Optimization of small coal pillar width and control measures in gob-side entry excavation of thick coal seams.

This study introduces an innovative optimized bolting support system specifically tailored for gob-side entry excavation in thick coal seams at a coal mine in southwestern Shandong, China. Employing theoretical analysis, numerical simulation, and field measurements, the research focuses on examining the failure characteristics of surrounding rock during gob-side entry excavation. The key innovation lies in the development of a 5-meter optimal coal pillar width, ensuring balanced stress distribution and structural integrity. Additionally, a lag time of at least 46 days between gob-side entry excavation and the upper working face retreat is recommended to mitigate roof subsidence and surrounding rock deformation. The optimized bolting support system, featuring increased bolt pretension, utilization of high-strength steel strips, and reinforcement of weak points, effectively reduces deformation of the roadway surrounding rock, meeting support requirements for normal production. This novel approach successfully addresses the support challenges in thick coal seam gob-side entry excavation, enhancing mining safety and resource recovery rates.

Reasonable layout range of lower mining roadways in the close and variable interval coal seams mining: a case study.

In the context of close-distance coal seam mining, the stress transfer mechanism following the extraction of the upper coal seam is crucial for the layout of roadways in the lower coal seam. This study utilizes ground-penetrating radar (GPR) to analyze the stability and plastic zone extent of residual coal pillars after upper seam extraction. A theoretical model for the stress distribution of multiple goaves and coal pillars was established. Based on a case study with variable inter-seam spacing, the stress distribution and transfer mechanisms of the floor were analyzed. The results indicate that during the transition from the center of the coal pillar to the goaf, stress concentration initially decreases significantly and then gradually returns to the original rock stress, forming a distinct stress gradient distribution. As the depth of the floor strata increases, the weight of the overlying strata is more evidently transferred through the coal pillars to the goaf. Finally, a reasonable layout range for the lower seam mining roadways under different inter-seam spacings is proposed, providing a reference for optimizing roadway layout in similar geological conditions.

Influence of different mining locations on deformation characteristics of overlying strata on gently anti-dip high-steep mining slope.

High-steep gently inclined mining slopes, prevalent globally, often suffer significant deformation, leading to landslides due to numerous goafs. This research investigates the critical role of goaf location in controlling deformation, failure mechanisms, and disaster evolution, vital for safe mining practices. Through field investigations and model generalization in Guizhou, a physical model test method was used to study three positions of goafs: under the shoulder and foot of the slope, under the slope shoulder, and within the slope. Findings highlight the stronger influence of the goaf's mutual position with the slope shoulder on slope and overburden deformation and failure compared to the slope toe. Deformation and failure modes evolve from collapse toppling to collapse slip and collapse settlement as the goaf shifts from the near slope surface to inside of the slope. Statistical analysis of fracture distribution in the goaf's overlying strata reveals damage increase with larger goafs, following a Gaussian distribution, with concentrated damage in the middle. The study identifies the maximum damage area, influenced by the horizontal distance between the goaf center and slope shoulder. These insights advance understanding of overburden rock deformation in gently inclined high-steep mining slopes.

Study on the improvement of grouting stone properties in coal mine goafs using combined denitrifying bacteria.

Grouting can effectively reduce residual deformation of coal mine goafs, but fly ash grouting materials suffer from poor flowability and slow early strength development. Microbially -induced calcite precipitation (MICP), with its high environmental compatibility and minimal disturbance to geotechnical bodies, effectively improves the injectability of grouting slurry in goafs. This study combined Castellaniella denitrificans and Sporosarcina pasteurii to induce calcite precipitation, preparing cement-fly ash slurry with varying water-solid ratios, solid ratios and denitrifying bacteria concentrations. The physical properties of the slurry and the mechanical properties of the grouted stone bodies under sealed curing conditions were measured. Results show that the dual-bacteria MICP improves stone body performance by enhancing cohesive, frictional and interlocking forces, so that the strength of the stone bodies cured by MICP increased rapidly within 7 days, and the strength reached the standard 2.03 MPa at 28 days under conditions of low solid ratio and high water-solid ratio, with the best compressive strength at a denitrifying bacteria concentration with an optical density of 0.8 at 600 nm wavelength. At a water-solid ratio of 1 : 1.2 and a solid ratio of 15%, initial and final setting times were 67.2 and 96 h, respectively, which prolonged the initial setting time and final setting time by nearly 70% and 110% compared with that of the slurry without MICP treatment, indicating that MICP enhances slurry fluidity, providing more time for grouting construction in goafs.

Study on stability evaluation of goaf based on AHP and EWM-taking the northern new district of Liaoyuan city as an example.

Throughout the history of coal mining in all countries of the world, large areas of goaf have been left behind, and sudden collapses and surface subsidence of large areas of goaf may occur, especially for mining areas with long mining cycles. The northern new district of the Liaoyuan mining area has been subjected to nearly half a century of mining activities, accompanied by a gradual accumulation of disasters, which have occurred frequently in recent years. In order to assess the stability of the goaf in the study area, this paper proposes a hybrid decision-making multi-factor integrated evaluation method. The distribution of underground goafs was determined using geophysical exploration techniques (seismic survey and transient electromagnetic method) and geological drilling exploration. First, an evaluation index system was established based on the specifications of the goaf, the ecological and geological environment, and the mining conditions; the system included 14 indicators. Two weight calculation methods, AHP-EWM, were employed to determine the comprehensive weight of each indicator by combining subjective and objective weights on the basis of improved game theory. Subsequently, the fuzzy comprehensive evaluation method was utilised to complete the stability rating of each block in the study area, and MapGIS and ArcGIS were employed to complete the drawing of the stability zoning map of the northern new district goaf. The study area was divided into three zones of stability, basic stability and instability, according to the critical value. These zones accounted for 23.03%, 36.45% and 40.52% of the total area of the study area, respectively. The comprehensive on-site investigation revealed a decrease in the size and number of collapse pits and the rate of damage to the houses from the unstable zone to the stable zone. This indicates that the results of the division are consistent with the actual situation. The classification results are consistent with the actual ground disaster situation, thus verifying the rationality and validity of the evaluation method. The results indicate that the stability of the study area is generally at the lower middle level.

Hazard evaluation of goaf based on DBO algorithm coupled with BP neural network.

China is rich in mineral resources, and problems of goaf formed in the process of resource exploitation are serious obstacle to the development of China's economic, so it is of great significance for the assessment and management of goafs. This paper introduces emerging dung beetle optimizer (DBO) algorithm and establishes DBO-BP (back-propagation) model, at the same time, it is compared with a series of heuristic algorithms coupled with BP neural network models: PSO (particle swarm optimization) - BP model, WOA (whale optimization algorithm) - BP model, and SSA (sparrow search algorithm) - BP model. Then they are applied to evaluate the hazard of goafs, the result shows that the DBO-BP model gets the highest train set accuracy, which is at least 2.7 % higher than other models, while the DBO-BP model obtains the highest test set accuracy, meanwhile its effectiveness and stability have also been proven. Finally we apply the established DBO-BP model to evaluate the hazard of the tungsten mine goaf of Yaogangshan in Hunan Province, and its excellent practicability was confirmed. This paper may provide a reference for the solution of nonlinear engineering problems.

Analysing the application of a flexible formwork pre-cast wall driving roadway along goaf in a large mining height face.

The technology of building a retaining roadway along goaf or a protecting roadway with a small coal pillar has been developed and applied for many years, and a satisfactory supporting effect has been obtained in medium-thick coal seam and thin coal seam mining. However, the gob-side roadway or small coal pillar mining in a thick coal seam is still subjected to technical problems occasioned by factors such as high roadway, high support pressure beside roadway, and waste of coal resources. To solve these problems, the author proposes an innovative technology of coal-free mining: the technology of driving roadway along goaf with a flexible formwork pre-cast wall. The article utilizes the 3503 and 3505 working faces of Wangzhuang Coal Industry Group as the research background, and comprehensively introduces the principle of the technology and the overburden rock movement law. Through theoretical calculations and numerical simulations, the support resistance and support parameters of flexible formwork pre-cast walls have been determined and successfully performed in industrial practice. The results indicate that the combination of the flexible mould pre-cast wall coal pillar-free mining technology and roof cutting process is more conducive to the maintenance of the roadway in the lower working face, and effectively reduces the stress and deformation of the surrounding rock. The roof and floor of the drivage roadway move, and the deformation of the two sides is small; furthermore, the overall roadway retention effect is satisfactory, which meets the requirements of mining in the lower working face. The coal pillar pertaining to the 20 m section of the 5 m high mining height face was recovered for Wangzhuang Coal Mine, and the recovery rate of the coal resources and the driving speed of the roadway were improved. The proposed method can be popularised and applied in this mine and even in the mining of 15# large-height coal seams in the two cities.

Feasibility experimental study on plugging leakage in goaf based on two-phase foam.

Air leakage in goaf often leads to coal spontaneous combustion (CSC), which not only directly affects the safety production of mines but also causes significant environmental damage. Therefore, effectively sealing the airflow in goaf is crucial for preventing CSC. Feasibility experiments on using two-phase foam to seal air leakage in goaf were conducted, leveraging the advantages of large flow rate, wide diffusion range, and good accumulation characteristics of two-phase foam. The research results indicate that continuous injection of foam into loose media with maintained ventilation can completely seal the air leakage, with the foam capable of withstanding wind pressures of nearly 600 Pa. When the foam is used for one-time sealing with a length of 2 m, it remains effective for 60 min, and the sealing effectiveness improves with longer distances sealed against air leakage. Numerical simulation analysis and field measurements of airflow leakage in mine working faces reveal that effectively sealing the airflow passage in the goaf behind the corner of the return airway is crucial for preventing CSC. Two methods are proposed for sealing external airflow during coal mining: foam injection using a point drilling method near the heading and an incremental buried pipe injection method. Finally, the feasibility of two-phase foam sealing technology for goaf airflow leakage is analyzed from multiple perspectives including sealing effectiveness, practicality, economy, foaming process, and engineering implementation. The research findings provide new insights into goaf sealing technology, aiding in addressing safety and environmental issues caused by spontaneous combustion in goaf areas.

Research on disturbance influence of goaf residual deformation on simply supported beam bridge.

In order to analyze the stability of the bridge above the goaf, the disturbance influence of goaf residual deformation on the bridge is studied. Firstly, an equivalent numerical simulation method of goaf residual deformation evolution process is studied by quantitative analysis the sensitivity of residual subsidence to the rock parameters using the OAT (one-variable-at-a-time). Then, the collaborative deformation of ground, pile, and bridge floor is studied under the condition of a simply-supported beam bridge above the goaf center. Finally, the mechanism of collaborative deformation of ground, pile, and bridge floor is revealed. The results show that the goaf residual deformation process can be obtained by weakening the elastic modulus of fractured rock in the caving zone. At the final residual deformation stage, the subsidence ratio of ground to pile is about 10, and the subsidence ratio of pile to bridge floor is about 2, while the ground horizontal movement ratio of ground to pile is about 7, and the bridge floor horizontal movement can be ignored. The bridge floor is always in the positive curvature influence zone, and the pile has an inhibitory effect on the curvature deformation of the bridge floor. The compression deformation occurs between the piles locations, while the tensile deformation occurs at the pile location. The evolution of negative frictional resistance derived from goaf residual deformation is the main reason for the change in the collaborative deformation law among the ground, pile and bridge floor. This research can provide scientific support and theoretical basis for the design, construction, and protection of the bridge above the goaf center, and can also provide reference for the stability evaluation of bridge above goaf under other conditions.

Study on compaction characteristics and mechanical model of dry crushing filling material under lateral confinement condition.

The compaction characteristics and bearing capacity of dry filling materials in goaf have a significant influence on stope control and surface stability. Through acoustic emission monitoring and mechanical model analysis, a series of confined compression tests were conducted on crushed waste with varying particle sizes and Talbot coefficients. The deformation, fragmentation, and acoustic emission characteristics under corresponding working conditions were determined. The results indicate that the stress-strain curves of crushed stone with different particle sizes and Talbot coefficients exhibit similar nonlinear behavior during confined compression. However, the strain response varies with changing stress levels. By analyzing the slope change rate of the stress-strain curve, the lateral uniaxial compression process of waste rock can be divided into three deformation stages: rapid compression, stable crushing, and slow compaction. The compressive deformation characteristics of gravel differ based on particle size and Talbot coefficient. Specimens with a higher Talbot coefficient demonstrate stronger compressive resistance and weaker deformation resistance during initial compaction loading. Notably, the internal pressure structure strength is influenced by factors such as maximum particle size D, grading coefficient n, and particle size distribution continuity, rather than solely by the proportion of large particles. The evolution of acoustic emission signals and energy-time curve during waste rock confined axial compression synchronizes with the compaction process. Overall, compaction plays a critical role in maintaining the stability of goaf in dry crushed waste filling.

Stability analysis of pumped storage hydropower plant in abandoned open-pit mine affected by dynamic surface subsidence of combined mining.

The construction of a pumped storage hydropower plant (PSHP) in an abandoned open-pit mine is a potential alternative to green mining and energy storage, which can increase the utilization rate of renewable energy and develop residual resources of abandoned mines. Dynamic surface subsidence affected by combined underground and open-pit mining (CUOPM) seriously affects the construction and operation of the PSHP and is one of the critical scientific issues that needs to be solved immediately. The stability of the PSHP was analyzed and treatment scheme of the goafs was proposed based on on-site measurement, theoretical analysis, and numerical simulation. First, the distribution of goafs in the Haizhou open-pit mining area was investigated and surface subsidence value was obtained using InSAR technology and ground monitoring. Secondly, the surface subsidence mechanism affected by CUOPM is analyzed and indicates the subsidence maximum values and scope of influence are greater than those of single underground mining. A dynamic surface subsidence prediction model for combined mining is established based on the Knothe time function model. Thirdly, based on the CVISC model, the numerical calculation models were established by using FLAC3D, and the characteristics and laws of surface subsidence in different periods of CUOPM were studied. The comparative analysis of the observation results shows that the proposed model and numerical simulation calculation method have excellent applicability and accuracy. Finally, a stability evaluation method of PSHP was established, and the results of the evaluation show that the affected areas are the semi-ground powerhouse (SGPH) and the west side of the lower reservoir. The method of grouting filling was used to treat the goafs, and the results showed that it effectively alleviates the dynamic surface subsidence affected by CUOPM, and provides a safety guarantee for PSHP.

Experimental study on mine water purification mechanism for broken coal and rock masses in the underground reservoir of ecologically vulnerable mining area.

Water-rock interaction mechanism and water purification capacity of broken coal and rock masses are very important for the efficient operation of the underground reservoir. In this paper, a water purification simulation device for an underground mine reservoir was designed. The experimental study on the dynamic interaction between broken coal and rock masses and mine water was carried out. The water purification mechanism is analyzed from the changes in rock mineral composition and mine water quality before and after the test. The results show that after the broken coal and rock mass purification, the water turbidity and the concentration of chlorides and suspended solids decreased obviously. The water purification capacities of mudstone and sandstone are stronger than that of coal samples. After 60 days of reaction between the working face sewage and the broken samples (mudstone, sandstone, and coal), the turbidity, chromaticity, and residual chlorine decreased by > 90%, 90%, and 60%, respectively; and COD decreased by 35.29%, 30.59%, and 28.99%, respectively. While the TDS and the total hardness increased by about 40%, 30%, and 10% for the mudstone, sandstone, and coal, respectively. It shows that coal also has the worst degradation performance. The water purification effect of broken coal and rock masses has a significant time effect. The early stage of water-rock interaction is dominated by mineral dissolution, and the middle stage is dominated by precipitation and adsorption. The pH value of the solution has a certain influence on the ion change. In the later stage, the water-rock interaction is weak in a dynamic equilibrium state, and the change in the mine water quality index is not obvious. Considering the influence of rock lithology on water quality and the law of water-rock interaction time, the construction site selection and water storage time optimization of underground reservoirs in Jinjie Coal Mine were carried out, respectively.

Solid waste filling and roadway retaining for longwall mining by numerical investigation.

Solid waste filling and roadway retaining can effectively control surface subsidence and alleviate solid waste accumulation pollution. In order to effectively evaluate the advantages of solid waste filling in deformation control of overlying strata and surrounding rock of retained roadways, this study used theoretical analysis and numerical simulation methods to analyze the factors affecting surface subsidence, as well as the deformation characteristics of surrounding rocks and retaining tunnels during backfill mining. By calculating the influence of factors such as the foundation coefficient and the filling rate on the subsidence of the roof, it is concluded that the filling rate is the main controlling factor affecting the subsidence of the roof. Through simulation and comprehensive analysis of the impact of different filling rates on overlying rock migration, it was found that when the filling rates are 70 % and 80 %, it can effectively control the subsidence of overlying rock in the mining area. By simulating the effects of these two filling rate conditions on the deformation of surrounding rock within the retained roadway zone, the results show that the optimal filling rate that can effectively control the subsidence of the overlying rock and improve the stability of the retained roadway is 80 %.

Analysis of characteristics of diffusion tortuosity of fine-grained gangue slurry in coal mine caving zone.

The diffusion characteristics of fine-grained gangue slurry are significant for gangue consumption and ecological environment protection, especially in underground mines. Therefore, based on the theory of porous medium transportation, this paper establishes a 3D caving zone model, proposes methods for reconstructing, statistical, and analytical voids, characterizes the distribution of voids in the caving zone and the diffusion characteristics of gangue slurry, and explores the influencing factors of the distribution of voids and the diffusion characteristics of gangue slurry. The diffusion rate and depth of the gangue slurry were calculated using Df and DT parameters. Finally, a fluidization filling method for gangue was proposed, providing a theoretical basis for solid waste treatment and environmental protection in coal mines.

Evolution analysis of research on disaster-causing mechanism and prevention technology of mine goaf disaster.

The goaf is an important factor that induces major accidents. Based on the quantitative analysis of the existing research results, summarize and sort out the research and prevention technology of the goaf disaster with the experience of experts. Temporally, the research on goaf disasters was divided into two stages: the embryonic stage and the rapid development stage. Spatially, a collaborative network with the United States, China, Germany, France, Turkey, and the United Kingdom as the core, including India, Japan, Belgium, Italy, South Korea, and Canada, was analyzed. By constructing a co-occurrence and clustering network of keywords and co-cited literature to explore the focus and hotspots of goaf disaster research, the hotspots of goaf disaster research are summarized into four main aspects, such as goaf detection technology, goaf disaster analysis, goaf risk assessment and goaf treatment technology, which grasp the content of goaf research from a macro perspective. The burst detection analysis of keywords and co-cited literature was conducted to obtain the research frontiers of goaf disaster research in different periods. At the current stage, the microstructural characteristics of surrounding rocks in the context of deep mining and complex goaf group effect and the mining technology of the integration of excavation, anchoring and supporting are the current frontier research directions. This combined qualitative and quantitative method is more helpful to grasp the development context of goaf disaster research and provides a new reference perspective for sorting out the process of goaf disaster research.

Long-distance migration law of radon in overburden of abandoned goaf during coal spontaneous combustion.

The surface isotope radon measurement method (SIRMM) is widely used in fire source detection in abandoned mines. However, studies on the long-distance migration of radon during coal spontaneous combustion are lacking, which hinders the further popularization of this technology in coal fire prevention and control. For this reason, the migration law of radon in overlying strata in fire areas was studied through experiments and numerical simulation. The radon exhalation concentration of coal was found to increase at first and then decrease in the range of 30-350 °C through experiments. The radon concentration reaches the maximum value (557.1 Bq/m3) at 150 °C, which is 6.3 times higher than that at 30 °C. Based on the radon source term obtained by fitting the experimental data, the radon migration model of coal spontaneous combustion in abandoned goaf was constructed, and the dynamic distribution characteristics of the airflow, temperature, and radon concentration fields in the overlying strata area were analyzed. The internal relationship between surface radon and underground fire source was discussed. The simulation results revealed the sharp change in the porosity of the overlying rock causes radon concentration at the interface between the caving and fissure zones to increase continually with the process of spontaneous combustion, providing material and energy support for the long-distance radon migration. When the maximum temperature of the coal pile reaches 70 °C, the concentration of radon released from the coal pile increases rapidly to 13696 Bq/m3, and the radon from the underground space appears on the surface at this temperature. In the range of 70-150 °C, with rapid increase in radon released from coal piles, the surface concentration of radon also increased rapidly to 225 Bq/m3. At the high-temperature stage exceeding 150 °C, the concentration of radon released from coal piles exhibited a downward trend, resulting in a decrease in the rate of increase of radon concentration on the surface. A close relationship between the surface radon concentration and underground fire source temperature in the process of coal spontaneous combustion was observed. In the spatial position, the peak position of radon on the surface was highly consistent with that of the fire source longitudinally, which ensures the accuracy of the SIRMM to determine the location of the hidden fire source. This suggests that the SIRMM can accurately evaluate the fire source's temperature and fire area's development trend.

Study on Preparation and Performance of Foamed Lightweight Soil Grouting Material for Goaf Treatment.

The harm goafs and other underground cavities cause to roads, which could lead to secondary geological hazards, has attracted increased attention. This study focuses on developing and evaluating the effectiveness of foamed lightweight soil grouting material for goaf treatment. The study examines the foam stability of different foaming agent dilution ratios by analyzing foam density, foaming ratio, settlement distance, and bleeding volume. The results show that there is no significant variation in foam settlement distance for different dilution ratios, and the difference in foaming ratio does not exceed 0.4 times. However, the bleeding volume is positively correlated with the dilution ratio of the foaming agent. At a dilution ratio of 60×, the bleeding volume is about 1.5 times greater than that at 40×, which reduces foam stability. Furthermore, an appropriate amount of sodium dodecyl benzene sulfonate improves both the foaming ability of the foaming agent and the stability of the foam. Additionally, this study investigates how the water-solid ratio affects the basic physical properties, water absorption, and stability of foamed lightweight soil. Foamed lightweight soil with target volumetric weights of 6.0 kN/m3 and 7.0 kN/m3 meet the flow value requirement of 170~190 mm when the water-solid ratio ranges are set at 1:1.6~1:1.9 and 1:1.9~1:2.0, respectively. With an increasing proportion of solids in the water-solid ratio, the unconfined compressive strength initially increases and then decreases after 7 and 28 days, reaching its maximum value when the water-solid ratio is between 1:1.7 and 1:1.8. The values of unconfined compressive strength at 28 days are approximately 1.5-2 times higher than those at 7 days. When the water ratio is excessively high, the water absorption rate of foamed lightweight soil increases, resulting in the formation of connected pores inside the material. Therefore, the water-solid ratio should not be set at 1:1.6. During the dry-wet cycle test, the unconfined compressive strength of foamed lightweight soil decreases, but the rate of strength loss is relatively low. The prepared foamed lightweight soil meets the durability requirements during dry-wet cycles. The outcomes of this study may aid the development of enhanced approaches for goaf treatment using foamed lightweight soil grout material.

Methane migration and explosive fringe localisation in retreating longwall panel under varied ventilation scenarios: a numerical simulation approach.

Methane-based inflammable underground coal mine environment has led to catastrophic losses in the past. Migration of methane from the working seam and desorption region above and below the seam causes explosion hazard. In this study, the computational fluid dynamics (CFD)-based simulations of a longwall panel in a methane-rich inclined coal seam of the Moonidih mine in India established that the ventilation parameters greatly influence the methane flow in the longwall tailgate and porous medium of the goaf. The field survey and CFD analysis revealed that methane accumulation on the "rise side" wall of the tailgate is attributable to the geo-mining parameters. Further, the turbulent energy cascade was observed to impact the distinct dispersion pattern along the tailgate. The numerical code was used to investigate the changes in ventilation parameters made to dilute the methane concentration in the longwall tailgate. Methane concentration in the tailgate outlet decreased from 2.4 to 1.5% as the inlet air velocity increased from 2 to 4 m/s. The oxygen ingress into the goaf increased from 0.5 to 4.5 lps as the velocity was increased, causing the explosive zone in the goaf to expand from 5 to 100 m. Amongst all velocity variations, the lowest level of gas hazard was observed at an inlet air velocity of 2.5 m/s. This study, thus, demonstrated the ventilation-based numerical method to assess the coexistence of gas hazard in the goaf and longwall workings. Moreover, it provided impetus to the necessity of novel strategies to monitor and mitigate the methane hazard in U-type longwall mine ventilation.

A Field Study Implementing New Monitoring Technology for Roof Caving and Systematic Monitoring for Gob-Side Entry Retaining via Roof Cutting in Underground Coal Mining.

The longwall mining method with gob-side entry retaining via roof cutting is a new underground coal mining method which has the characteristics of a high resource recovery ratio and environmental friendliness. Due to the complexity of this method, the research method of case-based dynamic on-site monitoring, analysis, adjustment, and optimization is usually adopted. Based on a roadway retaining via roof cutting project, in addition to the traditional indirect monitoring method of hydraulic support pressure, this study innovatively establishes a direct monitoring method for roof caving by monitoring the gangue pressure in the goaf, which provides data for the roof cutting effect and offers a new method for studying the overlying strata movement. In the project, a comprehensive monitoring and analysis system was established, including gangue pressure, cable bolt stress, bracket pressure, roadway deformation, and roof separation, which was used to dynamically analyze the effect of roof cutting and optimize the support design. The results show that the pressure of the hydraulic support close to the roof cutting is low, indicating that roof cutting is favorable in the roadway retaining mining method. The roadway deformation in the advanced abutment pressure area of the working face is small. The mining-induced stress caused by the collapse and compaction of the overlying strata in the goaf is the dominant factor affecting the effect of roadway retaining, especially in the 50-100 m range behind the working face, where the dynamic load causes high bearing capacity of the support elements, large roadway convergence, and roof separation. Temporary support and supplementary reinforcement should be added when necessary. The monitoring system presented in this study is highly comprehensive, simple, reliable, and low in cost, providing a reference for roof cutting roadway retaining projects and roof caving-related studies.

Numerical simulation and investigation of methane gas distribution and extraction in goaf with U-type ventilation of working face.

The accumulated methane in goaf during coal mining may leak into the working face under the airflow influence, which is possibly causing disasters such as methane gas excessive at the working face and seriously threatening the mine safety. This paper first established a three-dimensional numerical model of the mining area under U-shaped ventilation, introducing the gas state equation, continuity equation, momentum equation, porosity evolution equation, and permeability evolution equation to simulate the airflow field and gas concentration field in the mining area under the natural state. The reliability of the numerical simulations is then verified by the measured air volumes at the working face. The areas in the mining area where gas is likely to accumulate are also delineated. Subsequently, the gas concentration field in goaf under the gas extraction state was theoretically simulated for different locations of large-diameter borehole. The maximum gas concentration in goaf and the gas concentration trend in the upper corner were analyzed in detail, and the critical borehole location (17.8 m from the working face) was determined as the optimum location for gas extraction from the upper corner. Finally, a gas extraction test was carried out on-site to evaluate the application effect. The results show that the measured airflow rate has a small error with the simulated results. The gas concentration in the area without gas extraction is high, with the gas concentration in the upper corner being over 1.2%, which is greater than the critical value of 0.5%. The maximum reduction in gas concentration was 43.9%, effectively reducing the gas concentration in the extraction area after employing a large borehole to extract methane gas. The gas concentration in the upper corner and the distance of the borehole from the working face are expressed as a positive exponential function. The field engineering results show that the implementation of the large borehole at a distance of less than 17.8 m from the working face can control the gas in the upper corner to less than 0.5%, effectively reducing the risk of gas in the upper corner. The numerical simulation work in this paper can provide some basic support for the design of an on-site borehole to extract gas from the mining void and reduce the gas hazard in coal mines.