simulation analysis, the optimal cavity diameter was determined. A single-wing embedded over-excavated bit was developed and successfully applied in the #2302 workface of the Licun Coal Mine. [ABSTRACT FROM AUTHOR]

Water–rock interaction is an essential factor in the weakening of coal rock masses. Long-term soaking (LTS) as a type of water–rock interaction is widespread in mining engineering and shaft engineering. In this paper, progressive damage characterization of LST coal samples is obtained by a uniaxial compression experiment. Combining physical properties tests in coal samples and hydrochemistry tests in soaking solution quantitatively characterize changes in mineral compositions and microstructures of samples. The weakening mechanism of LTS coal is discussed at a micro-scale. The results show that mechanical parameters and acoustic emission (AE) signals of LTS coal samples continuously decline during loading. The crack initiation stress σCI is regarded as the threshold value for long-term stability of coal rock masses, and the crack initiation stress ratio σCI/σP in a sample soaked for 360 days decreases by 10.8% compared with a saturated sample. With increasing of immersion time, the total number of cracks decreases while the percentage of shear cracks increases. The deformation failure of a sample transforms from tensile failure predominantly to tensile–shear composite failure. The variation of physical properties in coal samples as well as Ca2+, Na+ and K+ concentrations in soaking solution indicates that the dissolution of albite and calcite as well as the swelling of illite are observed throughout immersion process, which leads to an increased pore-crack number and enhanced connectivity in coal samples. As physical properties continue to evolve, the cementation of pre-existing weaknesses between coal matrix is more easily weakened, promoting the slip between matrixes. Under the effect of axial load, the failure surface of a LTS sample will develop along internal pre-existing weaknesses which is most favorable to induce failure. The study results can provide help for reliability assessment and maintenance of underground bearing structure under LTS. Highlights: Long-term soaking of coal samples with mine water. Effect of long-term soaking on the progressive damage characteristics of coal samples. Quantitative characterization of the evolution process of mineral compositions and microstructures of coal samples during long-term soaking. Study on the weakening mechanism of coal under long-term soaking. [ABSTRACT FROM AUTHOR]

Liquid nitrogen (LN2) cooling can induce significant cracking damage in coalbed methane (CBM) reservoirs. To investigate the combined effects of LN2 cooling and bedding angles on the mechanical behaviors of bedded coal, coal specimens with different bedding angles (0°, 30°, 60° and 90°) were cooled. The variations in the surface features and P-wave velocity of the specimens were analyzed. The mechanical properties (tensile strength, peak strain and absorbed energy) were measured under Brazilian test conditions and compared. Furthermore, the crack propagation processes and the failure mechanisms were observed and compared. The results showed that LN2 cooling can induce considerable thermal damage inside the coal, resulting in the deterioration of the coal's mechanical properties. This deterioration led to easier cracking and a higher fragmentation degree under splitting load. In addition, the reductions in tensile strength, peak strain and absorbed energy exhibited obvious anisotropic characteristics. The thermal cracks induced by LN2 cooling participated in the formation of macrocracks. After LN2 cooling, the failure mode remained unchanged from that before cooling at bedding angles of 0°, 30° and 90°. Notably, a transition from mixed-mode failure (primarily caused by tensile failure) to tensile failure occurred at the 60° bedding angle. This transition caused the specimen to fail at a relatively lower load, resulting in maximum reductions in tensile strength (34.8%), peak strain (27.6%) and absorbed energy (55.0%) at the 60° bedding angle. This paper provides new insights into the LN2 fracturing of bedded coal seams. [ABSTRACT FROM AUTHOR]

The deformation and permeability of coal are largely affected by the presence and distribution of natural fractures such as cleats and bedding planes with orthogonal and abutting characteristics, resulting in distinct hydromechanical responses to stress loading during coal mining processes. In this research, a two-dimensional (2D) fracture network is constructed based on a real coal cleat trace data collected from the Fukang mine area, China. Realistic multi-stage stress loading is designed to sequentially mimic an initial equilibrium phase and a mining-induced perturbation phase involving an increase of axial stress and a decrease of confining stress. The geomechanical and hydrological behaviour of the fractured coal under various stress loading conditions is modelled using a finite element model, which can simulate the deformation of coal matrix, the shearing and dilatancy of coal cleats, the variation of cleat aperture induced by combined effects of closure/opening, and shear and tensile-induced damage. The influence of different excavation stress paths and directions of mining is further investigated. The simulation results illustrate correlated variations among the shear-induced cleat dilation, damage in coal matrix, and equivalent permeability of the fractured coal. Model results are compared with results of previous work based on conventional approaches in which natural fracture networks are not explicitly represented. In particular, the numerical model reproduces the evolution of equivalent permeability under the competing influence of the effective stress perpendicular to cleats and shear-induced cleat dilation and associated damage. Model results also indicate that coal mining at low stress rates is conducive to the stability of surrounding coal seams, and that coal mining in parallel to cleat directions is desirable. The research findings of this paper have important implications for efficient and safe exploitation of coal and coalbed methane resources. [ABSTRACT FROM AUTHOR]

Before a coal mining operation, it is necessary to carry out coal-bed methane (CBM) pre-extraction to prevent coal and gas explosions, causing accidents. However, gas extraction and coal mining will lead to coal damage, which results in a change in the gas migration law. Especially, under deep mining conditions, the gas migration mechanism is more complicated owing to a high ground temperature. Coal permeability is the most important constituent that determines gas flow properties. Therefore, the coal permeability evolutionary law related to damage-induced conditions under different temperatures should be further researched. In this paper, a series of triaxial seepage experiments during the whole stress-induced process, and in the changing of the effective stress process were carried out. The results have shown that during the whole stress–strain process, with an increase in axial strain, coal permeability gradually decreases to a minimum value at first, then increases sharply, and finally keeps nearly constant. A higher temperature resulted in a lower elastic modulus, peak strain, and peak strength, but it caused higher thermal damage. When the coal fractures, coal permeability increases with the increase in temperature. During a change in the effective stress process, higher temperatures resulted in higher permeability. Under higher effective stress, the impact of temperature on permeability was not significant. Based on the above results, a novel damage-based permeability model was developed to describe the permeability evolutionary law caused by damage-induced conditions under compaction, and in a fracturing situation. In the proposed model, an exponential function has been used for the combination between permeability and damage variable. The damage variable is composed of thermal damage and mechanical damage. In addition, the damage variable has been modified by introducing a modified function of the initial damage. Finally, the proposed model has been applied to fit two sets of experimental data available. The fitting results showed that the proposed permeability model could well reflect the permeability behaviors of damage-induced coal at different temperatures. [ABSTRACT FROM AUTHOR]

This paper presents the results of laboratory tests conducted on a series of mini-cylinder specimens to measure the uniaxial compressive strength of sub-bituminous coal, which is sparsely recorded in the public domain. The measurements were made on mini-cylinder specimens (non-standard) sized from 9.3 to 22.6 mm in diameter, sourced from the Surat–Clarence–Moreton Basin in Australia, an important region for coal mining and coal seam gas extraction. The tested specimens presented poorly developed cleats and were texturally classified as non to poorly banded, although the composition is rich in vitrinite group macerals. The inherent variability of experimental coal strength results was found to be related to fabric and texture. The results from the mini-cylinder specimens could be scaled to predict the behaviour of larger core sizes commonly used for testing mechanical variability and for estimating full seam rock mass strength. [ABSTRACT FROM AUTHOR]

The morphology of the water jet in the far field is a discrete flow consisting of multiple disorganized discrete single droplets. However, the stress distribution and dynamic evolution of the coal surface under the impact of a single droplet is still unclear, which makes it difficult to obtain the key factors affecting the coal stress. In this paper, a mathematical model for coal surface stress distribution and dynamic evolution under single droplet impact was established, the stress concentration characteristics and the key affecting factors of coal surface were studied, and the synergistic effects of these factors on the stress distribution and damage characteristics were explored. The results show that the impact velocity of the droplet is the main controlling factor in determining the coal surface's stress distribution and dynamic evolution. The principal stresses are concentrated by compressive stress in the center of the droplet-coal contact area, and the radial stress is concentrated by tensile stress in the edge of the contact area. The principal stresses have the same evolutionary trend with time and the stress attenuation decreases with increasing distance between the material point and the center of the contact area. The droplet’s impact velocity and the Poisson's ratio of coal synergistically affect the stress distribution and damage on the coal surface by changing the value of the principal stresses and the proportionality between the principal stresses, respectively. The results lay a theoretical foundation for clarifying the parameter adjustment and the coal damage mechanism under discrete flow impact. [ABSTRACT FROM AUTHOR]

The loading system stiffness (LSS) of a rock mechanics testing machine is important in laboratory testing because it dictates whether the coal or rock failure process is stable or unstable. It is also critical to ejection kinetic energy after coal specimen failure. When unstable failure accidents such as rock burst occur in rock engineering, the speed of ejected coal or rock may reach or even exceed 10 m/s. Such unstable failures, if unplanned, can result in injuries of workers, damage to equipment and delays in production in underground mines. To systematically study the influence of the LSS on mechanical characteristics and kinetic energy of coal specimens, tests are first carried out using an LSS-variable testing system named BGYD-600. PCI-2 acoustic emission (AE) test system, acceleration sensor and dynamic strain gages are used to monitor the AE signals of coal and deformation, displacement of test machine. And the kinetic energy is calculated by energy analysis method. The test results show that the LSS has influence on the stress–strain curves of coal specimens after the peak strength. The smaller the LSS, the steeper the post-peak curves. In addition, as the LSS decreases, the spalling of coal blocks increases considerably, and the powdery phase begins to appear, and the coal specimen is destroyed at a shorter time with more coal blocks being ejected. Moreover, the maximum AE energy occurred not at the peak strength using the BGYD-600, which is different from the large stiffness testing machines. Finally, considering the impact of the elastic deformation energy release of the testing machine on the coal specimens, the kinetic energy calculation formula when the coal specimen failure is derived. And the influence of LSS on the kinetic energy, elastic deformation energy, dissipation energy and total input energy is analyzed. As the LSS increases, the kinetic energy of the coal specimen shows a decreasing trend. The quantitative relationship between mechanical characteristics, kinetic energy and LSS given in this paper has certain reference value for mining and civil engineering projects. [ABSTRACT FROM AUTHOR]

The permeability of broken coal is not only mainly controlled by coal structure, but also unavoidably influenced by variations of stress, particle size, moisture content, and temperature. In this paper, a self-designed experimental device was utilized to investigate the permeability evolution of seven coal samples with different particle size ratios during the loading processes under the condition of temperature, water content, and pore pressure. The results show that both the permeability and the porosity are negative exponential functions of axial stress. Under the same stress, the porosity and the permeability decrease with the increase of the particle size and the particle size range of the coal samples; in the process of compaction, the larger the particle size and the particle size range of the coal samples, the higher the change rate of the porosity and the permeability. Under the same stress, any increase of temperature, water content, or pore pressure would cause the decrease of the permeability of the coal samples, i.e., they are negatively correlated. With the establishment of the permeability calculation model of the coal samples under different temperatures, water contents, and pore pressures during the loading processes, the permeability of different locations in different environments can be speculated. Based on the estimated data, the air leakage in different areas can be obtained to determine the spontaneous combustion zone in goaf, which is crucial for fire prevention and control in goaf. [ABSTRACT FROM AUTHOR]

Gas-bearing, coal-bearing rocks are affected by geological structures and mechanical disturbances. A stress environment exists in an unequal three-way pressure state. To determine the mechanisms of stress change that influence the fissure evolution in stratified coal under true triaxial conditions, different stratifications (vertical, horizontal and oblique stratifications) are experimentally studied based on coal gas permeability. The coal samples are investigated using scanning electron microscopy and transmission electron microscopy to analyze the microstructure differences of the coal samples before and after loading. The results show that the permeability of the different stratified coal samples is exponentially related to the maximum principal stress, the intermediate principal stress, the minimum principal stress and the effective stress. The initial permeability of the vertically stratified coal samples is only 13.5%, which is 22.2% of that for skewed bedding. The bedding direction has a significant effect on the seepage characteristics of the coal samples. In the past, most scholars ignored the influence of bedding when conducting permeability tests. The results of this paper have important theoretical and practical value for optimizing the parameters of gas drainage, increasing the gas drainage rate and reducing the "greenhouse effect" caused by gas emissions. [ABSTRACT FROM AUTHOR]

Because the mechanisms that cause hydraulic fracturing to increase coal seam permeability are not clear, the effective application of hydraulic fracturing has limitations. This paper presents hydraulic fracturing and gas seepage experiments using raw coal under different stress states. The results showed that fracture propagation in coal was influenced by the properties of natural cleats and their ability to induce complex fractures. Using complex fracture abstraction and a connectivity coefficient β, a permeability model of coal subject to hydraulic fracturing with a stimulated reservoir volume (SRV) was established. The SRV effect was optimal when the pressure coefficient (Rn) was 1.46. The value of the Biot coefficient α remained unchanged at approximately 0.854 as the net pressure coefficient was changed. As Rn was increased, the stress sensitive coefficient Cf decreased and the increase in permeability caused by SRV was higher. The permeability of coal subjected to SRV was about two to three orders of magnitude higher than before fracturing. Conventional fracturing only increased the permeability to twice that before fracturing. Therefore, the influence and benefits of SRV should be considered when predicting coal-bed methane production from the permeability of coal beds. [ABSTRACT FROM AUTHOR]

This paper presents results for using an irregular yield pillar for gate road stability in a split-level panel layout (SPL) at a coal mine. The mine extracts 8# and 9# coal seams with a mudstone interlayer. SPL is being used to improve caveability for top coal. The tailgate is driven along the floor of 9# coal seam while the headgate is driven along the roof of 8# coal seam producing a gradually elevated (or curved) section on one end of the panel. Therefore, the gate pillar is of irregular shape and consists of two coal seams and a sandwiched mudstone interlayer. The stability of the tailgate next to this type of irregular yield pillar was investigated using numerical modelling and validated through field measurements and observations. The results show that tailgate with a 6-m-wide pillar has the minimum deformation. The sandwiched mudstone interlayer increases overall stability of tailgate and the yield pillar. The yield zone is smaller especially for the roof of the tailgate which provides a better support condition for the tailgate. Roof-to-floor convergence is smaller than rib-to-rib convergence. The intact zone in the curved section contributes to the overall stability of the gate pillar. Field observations show that stability of the tailgate next to the 6-m SPL yield pillar was maintained along with reduction of other ground control problems. [ABSTRACT FROM AUTHOR]

Resin end-anchored rockbolts consist of three parts: (1) anchored section; (2) free section; and (3) faceplate and locking nut. Tensile failure of the bolt rod usually occurs in the free section. This paper presents a new model for end-anchored rockbolts loaded in tension by implementing a novel tensile failure criterion as part of a 3D continuum numerical modeling package. The proposed model is applied to the study of the behavior of end-anchored rockbolts in a deep coal roadway. The results are in close agreement with both experimental results and field observations in terms of load–displacement relationships and deformation characteristics. The results indicate that the use of rockbolts with high tensile strength and capable to undergo higher elongation would be a key controlling factor for designing excavations in deep coal mines, and the allowable elongation of the bolts in the ribs should be larger than the ones in the roof for deep coal roadways. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive analysis of roof strata conditions, shield pressure, and ground surface subsidence above Panel 42105 of the Buertai Coal Mine in Inner Mongolia, China. A 3D geological model and a 2D numerical simulation model are established to examine the roof strata structures with varying strata conditions. Numerical simulation results demonstrate that the hard rock 1 and 2 roof layers are, on average, likely to break 20 m and 57 m behind the coal face, respectively. It is concluded that when the immediate roof is relatively thin, a cantilever beam is formed, whereas a Voussoir beam is formed when the immediate roof is relatively thick. The patterns of shield pressure and surface subsidence under different roof conditions are also examined. The shield working pressure under the Voussoir beam structure appears to be lower than that under the cantilever beam structure. The average horizontal distance behind the working coal face at which the maximum surface subsidence occurs is 92.1 m. On-site monitoring data are used to explain the causes of the unusual shield pressure and its relation to surface subsidence. Finally, guidelines to predict the shield pressure under similar conditions are provided. [ABSTRACT FROM AUTHOR]

Effective stress laws and their application are not new, but are often overlooked or applied inappropriately. The complexity of using a proper effective stress law increases when analyzing stress variation in coal as a result of gas production or mining. In this paper, an effective stress law is derived analytically for coalbed methane reservoirs, combining the concepts of matrix shrinkage/swelling and external stress by including the effect of sorbing gas pressure on the elastic response of the reservoir. The proposed law reduces to that of Terzaghi when the compressibility of bulk material is sufficiently greater than the compressibility of the solid grain, and without the strain associated with matrix shrinkage/swelling effect. Moreover, it is shown that the Biot coefficient ( α) can have a value larger than unity for self-swelling/dilation materials, such as coal. The proposed stress-strain relationship was validated using experimental results. Overall, the effective stress law for deformation was extended for sorptive materials, providing a new and unique technique to analyze the elastic behavior of coal by reducing three variables, namely, external stress, pore pressure and matrix shrinkage/swelling along with the associated stress, down to one variable, 'effective stress'. [ABSTRACT FROM AUTHOR]

A coal and gas outburst is an extreme hazard in underground mining. The present paper conducts a laboratory simulation of a coal and gas outburst combined with acoustic emission analysis. The experiment uses a three-dimensional stress loading system and a PCI-2 acoustic emission monitoring system. Furthermore, the development of a coal and gas outburst is numerically studied. The results demonstrate that the deformation and failure of a coal sample containing methane under three-dimensional stress involves four stages: initial compression, elastic deformation, plastic deformation and failure. The development of internal microscale fractures within a coal sample containing methane is reflected by the distribution of acoustic emission events. We observed that the deformation and failure zone for a coal sample under three-dimensional stress has an ellipsoid shape. Primary acoustic emission events are generated at the weak structural surface that compresses with ease due to the external ellipsoid-shaped stress. The number of events gradually increases until an outburst occurs. A mathematical model of the internal gas pressure and bulk stress is established through an analysis of the internal gas pressure and bulk stress of a coal sample, and it is useful for reproducing experimental results. The occurrence of a coal and gas outburst depends not only on the in situ stress, gas pressure and physical and mechanical characteristics of the coal mass but also on the free weak surface of the outburst outlet of the coal mass. It is more difficult for an outburst to occur from a stronger free surface. [ABSTRACT FROM AUTHOR]

Analysis of the failure mechanism of inclined coal pillars is one of the complicated issues. The wide variability of dip angles of inclined coal pillars makes it more complex. The asymmetric stress distribution and the tendency of shearing along the bedding planes make the inclined coal pillars to behave differently from the flat coal pillars. There is a need for in-depth investigation of the failure mechanism for addressing the instability problems of the inclined coal pillars. Most of the literature quantifies only the magnitudes of the mean principal stresses by classical statistics. As the stress is a second-order tensor having six independent components, the classical statistics is not appropriate to calculate the mean and variability of the principal stresses at the onset of failure of the pillars. In this paper, a comprehensive analysis is done to understand the complex failure mechanism of the inclined coal pillar using numerical modelling as well as tensorial statistics and validated the results with field measurement data of failure cases. The failure mechanism is analysed by quantification of the characteristics of the inclined coal pillars by the principal-stress magnitude and its orientation, induced at the time of failure. Since the spatial variability of the magnitudes and orientations of the induced principal stresses exist within the inclined coal pillars, the mean induced principal stresses σ 1 ¯ , σ 2 ¯ and σ 3 ¯ are used to quantify the stress states within it. The failure stress states within the coal pillars having different dip angles are generated by the calibrated elasto-plastic numerical modelling with the ubiquitous joint model. Several statistical parameters are calculated to quantify the stress-tensor variability and the correlation among the stress-tensor components. It is found that the correlation coefficients among the shear components increase significantly with the increase of the coal pillar dip angle. Therefore, the inclined coal pillars are highly susceptible to shear failure. The magnitudes, as well as orientations of the mean induced principal stresses within the coal pillars obtained through numerical modelling, are quantified by the tensorial as well as classical statistics. It is found that the magnitude of the mean major induced principal stress σ 1 ¯ at the time of failure, i.e. the strength of the pillar decreases with the increase of the dip angles. The validation of the results with the actual stress measurement data shows that all the failed pillar cases are correctly predicted by the tensorial statistical approach whereas the classical statistical approach does not effectively predict the actual failed condition of the pillars. The study would help to characterise the behaviour of the inclined pillars and address the instability issues for safe and efficient mining of inclined coal seams. [ABSTRACT FROM AUTHOR]

Mining-induced seismicity is one of the dynamic energy sources that can trigger coal burst. This paper presents a new methodology to assess coal burst risks under different loading conditions by examining seismic energy attenuation and fracture size. Two new indices are proposed: (1) Dynamic Load Index (DLI) quantifies the magnitude of dynamic loading induced by seismic events, based on the relationships between the seismic energy, peak particle velocity and dynamic stress; (2) dynamic–static loading assessment index, I PD , links the DLI with passive velocity tomography (PVT) to assess the coal burst risks in a longwall panel under dynamic and static loading. A total of 3080 seismic records were examined to validate I PD in a typical burst-prone longwall panel in China. Based on the rate of occurrence of high-magnitude seismic events, I PD thresholds were determined to identify low-, medium- and high-risk coal burst zones. Using this new zoning approach, coal burst risks were assessed in the same longwall panel while mining through a fault structure. The proposed risk classes correlated well with the recorded high-magnitude seismic events with energies over 10 kJ. The analysis indicated that 69% of the high-magnitude events occurred in the high-risk zones, where I PD was between 0.35 and 1; and 31% of the events occurred in the medium-risk zones, where I PD was between 0.2 and 0.35. [ABSTRACT FROM AUTHOR]

The stress state of coal surrounding a coalbed methane (CBM) production well is affected by the bottom hole flowing pressure (BHFP). The permeability of coal shows a marked change under compression. The BHFP must be restricted to a specific range to favor higher permeability in the surrounding coal and thus higher productivity of the well. A new method to determine this specific range is proposed in this paper. Coal has a rather low tensile strength, which induces tensile failure and rock disintegration. The deformation of coal samples under compression has four main stages: compaction, elastic deformation, strain hardening, and strain softening. Permeability is optimal when the coal samples are in the strain softening stage. The three critical values of BHFP, namely, p, p, and p, which correspond to the occurrence of tensile failure, the start of strain softening, and the beginning of plastic deformation, respectively, are derived from theoretical principles. The permeability of coal is in an optimal state when the BHFP is between p and p. The BHFP should be confined to this specific range for the efficient drainage of CBM wells. This method was applied to field operations in three wells in the Hancheng CBM field in China. A comprehensive analysis of drainage data and of the BHFP indicates that the new method is effective and offers significant improvement to current practices. [ABSTRACT FROM AUTHOR]

The stability of underground parallelepiped coal pillars formed during trunk road development in inclined coal seams is very important for safe access to the mine workings. These protective coal pillars developed around the trunk roads have the longest life span in coal mines. Although these pillars are designed with high safety factors, their failures continue to occur especially in inclined coal mines. The acute corners of parallelepiped coal pillars are highly stressed and prone to failure. These failures may be attributed to the deterministic safety factor which does not consider field geotechnical uncertainties in their design parameters. This research work identified the geotechnical uncertainties in pillar designs and incorporated them in designing stable pillars in inclined coal seams. A probabilistic approach based on limit state function has been proposed for designing stable parallelepiped coal pillars and validated in an inclined coal mine. In this study, the working stresses of the inclined coal pillars are varied for evaluating their influence on pillar reliability using the three cases of the limit state functions namely, empirical, numerical average, and numerical maximum. The pillar reliabilities were estimated by Monte Carlo Simulation. The results indicate that the empirical and numerical average cases yielded stable pillars, whereas the numerical maximum case provided an unstable design. The correlation between safety factor and reliability has been established which can predict the reliability for a given safety factor of pillars with a similar range of design inputs. Further, the threshold values of pillar sizes, acute corner angles, and seam gradients for the reliable pillar design have been determined by sensitivity analysis. These findings can help in designing stable parallelopiped pillars, especially in inclined coal seams to reduce pillar failures and enhance mine safety. Highlights: Key geotechnical uncertainties in coal pillar stability parameters are identified A limit state function-based probabilistic design approach is proposed to include geotechnical uncertainties. The reliabilities of parallelepiped pillars in inclined coal seams are estimated using the Monte Carlo Simulation method. The correlation between pillar reliability and the safety factor of parallelepiped coal pillars is established. Threshold values of design parameters are determined for stable parallelepiped pillars using sensitivity analysis. [ABSTRACT FROM AUTHOR]

Coal pillars are often set up to protect roadways from rockburst hazards in the working face; however, the presence of coal pillars is a "double-edged sword" because unreasonable settings can increase the risk of coal mining. In this study, we used the 73upper11 working face in the Baodian coal mine as the engineering background to explore the instability characteristics of coal–rock mass in coal pillar areas. The failure types and focal mechanisms of 21 events (ML ≥ 1.2) in coal pillars were investigated using the moment tensor inversion method. Considering the limitation that moment tensor inversion assumes a point source, we inverted the stress field of the remnant coal pillar area on both sides of the working face to find the dominant controlling factor of seismicity. These inversion results were then used as input terms for a static Coulomb stress simulation to identify the characteristics of the coseismic stress caused by induced seismicity clusters and the influencing mechanism of subsequent seismic activities. Our results indicate that (1) the 21 high-energy (HE) seismic events were primarily compression failures, generally distributed on the floor, and the focal mechanisms of these events were primarily characterised by normal fault slips, whilst a few high-position events were reverse fault slips. Furthermore, (2) the stress field of the coal pillars differed from that of the in situ stress measurement; therefore, we inferred that these events were induced by mining, with their dominant control factor being the maximum compressive stress (σ1) extrusion with a high plunge. Lastly, (3) the locations of 37 microseismic (MS) events on August 1 and August 2, 2022 confirm that the mining-induced events triggered subsequent MS activities at a rate as high as 81%. Notably, 20 (54%) of these MS events were close to the isolines zero of the Coulomb stress changes. Highlights: Stability optimization of moment tensor inversion is proposed to analyze the focal mechanism of the coal pillar. Stress filed inversion reveals the main controlling factor and real fault plane information of the coal pillar rupture. The calculation of static Coulomb stress has excellent potential for predicting coal mine high-energy (HE) seismic events. [ABSTRACT FROM AUTHOR]

Coal cracks significantly impact the stability of underground coal mines, thus making it crucial to understand their properties for comprehending the mechanical properties of coal and ensuring mine stability. Natural and artificial coal samples were examined to investigate the effect of coal cracks on coal properties. Crack properties, such as volume, orientation, position, concentration, and fluctuation amplitude, were obtained via computerized tomography (CT) scans. In this study, the impact of these properties on coal strength and failure mode was examined after reconstructing the cracks in natural and artificial coal samples. Additionally, considering the impact of overly concentrated crack distribution in artificial coal, a numerical method was used to generate random cracks to further investigate the relationship between crack volume and coal strength. The results showed that, compared to crack volume and other factors, crack concentration had a significant impact on the strength of coal samples. As the crack volume increased, coal strength first rapidly decreased; when the crack volume reached approximately 5% of the coal sample volume, the coal strength would continue to decline with increasing crack volume, but the rate of decrease would significantly slow down. Crack curvature strongly influenced the relationship between crack orientation and coal strength. Therefore, the effect of crack shape should also be considered when discussing the relationship between crack orientation and coal strength. Highlights: Coal specimens containing artificial cracks were prepared using quartz sand, barite crystal powder, gypsum, cement, coal powder, and water-soluble cloth. Effects of crack properties, such as concentration, volume, position, and orientation on coal strength were examined via experiments and numerical simulations. Effect of cracks on the fractional tensile and shear failures of coal specimens was studied by calculating the numbers of tensile failure elements and shear failure elements [ABSTRACT FROM AUTHOR]

Viscoelastic strain rate-dependent behaviour of coal is critical in several subsurface engineering applications especially coal seams gas production. Such rate dependency is controlled by the interaction between coal bulk and gas sorption (a sorbing gas) or gas pressure (a non-sorbing gas). Despite the research conducted to date, the gas pressure effect (non-sorbing) on the viscous behaviour of sediments in particular coal remains unexplored. We, therefore, investigate the strain rate-dependent mechanical behaviour of coal under isotropic loading to specifically explore the effect of gas pressure (Helium) on its rate dependency eliminating the sorption effect. We perform a set of triaxial experiments on coal specimens at dry and pressurised gas (Helium) conditions under different strain rates under isotropic loading. The experimental results show that all coal specimens have viscoelastic strain rate dependency at a dry condition where viscous effect increases with strain rate. As a result, the bulk modulus of the specimens increases with the increase in strain rates. This strain rate dependency response, however, reduces with an increase in pore pressure and vanishes at a certain pore pressure under the same effective stress to that of dry specimens. We further employ X-ray micro-Computed Tomography (XRCT) to 3D scan a coal specimen saturated with Krypton gas undergoing different loading rates to shed light on the micro-mechanisms of gas pressure effect on specimens' rate dependency. The XRCT results show that gas can be trapped in small-scale fractures and pores during the loading process leading to a localised undrained response that can stiffen the specimen and reduce its ability to show viscous rate dependency. The obtained results are significant in optimizing coal seam gas production and coal seam gas drainage applications. Highlights: A series of triaxial tests have been conducted on coal specimens to investigate the strain rate dependency. The experimental results from triaxial tests show clear strain rate dependency of coal specimens under dry conditions. The strain rate dependency decreases significantly with introducing Helium pore pressure and vanishes at certain pore pressure. The XRCT results show that the gas can be trapped inside the microfractures and pores under drained isotropic loading, causing a decrease in rate dependency. [ABSTRACT FROM AUTHOR]

The plasma-induced crushing technology is an efficient method for coal seam cracking. To study the law of fracture development in coal during plasma-induced breakage, this study established an experiment platform of plasma-induced breakage. With the aid of this platform, the macroscopic fracture development characteristics of coal were obtained using a scanning electron microscope and the computerized tomography scanning technology. Furthermore, the process of plasma-induced coal breakage was numerically simulated and analyzed based on the energy-equivalent method. The experimental results suggest that the plasma-induced breakage leads to the formation of a crushing circle around the electrode and multiple penetrated fractures on the coal surface. The radius of the crushing circle increases with the rise of discharge voltage. As can be observed from the slices, the law of fracture development within coal resembles that on the coal surface, but the position of fracture initiation deviates from the position of plasma channel. According to the development degree, the fracture area along the axial direction can be divided into three parts in a descending order: the positive electrode influence area, the negative electrode influence area and the interior fluctuation area. The electrical explosion effect of the plasma channel causes the generation of a shock wave that propagates along the radial direction and attenuates continuously in the coal body. Resultantly, the coal body around plasma channel forms a crushing area and a fracture area. Highlights: A crushing circle and a fracture area are formed with the electrode as the center in the plasma-induced broken coal body. The fracture inside the coal body is smaller than that near the electrode. Fracture development is controlled mainly by the shock wave pressure produced by the electrical explosion. [ABSTRACT FROM AUTHOR]

Hydraulic fracture (HF) network at the meso-macro scale is crucial in the development of coalbed methane contained in coal seams with low permeability. The characterization of a real meso-macro HF network in coal mass helps to further understand its formation mechanism. The meso-macro morphology of different HFs in the HF network of coal mass is characterized by the X-ray micro-computed tomography according to different factors such as principles stress, joints, and natural cracks. The basic law of HF network growth under the stress disturbance of main hydraulic fracture (MHF) is discussed. The results show that: the MHF has an approximate shape of an oblate spheroid. The shear stress concentration and the deflection of the maximum principal stress emerge in the near field under the stress disturbance of MHF. Along the deflected maximum principal stress, the branching hydraulic fracture (BHF) laterally initiates. Induced by the water pressure and the stress disturbance of adjacent deflecting HF or penetrating HF, the bedding and cleats near the tip would be prone to open under the effect of a water wedge, resulting in the propagation of the HF network. The natural cracks may cause the deflection of approaching HF and the natural cracks linking with the HF network are found to continue to propagate at the tip, contributing to the extension of the HF network. The BHF, the opened bedding and cleats, the new HFs which connect with natural cracks and continue to extend, and the MHF form the HF network of coal mass at the meso-macro scale under the stress disturbance of MHF. Highlights: Morphology of meso-macro hydraulic fracture network is depicted by μ-CT. Deflected near-field stress induces the branching hydraulic fracture initiation. Basic law of hydraulic fracture network growth in coal mass is clarified. [ABSTRACT FROM AUTHOR]

True triaxial loading–unloading tests of coal samples were performed using a multifunctional true triaxial fluid–solid coupling experimental system. The deformation, seepage, damage, strength, and failure characteristics of the coal samples were analyzed. Based on this, the variation mechanism of the permeability and yield conditions under different gas pressures was investigated, and the true triaxial loading–unloading of the coal sample was verified using numerical simulation software. The results show that the volumetric strain of the samples exhibited a process of compaction, flatness, and re-compaction with an increase in time. The permeability was approximately negatively correlated with volumetric strain. With an increase in gas pressure, the threshold value of the damage factor increases gradually, and the initial damage point of the sample gradually moves backward. At a pressure of 2 MPa, the degree of damage to the sample was relatively large. After failure, the internal cracks in the specimen developed and expanded rapidly, resulting in an upward trend in permeability. At pressures of 2.5 and 3 MPa, although the yield failure generated new cracks, owing to a decrease in cementation strength between particles and particle separation, shear slip blocked some natural leakage channels, resulting in an unclear permeability variation range. In the post-peak stage, the increasing volumetric strain caused the sample to continue to densify, and the permeability of the sample further decreased. When gas pressure was applied, it was equivalent to the axial prestress of the sample. The greater the gas pressure, the smaller the shear stress of the sample when it entered the yield state. After the yield, the sample was continuously subjected to a larger three-way pressure, resulting in a longer plastic-stage duration. Highlights: The deformation, seepage, damage, strength, and failure characteristics of coal samples under true triaxial loading–unloading conditions were analyzed using various means. The reason why the permeability of the sample continues to decrease after damage was analyzed. The influence of gas pressure on the yield condition of the sample was analyzed. [ABSTRACT FROM AUTHOR]

At present, the horizontal distance between the surface subsidence boundary and the panel is typically selected as the width of the protection coal pillar with the buried pipeline at the gas–coal integrated mining area (traditional method), which causes abundant coal resources to be unrecoverable. To improve the recovery rate of coal resources, the protective coal pillar of the pipeline is optimally designed. First, the Gaussian function equation of the surface subsidence curve is investigated using the probability integral method (PIM). The elastic deformation limit of the pipeline within the subsidence basin was analysed. Then, the failure probability of the pipeline was calculated by analysing the multifactor indicators that affect it. The elastic deformation limit was modified by considering the time effect of the surface subsidence and the failure probability. Next, by analysing the pipeline deformation in the mining subsidence basins, a novel method for the optimal width of the protective coal pillars with buried pipelines in the thick loose layer undermining is proposed. Meanwhile, the verification method and protection measures for pipeline safety are proposed. Finally, theoretical analysis and engineering examples are used for analysis and verification. The results show that the surface subsidence curve caused by critical mining can be expressed by the Gaussian function when the buried depth/thickness ratio (DTR) of the flat coal seam is greater than 40–60 under thick loose layer. Using Panel 132201 as an example, the prediction method reduced the width of the protected coal pillar by 14 m and increased the panel recovery rate by 3.11% while ensuring the safety of the pipeline. This method effectively promotes coordinated mining between oil–gas and coal resources and provides a reference for the design of pipeline protection coal pillars in gas–coal integrated mining areas. Highlights: A novel method for the optimal width of the protective coal pillars with buried pipelines in the thick loose layer undermining was proposed. Under the premise of considering pipeline safety, this method reduced the width of protective coal pillars and increased the panel recovery rate. The elastic deformation limit was corrected while considering the pipeline failure probability. [ABSTRACT FROM AUTHOR]

Several experiments were conducted using self-developed equipment for visual gas-solid coupling mechanics. The raw coal specimens were stored in a container filled with gas (99% CH4) under different initial gas pressure conditions (0.0, 0.5, 1.0, and 1.5 MPa) for 24 h prior to testing. Then, the specimens were tested in a rock-testing machine, and the mechanical properties, surface deformation and failure modes were recorded using strain gauges, an acoustic emission (AE) system and a camera. An analysis of the fractals of fragments and dissipated energy was performed to understand the changes observed in the stress-strain and crack propagation behaviour of the gas-containing coal specimens. The results demonstrate that increased gas pressure leads to a reduction in the uniaxial compression strength (UCS) of gas-containing coal and the critical dilatancy stress. The AE, surface deformation and fractal analysis results show that the failure mode changes during the gas state. Interestingly, a higher initial gas pressure will cause the damaged cracks and failure of the gas-containing coal samples to become severe. The dissipated energy characteristic in the failure process of a gas-containing coal sample is analysed using a combination of fractal theory and energy principles. Using the theory of fracture mechanics, based on theoretical analyses and calculations, the stress intensity factor of crack tips increases as the gas pressure increases, which is the main cause of the reduction in the UCS and critical dilatancy stress and explains the influence of gas in coal failure. More serious failure is created in gas-containing coal under a high gas pressure and low exterior load. [ABSTRACT FROM AUTHOR]

The application of microwave irradiation in low permeability coal seams is considered to be an effective method for increasing the permeability of coalbed methane (CBM). Microwave irradiation not only improves the pore structure of coal, but also affects the mechanical properties. To evaluate the impact of microwave irradiation on the mechanical behavior of coal, a series of uniaxial compression tests of coal after microwave irradiation with different parameters were carried out. The results suggested that the mechanical properties of coal were significantly reduced by microwave irradiation with different parameters. Microwave irradiation changed the deformation behavior and fracture properties of coal. After microwave irradiation, the stress–strain curve of coal tended to be flat, and the failure mode changed from shear failure to tensile failure. Peak strength, initial modulus, and elastic modulus were negatively correlated with microwave energy, while peak strain was positively correlated. The energy evolution of coal under uniaxial compression was closely related to the deformation behavior. The energy absorption, storage and dissipation capacity of coal showed a downward trend with the increase in input microwave energy. Moreover, the energy distribution ratio ( U p e / U p d ) of the stress peak point was changed after irradiation. Under compression, the deformation and failure characteristics of coal showed significant enhancement of plasticity and ductility. The brittleness index (B1, B2) first increased and then decreased with the increase of microwave energy, and the microwave energy of 90 kJ was taken as the threshold boundary. It was observed that the mechanical behavior of coal after microwave irradiation is related to the degree of microwave thermal damage. The average decay rate of normalized mechanical parameters with microwave energy is 0.0026 kJ−1. Highlights: The effect of microwave irradiation on coal deformation and failure behavior is analyzed. The energy evolution law and distribution characteristics of coal after irradiation with different microwave parameters are compared. The brittleness characteristics are analyzed to evaluate the failure easiness of coal after microwave irradiation. The decay rate of coal mechanical characteristics after microwave irradiation is quantitatively characterized by normalizing mechanical parameters. [ABSTRACT FROM AUTHOR]

Coal seams are often affected by gas effects in addition to static and dynamic superimposed loads in the process of coal mining. Understanding the mechanical properties and energy dissipation of impacted coal under different initial gas pressures is extremely important. Therefore, dynamic compression experimentation of coal samples was conducted using a self-developed observable combined dynamic and static loading test system of gas-bearing coal. Furthermore, based on the crack volume increment, the crack energy dissipation density (CEDD) was defined to measure the energy required for a unit volume of cracks under different gas pressures, and the following conclusions were drawn: the deformation of gas-bearing coal can be divided into the elastic stage, elastic‒plastic stage, plastic stage and failure stage, and the dynamic strength and secant modulus decrease with increasing initial gas pressure. The impacted coal contains annular parallel cracks that run through the cross section and axial splitting/thoroughgoing cracks, and the two forms of cracks become more obvious with increasing initial gas pressure. The reflected energy of gas-bearing coal increases and the transmitted energy, dissipated energy and energy dissipation ratio decrease with increasing gas pressure. In addition, the CEDD decreases with increasing gas pressure because free gas and desorbed gas participate in external expansion work at the moment of impact, which results in a decrease in the CEDD required for gas-bearing coal failure. These conclusions enrich the basic theory of the dynamic coal–rock–gas disaster induction mechanism and can provide theoretical support for the monitoring, early warning and prevention technology of dynamic disasters in composites. Highlights: An observable combined dynamic and static loading test system of gas-bearing coal is successfully developed. Considering the strain–stress-dissipated energy curve of gas-bearing coal, the time-dissipated energy curve can be divided into the elastic stage, elastic‒plastic stage, plastic stage and failure stage. The relationship between the crack energy dissipation density (CEDD) of impacted coal samples and the initial gas pressure is clarified. The reason for the decrease in the CEDD with increasing gas pressure is revealed. [ABSTRACT FROM AUTHOR]

The utilization of deep mineral resources is severely restricted by dynamic disasters, such as rock burst, caused by mining disturbances. This study is aimed at investigating the instability failure mechanism and precursory response of coal specimen, with the aid of acoustic emission (AE) monitoring, under triaxial loading and unloading disturbance. The AE time-varying characteristics were analyzed, the crack propagation mode was explored, and the statistical b value and βt value (damage parameters) of AE events were introduced to evaluate the damage state of the loaded coal under disturbance. The results show that: the dynamic disturbance loads will accelerate the deformation process of coal specimen under high static confining stress, resulting in a positive relation between the AE characteristic parameters (AE counts and energy) and the increasing disturbance amplitude. The proportion of low-frequency and high-amplitude AE signals increases with the disturbance amplitude, indicating the disturbance with large amplitude will promote the formation of large-magnitude cracking events. Those stress-induced cracks, judged from the RA-AF value of AE signals, are mainly tensile mode (i.e., mode-I) under the applied disturbance load, but shear cracks are the critical factor affecting the instability and failure of the loaded coal specimen. Both the b value and βt value can reflect the time-varying evolution process of internal damage for the loaded coal specimen. The sudden decreasing of the b value together with the exponential increasing of the βt value can be regarded as a short-impending precursor indicator for the coal instability and failure. The results of this study will provide some experiment basic and theoretical guidance for dynamic disaster monitoring and early-warning during the deep coal mining. Highlights: The instability failure mechanism and the precursory response of coal specimens under triaxial loading and unloading disturbance were investigated. By analyzing the AE characteristic parameters, the internal damage evolution processes, crack propagation mode and the mechanisms of disturbance were studied. The damage accumulation process and precursor warning information of coal instability failure are extracted by the b and βt values. [ABSTRACT FROM AUTHOR]

Studying the anisotropic seepage characteristics of coal is of great significance to the safe and efficient extraction of coalbed methane (CBM). The effective stress law (σe = σ – αp) is often used to solve the fluid–solid coupling problem of rocks. However, the Biot's coefficient α = 1 is often assumed in the current study, and the remaining researches on the α evolution of coal are mainly focused on the conventional triaxial stress state and the elastic range, which cannot reflect the anisotropic evolution of α of damaged coal under true triaxial stress state in actual formation. In the current study, an experimental study on the effect of induced damage on α and permeability of coal subjected to different true triaxial stress states was carried out, and the anisotropy of coal structure was also considered. The experimental results show that αij and permeability are closely related to the fracture evolution, and both increase with the increase in fracture density and opening. Influenced by the induced damage, αij exhibits a significant anisotropy. α1 achieves larger than α2 and α3, and α3 > α2 occurs in the horizontal direction, and the difference between α3 and α2 increases with the increase in the horizontal stress difference. Under different initial stress states, αij for high horizontal stress is lower than αij in the low due to the restraining effect of horizontal stress on fracture propagation. Under different flow directions, αij increases the most with the increase in axial strain under the gas flow perpendicular to bedding, indicating that the natural fractures distributed along the bedding face are more likely to propagate under the external stress drive compared with the face and butt cleats. At the initial plastic stage, αij exhibits αface cleat > αbutt cleat > αbedding, and the permeability also shows kface cleat > kbutt cleat > kbedding, indicating that the natural fractures distributed along the cleats are more than those near the bedding. In this study, a new coal permeability model considering the damage effect and anisotropy of Biot's coefficient is established, which shows good agreement with the experimental permeability of simulating mining stress disturbance, and considering anisotropic αij in the model is closer to the measured permeability than assuming αij = 1. Highlights: Biot's coefficient αij and permeability of damaged coal increase with fracture density and opening increase, and always show α1 > α3 > α2. Considering anisotropic coal structure, αface cleat > αbutt cleat > αbedding, and αij increases the most under gas flow perpendicular to bedding. New permeability model considering damage and αij is established, considering αij in the model is closer to the permeability than assuming αij = 1. [ABSTRACT FROM AUTHOR]

The instability and failure of water-bearing coal mass can lead to serious coal–rock dynamic disasters, such as mine water inrush. It brings severe challenges to the safety produce during coal mining activities. Facing to this, the fine monitoring and accurate prediction are the direct and effective solution. To explore the influence of water content on the infrared radiation (IR) and acoustic emission (AE) signals during coal destruction, uniaxial compression experiments of dry and water-saturated coal samples were carried out. The time–frequency response of AE and the change of three quantitative IR indexes after the head–tail difference and the first order difference processing are investigated, to further analyze the precursory information of the IR and AE response characteristics. The results show that the influence of water on damage and failure modes play significant role to promote the thermal energy value generated by thermoelastic effect and frictional thermal effect, which also changing the AE time–frequency characteristics. Water role promotes the increase of the average infrared radiation temperature (AIRT) in elastic stage and plastic stage, and weakens the AE activities obviously. The first order difference infrared variance (VIRTb), first order difference infrared skewness (SKIRTb) and first order difference infrared kurtosis (KUIRTb) show significant abrupt changes simultaneously with the damage variable during different water-bearing coal rupture damage. For the sensitivity of abnormal precursor, SKIRTb have good effect similar to VIRTb, while it is weaker for KUIRTb, and water can promote their sensitivity. The surface damage measured by IR is local characterization of the overall damage localized by AE. The combination of SKIRTb and b value for accurate early warning can play an excellent effect. The results can reveal the monitoring and forecast information for coal–rock dynamic disaster prevention under water involvement. Highlights: The IRT and AE characteristics and differences of dry and water-statured coal samples are studied. The variance, skewness and kurtosis of infrared radiation temperature field can characterize coal damage state, compared with damage variable. The precursory characteristics of first order difference infrared radiation skewness and b value of dry and water-saturated coal samples are analyzed. [ABSTRACT FROM AUTHOR]

Effective methane extraction in underground coal seams can improve the efficient utilization of fossil energy and reduce the risk of safety accidents in coal mines. Pulsating nitrogen fatigue fracturing technology is proposed as a novel and effective method to enhance gas production in low-permeability coal seams and improve gas extraction efficiency. In this study, a pulsating gas test system was established to apply fatigue fracturing of pulsating nitrogen to low-permeability coal. Mercury intrusion tests, wave velocity tests, and triaxial compression tests were used to reveal the changes in microstructure and mechanical properties of low-permeability coal under fatigue fracturing tests. Results show that the residual deformation of the coal changes considerably under fatigue fracturing. The strain of the coal is characterized by a periodic "expansion–contraction" variation with the intrusion and discharge of the pulsating nitrogen, and the residual strain increases gradually in this process. After fatigue fracturing, facilitates the seepage of gas and the development of micropores and transition pores toward mesopores and macropores enhances the permeability of the coal. The specific surface area of the pores is considerably improved in the transition pores and mesopores. The pore fractal dimension of the coal tends to decrease under fatigue fracturing, resulting in a more uniform distribution of pores and enhanced interpore connectivity within the coal. The fatigue period is negatively correlated with the strength and wave velocity evolution of the coal, and the peak intensity first decreases rapidly and then gradually stabilizes. The fatigue fracturing results in a substantial increase in the spatial complexity and connectivity of the fracture distribution throughout the specimen. On the basis of the evolution characteristics of residual volumetric strain, a fatigue damage model was constructed to analyze the characteristics of fatigue deformation and failure of coal. Highlights: The pulsating nitrogen fatigue test of low-permeability coal was carried out. The change of residual deformation of coal under pulsating nitrogen fatigue is obtained. The influence mechanism of pulsating nitrogen fatigue on pore structure is analyzed. The macromechanical parameters and damage evolution characteristics under pulsating nitrogen fatigue are evaluated. [ABSTRACT FROM AUTHOR]

The early prediction of violent coal-bearing strata failure using effective monitoring is crucial to avoid losses due to catastrophic failures and geological disasters to ensure safe and efficient underground deep coal mining. In this study, the early coal failure precursor was established by researching the application of critical slowing down theory (CSDT) using two infrared radiation (IR) indexes, i.e., variance IR temperature (VIRT) and variance of differential infrared image temperature (VDIIT) under different loading rates. The CSDT parameters: variance and autocorrelation, are evaluated using both indexes in different time window and lag step lengths. The test results revealed that the abrupt and significant fluctuations in variance and autocorrelation for both indexes occurred during rock deformation and before the violent damage. The autocorrelation comparatively shows an obvious reliable fluctuation due to stationarity (show no change in fluctuation before the inflection point), which can be used as a precursor for violent rock failure. It has been shown that the stress level of autocorrelation at the inflection point decreases inversely with the loading rate for both indexes. These stress levels for VIRT are 0.920, 0.890, 0.865, and 0.813 of the σmax under the corresponding loading rates of 0.1, 0.4, 0.7, and 1.0 mm/min, respectively. For VDIIT, at loading rates of 0.1, 0.4, 0.7, and 1.0 mm/min the stress levels are 0.930, 0.892, 0.870, and 0.815 σmax, respectively. Therefore, it has been recommended that the prediction performance of precursory characteristics of IR can be improved by applying the CSDT for an early prediction of rock failure. Highlights: In this study, the early coal failure precursor was established by exploring the application of critical slowing down theory (CSDT) under different loading rates. During different loading rates, the CSDT parameters variance and autocorrelation are evaluated using IR indexes, i.e., variance infrared radiation temperature (VIRT) and variance of differential infrared image temperature (VDIIT) under different window and lag step lengths. The test results revealed that the abrupt and significant fluctuations in variance and autocorrelation for VIRT and VDIIT occurred during rock deformation and violent damage. The autocorrelation comparatively shows a reliable fluctuation due to stationarity, which can be used as a precursor for rock failure. The precursor information based on autocorrelation stress level decreases (about 3.4% for VIRT and 4.2% for VDIIT) with an increase in loading rate. [ABSTRACT FROM AUTHOR]

To examine the influence of lithology on the microseismic response mechanism during the process of coal and rock hydraulic fracturing (HF), we selected four lithological samples of coal, sandstone, shale, and mudstone, which are commonly found in coal-measure strata. The acoustic emission (AE) cumulative counts, AE energy, crack classification, AE peak frequency, and AE position of the four samples under natural conditions were studied, and the laws of crack propagation and evolution in the process of coal and rock HF were explained. We discovered the phenomenon of "bimodal frequency bands" during the HF of coal and rock samples. The results showed that the pressure curves and AE cumulative counts of the coal samples exhibited the largest fluctuations. The power-law distribution of AE energy has a discrete effect in the high-energy region and a plateau effect in the low-energy region. The AE energy power-law indexes of the mudstone samples are the largest, with the values of 0.64 and 0.61, whereas those of the shale samples are the smallest, with the values of 0.44 and 0.45. The crack classification of the samples of coal is mainly shear cracks, and the crack classification of the samples of sandstone, shale and mudstone is mainly tensile cracks. It is proposed to regard frequency band II as the dominant frequency band for HF in the coal, sandstone, and mudstone samples and frequency band I as the dominant frequency band for HF in shale samples. Highlights: The phenomenon of "bimodal frequency bands" during hydraulic fracturing of coal and rock samples was discovered. The AE energy power-law indexes and damage patterns during hydraulic fracturing of coal and rock were revealed. The dominant frequency bands of different coal and rock rupture were found. [ABSTRACT FROM AUTHOR]

During mining and underground excavation, when particle jets assist in coal rock breaking, the impact frequency and energy of the particle jets can significantly affect the strain and strain rate of the coal rock, which determines the fragmentation of the coal rock. However, studies on the particle jets impact frequency and the evolution development of coal rock strain and strain rate under dynamic impacts are limited. Therefore, the impact frequency of particle jets was measured using a vibration sensor, and then E (the ratio of particle jet energy to impact frequency) was calculated. Based on E, the strain field of coal was measured through the digital image correlation (DIC) experiments. The strain rate was then calculated, and the failure characteristics of coal were analyzed. The main conclusions are as follows. The impact frequency increases as jet pressure, but the rang of frequency is concentrated in 500–1500 Hz. The E increases as jet pressure, but the uptrend slows down. The E increases and then decreases as the particle mass flow rate increases. As impacting time increases, the strain field can be divided into the stages of erosion pit formation, stress wave propagation, and strain accumulation. The transition time for each phase shortens with increasing E. The strain rate can be divided into slowly increasing, stable and rapidly decaying as impacting time increases, and the strain rate values decreases along the direction of the jet axis. As the E increases, the strain ratio increases, but the trend is similar under different E. The failure characteristics of coal are determined by the local strain and strain rate. When the local elastic potential energy exceeds the limit of the potential energy under this strain rate, local failure occurs. [ABSTRACT FROM AUTHOR]

Supercritical CO2 (SC-CO2) fracturing is a water-less fracturing technology that has attracted increasing attention in the coalbed methane mining industry. However, the mechanisms of fracture initiation and propagation have not been studied in detail. In this study, SC-CO2 and hydraulic fracturing of coal was conducted using a true tri-axial testing system. The change in injection pressure and in fracture morphology, both before and after fracturing, was analyzed and results presented. In comparison to hydraulic fracturing, using SC-CO2 as a fracturing fluid reduces the pressure required to initiate fractures by approximately 32–41%. This reduction is due to the increased percolation and pore pressure effects arising from the use of SC-CO2. The low viscosity and high diffusivity of SC-CO2 in conjunction with the highly developed pore/fracture of coal result in the formation of a complex fracture network. The fractal dimension is positively correlated with the injection rate and temperature during SC-CO2 fracturing, indicating that using SC-CO2 as fracture fluid under high injection flows and temperature can achieve increased Coalbed methane production. Highlights: SC-CO2 and hydraulic fracturing experiments were conducted on coal utilizing a true tri-axial testing system. The variation law in the initiation pressure of coal in SC-CO2 and hydraulic were obtained. 3D reconstruction were performed to study the internal fracture distribution patterns of the specimens. Based on the unique physicochemical properties of SC-CO2 and its interaction with coal, the formation mechanism of fracture network was revealed. [ABSTRACT FROM AUTHOR]

Understanding coal rib geomechanics is essential for improving rib stability and eliminating fatality and injury trends due to rib failures. There are presently no standardized rib control practices available in most countries. In light of this observed dearth, this investigation aims to improve understanding of rib failure mechanisms using the distinct element modeling (DEM) technique. DEM is chosen because of its superior advantage to explicitly represent discontinuities and their constitutive behaviors, besides that of the intact rock matrix. To analyze the rib stability, a numerical monitoring protocol is implemented to monitor the deformation characteristics of the coal rock mass as its strength is gradually reduced and the deformation and safety factors are established. A number of scenarios were considered in the modeling process, including a non-cleated rib, a cleated rib, and the interaction between support and coal mass. The main conclusions drawn from the study were that the rib failure process initiated with tensile and shear cracks which coalesced to form predominantly sub-parallel tensile fractures to the rib line; and joints and defects in the rib limit fracture development and propagation. The depth of fracture was found to be ~ 1.14 m for the cleated rib and ~ 1.40 m for the non-cleated rib. The depth of softening (DOS) for the cleated and non-cleated ribs was ~ 1.80 m and ~ 1.60 m, respectively. Also, the results demonstrated the capability of DEM-based bonded block models (BBM) in explicitly capturing the rock and support interactions which makes this solution to be suitable for investigating coal rib stability and support requirements. [ABSTRACT FROM AUTHOR]

Irregular coal pillars left in longwall working faces are prone to stress concentration, resulting in failure and instability of coal pillar. Revealing the failure mechanism of the coal pillars is essential for the accurate prevention of coal pillar-type rockburst. Based on the geological conditions and the residual coal pillars in the 14320 working face of the Dongtan coal mine, this study investigates the failure mechanism and stress evolution characteristics in the abnormal area with irregular coal pillars through microseismic (MS) monitoring, moment tensor inversion, and velocity tomography of MS. The results show that (1) the MS events at the edge of the coal pillars are significantly more greater than those in the core area, in which, failure types of tension and compression occur in the edge and core areas, respectively; (2) the spatial parameters (strike φ, dip angle δ, slip angle γ) of the failure plane in the irregular coal pillar area were determined. The boundary of the irregular coal pillars was dominated by reverse fault sliding, and the core area is dominated by normal fault sliding; and (3) the stress field distribution characteristics of the working face and irregular coal pillars were determined using P-wave velocity tomography. The research findings provide a reference for analyzing the mechanisms of coal pillar failure, instability and the induced rockbursts. Highlights: Moment tensor theory is used to analyze the fracture mechanism of coal pillar in mines. Moment tensor theory reveals rupture types and occurrence of the rupture face in different regions of coal pillar. Stress evolution law of coal pillar is obtained by microseismic velocity tomography. [ABSTRACT FROM AUTHOR]

Coal and gas outburst have always been one of the principal gas energy disasters in coal mines. Pre-extraction of coal-bed methane (CBM) is an effective method to reduce the outburst risk. However, during coal mining and drilling for CBM extraction, coal will inevitably undergo plastic deformation or even failure. This will result in a change in gas's migration behavior, bringing severe challenges to coal mine gas disaster prevention and effective CBM extraction. Therefore, it was considered necessary to study the evolutionary law of permeability characteristics, and the mechanical property response of coal under engineering disturbance. Based on generalized plastic theory, the plastic strain in coal is calculated by using the non-associated flow rule. The damage variable was modified with reference to the stress–strain constitutive relationship, that was introduced into a simplified permeability model to successfully construct an original coupled damage–permeability model based on the elastoplastic mechanics in coal (D–P coupling model). The proposed model has been verified by carrying out a tri-axial compression-seepage experiment (mining simulations) under different confining pressures and a tri-axial seepage experiment (extraction simulations) under different effective stresses and under different pore pressures. The results showed that during the whole stress–strain process, the new model could well reflect the seepage behavior of CBM on whether coal permeability decreased before the yield point or increased sharply after peak failure. Coal permeability decreased with an increase of effective stress and pore pressure, and the new model corresponded well with the experimental results. Finally, the relationships between plastic strain, damage variables and mechanical properties in coal were discussed. The proposed model has provided a theoretical basis for coal mine gas disaster prevention and CBM extraction. Highlights: Tri-axial compression-seepage experiments under different confining pressures (mining simulations) were conducted. Based on the generalized plastic theory, the plastic deformation of coal was quantified. The mathematical model from elastoplastic deformation to damage and seepage of coal was established. The relationships between plastic strain, the damage variable, and mechanical properties of coal were analyzed. [ABSTRACT FROM AUTHOR]

After rock burst and spontaneous combustion, a coal seam is exposed, and this form of compound disaster shows new characteristics. Therefore, basic research on oxidized coal is important to the theoretical research and prevention of compound disasters. Based on temperature-programmed and mechanical experiments and the first law of thermodynamics, as the degree of the oxidation of coal increases, the volume expands slightly and then shrinks, and the mass, density and wave speed decrease. Raw coal and 70 ℃ oxidized coal have a strong impact, 135 ℃ oxidized coal and 200 ℃ oxidized coal have a weak periodic impact, and 265 ℃ oxidized coal has no impact. Additionally, wave velocity is positively correlated with compressive strength and the elastic strain energy index but is negatively correlated with the dynamic fracture duration. Compressive strength is positively correlated with the elastic strain energy index and negatively correlated with the dynamic fracture duration. As the degree of oxidation increases, the post-peak dissipated energy release of coal changes from linear to stepped, and its release intensity basically corresponds to the results of bursting liability. Finally, high stress is likely to cause coal fractures to develop, thereby inducing the spontaneous combustion of coal. Spontaneous coal combustion further weakens the bursting liability of coal, but the danger of rock burst increases significantly. [ABSTRACT FROM AUTHOR]

Along with the advance of the working face, coal experiences different loading stages. Laboratory tests and numerical simulations of fracture and damage evolution aim to better understand the structural stability of coal layers. Three-dimensional lab tests are performed and coal samples are reconstructed using X-ray computer tomography (CT) technique to get detailed information about damage and deformation state. Three-dimensional discrete element method (DEM)-based numerical models are generated. All models are calibrated against the results obtained from uniaxial compressive strength (UCS) tests and triaxial compression (TRX) tests performed in the laboratory. A new approach to simulate triaxial compression tests is established in this work with significant improved handling of the confinement to get realistic simulation results. Triaxial tests are simulated in 3D with the particle-based code PFC3D using a newly developed flexible wall (FW) approach. This new numerical simulation approach is validated by comparison with laboratory tests on coal samples. This approach involves an updating of the applied force on each wall element based on the flexible nature of a rubber sleeve. With the new FW approach, the influence of the composition (matrix and inclusions) of the samples on the peak strength is verified. Force chain development and crack distributions are also affected by the spatial distribution of inclusions inside the sample. Fractures propagate through the samples easily at low confining pressures. On the contrary, at high confining pressure, only a few main fractures are generated with orientation towards the side surfaces. The evolution of the internal fracture network is investigated. The development of microcracks is quantified by considering loading, confinement, and structural character of the rock samples. The majority of fractures are initiated at the boundary between matrix and inclusions, and propagate along their boundaries. The internal structure, especially the distribution of inclusions has significant influence on strength, deformation, and damage pattern. [ABSTRACT FROM AUTHOR]