Coalbed methane is not only a major source of disaster, but also a green energy source. The migration pattern of methane in coal serves as the theoretical basis for disaster prevention and resource utilization, while diffusion is the key link connecting microscopic adsorption and macroscopic flow. In this study, a methane multi-mechanism diffusion model combining Fick diffusion, transitional diffusion, Knudsen diffusion, and surface diffusion is established. The governing equations are derived theoretically and solved numerically using the finite difference method. The model is verified using the gas desorption experimental data of three typical types of coal samples (high, medium, and low grades of metamorphism) published by the authors. The results show that the calculated methane diffusion quantity is in highly consistent agreement with the experimental data. The relative error of the calculation data ranges from -12.46 % to 15.50 %, indicating that the model has good predictive accuracy. Based on the model, the influence laws of pore structure parameters (pore diameter and diffusion distance) on gas diffusion in coal are systematically analyzed. The dynamic conversion characteristics of the diffusion mechanism in different scale pores are revealed, and the control mechanism of pore structure on gas diffusion is clarified. This research provides theoretical support for accurately describing the gas migration behavior of coal.

The deformation of deep roadway is more severe under the influence of high stress, especially in the complex stress field, which is difficult to support and seriously affects the safety. Leveraging the engineering context of deep complex stress fields, this study constructs a three-dimensional mechanical model for roadway excavation. It employs numerical simulation to analyze the deformation characteristics of surrounding rock and the distribution of deviatoric stress, identifying critical areas for roadway support and clarifying the support methodology. The collaborative support technology of ‘multi-level pre-tightening of anchor cables + multi-level shotcrete sealing of surrounding rock surface + high-pressure grouting reinforcement of surrounding rock cracks’ in deep complex stress field is proposed. The parameters of each specific support method are calculated, which effectively resists the large deformation of roadway in deep complex stress field. The deformation of roadway and the stress of support body are within the allowable range of safety, and the effective control of roadway in deep complex stress field is realized. It provides reference for roadway support under similar deep complex engineering conditions.

To address the application limitations of traditional weakening techniques under complex geological conditions such as hard coal mine roofs, directional hydraulic fracturing (DHF) technology has become a key technical measure to ensure safety production by virtue of its core advantages of directional rock breaking. This paper systematically reviews the research status and development trends of underground DHF technology in underground coal mines, focusing on an analysis of the three key dimensions: directional fracturing methods, processes, and equipment. Regarding fracturing methods, three mainstream technologies based on manual slotting, linear arrangement drilling, and high-pressure water jet slotting have been sorted out. The paper compares their principles, advantages, and applicable scenarios, pointing out that a linear synergistic fracturing method using multiple fracturing holes with high-pressure water jet slotting demonstrates both precision and scalability, making it the most promising technological path at present. For fracturing processes, it elaborates on the standardized progress of the four core procedures: drilling construction, pre-treatment, high-pressure water injection, and effect verification, and analyzes the key bottlenecks in process optimization under complex geological conditions. In terms of fracturing equipment, technical characteristics and existing issues of drilling, slotting, high-pressure water injection, and monitoring devices are summarized. Aligning the development trends of mining engineering technology, the paper proposes that future directional hydraulic technology will evolve towards intelligent directional fracturing, multi-field coupled fracturing, and miniaturized precision fracturing. At the process level, it will develop towards integrated efficiency, adaptive dynamics, and green low-carbonization, while equipment will focus on breakthroughs in intelligent automation, high efficiency and reliability, and miniaturization and integration. These research results provide a reference for theoretical study, equipment development, and engineering applications of underground DHF technology, contributing to safe, efficient, and sustainable coal mining practices.

Composite thick-hard roof(CTHR) is characterized by substantial thickness, high mechanical strength, excellent load-bearing capacity, and remarkable elastic energy storage potential. Its sudden collapse tends to induce rock bursts due to the abrupt release of accumulated elastic energy. This study analyzes the layered bearing characteristics of the CTHR and reveals its stress-oriented fracturing mechanism. The bearing strength of the CTHR decreases significantly with the increase of the hanging roof length and the number of layered strata. After the implementation of stress-induced fracturing treatment, the CTHR is fragmented into small-sized rock blocks, and the "three-zone" structure of the overlying strata is clearly formed. Meanwhile, the total energy released by microseismic events is reduced by 36.98% compared with that before. The average periodic weighting interval is shortened from 35.24 m to 16.57 m, representing a reduction of 52.98%, and the peak weighting pressure is reduced from 8179.58 kN to 5618.49 kN, a decrease of 31.32%. The bearing strength of the CTHR has been effectively reduced, and the hazard of ground pressure impact has been significantly mitigated.

To address the problem of strong mine pressure caused by ultra-thick overlying strata (UTOS), this paper analyzes the distribution law of mining stress field in different layers of UTOS, and the expression of mining stress concentration coefficient of UTOS is given. The law of hydraulic fracture propagation in stress concentration area and original stress area of stope roof is expounded. The stress-oriented fracturing mechanism of UTOS is revealed. The results show that: (1) Along the advacned direction, the stress concentration value in the UTOS increases first and then decreases, exhibiting a “core” distribution pattern. As the buried depth of the roof increases, the range of the stress concentration core gradually decreases; (2) When hydraulic fracturing is carried out in the stress concentration area and the original stress area respectively, vertical hydraulic fractures and horizontal hydraulic fractures will be formed in the roof respectively; (3) The fracture propagation pressure in the stress concentration area is significantly greater than that in the original stress area. After the stress-oriented fracturing of the UTOS, the support resistance of the working face is significantly reduced, which is beneficial to the mine pressure management of the working face.

Crack propagation in gas-bearing coal-rock bodies under dynamic disturbance plays a crucial role in coal mine safety. Impact-induced damage can significantly alter the permeability of coal-rock strata, thereby increasing the risk of gas-related hazards. In this article, the dynamic crack propagation and permeability jump characteristics of gas-bearing coal-rock combinations under different strain rate loading conditions were systematically investigated. Triaxial dynamic compression experiments were conducted to obtain the stress-strain responses and permeability evolution patterns of the samples. Further analysis of the failure modes and crack types of the coal-rock combination was conducted using industrial computed tomography scanning equipment, and the three-dimensional crack network was reconstructed. On this basis, a fractal seepage model was developed by integrating the pore network modeling approach. And the intrinsic mechanism underlying the permeability jump in gas-bearing coal-rock combinations is clarified. The results indicate that the dynamic stress-strain curve of coal-rock combinations exhibits dual linear elastic characteristics and elastoplastic instability behavior. The dynamic mechanical response of coal-rock combinations demonstrates pronounced strain-rate dependence. The cracks in the coal-rock combination are induced by compressive-shear failure, tensile-strain failure, and unloading failure, respectively. The crack propagation and failure extent in coal-rock combinations become increasingly severe with rising strain rates. The continuously increasing crack spatial complexity directly leads to a permeability jump. Relative to quasi-static compression failure, the coal-rock combination subjected to dynamic compression failure exhibits gas seepage hysteresis prior to the permeability jump.

To address the problem of strong mine pressure caused by huge thick overlying strata(HTOS), this paper analyzes the distribution law of mining stress field in different layers of HTOS, and the expression of mining stress concentration coefficient of HTOS is given. The law of hydraulic fracture propagation in stress concentration area and original stress area of stope roof is expounded. The stress-oriented fracturing mechanism of HTOS is revealed. The results show that: (1) Along the advacned direction, the stress concentration value in the HTOS increases first and then decreases, exhibiting a "core" distribution pattern. As the buried depth of the roof increases, the range of the stress concentration core gradually decreases;(2) When hydraulic fracturing is carried out in the stress concentration area and the original stress area respectively, vertical hydraulic fractures and horizontal hydraulic fractures will be formed in the roof respectively;(3) The fracture propagation pressure in the stress concentration area is significantly greater than that in the original stress area. After the stress-oriented fracturing of the HTOS, the support resistance of the working face is significantly reduced, which is beneficial to the mine pressure management of the working face.

Huge-thick overlying strata(HTOS) is characterized by substantial thickness, high mechanical strength, excellent load-bearing capacity, and remarkable elastic energy storage potential. Its sudden collapse tends to induce rock bursts due to the abrupt release of accumulated elastic energy. This study analyzes the layered bearing characteristics of the HTOS and reveals secondary fracturing and weakening mechanism. The bearing strength of the HTOS decreases significantly with the increase of the hanging roof length and the number of layered strata. After the implementation of the secondary fracturing and weakening treatment, the HTOS is fragmented into small-sized rock blocks, and the "three-zone" structure of the overlying strata is clearly formed. Meanwhile, the total energy released by microseismic events is reduced by 36.98

Accurate prediction of the water-conducting fracture zone height is essential for water inrush prevention and safe production in coal mining. Based on extensive in-situ measurements collected from longwall panels in different mining districts, five key indicators-mining thickness, mining depth, coal seam dip angle, panel length along dip, and the hard-rock lithology ratio coefficient-were analysed. Using regression analysis, the empirical formula for predicting the water-conducting fracture zone height was refined and a multivariate nonlinear regression model was fitted. An optimal BP neural network model with the Levenberg-Marquardt algorithm and a 5:8:4:1 topology was validated, and subsequently an LWMA-PSO-BP neural network model was developed by jointly introducing the mutation operator from genetic algorithms and a linearly decreasing inertia weight (LDIW) strategy. Model fitting accuracy and generalisation were evaluated; the results indicate that the LWMA-PSO-BP model achieved the best overall performance, with a mean absolute error of 2.40 m and a mean absolute percentage error of 4.27%. In the Hebi mining district, a joint geophysical investigation integrating a microtremor survey, borehole coring, and drilling fluid loss measurements was conducted, and the water-conducting fracture zone heights for Panels 2301, 2302, 2303, and 2304 at Hemei No. 5 Mine were determined as 129.05 m, 134.21 m, 141.50 m, and 138.20 m, respectively. Field validation shows that the relative errors of the multivariate nonlinear regression and BP neural network models were 5.52% and 4.85%, respectively, whereas the LWMA-PSO-BP model yielded a relative error of only 2.99%. These results provide a reference for predicting the water-conducting fracture zone height under varied coal mining conditions.

Abstract Taking fly ash-doped high-water filling materials with equal substitution as the research object, this paper systematically investigates the influence laws of different fly ash dosages on the initial setting time, density, and 28 day mechanical properties of the materials under standard curing and natural weathering conditions. Fly ash replacement within 0–15% barely disturbs hydration and maintains stable mechanical performance, defined as the optimal doping range. Combined with microscopic testing methods such as X-ray diffraction (XRD) and scanning electron microscopy (SEM), the hydration products and microstructure evolution are analyzed to reveal the hydration mechanisms. The results show that equal substitution of fly ash prolongs the initial setting time of the material approximately linearly, and the mechanical properties of the material decrease overall with the increase of the fly ash dosage. For the adopted ultrafine fly ash, mechanical indicators drop sharply when the replacement ratio exceeds the critical threshold of 30%. When the dosage exceeds 20%, the strength, elastic modulus, and deformation modulus drop sharply. The fly ash dosage below 15% has little effect on the hydration reaction, while an excessive dosage inhibits the growth of ettringite crystals. Under a natural weathering environment, high-content fly ash can reduce the carbonization of ettringite but results in a low degree of hydration reaction. At 30% fly ash replacement, the compressive strength and elastic modulus exhibit a cliff-like attenuation of over 40% compared with the pure matrix specimen. The effects of fly ash on high-water filling materials are mainly physical filling and dilution and the formation of cementitious hydrated calcium silicate and hydrated calcium aluminate gels in the later alkaline environment, which can improve the internal cohesion and deformability of the material.

This article takes the transportation channel of the 2507 working face as the engineering background to carry out the analysis of the surrounding rock failure mechanism of the roadside filling in gob-side entry retaining of large mining height panel, and summarizes three common forms of instability failure of the filling wall. The method and idea of using a combination of “strengthened support + roof cutting pressure relief” to control the surrounding rock of gob-side entry retaining of roadside filling is determined. In theory, the optimal range of roof cutting height and roof cutting angle was obtained, and an innovative introduction of deviatoric stress analysis index was made. Numerical simulation research was conducted on the distribution characteristics of deviatoric stress in the surrounding rock of the gob-side entry retaining under different roof cutting heights, roof cutting angles, and wall width conditions. The optimal parameters for roof cutting and pressure relief of the gob-side entry retaining of roadside filling in the high mining face were obtained. The on-site engineering practice shows that after adopting the joint control technology of “strengthening support + roof cutting pressure relief”, the displacement of the surrounding rock of the retained roadway, the force of the anchor cable, and the bearing capacity of the flexible formwork wall are all within the normal range, ensuring that the retained roadway can continue to be used by the next working face.

To investigate the pressure-relief and permeability enhancement mechanisms of hydraulic flushing (HF), this study conducted the triaxialseepage experiments on raw coal; the permeability changes during the loading process were analyzed, and the coal permeability evolution model was derived considering the damage effect. Given the boundary conditions of gas seepage and stress fields, a multifield coupling model was developed to reflect coal seam methane (CBM) extracted by the HF borehole. Based on the actual geological conditions of Yi'an Coalmine, the three-dimensional (3D) numerical simulation was implemented to investigate the effects of coal discharge amount on the relief range, coal permeability, and effective extraction radius of the HF borehole. Moreover, the validity of the coupling model was confirmed through field tests on hydraulic flushing. Then, the interactions of various factors, including coal discharge amount, borehole spacing, initial CBM pressure, and coal permeability, were investigated using the response surface methodology (RSM). Through the results obtained, it was found that the interactions between the factors involved were very significant and caused an obvious time effect on CBM extraction. The interaction between coal discharge amount and borehole spacing increasingly affected the extraction effect of HF boreholes, whereas the interaction between initial CBM pressure and coal permeability gradually weakened. Then, the RSM regression model was established to optimize the borehole placement parameters using predicted extraction time and coal discharge amount as variables. The regression prediction data were consistent with the simulation results and mutually validated. Therefore, based on the elimination of gas outbursts and reduced construction costs, the modeling approach and the RSM model introduced in this study can be used to optimize the precise placement parameters of HF boreholes for enhancing CBM recovery within the expected extraction period.

In the tunneling of frozen soil section of shaft sinking by freezing method, the traditional air pick excavation has large consumption of pick and drill rod, low excavation efficiency, and high labor intensity. The drilling explosive blasting method has a poor working environment and potential safety hazards. The safe and efficient excavation of frozen soil section is a difficult problem faced by freezing method shaft sinking. In this paper, a directional fracturing method by liquid CO2 phase change for freezing shaft sinking is proposed. Based on the traditional CO2 phase change fracturing device, a liquid CO2 phase change fracturing device with directional function is designed. It can not only fully crush and weaken the frozen soil in the shaft, but also can not disturb the shaft wall structure. The liquid CO2 phase change directional fracturing technology is firstly applied to the assisted tunneling in freezing shaft sinking. The field test results indicate that the directional liquid CO2 phase change generator has a good directional fracturing function. There are many obvious cracks formed around the directional fracture drilling hole, and the cracks are all extending toward the center of the protection circle. The angle between the two outermost cracks is close to the phase angle of directional discharge hole of liquid CO2 phase change generator. According to the statistics of field construction, the average daily excavation speed of frozen soil section is increased from 1.67 m/d to 2.45 m/d after directional fracturing by phase change of liquid CO2. Compared with explosive blasting weakening, liquid CO2 phase change fracturing has the advantages of no spark in the phase transformation process, easy control of pressure, convenient storage and transportation, simple operation, and relatively safe construction process. Liquid CO2 phase change fracturing shows a good application prospect in some fields which have high safety requirements. It is expected to be popularized and applied in slope excavation, tunnel chamber excavation, and other fields in the future.

To examine the influence of borehole parameters on the instability and precursor characteristics of large-diameter boreholes in coal seams, this study conducts instability failure tests on coal samples with varying diameters, spacing, and row spacing. The investigation explores the effects of borehole parameters on the damage and failure of coal samples from the perspectives of stress distribution, crack propagation, failure modes, and acoustic emission characteristics. The following three points are addressed: (1) Increasing the borehole diameter and reducing the row spacing significantly enhances the pressure relief effect of large-diameter boreholes, necessitating rational adjustment of borehole spacing parameters for optimal performance. (2) The damage mechanism induced by borehole parameters primarily manifests as shear cracks, with minimal impact on tensile cracks. (3) A sudden decrease in the b-value indicates imminent rupture, and when the b-value reaches its minimum, the final failure of the coal sample occurs. The research findings enhance the understanding of the pressure relief process in large-diameter boreholes and provide a scientific basis for parameter selection and optimization in large-diameter borehole operations.

In order to investigate the essence of CH4/CO2 adsorption in coal for CO2-enhanced coalbed methane recovery (CO2-ECBM), this study established the coal structure models from the chemical composition and structure information on different rank coals to conduct CH4 and CO2 adsorption simulation under different environmental conditions. Thus, the differences and connections between integral heat and isosteric heat of CH4/CO2 adsorption in coal and its micro-mechanism were discussed. The results show that as the coal metamorphism degree deepens, the integral heat of CH4/CO2 adsorption, similar to adsorption capacity, presents a decreasing first and then increasing trend. While the adsorption equilibrium time of high-rank coal gives a significantly decreasing characteristic with pressure. Then, on the basis of adsorption simulation behavior, it finds that because complex functional groups exist in the coal macromolecular structure, the adsorption capacity shows a different characteristic compared with the experimental results; that is, it decreases with the coal metamorphism degree. Meanwhile, compared to CO2 adsorption, the isosteric heat of CH4 adsorption appears to have an obvious downward trend with increasing pressure and then gradually stabilizes. Further, there is always a clear linear relationship between CH4 adsorption capacity, and isosteric heat for aromatic pores in different rank coals. While for slit pores, both CH4 and CO2 molecules exhibit significant parabolic relationships between adsorption capacity and isosteric heat. In addition, on the one hand, except for the obvious chemical adsorption of low-rank coal in the high-pressure stage, affected by pore morphology and size, the isosteric heat of CH4 or CO2 adsorption manifests lower values in slit pores and large pore sizes. On the other hand, based on the adsorption systems of similar structural fragments with different functional groups, -OH has been identified as the functional group with the strongest adsorption effect on gas molecules and is also the main functional group causing CO2 chemical adsorption.

Taking the No.4 coal seam in the return air roadway in the upper formation No.1 coal mining area of a mine as the background, based on the energy dissipation theory, the energy dissipation mechanical model of arch roadway was established based on the energy balance equation and the energy analysis theory of surrounding rock loose circle, and the failure range of surrounding rock loose circle was positively correlated with the energy dissipation. Based on the Moore-Coulomb strain softening constitutive model, the energy dissipation fish program based on FLAC3D software was developed and numerical simulation was carried out, and the average energy dissipation range of the roadway in the model was determined to be 5.9 m. Combined with the field drilling imaging test, the failure range of the loose ring of the surrounding rock of the roadway was determined to be 5.9 m and 6.04 m respectively. The above content proves that the energy dissipation theory can be well applied to the analysis of the loose zone of surrounding rock.

To investigate the mechanisms of instability and precursor information related to coal seam large diameter borehole depressurization. Based on the fine-scale numerical experiment with Universal Distinct Element Code (UDEC) and the macro failure experiment in the laboratory, the influence of borehole parameters on coal sample damage and failure was studied from the perspectives of stress, micro cracking, failure mode, and acoustic emission (AE) information. The results show that the borehole parameters mainly affect the micro damage mechanism of coal samples in the form of shear cracks. The secant modulus and compressive strength of coal sample decrease with the increase of borehole diameter, the shear crack decreases rapidly, and the degree of cavity deformation increases. The compressive strength and impact properties of coal can be effectively reduced after borehole. According to the failure pattern of coal samples, it is found that adjusting the borehole parameters can control the fracture development of coal samples and determine the final failure pattern to a certain extent. The continuous expansion of small energy micro fracture events will lead to large energy local fracture events, and then produce high stress concentration around boreholes, accelerate the intersection of local fracture areas, and lead to the overall instability of coal samples. The research provides a scientific basis for the selection and optimization of the parameters of large-diameter depressurization borehole in coal seam, and has a good guiding significance for the prevention and control of coal burst and the monitoring and early warning.

Rockburst with structural instability is prone to occur during mining and excavation in the bifurcation area of coal seam. It is crucial to explore the failure and instability characteristics and mechanism of coal–rock parting–coal structure (CRCS) in order to prevent rockburst occurrences. The following four points are addressed: (1) The failure and instability of the CRCS involve slip and fracture characteristics, which are influenced by factors such as the strength of the coal and rock parting, inclination angle, friction coefficient, and surrounding rock pressure. (2) The failure and instability of the CRCS encompass fracture instability (FI), single and double contact surface slip and fracture instability (SSI and DSI). Rockburst manifests as strain instability in the form of FI, along with structural instability in the form of SSI and DSI. (3) Crack development is primarily characterized by shear cracks supplemented by tensile cracks. However, it is the connecting effect of tensile cracks that serves as a primary cause for macro-instability within the CRCS. (4) A larger inclination angle on the contact surface results in more pronounced slip phenomena, leading to greater energy release. This demonstrates low strength but high release energy characteristics, making it difficult to predict rockburst occurrences. The research results have important theoretical significance for preventing rockburst in the bifurcation area of coal seam.

Enhanced coalbed methane (ECBM) recovery by gas injection is regarded as a feasible method for ECBM recovery. To investigate the mechanism of CH4 displacement by N2 injection, a series of physical experiments were conducted in the laboratory under different N2 injection pressures. The experimental results showed a continuous increase in the N2 volume fraction and a decrease in the CH4 volume fraction with the N2 injection pressure. The CH4 displacement efficiency increases rapidly in the initial stage and then gradually stabilizes. Given the effect of matrix shrinkage and effective stress on coal permeability, a dynamic multi-field coupling model of ECBM recovery by N2 injection (N2-ECBM) was proposed in this work. Then, numerical simulations were implemented by the coupling model to analyze the key factors affecting coal permeability, N2 injection pressure and CH4 pressure during the N2-ECBM process. Furthermore, the Response Surface Methodology (RSM) was used to investigate the interactions of multiple factors, the results reveal that an increase in one factor would weaken the influence of another factor. In addition, a RSM regression model was obtained and verified by the experimental data, with a well-fitting outcomes. Therefore, the RSM model can be used in practical engineering applications to optimize specific extraction parameters for ECBM recovery at minimum cost in a limited extraction period.

The extraction of extremely thick coal seams through slicing mining requires leaving behind several island coal pillars in the top slicing, which could lead to rockburst in the bottom slicing. This article proposed a method of alternate exterior entry (AEE) for rockburst prevention based on the stress distribution of bottom slicing. The influences of coal pillar width ( B ) and stress concentration factor ( K ) on stress distribution were explored using two-dimensional stress imaging. The results indicated that K value had a greater effect on stress distribution comparing with B value. Both stress relaxed and stress rate angles increased linearly with an increase in K value. In order to get the optimized location of AEE in the coal mine stress rate angle was chosen as the stress extension angle, which was further verified by field data from 1210 coal faces. Finally, numerical simulation was used to explore the optimized location of special tail entry under dip island coal pillars for rockburst prevention. It was revealed that plan first was found to be most effective due to its presence in a stress-relaxed area, thus verifying the effectiveness of the AEE method for rockburst prevention.