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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]

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]

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]