Search

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]

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]

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]

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]

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]

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 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]

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]

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]

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]

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]

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]

This coal face is located between No. 3104 coal face and No. 3106 coal face. Keywords: Coal face; Aquifers; Overlying strata; Fractured zone; Tracer isotopes; Millesimal deviation value EN Coal face Aquifers Overlying strata Fractured zone Tracer isotopes Millesimal deviation value 4379 4391 13 08/06/21 20210801 NES 210801 Introduction Underground coal mining activities cause redistribution of the stress and deformation fields in surrounding rocks, resulting in different degrees of failure to the overlying strata above the coal face (Liu et al. [16]; Wu et al. [37]; Wang et al. [32], [33]). The water-rich aquifers are medium sandstone (6.83 m thick, 15.48 m from No. 3-2 coal seam), fine sandstone (10.65 m thick, 33.74 m from No. 3-2 coal seam), medium-coarse sandstone (5.39 m thick, 50.6 m from No. 3-2 coal seam), and conglomerate coarse sandstone (5.82 m thick, 64.62 m from No. 3-2 coal seam). Excavating No. 2106 coal face, No. 2107 coal face and No. 2108 coal face in turn, and iterating to the stress equilibrium state. [Extracted from the article]

It is very important and effective to characterize the failure mechanism of coal and rock materials from the perspective of energy. Energy dissipation and release are important causes of rock mass failure. To qualitatively and quantitatively characterize the energy evolution process of gas-bearing coal and the energy evolution mechanisms of coal and gas outburst, experimental studies were conducted on gas-bearing coal under different stress paths. The research results demonstrate that the energy evolution of gas-bearing coal has nonlinear characteristics and that the energy density is a quadratic function of stress. The total energy density is mainly stored as elastic energy density and is unrelated to loading paths. According to the distribution characteristics of abutment pressures in a stope, the energy distribution can be divided into an energy dissipation and release zone, an increasing-energy zone, and a stable energy storage zone. During coal mining, abutment pressure in front of the coal wall is an important factor causing energy accumulation, and the stress concentration area is a key area to prevent and control dynamic disasters of coal and rock masses. Furthermore, based on the principle of energy conservation, this study established an energy balance model for coal and gas outburst and derived energy criteria for coal and gas outburst. When Υ > 1 , there is a risk of coal seams have outburst. If Υ = 1 , the system is in a state of limit equilibrium, and mining disturbances could trigger outburst. [ABSTRACT FROM AUTHOR]

From the perspective of spontaneous combustion control in coal mining, this study conducted a progressive axial loading test of broken coal using a self-designed multi-field coupling oxidation test system. The evolutionary characteristics of the bulking factor under different loading conditions of stress, stress–temperature, and stress–moisture were obtained. Subsequently, factors affecting the bulking properties of broken coal in the synchronous compression and deformation stage were analyzed theoretically using the four-particle accumulation model and Hertzian contact deformation principle. The research results are summarized as follows: (1) in the process of axial loading, the bulking factor of broken coal generally decreased, but showing three stages; (2) under the stress-moisture coupling effect, when the axial force was 0 kN and the external water content increased from 0 to 6.75%, the water wedge effect increased the bulking factor of broken coal by 2.17%; (3) under the stress-temperature coupling effect, when the axial force was 47.1 kN and the temperature increased from room temperature (23.3 °C) to 100 °C, the bulking factor of broken coal decreased by 5.08%; and (4) in the synchronous compression and deformation stage (at the axial stress of 94.2 kN), the theoretical analysis revealed that the bulking factor decreased gradually with increasing stress, temperature, and external water content, which was consistent with the test results. The abovementioned test results established the basic theory and provide practical support for subsequent recognition of the spontaneous combustion of confined coal. [ABSTRACT FROM AUTHOR]

In underground coal mining, coal failure generally occurs due to the relatively weak strength of the coal and the high applied mining-induced stresses. The outer complex geological conditions (i.e., tectonic structure and water intrusion) and internal structural anisotropy of the coal introduce uncertainty in predicting its mechanical properties and failure behavior. In this study, laboratory investigations of the mechanical properties and failure behavior of dry and water-saturated anisotropic coal samples subjected to different true-triaxial loading stresses were conducted. The effects of water weakening, intermediate stress, and structural anisotropy on the mechanical properties and failure behavior of the coal were systematically studied. The results indicate that the presence of water significantly reduced the strength, elastic modulus, and strength anisotropy of the coal. The maximum stress at failure first increased and then decreased with increasing intermediate stress. The residual strength-to-peak strength ratios and failure plane angles of the coal showed a linear increase with increasing intermediate stress. When the coal samples were loaded in the bedding plane direction, the brittleness of the coal was higher than when they were loaded in the other two cleat plane directions. In addition, when the coal samples were loaded in the butt cleat plane direction, the brittleness of the coal decreased with increasing intermediate stress. Two typical failure modes of the dry and water-saturated coal samples were observed: shear and mixed splitting and shear failures. The dominant failure mode of the coal also varied with the loading direction relative to the weakness planes, which could be well recognized and predicted by the acoustic emission (AE) characteristic curves. To further reveal the fracture mechanism, the microcrack patterns of the coal were further identified based on the AE parameters. [ABSTRACT FROM AUTHOR]

Knowledge regarding microseismic characteristics associated with longwall coal mining is crucial in evaluating the potential for underground mining hazards. Although microseismicity is induced by mining activities, it still remains uncertain as to what extent mining activities influence the spatial, temporal, and magnitude characteristics of microseismicity. To establish a thorough understanding of the relationship between microseismic characteristics and mining activities, a 27-month long microseismic monitoring campaign was conducted around a highly stressed coal zone and eight producing longwall panels at Coal Mine Velenje in Slovenia. Each microseismic event was classified to be associated with the producing longwall panel that triggered it, and the microseismic response to multi-panel longwall top coal caving face advance was analysed. Monitoring data have shown that locations of microseismic events coincided with stress concentrated regions. It was established that both seismic count and energy-intensive regions associated with coal mining in different panels are spatially connected, but they do not fully overlap with mined-out or stress concentrated areas. In addition, microseismic event counts frequency was found to be well correlated with mining intensity, while seismic energy magnitude and spatial distribution are poorly correlated with the same. Therefore, microseismic characteristics could not be explained solely by the mining-induced stress transfer and mining intensity, but are believed to be dominated by pre-existing natural fractures throughout the coal seam. Analyses of these observations helped the development of a conceptual seismic-generation model, which provides new insights into the causes of microseismicity in coal mining. [ABSTRACT FROM AUTHOR]

To obtain a comprehensive understanding of the difference between deep and shallow rocks, and the damage evolution laws of a rock mass at different depths, a case study was conducted on the Pingdingshan Coal Mine. Mechanical behavior and real-time acoustic emission (AE) testing were carried out to investigate the spatiotemporal evolution of the damage in coal at different depths under triaxial compression conditions. Coal samples from different depths (300, 600, 700, 850, and 1050 m) of the same coal seam were collected, and the geostresses measured at each depth were considered in the two-factor simulation method. The AE characteristics and the spatiotemporal evolution of the coal damage at different depths were also obtained. The testing results show that under the influence of the confining pressure caused by increased geostress, the AE activity and the average scale of the cracks in the coal decreased with increasing depth. The deeper coal developed more small cracks and more plastic strain. With increasing depth, the fractal dimension reduction mode of the spatial distribution of AE under continuous loading changed from dropping suddenly from a higher dimension level (2.97) at a higher stress level (70%) to dropping slowly from a lower dimension level (2.63) at a lower stress level (20%). The generation of AE events was more uniform in the time dimension, while the spatial distribution became more uneven and clustered. With continuous loading, the number of AE events and damage in the shallow coal increased, and the dimension of the spatial distribution of AE decreased sharply during the failure stage before the peak stress was reached. With increasing depth, the damage initiation of coal shifted to an earlier time, while the damage evolution process was more stable and orderly, and ended with a higher damage degree. The sudden damage increase of deep coal was not obvious when approaching to failure. The shallow coal was more brittle and prone to sudden failure with the centralized release of AE energy after reaching the peak stress. However, the deeper coal exhibited a more plastic behavior and the plastic deformation became more obvious during the loading process with the gradual development of damage. These research results deepen the understanding of rock mechanics at different depths, promote the study of the difference in the damage laws of deep and shallow rock masses, and provide a meaningful reference for microseismic monitoring and disaster prevention in deep rock engineering. [ABSTRACT FROM AUTHOR]

The inclined coal pillars, formed during the underground extraction of the inclined coal seam, are different from the flat coal pillars due to the high strength anisotropy of the inclined bedded rock, and the asymmetrical stress distribution. To ease the manoeuvring of the men and machinery in the inclined coal mine, the pillars are developed along the apparent dip. Thus, the pillars become rhombus-shaped with acute-angled corners which crush rapidly due to the high stress concentration. No suitable formula is found to estimate the strength of the inclined coal pillar that incorporates all these factors. Thus, the use of the available coal pillar strength formulae may endanger the extraction of the inclined coal seams. This study elucidates the procedures to estimate the strength of the inclined coal pillars by the numerical modelling technique. The ubiquitous joint model is used to simulate the shearing characteristics of the inclined strata. The parametric study shows that the strength of the inclined coal pillars decreases with the increase of the dip of the coal seams. It is also obtained that the strength of the pillars decreases with the decrease of the value of the acute angle of the corners. The peak stress distribution and the strain accumulation over the inclined pillars at the time of failure are plotted in three-dimensional graphs which show the asymmetrical characteristics of the inclined coal pillars. The analysis of variance shows that the dip of the coal seams and the acute angles of the corners have a statistically significant effect on the strength of the inclined coal pillars. Based on the results of the simulation, the best fit relation is established by the multivariate non-linear regression technique to estimate the strength of the inclined coal pillars. The coefficient of determination (R2) and the root mean square error of the model are obtained as 0.92 and 0.065, respectively. The validation of the developed model has been carried out by the stable and failed inclined pillar cases of different underground inclined coal mines in India. Comparisons of the safety factors, obtained from the developed model and the flat pillar strength formula, indicate that the flat pillar strength formula overestimates the strength of the inclined coal pillars. The developed model can be used to design the inclined coal pillars for safe extraction of inclined coal seams. [ABSTRACT FROM AUTHOR]

Rock burst is one of the main coal and rock dynamic disasters in steeply inclined and extremely thick coal seams (SIETCS). Timely identification of potential precursor information enables effective and specifically targeted measures to mitigate hazards. To provide a reference for the determination of precursor information in SIETCS with similar conditions, an in situ investigation that lasted for more than 1 year was conducted on the + 450 horizontal no. B3 + 6 fully mechanized top-coal caving face of the Wudong coal mine. This mine has an average dip angle of 87° and experienced a total of three rock bursts. The investigation focused on the consequences of rock burst (including the source location, energy, damage range, and modes of ensuing damage), the temporal and spatial evolution laws of the monitoring parameters of microseismic (MS) and acoustic emissions (AE), and the relationship between precursory information of both the AE and MS monitoring systems. Furthermore, a comparison of the evolution laws of the precursory characteristics between steeply and gently inclined coal seams was conducted. The obtained results indicate that rock burst failure mainly shows a directionality from south to north of the roadway, and the main factor causing the rock burst was the suspended rock pillar. Prior to the rock burst, sources of MS events gradually clustered and congregated around the rock pillar or coalface, and high-energy events (energy > 103 J) noticeably increased. Similar variations of sharp rise–sharp drop were found for both daily total energy and event counts. Prior to rock burst or large energy mine tremors, the AE energy deviation continued to rise, a peak was observed in the total number of high-value daily AE deviations, and the precursory information of AE was ahead of the precursory information of the MS monitoring system. The evolution laws of MS precursor information prior to rock burst differed between SIETCS and gently inclined coal seams. These evolution laws can be used as precursory warning for rock burst in SIETCS. Integrated monitoring of the AE and MS monitoring systems realizes the real-time monitoring and early warning from the germination stage to the occurrence of rock burst, and can determine the danger area in space. This can improve the early warning efficiency and provides a solid foundation for safe mining in SIETCS. [ABSTRACT FROM AUTHOR]

Considering the complex geological conditions of mining in the fully mechanized 828 working face of the Qingdong Coal Mine, a mining plan was developed for premining the upper No. 7 coal seam to control the hard roof and gas of the No. 8 coal seam. Failure depth of the No. 7 coal seam floor and the caving interval of the hard roof were analyzed using a proposed model based on the Terzaghi's principle and rock beam theory, respectively. The movement rules of the overlying strata under premining (No. 7 coal seam) and non-premining conditions were analyzed using physical material similarity simulations experiment. Furthermore, field monitoring was performed to determine the working resistance of the hydraulic supports. The estimated results from the proposed method showed that the failure depth of the floor of the 726 working face was 13.8 m, which is in agreement with that noted during the field observation. The caving interval of the hard roof decreased from 24.9 m to 10.6 m after premining the No. 7 coal seam. In the simulation, the caving interval of the hard roof of the No. 8 coal seam decreased from 20 m to 10 m after premining the No. 7 coal seam. The hydraulic supports were effective in the actual production process, and the initial and periodic weighting intervals were 20.4 m and 10.4 m, respectively. These findings demonstrate that after premining the No. 7 coal seam, the initial and periodic weighting intervals decreased, thereby reducing the pressure intensity of the roof. This method effectively weakens the hard roof of the No. 8 coal seam. The issues pertaining to the hard roof during the mining of the 828 working face could be overcome by premining a protective coal seam of the upper No. 7 coal seam. [ABSTRACT FROM AUTHOR]

Thick hard rock strata (THRS) are widely distributed among coal-bearing strata in China. The dynamic THRS failure may induce the coal-rock dynamic hazards during underground coal mining. In this study, the size, stress and energy characteristics of dynamic THRS failure were analyzed; the Yangliu coal mine, Huaibei coal basin, China, was taken as a case study. Then the apparent-depth effects of the dynamic THRS failure on the underlying coal mining were researched. The results revealed that the rock roof would be caved, fractured and deformed and even develop the bed separation during underlying coal mining. A large quantity of elastic energy is stored in the bent and compressed THRS, the amount and scope of the vertical stress concentration in front of the working face increases significantly before THRS breaking. THRS generally exhibit the square-form structure failure (SSF), after which the elastic and kinetic energy is attenuated in power form with the energy spreading distance. The shock force and energy effects of the dynamic THRS failure on the underlying coal mass were greater than those observed for the common lithological rock strata. Thus the mining coal mass below THRS bears similar mechanical conditions as the deeper coal mass and is of greater apparent depth, which is called the apparent-depth effects of the dynamic THRS failure. The apparent-depth effects of the dynamic THRS failure significantly influenced the coal-rock dynamic hazards according to in situ measurements of the mine earthquake energy, in situ stress, roof pressure and gas emission parameters. As a result, reasonable measures should be taken to avoid stress concentration and energy accumulation and to reduce the apparent depth of the mining-disturbed coal mass for preventing coal-rock dynamic hazards. The research results provide significant value for the control of coal-rock dynamic hazards below THRS. [ABSTRACT FROM AUTHOR]

Studying the mechanical properties of and crack propagation in coal after water intrusion is necessary to tackle a number of geological engineering problems such as those associated with underground water storage in collieries and support for underground roadways in coal mines. To study the mechanical properties and crack development, 12 coal samples with moisture contents of 0, 2.37, 3.78 and 5.29 % were prepared for acoustic emission tests under uniaxial compression. Over about 6 days, the coal samples absorbed moisture from a humidifier in three different phases. In this period, uniaxial tests show that the peak stress, elastic modulus, strain softening modulus and post-peak modulus decreased with rising moisture content in the samples while the peak strain increased. It was further found that, by analysing the relationship between the stiffness and stress and the accumulated acoustic emission counts, all the phases of crack development can be evaluated. This is useful for studying the effect of water intrusion on crack propagation and for calculating the mechanical properties of the coal such as the elastic modulus. This investigation also quantifies the percentage of the stress thresholds for crack closure, crack initiation, and crack damage that constitutes the peak stress. These stress thresholds do not change with moisture content. Our results are of great significance for water storage in coal mines, for determination of pillar dimensions in coal mines, and for expanding the knowledge base of the mechanical properties of coal and the characteristics of crack propagation. [ABSTRACT FROM AUTHOR]

To comprehensively understand the mining-induced coal permeability change, a series of laboratory unloading experiments are conducted based on a simplifying assumption of the actual mining-induced stress evolution processes of three typical longwall mining layouts in China, i.e., non-pillar mining (NM), top-coal caving mining (TCM) and protective coal-seam mining (PCM). A theoretical expression of the mining-induced permeability change ratio (MPCR) is derived and validated by laboratory experiments and in situ observations. The mining-induced coal permeability variation under the three typical mining layouts is quantitatively analyzed using the MPCR based on the test results. The experimental results show that the mining-induced stress evolution processes of different mining layouts do have an influence on the mechanical behavior and evolution of MPCR of coal. The coal mass in the PCM simulation has the lowest stress concentration but the highest peak MPCR (approximately 4000 %), whereas the opposite trends are observed for the coal mass under NM. The results of the coal mass under TCM fall between those for PCM and NM. The evolution of the MPCR of coal under different layouts can be divided into three sections, i.e., stable increasing section, accelerated increasing section and reducing section, but the evolution processes are slightly different for the different mining layouts. A coal bed gas intensive extraction region is recommended based on the MPCR distribution of coal seams obtained by simplifying assumptions and the laboratory testing results. The presented results are also compared with existing conventional triaxial compression test results to fully comprehend the effect of actual mining-induced stress evolution on coal property tests. [ABSTRACT FROM AUTHOR]

Dynamic collapses of deeply mined coal rocks are severe threats to miners. To predict the collapses more accurately using electromagnetic radiation (EMR), we investigate the time-varying multifractal characteristics and formation mechanism of EMR induced by underground coal mining. A series of uniaxial compression and multi-stage loading experiments with coal samples of different mechanical properties were carried out. The EMR signals during their damage evolution were monitored in real-time; the inherent law of EMR time series was analyzed by fractal theory. The results show that the time-varying multifractal characteristics of EMR are determined by damage evolutions process, the dissipated energy caused by damage evolutions such as crack propagation, fractal sliding and shearing can be regard as the fingerprint of various EMR micro-mechanics. Based on the Irreversible thermodynamics and damage mechanics, we introduced the damage internal variable, constructed the dissipative potential function and established the coupled model of the EMR and the dissipative energy, which revealed the nature of dynamic nonlinear characteristics of EMR. Dynamic multifractal spectrum is the objective response of EMR signals, thus it can be used to evaluate the coal deformation and fracture process. [ABSTRACT FROM AUTHOR]