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Coalbursts are one of the most serious disasters in coal mines. Exploring the critical conditions of coalbursts from a theoretical perspective is the basis for realizing early prediction and quantitative prevention of coalburst. Based on the principle of virtual displacement and disturbance response instability theory, we proposed the energy extreme point method for identifying the instability of coal–rock systems and determined the critical conditions (the length of the critical plastic zone and critical load) for coalburst. Based on actual engineering field data, the critical values of the above two indexes were calculated for 15 coalburst mines in China. Finally, we analyzed the effects of coal bursting liability, roadway section size, roof stiffness, coal–rock interlayer property, and support measures on the length of the critical plastic zone and critical load. The results suggest that the calculated length of the critical plastic zone and critical load can be used as early indicators of coalburst. In addition, the length of the critical plastic zone can be used to calculate the elastic deformation energy, allowing the magnitude of the coalburst to be predicted. The critical load can be used to obtain the critical mining depth and safety factor to facilitate mine safety evaluation. The length of the critical plastic zone and critical load obviously decrease with increasing coal bursting liability. As the width-to-height ratio of the roadway increases, the length of the critical plastic zone decreases, while the critical load increases. Increasing roof stiffness leads to increases in both the length of the critical plastic zone and critical load. Increasing the friction coefficient between the coal and rock layers causes the critical plastic zone to decrease and the critical load to increase. Finally, support measures can obviously increase the critical load. Highlights: The energy extreme point method for identifying instability in coal-rock systems was proposed. An analytical model of coalburst in a rectangular roadway considering the mechanical behavior of the coal-rock interface was established. Critical conditions of 15 coalburst mines in China was calculated based on the energy extreme point method. Influencing factors of critical conditions of coalburst were analyzed. [ABSTRACT FROM AUTHOR]

Coal and gas outburst (referred to as "outburst") is induced by damage and failure of loaded outburst coal under internal and external stress. To further clarify the specific role of the elastic strain energy generated by the principal stress in the process of coal damage and fracture, from the perspective of the energy relationship between uniaxial compression test and outburst test, taking the outburst coal seam of Pingmei No. 11 Coal Mine as the research object, uniaxial compression test and true triaxial outburst simulation test were carried out. The damage evolution law of outburst coal is clarified using the characteristic parameters of acoustic emission. According to the characteristic stress and in-situ stress, the energy calculation of uniaxial specimen and outburst coal is carried out, and the specific damage degree of elastic strain energy to outburst coal is clarified. The results indicate that: (1) When the stress level reaches the crack initiation stress σci, the damage begins to accumulate. When the stress level reaches the damage stress σcd, the coal sample strength begins to deteriorate. (2) The strain in the process of outburst coal damage and fracture can be described by referring to the relationship equation between strain and gas pressure and the variation of test gas pressure. (3) It is determined that the degree of damage and fracture caused by the outburst elastic strain energy to the coal sample is at the end of the stable crack propagation stage. Highlights: Judging criterion of the initiation of coal body damage accumulation and strength deterioration have been identified. A relationship equation between strain and gas pressure has been established and validated. A quantitative analysis of the role of elastic strain energy in the formation and propagation of fractures in outburst coal body has been conducted. [ABSTRACT FROM AUTHOR]

The bursting liability is very important to evaluate the risk of rock burst in coal mine. Conducting uniaxial compression test is the main method to study the bursting liability of rock (coal). To establish an accurate uniaxial constitutive model, the structure ensemble dynamics theory is adopted. The model of rock (coal) specimen compression by testing machine is simplified. Through dimensional analysis, the stress–strain relationship of the specimen is a power model. The mechanical properties of rock (coal) vary with different levels of deformation and can be divided into four stages. The different states can be considered as different ensembles. By using the analysis method of structural ensembles dynamics, one formula completely represents the four stages accurately. The parameters in the formula are physically meaningful and can be uniquely determined based on experimental data. A criterion for instability criterion of the system of the specimen and the testing machine is derived, and a theoretical explanation of instability failure related to uniaxial compressive strength is given. Parameters sensitivity analysis and verification are conducted on the theoretical results. The theoretical results are highly accurate and can effectively reflect the stress–strain relationship of rock (coal) under uniaxial compression. A bursting liability index which contains the information of the peak and post-peak of the stress–strain relationship is proposed. The new index is positively correlated with the compressive strength and the bursting energy index and a classification of bursting liability levels for the new index was determined based on them. Highlights: The structure ensemble dynamics theory is used to establish a constitutive model. One formula completely represents the stress-stain relationship accurately. The parameters are physically meaningful and can be uniquely determined. A theoretical explanation of instability failure related to strength is given. A new bursting liability index based on the new constitutive model is proposed. [ABSTRACT FROM AUTHOR]

Borehole destressing is one of the most widely used techniques for mitigating coal bursts. Understanding the working principle of destressing boreholes, exploring the main efficiency factors, and proposing a quantitative method for determining the drilling parameters for coal burst control are essential for maximizing coal burst control. This study establishes a unified damage mechanics model of borehole bursts (that is, dynamic collapse in boreholes) and roadway coal bursts, deduces the critical indices of borehole bursts, and identifies the main factors. The theoretical relationship between the critical stresses of borehole burst and roadway coal burst is quantitatively explored, and a theoretical criterion for borehole effectiveness in coal burst control is proposed using the critical indices of borehole bursts. Finally, a design principle, determination method, and calculation formulae for the drilling parameters, which are directly related to the roadway burst risk state, coal seam thickness, coal properties, and drilling size, are proposed. The optimized drilling parameters of typical coal burst mines in Hebei and Inner Mongolia verify the engineering applicability and effectiveness of the proposed design method. This study proposes a quantitative determination method for destressing drilling parameters that supports coal burst occurrence theory. Highlights: Critical conditions for borehole bursts, directly related to the mechanical properties of coal and borehole size, are theoretically provided. Theoretical relationship between boreholes and roadway coal bursts is explored. Quantitative description of the borehole destressing principle to mitigate coal burst is provided. Theoretical criterion for boreholes to mitigate coal bursts, which is directly related to the critical conditions of borehole bursts, is provided. Quantitative method for determining the drilling parameters to mitigate coal bursts is proposed. [ABSTRACT FROM AUTHOR]

Risk evaluation of roadway rockburst refers to quantitatively dividing the rockburst-prone area into multiple sections with different risk levels at the mining design stage, according to the geological and mining technical factors influencing the rockburst occurrence, for partitioned and graded management of rockburst. Based on this, a targeted rockburst monitoring and early warning system can be formulated for mining working faces, and it is the premise of rockburst management and control. To determine the theoretical criteria of rockburst risk evaluation and improve the accuracy of the evaluation results, a mechanical analysis model of roadway rockburst with the structure of "elastic zone–plastic softening zone–fracture zone" was established. Theoretical calculation formulas for critical ground stress, critical mining stress, critical depth of the plastic softening zone, and fracture zone of rockburst occurrence were obtained. These were related to the mechanical parameters of the coal and rock mass, geometric parameters of the roadway, ground stress, and support strength. Accordingly, a critical mining stress index was defined (the ratio of actual mining stress to critical mining stress), a quantitative evaluation method for rockburst risk based on the critical mining stress index was proposed, and the process and steps of a critical mining stress index method for rockburst risk evaluation were explained. A rockburst risk evaluation was performed for the 302 (upper) working face in a coal mine in Shandong, China. The critical mining stress index method for rockburst risk evaluation was validated by comparing results with the monitoring data obtained using the drill cutting method, microseismic monitoring method, and comprehensive index evaluation method. Highlights: Critical indexes and criterion for rockburst risk evaluation were explored. The definition and calculation method of mining stress index were proposed. A quantitative evaluation method of rockburst risk was proposed. Process and steps of the new evaluation method for rockburst were elaborated. [ABSTRACT FROM AUTHOR]

The process of cable bolt anchoring the roof of a coal mine roadway involves the mixture of different types of resin cartridges. However, this procedure presently faces several shortcomings including insufficient cartridge breakage, non-ideal viscosity and annular thickness values of the anchoring body, short effective anchorage length, incomplete hole walls in the surrounding rock, the resin cartridges easily roll, bend, overlap, and pile up on the hole wall and do not properly react. This study presents a systematic analysis of the anchoring characteristics of roof cable bolts in a roadway with weak interlayers based on the research and development of synergistic components. The synergistic mechanism is identified. Numerical simulations are performed to demonstrate the dynamic flow field characteristics of the resin cartridge mixing, and laboratory tests are performed for comparison to determine a reasonable setting for the synergistic components. Field tests are conducted to comprehensively verify the working performance of the synergistic technology. The results show that the synergistic component can greatly improve the fluidity and uniformity of the resin cartridge reaction. The anchoring performance of the synergistic anchorage cable bolt used in the field is also significantly greater than that of ordinary anchoring cable bolts and higher than or equal to the engineering requirements. When an anchor hole is broken or collapses, the spacing of the synergistic components can be reasonably adjusted (i.e., by shortening the anchorage length) to allow the resin cartridge to evenly and densely fill the anchoring area, which greatly improves the anchoring system's bearing capacity. Highlights: Cable bolt resin anchoring on roadway roofs with weak interlayers is analyzed under typical on-field construction conditions. Anchoring synergistic components greatly improve the bearing capacity and energy absorption capacity of an anchoring system. On-field dynamic mixing guidance parameters are proposed for different types of resin cartridges in combination or used alone. Synergistic component spacing adjustments effectively improve the density and uniformity of an anchoring body, thus improving the anchoring quality. [ABSTRACT FROM AUTHOR]

In this study, the typical modes, hazards, and causes of rebar failure were analyzed under the condition of large deformation around deep roadways induced by dynamic disasters such as rock burst. We found that the fundamental causes of the tail failure of rebars are their low elongation and poor energy-dissipation performance. To solve this problem, we designed axial splitting components as energy absorbers, and developed anti-impact and energy-absorbing tail bolts, and in this paper, we elaborate on their working principles. The behavior of energy absorbers of a specific size (wall thickness of 4 mm and inner diameter of 26 mm) was experimentally investigated under both static and dynamic loading conditions. The self-protection performance of these novel bolts was studied using quasi-static tensile tests. The results show that they deformed stably with a constant resistance under both static and dynamic loading. The constant resistance value of the splitting energy absorbers was as high as 125–150 kN, while the energy-absorption efficiency was 0.111–0.127 kJ/mm. The energy absorbers effectively increased elongation, delayed yielding, and enhanced the energy-dissipation capacity of rebars, which resulted in the self-protection of bolts subject to large deformation loading. [ABSTRACT FROM AUTHOR]

The process of coal and gas outburst (hereinafter referred to as “outburst”) is inevitably accompanied by temperature variation. To further reveal the temperature evolution law and proposal of risk indicator of deep outburst, taking the outburst coal seam of Pingmei No.11 Coal Mine as the research object, the outburst tests with simulated buried depths of 600 m, 800 m, 1000 m, 1200 m, 1400 m, and 1600 m are carried out. The relationship between outburst parameters (critical gas pressure, test-effective stress), acoustic emission signal, and temperature variation is analyzed. The approximate range of the critical buried depth of Pingmei No.11 Coal Mine is given. According to the characteristics that the temperature variation decreases earlier than the acoustic emission signal, a new outburst risk indicator is established. The results show that the coal-seam temperature variation in the outburst process is determined by gas pressure and gas content. When the buried depth is higher than the critical depth, the influence of gas content on outburst is intensified, and the sensitivity of outburst to gas content is greater than that of gas pressure. The test-effective stress is negatively correlated with temperature, and the gas-expansion work is positively correlated with temperature. The development of outburst leads to the variation in internal temperature of coal seam. The temperature variation eventually reacts to the development of outburst. The risk indicators P2, N1, N2, and S are defined based on acoustic emission signal and temperature variation. According to the change of risk indicators, the outburst risk is divided into four grades: “no, weak, moderate, and strong”, which is of great significance for the prediction and early warning of outbursts. [ABSTRACT FROM AUTHOR]

During the coal-mining process, high-energy mining microseismic events constrain the safety production of coal mines. To address the issues of low accuracy in predicting the time, space, and intensity (energy) of mining microseismic events, as well as insufficient feature extraction of mining microseismic monitoring data, the Combined Hierarchical Information Modeling (CHIM-Net) is proposed, integrating deep learning theory and technology. This model consists of a data decomposition module, a data partitioning module, and two prediction branch modules. The original mining microseismic monitoring data is decomposed and segmented, and then input into different prediction modules for training to obtain the trained model, ultimately obtaining the prediction results. To evaluate the model, mining microseismic monitoring data collected from coal mines in different provinces were used to train and test the model. Compared with several models such as Autoformer and Informer, the effectiveness of this model in the field of mining microseismic prediction is verified. The ablation experiment demonstrated the effectiveness of the data decomposition module in improving prediction performance. The results show that when at a prediction length of 48, the model achieves an average reduction of 18.92% in mean squared error and 10.57% in mean absolute error compared to other models, while at a prediction length of 96, it reduces by an average of 8.46% and 10.23%, respectively. Experimental results demonstrate that the proposed model performs well in predicting high-energy mining microseismic events, providing valuable insights for mining microseismic prediction and early warning. [ABSTRACT FROM AUTHOR]

Brittleness index is a significant mechanical parameter of rocks, widely applied in various geotechnical engineering. However, existing methods for evaluating brittleness indices have their limitations. In this study, we analyzed the energy evolution characteristics before and after peak stress in both radial and axial directions and proposed a new brittleness index considering radial strain energy evolution. The proposed method was validated through experiments. The results indicate that as confining pressure increases, rock brittleness gradually decreases. The widely used rock strength-based, elastic parameter-based, and modulus-based brittleness indices fail to reflect this trend. Only energy-based brittleness indices and the new brittleness index proposed in this paper align with this trend. However, existing energy-based indices do not consider radial energy evolution. For rocks with similar axial energy evolution characteristics but significant radial energy evolution differences, the energy-based brittle index will give the same brittleness index, which fails to evaluate brittleness characteristics accurately. The brittleness index proposed in this paper can provide accurate value for brittleness index and objective evaluations. Therefore, this new method in this study is more precise and exhibits significant advantages, compared with other mentioned brittleness index evaluation methods. [ABSTRACT FROM AUTHOR]