Mining Hazards Research Articles

Development of an in-situ gel from CO2-captured complex solution and inhibiting coal spontaneous combustion: A case study in thermal engineering

The safety of underground coal mining is threatened by coal spontaneous combustion, making it essential to develop effective inhibitors to prevent such incidents. To address this, a new in-situ gel has been proposed, which is formed in coal pores from a fire-prevention aqueous solution (FPAS) that reacts with atmospheric CO2. To determine the appropriate concentrations of polyvinyl alcohol (PVA) and sodium-metasilicate nonahydrate (SN) for FPAS, 24 groups of wire-mesh basket tests were conducted on Baiyinhua coal. The gelling mechanism and performance on fire inhibition of FPAS were then investigated using temperature-programmed tests, scanning electron microscopy, energy dispersive spectrometry, fourier transform infrared tests, thermogravimetric differential thermal analysis tests, and fire-extinguishing tests. The results indicate an inhibition rate of up to 43.3% and effective protection of active groups from oxidation. The in-situ gel demonstrated excellent fire-extinguishing properties. Additionally, the self-ignition temperatures of residual coals with varying thicknesses were estimated using the Frank-Kamenetskii theory. The combined method of CO2 injection and FPAS spraying is recommended for achieving better fire-prevention effectiveness in coal mine gob. The findings are expected to provide guidance on the application of FPAS to address thermal hazards in coal mines.

Study on the evolutionary characteristics of coal burst proneness under the action of multiple mining disturbances

AbstractCoal burst is a serious hazard in deep coal mining. To investigate the coal burst proneness evolution during multiple mining disturbances (MMD), this paper implemented a range of uniaxial cyclic loading–unloading tests coupled with acoustic emission (AE) measurement to simulate the loading–unloading effects of the MMD. The maximum load was used to simulate the highest load that the coal has endured in history, the loading–unloading rate was used to simulate the mining speed, and the loading–unloading cycles were used to simulate the number of mining disturbances. Finally, the burst proneness indexes of the damaged coal sample are measured and compared with the origin coal sample. Analysis on the experiment results suggest that, the burst proneness increases when the maximum load is 30% of the uniaxial compressive strength (UCS), interestingly, when the low loading–unloading rate is applied, 50% of the UCS enlarge the burst proneness as well. However, it decreases when the maximum load is 70% of the UCS. Sensitivity analysis indicates that the maximum load, loading–unloading rate, and loading–unloading cycles exhibit decreasing influence sensitivity to the elastic modulus, as well as to the UCS and duration of dynamic fracture. Similarly, the influence sensitivity of maximum load, loading–unloading cycles, and loading–unloading rate to the elastic strain energy index decreases, as well as to the bursting energy index. To ensure the safety of coal mining, it is crucial to avoid the disturbances that would put the coal under low loading and low loading rate.

Determinants of prioritised influencing factors on coal spontaneous combustion propensity – A Fuzzy-Delphi-geometric mean analytic hierarchy process

One of the most common hazards in coal mines is spontaneous combustion, a significant cause of coal loss in underground mines. Specifying a suitable approach for the determinants of prioritized influencing factors on coal spontaneous combustion propensity is challenging. There needs to be more research that directly addresses the uncertainty of human judgement in assessing the factors influencing coal propensity to self-combustion. This is the first attempt to address the aforesaid problem by ranking the factors influencing spontaneous coal combustion. The ranking was performed using the Fuzzy Delphi Analytic Hierarchy Process (FDAHP) with two variants of the Geometric Mean Method (GMM), which enabled the analysis of both quantitative and qualitative data related to the rating of each factor influencing spontaneous combustion risk and the relative importance of each factor related to coal seam characteristics, geological aspects, mining practices, and environmental aspects. The results show that ‘GMM as a whole’ is a comparatively best-aggregating approach to the AHP process. The presented case study of using this methodology in underground mine conditions confirms its efficacy in assessing susceptibility to coal spontaneous combustion. This mainly concerns the pace and reliability of the obtained results and the determination of risk evaluation system based on intrinsic and extrinsic factors influencing susceptibility to spontaneous combustion of coal. Finally, this systematic technique was validated in the research area by utilizing spontaneous combustion incident that occurred in Jharia and Raniganj coalfields, which revealed rather excellent concordance in Jharia and Raniganj coalfields. The study can be used in mine planning to develop engineering data to select the operational parameter and prevent the initiation of spontaneous heating by identifying intrinsic factors and optimizing extrinsic factors.

Heat Transfer Model and Thermal Insulation Characteristics of Surrounding Rock of Thermal Insulation Roadway in a High-Temperature Mine

The thermal insulation method is one of the effective methods for controlling the thermal environment of a high-temperature mine. In order to explore the thermal insulation mechanism and characteristics of thermal insulation roadways in high-temperature mines, a heat transfer model for the surrounding rock of the thermal insulation roadway was established based on the steady heat transfer theory. The temperature field of the surrounding rock of the thermal insulation roadway was studied, and the effects and sensitivities of thermal insulation layer thickness and thermal conductivity, convective heat transfer coefficient between roadway wall and airflow, and roadway radius on the thermal insulation performance of thermal insulation roadway were discussed. The results suggest the following: (1) The temperature gradient inside the thermal insulation layer is greater than that inside the surrounding rock. The thermal insulation roadway reduces the temperature difference between the original rock and the outside surface of the thermal insulation layer, thereby reducing the heat dissipation of the surrounding rock. (2) As the thermal insulation layer thickness increases, the thermal insulation capacity gradually increases, but its enhancement rate gradually weakens; as the thermal conductivity of the thermal insulation layer or the roadway radius increases, the thermal insulation capacity gradually decreases and its decline rate gradually weakens; and the convective heat transfer coefficient between the roadway wall and airflow has almost no effect on the thermal insulation capacity. (3) The thermal insulation performance of the thermal insulation roadway is highly sensitive or above the thickness and thermal conductivity of the thermal insulation layer, as well as the roadway radius. The sensitivity of thickness and thermal conductivity of the thermal insulation layer is greater than that of roadway radius. Therefore, the research results have guiding significance for the application of thermal insulation methods in the prevention and control of thermal hazards in mines.

Downward counterfactual analysis of multi-risk cascades

Loss outcomes from geohazards are compounded by an array of human risk factors. The combination of geohazards and human risk factors can generate multi-risk cascades. In the historical record, disasters arising from such multi-risk cascades are comparatively rare. However, far more common are near-misses, where a disaster tipping point to massive destructive energy release and expanding losses was narrowly averted. What happened historically is only one realization of what might have happened. Due to psychological outcome bias, people pay far less attention to near-misses than to actual losses. A downward counterfactual is a psychological term for a thought about the past, where things turned for the worse. Exploration of downward counterfactuals enhances risk awareness and can contribute to risk preparedness. There are no databases of multi-risk cascade near-misses, but insights can be gained from downward counterfactual analysis. Geohazard examples of multi-risk downward counterfactuals are given, including cases of critical infrastructure damage. A downward counterfactual can drive a minor hazard event beyond the disaster tipping point boundary, and turn a disaster into a major catastrophe. To illustrate the latter, a downward counterfactual analysis is presented of the Fukushima nuclear accident of 11 March 2011, which might have crossed the tipping point boundary into a multi-risk cascade catastrophe.

An improved model to predict the water-inrush risk under an unconsolidated confined aquifer based on analytic hierarchy process and information value method

To prevent water inrush hazards of mining under the Quaternary unconsolidated confined aquifer in North China coalfield, it is necessary to build a water inrush prediction model to guarantee coal safety production. The current method of evaluating mining hazards under unconsolidated confined aquifers is easy to fall into the problems of simple superposition of evaluation factors, large subjectivity of the factor selection, and low accuracy of prediction results. In this paper, an analytic hierarchy process (AHP) is improved according to contribution of information value method (IVM) to analyze the impact of water inrush factors. The factor weight values are considered in calculating the total information value of IVM to build a coupled AHP-IVM model. The information value of each factor is calculated through IVM, and area under curve (AUC) value is used to rank the factors, to exclude the subjective judgment of the importance of each factor. The geological and hydrogeological data of Qinan coal mine in Huaibei coalfield are analyzed and organized. The results suggest that the coupled AHP-IVM model has an AUC value of 0.8309, indicating its great potential and practical significance for future mining operations.

Оценка влияния тяжелых углеводородов на аэрологические риски аварий в угольных шахтах

Introduction. An analysis of the dynamics of the increase in the number of functioning coal mines in the Russian Federation shows that over the past 22 years, the maximum number of new mines has increased dramatically and amounted to (over a decade) 11, which is 1.6 times more than the same maximum value of 7 (new mines commis-sioned over previous decades), for the entire period from 1910 to 2000. The largest number of mine seams currently being developed in the Russian Federation is represented by grades: D, G, GZh, Zh. It has been established that the content of heavy hydrocarbons in the residual gases of Kuzbass and Donbass coals is more than 80%, reaching 98% for Zh and KZh grades. Heavy hydrocarbons pose a great explosion and fire threat to coal mines, which necessitates the study of their composition and content and taking into account the impact on the aerological risks of mines. Objective. Study of the composition and content of heavy hydrocarbons in the residual gases of coal and coal dust and assessment of their impact on the aerological risks of accidents in coal mines. Methodology. To assess the impact of heavy hydrocarbons on the aerological risks of coal mines, the methodology developed by the authors was used to find the area of intersection of hazardous factors in mining and the vulner-abilities of ventilation schemes and methods, characterized by the proportion of probabilities of undesirable events. This methodology has been further developed in assessing and taking into account the impact of heavy hydrocarbons on aerological risks in excavation areas and development workings. The previously assessed mining hazards were supplemented by taking into account the high methane content of the seams, the presence of a critical content of heavy hydrocarbons, and the depth of mining operations. The vulnerability of the schemes and methods of ventilation of the working areas and development workings was manifested at high loads on the stopes, high values of the absolute methane content of the working areas, high speeds of advancement of the working and development faces. Results and discussion. A general pattern has been obtained for the two basins, consisting in the fact that the content of heavy hydrocarbons in the residual gases of coal is more than 80%, reaching 98% for Zh and KZh grades. It was determined that the residual gases of coal dust contain a greater amount of heavy hydrocarbons (up to 91%) compared to the residual gases of coals (87%), with propane and butane predominating. The presence of heavy hydrocarbons in the residual gases of coals under favorable conditions for mining coal seams increases aerological risks by 3.3 times; for the worst conditions, aerological risks due to the influence of heavy hydrocarbons increase only 1.7 times, which is due to the large influence on risk assessment of such factors as the load on the stope, absolute methane abundance, the rate of advance of the face, the depth of mining operations. For development workings, aerological risks due to the influence of heavy hydrocarbons are 1.5 times greater than for extraction areas. The research results can be useful in assessing the impact of heavy hydrocarbons on the aerological risks of coal mines. Conclusions. Heavy hydrocarbons contained in the residual gases of coal and coal dust, under certain conditions, increase the explosion and fire hazard of coal mines. It is possible to imagine the following scheme of methane ignition processes followed by an explosion of coal dust: methane ignition, heating of coal dust, release of combustible components from dust, ignition of combustible components, combustion of coal dust, explosion.

Electrical resistivity tomography technique coupled with numerical modelling: A case study for stability analysis

AbstractCavity due to underground old mine workings is one of the major threats to the coal mines and the overlying subsurface and surface properties, which need to be protected. The detection of old mine workings and stability assessment of overlying strata are common problems in most of the Indian coalfields. Several coal mines in India are loss‐making, mainly due to different types of mine hazards. Khandra mine is one such mine at Raniganj Coalfield, Eastern Coalfields Ltd., a subsidiary of Coal India Limited. In the present study, 2‐dimensional and 3‐dimensional electrical resistivity tomography were carried out for detailed subsurface characterization. It supports delineating underground workings, including the nature of voids/cavities (air or water‐filled). Excessive distortions were reported in electrical resistivity tomography application, especially at the near‐surface, owing to large resistivity variations. Refinement of the model by half‐unit electrode spacing was attempted here to reduce the distortions with minimum possible absolute errors. 3‐Dimensional resistivity volumetric model was also developed with the help of five electrical resistivity tomography parallel profiles for better apprehension of the subsurface. Analysis provided important inputs for stability analysis using 3‐dimensional numerical modelling. The physico‐mechanical properties of the overlying strata, pre‐excavation in situ stresses, boundary conditions and the mine geometry simulation were incorporated for understanding the stability analysis. Stability analysis was carried out using the finite difference technique. The analysis of 3‐dimensional numerical modelling indicated that two distinct layers comprising (i) laterite/part of the course to medium‐grained sandstone and (ii) developed galleries of R‐IX seam exhibited a very low safety factor below 1.0, indicating potholing/subsidence susceptibility. The other three layers comprising parts of fine‐grained sandstone exhibited a relatively higher safety factor of around 2.0, indicating moderately stable zones, but not on a long‐term basis. Parts of Siduli stream embankments need suitable retaining walls to avoid water inundation for the stability of the area.

Research and Application of Rock Burst Hazard Assessment of the Working Face Based on the CF-TOPSIS Method

Coal mining activities have intensified, and underground mining depths have increased, posing significant challenges for mitigating rock burst hazard. To address this issue, this paper proposes an improved comprehensive weighting prediction (CF-TOPSIS) method that predicts weight and grade indices for rock burst evaluation. The study introduces a novel variable coefficient method to optimize the conflict response problem and employs correlation forward and other techniques to minimize errors caused by large data differences. Based on these, the CRITIC objective weight algorithm model is established, and FAHP is used to optimize weighting factors to make the index weight of rock burst more reasonable, thus enhancing the accuracy of hazard assessment for working faces. Results from engineering examples show that the prediction results combined with field drill cutting methods and microseismic monitoring data verify the accuracy of the proposed method, indicating its feasibility and effectiveness in predicting rock burst hazard in underground coal mines.

Experimental investigation on coal desorption characteristics and spontaneous combustion properties evolution under the coupled effect of temperature and pressure

Gas desorption patterns and microscopic pore structure morphology in coal are influenced by the coupled effects of temperature and pressure, and their effects on the spontaneous combustion (SC) characteristics of coal cannot be ignored. In this paper, two types of coal samples, lean coal and fat coal from high gas-prone SC mines, are used as test objects. A self-designed experimental platform for coupling gas adsorption and desorption with coal spontaneous combustion (CSC) and heating and oxidation was used. Firstly, orthogonal tests on the desorption of methane from coal under the coupling effect of different temperatures and pressures were carried out to obtain a gradual decrease of methane desorption from coal with increasing temperature under the same pressure conditions. The results show that the pore volume and specific surface area of coal show a monotonic decreasing pattern with increasing temperature and pressure, while the percentage of micropores and mesopores, which promote the oxygen reaction of coal, show an increasing trend. In addition, a comparative analysis of the characteristic temperature points of the first stage showed that the characteristic temperature points of the desorbed coal were higher than those of the original coal, reflecting the higher SC activity of the desorbed coal; and the crossover temperature points of the lean coal and the fat coal were 79 °C and 88 °C, respectively, after desorption during the heating and oxidation process, which also confirmed the high SC activity of the desorbed coal. Based on the above findings, the methane concentration calculation model of the mining area was applied to redraw the oxidation zone using the mining area of the 41,201 working face of Hongyang No. 2 Mine as an example, and the area was reduced by 39.5%. The research results of the paper can provide guidance for the prevention and control of fire hazards in mines with high gas-prone SC.

Numerical analysis of the distribution of combustion products from methane explosions in a full-scale tunnel using all-speed CFD code GASFLOW-MPI

ABSTRACT Methane explosions are among the main hazards in coal mines. Shock waves from methane explosions can cause damage near the explosion site, and combustion products can spread along the tunnel to locations far from the explosion source and endanger the lives and health of personnel. Therefore, the study of the propagation patterns of methane explosion shock waves and the distribution of high-temperature combustion products in tunnels. has significance for emergency decision-making in the event of methane explosions in a mine. This study uses the 3D Computational Fluid Dynamics (CFD) program GASFLOW-MPI, which models the one-step methane combustion mechanism with the addition of a heat transfer model. The methane explosion process is simulated and reproduced at the Lake Lynn Experimental Mine (LLEM) to analyze the process of gas deflagration. The results reveal that the overpressure in the tunnel after the methane explosion oscillates and decays with time. Gaseous products of the explosion “expand and compress” and flow back and forth in accordance with the oscillation of overpressure. The maximum expansion ratio of the CO2 concentration isosurfaces of 0.5% in the heat transfer simulation is 6.21, whereas the volume expansion ratio is 3.78 once the flow field stabilizes. The distribution of combustion products along the alleyway exhibits a Gaussian decay trend. The range of gaseous product distribution and temperature fields in the adiabatic tunnel is significantly higher than that in the heat transfer simulations, thus indicating that heat loss significantly influences the temperature characteristics and distribution pattern of combustion products in the full-scale tunnel.

Study on MICP dust suppression technology in open pit coal mine: Preparation and mechanism of microbial dust suppression material

With the recent increases in energy demands, the dust hazards of coal mining caused by transportation, loading and unloading and other processes are becoming increasingly serious. To control dust in open pit coal mines more environmentally friendly and efficiently, and to promote the use and development of non-in situ high-yield urease microorganisms for dust suppression in coal mines, Bacillus pasteurii was selected for dust suppression experiments in this article. Additionally, the growth of microorganisms in the coal dust microenvironment was simulated, and the effect of microbial mineralization products on the calorific value of upper coal dust was further studied. Our findings indicated that Bacillus pasteurii induced dust suppression by forming a calcite precipitate with non-uniform particle size to coal dust cementation. Moreover, after a single spray, the wind erosion resistance efficiency was 84% when the wind speed was set at 10 m/s. The growth of microorganisms and urease activity in the coal dust leachate were largely equal to those in the control group, reaching a peak at approximately 24 h, that the maximum growth quantity of OD600 was about 1.5, and the maximum urease activity was 11 mmol·L−1·min−1. The difference between the peak heat release rate of mixed coal dust and pure coal was only 4.82 kW/m2, which would not affect the value of coal products. Non in-situ Bacillus pasteurii can be growth metabolized normally in the microenvironment of coal dust. Finally, the mechanism of coal dust suppression by mineralization of microbial bacterial solution to form calcium carbonate was described by a reaction equation, which is important for further application and development of microbial dust suppressants.

Evaluation of Control Effect of Confined Water Hazard in Taiyuan Formation Coal Seam Mining in Huanghebei Coalfield

The shallow-layer resources in the Huanghebei Coalfield have been depleted, and the deep coal seam of the Taiyuan Formation (lower coal group) is the main continuous resource in mining at this stage. With the annual increase in mining depth, the exploitation of lower coal groups is being met with the influence of high ground stress, high water pressure, high temperature, and strong mining disturbances, as a result of which the threat of water inrush is particularly serious. Based on the grouting data of the coalface floor aquifer, this paper proposes an evaluation method for the control of water rushing into the coalface floor. By comparing the test data with mine electrical exploration data through ArcGIS, the results show that the water pressure threshold based on ArcGIS is twice the water pressure of the grouting reconstruction layer as the optimal solution. The research results can provide a reference for the prevention and control of water inrush in the lower coal group of Huanghebei Coalfield.

Classification of Water Source in Coal Mine Based on PCA-GA-ET

In recent years, inrush water has hampered the regular mining of coal mines, and the proper identification of the source of inrush water is critical to the prevention and management of water hazards in mines. This paper extracts the standard water chemistry discriminating ions Na++K+, Ca2+, Mg2+, Cl−, SO42−, and HCO3− from observed water samples. An improved water source discrimination model is proposed which combines algorithms from data mining, classification models, and learning reinforcement. According to the Pearson correlation coefficient, Na++K+ has a strong correlation with HCO3−. To identify the major metrics, we performed principal component analysis (PCA), and the adaptive differential evolutionary genetic algorithm (GA) was utilized to optimize the depth of the extreme tree (ET) and the number of classifiers. Finally, the model distinguished 25 sets of studied samples from various water sources in the Pingdingshan coalfield. Comparative analysis demonstrated the efficacy of each stage of our work. PCA-GA-ET outperformed the conventional approaches, such as the support vector machine, BP artificial neural network, and random forest. The studies revealed that PCA-GA-ET can eliminate the information overlap between data and simplify the data structure and thereby improve the efficiency and accuracy of water source detection. We discovered that by utilizing the evolutionary algorithm to optimize parameters such as the depth of the extreme trees and the number of decision trees, we could get the model to converge faster and to be more stable and more accurate. The results suggest that PCA-GA-ET has good robustness and accuracy and can meet the needs of water source identification.

Post-Mining Multi-Hazard Assessment for Sustainable Development

Today, most mines (coal, iron, and others) in Europe are already closed due to economic, environmental, and societal issues. Therefore, post-mining risk assessment and management remain crucial for mining authorities, policymakers, and planners. In the post-mining period, several hazards are likely to affect the surface areas in the closed mining sites. The impact of closed mines can lead to potentially damaging changes in surface and/or underground water flow, as well as the development of surface instabilities that can affect people or infrastructure, sometimes dangerously. The assessment of the different hazards must consider the interaction between the mining hazards and other risks (natural and technological). Thus, land use planning, particularly the rehabilitation of former mining sites, requires better tools to apprehend the multiplicity of hazards and their constraints. The paper presents a methodology considering the interactions between hazards around closed mines. After recalling the advantages of this multi-hazard analysis, the work consisted of, almost exhaustively, describing the three prominent families of hazards: mining, natural, and technological. Then, the possible interactions between hazards were described according to their nature (trigger or aggravating), their category (technical or regulatory), and their typology (dependent or independent). Finally, an attempt was proposed to evaluate the type and intensity of interactions between hazards. The multi-hazard assessment methodology was applied to a coal mine and showed the complexity and the utility of such a risk assessment analysis to improve risk management in closed mines.

Corrigendum: Study on prevention and control mechanism of runoff water hazard in thick coal seam mining in valley terrain

Corrigendum: Study on prevention and control mechanism of runoff water hazard in thick coal seam mining in valley terrain

Research and Application of an Intelligent Prediction of Rock Bursts Based on a Bayes-Optimized Convolutional Neural Network

Intelligent prediction of rock bursts has great significance in rock mechanics research and a high value in engineering applications. An intelligent rockburst prediction method based on a Bayes-optimized convolutional neural network (BOCNN) was proposed. First, an exploratory analysis of data was conducted using joint distribution diagrams and the heat map of the correlation matrix to establish a high-quality data set of rockburst engineering cases and a parameter system for rockburst prediction. Second, six rockburst prediction models were built by combining machine learning algorithms, such as random forest, k-nearest neighbor (KNN), and Bayes, deep learning (CNN1d and CNN2d), and BOCNN. In addition, we used accuracy, precision, recall, F1 score, receiver operating characteristic curve, Taylor diagram, and the probability indicator of prediction results as indicators to evaluate the accuracy of the models. A comparative analysis of the six rockburst prediction models was conducted to explore rockburst prediction models with good robustness, generalization performance, and high accuracy. Moreover, 11 comparative models were established for comparative analysis. Then, we used the MATLAB tool to build an intelligent rockburst prediction system and applied the findings to the Jiangbian Hydropower Station in Sichuan Province, China. The results of the study show that the intelligent rockburst prediction system can provide technical support for predicting rockburst hazards in mining, transportation, and water conservancy and hydropower projects and a scientific basis for later construction and the design of support structures.

Impact of Natural Hazards on the Operation of Powered Roof Support in Polish Hard Coal Mines

The powered roof support is mainly used in natural hazard conditions. Natural hazards in Polish mining include methane hazards, gas and rock ejections, coal dust explosions, climate hazards, water hazards, landslides, eruptions, hydrogen sulphide hazards, radioactive substances hazards and rock mass tremors. The latter has a significant impact on the operation of powered roof support. The work analyses rock mass tremor risks in the last ten years. The study showed an intense increase in this hazard, which significantly affects the work safety of people operating machines and devices in mining excavations. The paper takes into consideration surveys carried out among employees working in conditions of rock mass shock risk. The purpose of the study was to obtain information on the safe operation of powered roof supports. The data allowed us to draw conclusions regarding the possibility of increasing work safety regarding rock mass shock risk conditions.

Hydrogeological Hazards in Open Pit Coal Mines–Investigating Triggering Mechanisms by Validating the European Ground Motion Service Product with Ground Truth Data

This research focuses on the investigation of hydrogeological hazards in open pit coal mines. The study area is the Amyntaio sub-basin area, located in West Macedonia prefecture, Greece. A major part of the SE of this area is occupied by the Amyntaio open pit coal mine. In recent decades, the Amyntaio basin’s aquifer has been overexploited both by dewatering wells of the open pit coal mine and irrigation wells, triggering extensive land subsidence in an area that extends 3 km around the mine. Additionally, one of the biggest mining landslides worldwide occurred on the South-West slopes of the open pit on 10 June 2017. The current study investigates the land subsidence phenomenon and the landslide, highlighting the influence and the interaction of their causal factors which were strongly affected by the groundwater management. To estimate ground surface movement, Earth Observation data from the European Ground Motion Service, of the Copernicus European Union′s Earth observation program, were used for the period 1 January 2016–31 December 2020. The geologic, geotechnical and hydrogeologic data coming from the extensive ground truth survey have been incorporated with the Earth Observation data, highlighting the opposing mechanisms of the interacting geohazards.

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 (17.8m 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 17.8m 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.