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The existence of heterogeneous rock mass in the goaf easily causes surface deformation, subsidence, and even collapse, which restricts the development of urbanization of the buildings above the goaf. Because of the saturation of the city and the shortage of power supply, the idea of building the Yingbin substation was proposed on the old coal mine goaf in Xinhua District, Pingdingshan City, Henan Province, China. To ensure the safety and stability of the foundation of the proposed substation, the comprehensive detection method was proposed, which is mainly based on transient electromagnetic detection and supplemented via drilling detection. It was used to determine the spatial position and location of heterogeneous rock mass in the goaf underlying the proposed substation. The results show that 1) after eliminating the interference to the transient electromagnetic detection, it is found that there are low-resistance anomalies near the depth of −160 m at points 1 to 2 and point 8 of the X2 line and at points 1 to 3 of the Y3 line, which are preliminarily presumed to be the influence of the heterogeneous rock mass. 2) The apparent resistivity anomalous areas of −160 and −200 m depth are highly consistent, and the location and scope of the heterogeneous rock mass were basically determined. 3) The abnormal depth range of core rate obtained by drilling is in good agreement with the abnormal range of apparent resistivity at the same location obtained by transient electromagnetic, which indicates the reliability of the detection results. 4) Combining the detection results of the two methods, the location and scope of the heterogeneous rock mass were basically determined, which is located in the western part of the proposed area. The detection results can provide a basis for the safety and stability of the proposed substation foundation. To ensure the safe and stable operation of the proposed substation foundation, effective governance measures for the heterogeneous rock mass of the goaf should be taken.

Based on the red clay slope in the northern part of Pingshuo East Open-pit Mine, the mechanism of red clay slope instability under the influence of goaf is studied. The arch effect, stress distribution and failure rule of red clay slope under the influence of goaf are revealed by numerical simulation analysis and limit equilibrium theory. The results show that the stability of red clay slope has a “step-like” development and change law from stability to failure, and the failure mechanism is “shear-tension crack-slip-collapse” .

In order to study the effect of nitrogen injection on the gas explosion danger zone in the goaf of high-gas mines, taking the 3235 working face of a coal mine in Anhui Province as an example, the COMSOL multiphysics coupling simulation software was used to establish a model of nitrogen injection in the goaf, and the gas migration laws of goaf under different nitrogen injection flow rates and nitrogen injection positions were analyzed. According to the mathematical principle of linear superposition, the gas explosion hazard area of goaf is divided and its distribution characteristics are analyzed. Through comparative study, it is found that with the increase of nitrogen injection flow in the goaf, the oxygen with the lowest concentration required for the explosion and the methane within the explosion limit have the tendency to move towards the working face and the goaf respectively, and the maximum width and area of the gas explosion hazard area show a trend of decrease. The nitrogen injection position and the hazard area of the volume fraction of oxygen required for a gas explosion showed a negative correlation.

Aiming at the situation that the top coal of the close distance coal seams is goaf and the remaining 30 m coal pillar, taking the 2-1 coal seam and 2-2 coal seam of Hulusu Coal Mine as the research background, the mechanical model of the upper coal goaf and the remaining coal pillar under force is established, the stress distribution state of the strata under the coal pillar in the upper coal section is analyzed, and the reasonable spacing of the stoping roadway under the close distance coal seams goaf is studied. The research shows that the optimal offset distance should be 80 m after the protective layer is fully discharged; from the influence range of the overlying coal pillar, the optimal offset distance is 40-80 m; considering the degree of influence on the upper coal roadway, the optimal offset distance should be greater than 80 m; when the offset distance is 160 m, the lower working face mining has the lowest influence on 21101 return air alley. By combining 4 different angles and calculating the intersection of the influence range, the optimal offset distance of the first working face of 22103 was finally determined to be 80 m.

Low oxygen and high carbon monoxide often appears in the upper corner of the surface of shallow buried coal seam mining. This paper analyzes in detail the reasons for the phenomenon of low oxygen and high carbon monoxide in the upper corner of the comprehensive mining surface of Sandaogou Coal Mine. By adopting measures such as forecasting of spontaneous combustion, adjusting the ventilation system of the working surface, reducing the ventilation resistance of the mine, reducing the air pressure of the main ventilators, strengthening the plugging up and down the well, reducing coal retention in the empty mining area, accelerating the speed of the working surface, and strengthening the sealing management of the empty mining area and nitrogen injection in the empty mining area, it effectively solves the phenomenon of low oxygen and high carbon monoxide in the upper corner during the recovery of the comprehensive mining face, and ensures the safe recovery of the working face.

The particularity of the occurrence conditions of the ore body in Xianglushan Tungsten Mine determines the mining form of the ore body and the particularity of the ground pressure distribution after mining. A large number of mined-out areas, supporting pillars, and natural and human factors have formed a comprehensive disaster environment. This can lead to frequent disasters, great harm, serious economic losses, and the necessity of severe environmental protection operations in the mine. This study aims to establish a microseismic monitoring system according to the actual needs of the site and to reveal the law of ground pressure manifestation by analyzing the distribution characteristics of microseismic events; to analyze the occurrence stability of the goaf; further verify it laterally; and finally, demonstrate the feasibility and effectiveness of the microseismic monitoring sensor system. In view of the current ground pressure problem in Xiangxuoshan tungsten mine, the stress change characteristics during dynamic mining and filling were obtained through comparative analysis of different perspectives such as surface change, energy release, and mining loudness, and key areas were identified to improve the reliability of underground ground pressure monitoring. The results show that the process of deposit destabilization caused by ore body mining can be further analyzed by microseismic monitoring, and the combination of surface settlement, mining intensity, and energy release can verify the accuracy of stress distribution and ground pressure transfer. In turn, the general reliability of underground ground pressure hazard warning is empirically improved.

The inerting technology is a common means to prevent fires in goaf of coal mine, in which adsorption capacity is the key index to measure the inerting effect. Therefore, this paper obtained the coal spontaneous combustion (CSC) stage of Qianyingzi coal mine through programmed heating experiments, and determined the physical adsorption stage temperature (below 323.15 K) matching the actual environment. The oxidation characteristics of coal samples with different adsorption degrees were further studied by adsorption experiment and TG-DSC test. The results indicate that the combustion activation energy of coal samples follows the sequence of second-stage adsorption coal > dried coal > raw coal > first-stage adsorption coal. This implies that the CO 2 saturated adsorption can exert a specific inhibitory impact on coal combustion. Therefore, the index of the saturation adsorption capacity (SAC) was proposed to evaluate the inerting effect. We further studied the adsorption capacity of coal at different ratios of CO 2 /N 2 by molecular simulation under different pressures (0–5 MPa). The results show that CO 2 dominates in both single-component adsorption and competitive saturated adsorption. The average of ratios (AOR) and average of differences (AOD) threshold are determined. The priority of SAC is obtained in the order of single-component (C S), CO 2 /N 2 mixture ratio at 4:6 (C 4:6), CO 2 /N 2 mixture ratio at 2:8 (C 2:8) from largest to smallest. Furthermore, a fuzzy optimization method for injection ratio of CO 2 /N 2 under different pressures was established. During dynamic changes in environmental pressure, it is appropriate to inject CO 2 separately at 0.2–2.5 MPa. At 3 MPa, a mixture of CO 2 /N 2 at a ratio of 2:8 is the best for prevent fires in goaf. Similarly, an inert gas with a CO 2 /N 2 ratio of 4:6 is injected at 3–5 MPa. [ABSTRACT FROM AUTHOR]

In hilly regions, landslides or slope failures are very common phenomena, when underground mineral resources are excavated. In this study, some landslide disasters in a mountain mining area were analyzed. The engineering geological and instability reason were investigated. The numerical simulation of a high steep rock slope disturbed by a room and pillar mine was established. The failure process of a high steep rock slope induced by the pillar deterioration was analyzed to reveal the characteristics of deformation and sliding. The results show that the pillar plays an important role in maintaining the stability of the slope, if the pillar can support the overlying rock mass, only a tiny deformation will be induced. When the pillar fails and the roof caves, the overlying rock mass above the room and pillar goaf will rapidly subside, and the crack evolution of slope is induced, forming the potential slip surface. The landslide mass gradually moves. When the rock mass at the middle and lower of the slope is squeezed out, slope sliding will be induced. The failure process can be divided into four stages as follow: tiny displacement is caused by the mining, roof collapse is caused by the pillar failure, the potential slip surface is formed from the crack evolution; the slope sliding is induced by the fracturing of rock mass at the middle and lower of the slope.

Underground goaves were left in many mining areas due to the continuous exploitation of coal resources. These mining areas seriously affect the production safety of the mines and the safety of life and property of the surrounding residents. Enormous safety hazards will be generated if the goaf range is not accurately controlled. In this study, we proposed a method for the detection of goaves in coal mines with a complex terrain by combining controlled source audio-frequency magnetotellurics (CSAMT) and an activated-carbon method for radon measurement. The disadvantage of failing to interpret goaf depth for the activated-carbon method for radon measurement was compensated by the advantage of the capability of goaf-depth sounding for CSAMT. Subsequently, the reference for CSAMT data was provided by the immunity of the activated-carbon method for radon measurement to the influences of terrain, earth electricity, and EMF. On this basis, the proposed method was employed to detect the goaf of Houjiagou Coal Mine in Liulin County, China, and obtained reliable detection results. The feasibility of the comprehensive geophysical prospecting method in the complex terrain was verified and it provides a new reference for the detection method of goaves with other conditions.

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In view of the complexity and concealment of goafs, numerical simulations have become an important means for studying the coupling disasters of spontaneous coal combustion and gas. Porosity is an important parameter in the numerical simulation, but this is difficult to detect directly on site, as the current porosity model does not fully reflect the characteristics of "three horizontal areas" and "three vertical zones" in the goaf. To establish a more accurate porosity model which is based on a theory of overlying strata movement, the Sigmoid function was introduced to reflect the distribution characteristics of the "three horizontal areas" in the goaf, and subsidence models of the main roof and overlying strata were established. A model that measures the porosity of the goaf was formulated, which reflects its O-shaped circle characteristics. The accuracy of the theoretical model was verified by obtaining experimental data. The results showed that the main roof subsidence in the natural accumulation area was approximately zero, there was an exponential decrease in the load-affected area, and the maximum subsidence appeared in the compacted area. The descending displacement was very small around the impacted area along the roadway wall. The movement of overlying strata in the fractured zone was controlled by the main roof, and the subsidence of the strata near the roadway wall and the working face was very small. In the vertical direction, the subsidence of the overlying strata decreased as the distance from the mining coal seam increased. The distribution of porosity in the goaf was shaped like a dustpan. In the horizontal direction, the porosity of the natural accumulation area and the impacted area of the roadway wall was the largest. Deep within the goaf, the porosity gradually decreased. The porosity in the fractured zone decreased in the vertical direction in a logarithmic manner. The porosity model proposed in this paper fully reflects the characteristics of the goaf, and provides a more accurate porosity model for the numerical simulation of spontaneous coal combustion and gas coupling disasters in the goaf. [ABSTRACT FROM AUTHOR]

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Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Hydrogeology & Engineering Geology / Shuiwendizhi Gongchengdizhi is the property of Hydrogeology & Engineering Geology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Aiming at the deformation and failure of roadway surrounding rock and its control problems under goaf in upper and lower coal seam mining of Buertai Coal Mine, the distribution characteristics of surrounding rock stress in different mining stages of goaf were analyzed, the evolution law of plastic zone of roadway surrounding rock under goaf was analyzed, and the failure mechanism of roadway surrounding rock was revealed by theoretical calculation and numerical simulation.The results show that: the principal stress of the mining roadway under the goaf is smaller than that of the original rock stress. The plastic zone of surrounding rock is small during excavation. Affected by the first mining, the maximum principal stress is 1.24 times of the original rock stress, and the minimum principal stress is 1.49 times of the original rock stress. Affected by the secondary mining, the maximum principal stress is 1.52 times of the original rock stress, and the minimum principal stress is 1.74 times of the original rock stress. In order to control the malignant development of surrounding rock plastic zone, the staged support technology in different mining stages is proposed, and the stress environment of surrounding rock is improved by hydraulic fracturing, so as to reduce the plastic zone of surrounding rock and maintain the safety and stability of surrounding rock. [ABSTRACT FROM AUTHOR]

Taking the investigation of the mined-out area of gypsum mine in Mengxi-Huazhong railway Jingmen-Yueyang section as an example, this paper introduces the comprehensive survey method of the mined-out area by the combination of remote sensing interpretation, investigation and visit and exploration verification. In view of the problems of railway route selection through goaf, the working idea of goaf survey is put forward, and the remote sensing interpretation method of goaf based on satellite photograph, aerial photograph and multi-temporal analysis is expounded. This paper puts forward the problems that should be paid attention to in the investigation and visit of goaf and how to carry out comprehensive analysis. It is also proposed that the exploration verification method should be reasonably selected according to the regional geological conditions, the development characteristics of the goaf, the applicability of exploration methods, site conditions and so on. Finally, the comprehensive survey of goaf in Jinling and Baoan gypsum mines is introduced, which can provide reference for similar goaf investigation.

In order to explore the law of CO generation in the oxidation process of coal leftover in shallow-buried comprehensive and mechanized mining areas, This article takes the shallow coal seam Gaojialiang Mine as the research object, and compares it with the deep coal seam Yangquan mine. Using the oil bath temperature-raising oxidation system, a temperature-increasing oxidation test was carried out on the coal samples of the fully mechanized coal mining face in different coal seams of Gaojialiang Mine and Yangquan Mine. Research shows: Each coal sample in the shallow-buried Gaojialiang Mine consumes O2 at a low temperature of 40 ℃ to produce CO volume fraction of 1×10-4; The oxygen consumption rate, CO production rate and heat release intensity of each coal sample during the oxidation and heating process gradually increase with the increase of temperature; The coal sample of the shallow-buried Gaojialiang Mine produced two critical temperatures of 40 ℃ and 130 ℃, corresponding to the starting point of accelerated oxidation reaction and the starting point of violent oxidation reaction, respectively. However, the Yangquan coal sample in the deep-buried coal seam has only one insignificant critical temperature, and relatively lagging behind, reaching 100 ℃~120 ℃. At the same coal temperature, the CO production and production rate of each coal seam in the shallow-buried Gaojialiang Mine, and the consumption and consumption rate of O2 in each coal seam are significantly greater than those of the deep-buried coal seam in Yangquan Mine. The coal seams of the shallow-buried Gaojialiang Mine enter the accelerated oxidation stage earlier than Yangquan coal seams, and the required temperature is lower. The shallower the coal seam is buried, the faster the coal's oxygen consumption rate and CO production rate during heating oxidation., The increase in temperature promotes the oxidation exotherm of coal samples at shallow depths more strongly.

Aiming at the problem of delayed repair of cracks in the airtight wall of the goaf, a method to initiate crack repair using microcapsules in the initial stage of crack evolution of the airtight wall (cracking period) is proposed. This method makes use of the stress at the crack tip. The microcapsule wall is torn to allow the repair agent wrapped in the wall of the capsule to flow out and fill the gap. After the curing reaction, crack repair is completed. During the crack repair process of the airtight wall, the particle size of the microcapsule affects the strength of the capsule wall and directly affects the repair effect. In order to study the influence of particle size on the strength of microcapsules, eight kinds of microcapsules with different particle diameters and the same wall thickness were prepared by orthogonal experiments. The particle size and wall thickness were characterized by SEM, and the strength of the capsule wall was measured using nanoindentation technology. The quadratic function relationship that the strength of the capsule wall decreases with the increase of the particle size is obtained.

In order to determine the maximum plastic zone width of coal pillar under the action of self-weight of overlying coal rock mass near the goaf, the method of combining theoretical derivation, numerical simulation and on-site monitoring was adopted. By considering the arching effect of overlying coal rock mass in the goaf, the elastic-plastic analysis of coal pillar was carried out. According to the engineering conditions of Yuhua Coal Mine, the ANSYS simulation mining depth was 500 m and 600 m, the mining height was 4 m, 5 m, 6 m and 7 m, and the maximum plastic zone of coal pillar in the roadway of Yuhua 2410 working face was monitored. The results show that the results of theoretical calculation, numerical simulation and field monitoring of the maximum plastic zone width of coal pillar are consistent. The theoretical formula of the maximum plastic zone width of coal pillar in medium mining height is given, which can provide the basis for the design of coal pillar width.

Under the condition of coal seam group occurrence, the gas sources in goaf cannot be determined when the protective coal seam is mined. To achieve the purpose of efficient gas control, it is necessary to analyze the gas sources in goaf in advance and carry out gas control for different gas sources. In this paper, the composition and stable carbon and hydrogen isotopes of desorption gases in main coal seams were tested and analyzed. Also, the gas sources and gas composition in the goaf of the protective coal seam are analyzed by establishing a quantitative identification model of mixed gas volume ratio. Gas source of the protective coal seam goaf is mainly M3 coal seam, accounted for 90%, M2 coal seam accounts for 9% on average, while M1 coal seam accounts for 1% on average, which indicates that the pressure-relief gas of M1 coal seam cannot enter goaf of M2 coal seam during M2 coal seam mining. This phenomenon makes M1 coal seam gas content constant and poses a safety risk of gas emission to the M1 coal seam. Therefore, the pressure-relief gas of the upper M1 coal seam has to be extracted to eliminate the hidden gas hazards when mining M2 coal seam (protective coal seam). Through the analysis of gas extraction concentration, it is found that after the formation of gas migration channels that used for gas flowing in the M1-protected coal seam, the migration channels can at least maintain until the working face of M2 protective coal seam is mined 350 m; this study can provide a certain reference for the gas control technology in protected coal seam under the condition of coal seam group mining. [ABSTRACT FROM AUTHOR]

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Numerical simulation is an important method of studying the flow, temperature, and concentration fields in the goaf. In a simulation, the permeability and inertial resistance coefficient are the key factors. For the verification of goaf permeability model, scholars usually use field data for verification. However, the site conditions of coal mine are complex, and the gas emission and air leakage in the goaf are difficult to control, which will lead to inaccurate verification of permeability model. Therefore, this paper verifies the goaf permeability model under experimental conditions, which can ensure that the experimental conditions are consistent with the numerical simulation conditions and realize the accurate verification of the model. An experimental scenario of a thermodynamic disasters in the goaf can be built, based on gas flow, and heat and mass transfer in porous media. The permeability and inertial resistance coefficient models can then be experimentally verified. Experimental results show that methane concentration increases with increased distance from the working face, hence the methane concentration in the return roadway side is higher than that in the intake roadway side. As distance from the coal seam floor increases, the methane concentration shows a downward trend. After releasing methane from the floor for 120 min, the concentration field inside the goaf stabilizes; therefore, coal spontaneous combustion experiments are started after a 120 minute lapse time period. In a numerical simulation, the goaf is the porous media seepage model, and the boundary conditions of the numerical model are set according to experimental conditions. The numerical simulation results concurred well with the experimental results; verifying the suitability of the authors’ proposed goaf permeability and inertial resistance coefficient model. [ABSTRACT FROM AUTHOR]

The spontaneous combustion of coal in the goaf during the coal mining process is an important problem that restricts the safe production of coal mines and causes the pollution of the underground operation environment. Therefore, the fire prevention and extinguishing process in the goaf is related to the safety of underground operators and the underground operating environment. In this paper, the numerical simulation method is used to analyze the influencing factors of the fire spread in the process of coal spontaneous combustion in the goaf. The research results show that the temperature of any point in the goaf will gradually increase in the natural process, and the temperature of this point will gradually decrease as the distance from the fire point in the goaf becomes farther at the same time. At the beginning, the temperature at the rightmost end of the goaf was 35°C, and at the end, the temperature at the rightmost end of the goaf had risen to 821°C, which was mainly due to the long-term heat radiation from the spontaneous combustion of the fire point. In addition, the study also found that the spread of coal spontaneous combustion will continue to accelerate with the increase of oxygen concentration in the goaf. When the oxygen concentration is 14%, the temperatures at points E and F in the goaf are only 95°C, while when the oxygen concentration rises to 28%, the temperatures at points E and F in the goaf can reach 840°C. This shows that oxygen as a combustion-supporting material can aggravate the spontaneous combustion process of coal. For the fire prevention and extinguishing process of spontaneous combustion in the goaf, it is recommended to design the thickness of the fireproof material in the goaf to 1.5 cm and the power of the ventilation equipment to be 200 KW. [ABSTRACT FROM AUTHOR]

Copyright of Railway Investigation & Surveying is the property of Railway Investigation & Surveying Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Surface subsidence threatens the structural stability of ground facilities located in mining-induced subsidence areas. Clarifying and evaluating the influence of surface subsidence can inform the construction and maintenance of various ground facilities, such as buildings, roads, and bridges. In this paper, we investigated mining-induced surface subsidence and areas of potential damage in Yangquan City, Shanxi Province, by exploiting small-baseline set interferometric synthetic aperture radar (SBAS-InSAR) monitoring and geographic information system (GIS) techniques. More specifically, we first investigated the distributions of subsidence areas and subsidence rates in Yangquan City from June 16th, 2016, to December 1st, 2016, by exploiting SBAS-InSAR monitoring. We then classified ground facilities, such as buildings, highways and railways, and identified their distributions using spatial analysis using GIS. Finally, we integrated the results of the two techniques to evaluate the potential damages induced by surface subsidence for various ground facilities. We found that, overall, (1) surface subsidence has seriously developed in the Yangquan Mine and (2) some of the subsidence areas exist in facilities with high-level restrictions, such as high-rise buildings, highways, and railways, which may cause potential damage. Our work presented in this paper could be referred to and applied to other similar cases. [ABSTRACT FROM AUTHOR]

Blast fragmentation size distribution is one of the most critical factors in evaluating the blasting results and affecting the downstream mining and processing operations in open-pit mines. Image-based methods are widely applied to address the problem but require heavy user interaction and experience. This study deployed a deep learning model Mask R-CNN to develop an automatic measurement method of blast fragmentation. The model was trained using images captured from real blasting sites in Nui Phao open-pit mine in Vietnam. The trained model reported high average precision scores (Intersection over Union, IoU = 0.5) 92% and 83% for bounding box and segmentation masks, respectively. The results lay a solid technical basis for the automated measurement of blast fragmentation in open-pit mines. [ABSTRACT FROM AUTHOR]

An innovative scheme of induced caving by blasting was adopted in the depillaring continuous miner panel CM1 of Jhanjra Colliery of Eastern Coalfields Limited, a subsidiary of Coal India Limited. Initially, the blasting pattern was designed with respect to underground workings and later with respect to the surface, vertically above the panel, to facilitate effective caving. The natural caving was delayed due to the presence of three hard-rock beds of more than 5000 cavability index in the immediate roof of the R-VI seam in addition to the oversized left-out standing pillars kept during coal extraction in the panel, which provided extra supports to the hanging roof-rock in the goaf. To mitigate the situation, the design parameters were meticulously planned and implemented for successful blasting operations, which led to effective caving in the goaf and enabled successful extraction of the coal from the panel maintaining the required level of safety and productivity. [ABSTRACT FROM AUTHOR]

Taking No.1305 working face equipment recycling period of Dananhu No.1 mine of Shenhua Guoneng Hami Coal Power Co., Ltd. as the research object, based on the data of the surface coal temperature of the goaf collected by the infrared thermal imager and the gas volume fraction of the goaf analyzed by the gas chromatograph, the OriginLab data analysis software is used to systematically analyze the changes in the surface coal temperature and the gas volume fraction of the goaf before and after full-section shotcrete plugging. The results show that after full-section shotcrete plugging of surface coal in the goaf, the heat dissipation period is about 5 days and the heat stabilization period is about 5 days. Then, the surface temperature of the coal increases by about 0.5 to 1 ℃ every 2 days; the volume fraction of O2 decreases about 3%~4% and N2 increases about 3%~4% in the goaf.

The extended tunneling area of the return-air roadway in the second mining area of Weiqiang Coal Mine was taken as the research object. Through the analysis of mine geological and hydrogeological conditions, combined with the previous exploration of goaf by means of geophysical exploration and drilling, the main water-filling threat of roadway tunneling was determined. Then, in the process of roadway tunneling, advanced drilling and hydro-chemical analysis were carried out to ensure the safety of roadway tunneling. The results show that: in the early stage of roadway tunneling in the second mining area of the mine, a number of suspected goaf abnormal areas and rich water area were delineated by surface 3D seismic and transient electromagnetic exploration. Through the verification of the goaf area by drilling engineering and TV logging, it is found that there are goaf and water areas in the south boundary of the second mining area of the coal field. According to the delineation range, the roadway layout was designed. Combined with the geophysical exploration results, advanced drilling was carried out in the underground during the heading stage of the return-air roadway on the southernmost side. At the same time, the abnormal water from the borehole was sampled and analyzed, and it was found that there was no goaf in the explored area.

To optimize the control effect of nitrogen injection fire-fighting technology, the No.8401 working face in Madaotou Coal Industry Company of Datong Coal Mine Group is taken as the research object. The numerical simulation method is used to study the influence of ventilation and advancing degree of working face on the scope of spontaneous combustion oxidation zone in goaf under nitrogen injection and non-nitrogen injection. The results show that whether nitrogen injection or not, the larger the air volume in the working face is, the wider the oxidation temperature rise zone is, and the greater the spontaneous combustion risk of goaf is. From the view point of preventing the goaf from spontaneous combustion, the air volume of the working face should be reduced as much as possible, i.e. the reasonable ventilation technology should be adopted. The reasonable range of air supply in No.8401 fully mechanized caving face is 1 400~1 755 m3/min. At the same time, it is concluded that the nitrogen injection measures must be taken in No.8401 fully mechanized caving face, and the monthly advancing degree of the working face should be no less than 36 m. Under the above conditions, the application effect of nitrogen injection fire-fighting technology is the best.

To solve the sign problem of coal spontaneous combustion in the mining operation of 1308 working face in Guojiahe Coal Mine, the characteristic parameters of coal spontaneous combustion are analyzed by the on-site coal sampling experiment, CO and C2H4 are determined as the index gas of coal gas spontaneous combustion. The causes of coal spontaneous combustion and the prominent process of spontaneous combustion in goaf under the influence of high pumping alley without collapsing in time are analyzed. The site adopts the collaborative prevention and control method of plugging, cooling and high pumping alley releasing pressure to control oxygen. Through the detection of index gas in the high pumping alley, the result shows that the concentration of CO in 1308 working face is effectively controlled to ensure the safe mining for the working face.

Under the mining condition of U-type ventilation and spontaneous combustion coal seam, the air leakage in goaf may create an environment with explosion hazard due to the mixing of oxygen and gas. Through the observation by embedding methane and temperature sensor in the goaf, the Fluent method is used to perform analog computation on gas distribution regularities of goaf, and the possibility of gas combustion in the dangerous area of spontaneous ignition is analyzed. Meanwhile, the hazard judgment and analysis is conducted on the explosion coupling area resulting from the interaction of gas and oxygen in the goaf, which provides the basis for field gas control.

In order to effectively control and prevent coal spontaneous combustion, to ensure safe mining of coal mines, aiming at the problems of mining spontaneous combustion coal seam in Daxing Coal Mine, complicated geological structure and serious igneous rock intrusion disaster, the idea of nitrogen and carbon dioxide inert coupling gas fire prevention and extinguishing is proposed, and the law of coupled gas migration in goaf is studied by numerical simulation. The results are as follows: when the ratio of nitrogen to carbon dioxide is 3∶1, the inerting effect of goaf is the best; when the injection point of coupling gas is located 45 m away from side goaf of transport roadway, the inert coupling gas injected at this location has the best inerting effect on goaf.

In order to explore the influence of different oxygen concentrations in goaf on the micro-characteristics of low-temperature oxidation process of coal, a self-built low-temperature oxidation simulation experimental system was used to simulate the natural oxidation state of coal at different depths of goaf. Based on electron spin resonance spectroscopy(ESR) technology, the free radicals in coal oxidation at low temperature under different oxygen concentrations were analyzed, and the release of gaseous products in coal oxidation process was determined. The results show that at the temperature node of 130 ℃, the change is from slow to fast. Coal body changes from slow oxidation to fast oxidation, but the g factor is between 2.001 41 and 2.002 034, with little change. With the increase of oxygen concentration, the variation trend of each parameter with oxidation temperature remains unchanged, but the overall variation increases gradually. When the oxygen concentration is less than 9%, the reaction of free radicals is slow at low temperature, and the generation of CO lags behind obviously. When oxygen concentration is more than 9%, the probability of coal-oxygen combination increases. The increase of temperature is more likely to stimulate the free radical reaction, and the CO concentration increases exponentially.

To study the delayed failure characteristics of deep coal seam floor in goaf, the deformation and failure characteristics of the locking section between the crack surfaces are analyzed by using the cantilever beam model. The relationship between volumetric strain and partial stress in the unloading and reloading process of rock samples after loading into the plastic stage is analyzed by laboratory test. Finally, the change of permeability of deep coal floor rock mass in the process of goaf recompaction is analyzed with the measured data of water injection test in coal seam floor. The results show that the deformation and failure degree of the interlock section between the crack surfaces increases gradually in the process of mining shear slip and mining unloading; the rock sample in the plastic state is loaded to the unloading point of about 81 MPa after the axial stress unloading, and the volume strain increases from 0.000 56 mm to 0.001 1 mm. Through the field measurement, it is found that the crack propagation and deformation failure appears in the rock mass 18.19 m of the coal floor in the process of the working face pushing through the measuring point from 3.6 m to 15.8 m. Meanwhile, the deformation failure of the coal floor appears again in the process of the working face pushing from 20.6 m to 30.9 m, which indicates that the deep coal seam floor still has the possibility of deformation and failure in the process of unloading and reloading after loading to the plastic state.

To solve the space and time limitations of artificial spontaneous combustion “three belts” measuring points, the dynamic and automatic analysis of spontaneous combustion “three belts” is realized by relying on KJ428 mine distributed laser fire monitoring system, beam tube maintenance technology, measuring point location marking technology and “three belts” visual curve drawing technology. The range of non-spontaneous combustion zone, spontaneous combustion zone and suffocation zone on the return air side of the goaf in 31202 obtained by using this technology is respectively: 0-60 m behind the mining face, 60-130 m behind the mining face and 130 m behind the mining face to the deep part of the goaf, which is basically consistent with the artificial measurement results.

To study the method of using transient electromagnetic method to detect the goaf of the integrated mine, on the basis of studying the principle of transient electromagnetic method and investigating the background noise, the goaf in the industry square and the production area in Tonghe No.2 coal seam are detected by transient electromagnetic method, respectively. Through data interpretation, the results indicate that there are three broken regions of goaf in the industry square and eight broken regions of goaf in the production area.

In view of problem that current numerical simulation of combined hazards of gas and coal spontaneous combustion in goaf only considers fluid effects, and does not consider influence of other physical fields, multi-field coupling numerical simulation software Comsol-Multiphysics is used to establish coupling model of gas and coal spontaneous combustion in goaf. Distribution law of gas and O2 in stope of working face and goaf is analyzed, and influence of drainage volume and air intake volume on gas concentration in high-level drainage roadway and O2 concentration in floor of goaf is explored to comprehensively determine the optimal drainage volume and air intake volume. The results show that with the increase of drainage volume, gas drainage concentration increases first and then decreases, and the width of oxidation temperature rise zone in goaf has a positive correlation with growth,comprehensively considering gas drainage effect and natural ignition prevention, the optimal drainage volume 90 m3/min is recommended for high-level drainage roadway; with the increase of air intake volume, gas concentration and scalar quantity of the high-level drainage roadway first increase and then decrease, the width of oxidation temperature rise zone on the air intake side of the goaf obviously increases, and the maximum at 109.3 m, the width of oxidation temperature rise zone on the return air side has a small change,comprehensively considering gas drainage effect and natural ignition prevention, the optimal air intake volume at the test face is 1 500 m3/min.

Abstract The abnormal emission of gas in coals is the main factor that induces serious gas disasters in coal mines. During the coal mining process, roof movement has important effects on the gas emission law and gas flow characteristics. In this study, an extremely thick coal seam under fully mechanized sublevel caving mining conditions from the Tashan coal mine in China was used as an example; the effect of roof weighting on gas emission in the goaf was studied. Then, the large roof weighting step and the small roof weighting step caused by the different thicknesses of lamprophyre breakage were used as the objects and a numerical simulation study on the gas flow characteristics in the goaf was carried out. The results indicate that the mining coal seams, the overlying adjacent unmined coal seams, and the goaf are the main contributors to the large gas emission capacity, and even situations exist where the gas concentration exceeds the limit in the working face. During roof weighting, the gas concentration and emission capacity in the return airway, upper corner, and high drainage roadway increase substantially. The results of the numerical analysis are in good agreement with the observed data, which indicates that it is effective to control the gas in coal seams by designing the location of high drainage roadway according to the mining characteristics of coal seam and the gas emission law.

To study the distribution law of spontaneous combustion "three zones" under the condition of gas extraction by buried pipe in goaf during coal mining, taking the goaf of 81307 fully mechanized caving face in Baode Coal Mine as the research background, the “three zones” of spontaneous combustion were studied by numerical simulation and field measurement. The research results show that on the inlet side of 81307 fully mechanized caving face, from 0 to 175 m is the heat dissipation zone, from 175 to 330 m is the oxidation zone, and more than 330 m is the asphyxiation zone; on the side of the rubber conveyor, from 0 to 110 m is the heat dissipation belt, from 110 to 265 m is the oxidation zone, and more than 265 m is the asphyxiation zone.

Taking II020803 working face of coal mine as the background, the theoretical analysis, numerical simulation and field practice for gas and residual coal spontaneous combustion coupling disaster prevention and control in goaf are studied. The results show that the gas explosion triangle area is mainly located in the working face and the area along the goaf, and the mixed gas concentration in this area is suitable, and an explosion may occur when an ignition source of sufficient energy is encountered. When the oxidation reaction occurs in the area where the oxidation zone of goaf in the same layer and the explosion zone coincide, the temperature of the coal body and its surroundings will rise. Once the temperature exceeds the CH4 ignition point, a gas explosion will occur. The integration of high gas drainage and fire prevention drilling and the dynamic isolation control technology of goaf can make the CO concentration and CH4 concentration in the corner of the working face decrease, which can effectively prevent and control the coupling disaster of gas explosion and coal spontaneous combustion in the working face.

To study the distribution law of coal spontaneous combustion zone in goaf of two intake and one return ventilation working face, the oxygen concentration under different working conditions during the mining of the 8301 working face of Tashan Coal Mine was simulated, the danger zone of spontaneous combustion of coal was determined and corresponding fire prevention measures were proposed. The spontaneous combustion danger zone of coal in the goaf of two intake and one return ventilation working face is large, and the spontaneous combustion zone reaches 96 m when the mining distance is 150 m; nitrogen injection can greatly change the distribution of spontaneous combustion zones and reduce the spontaneous combustion of coal in the goaf. For the two intake and one return ventilation working face, nitrogen injection should be considered on both sides of the goaf; increasing air volume allows the boundary of the spontaneous combustion zone to extend deep into the goaf and increase the distance of the front end from the working face.

In view of the danger of fire and gas coupling disaster in the upper adjacent goaf of the coal seam caused by the tunneling process of 2274 working face in Baijiao Coal Mine, by means of identifying fire and gas coupling areas in goaf where the flammable explosive concentration range and the width of spontaneous combustion zone overlap, based on the data from the site monitoring, the superimposed area of the spontaneous combustion zone and the CH4 explosive zone is obtained from the working face of the goaf in the direction of the return air side 76 m to 100 m. The safety of mine production is ensured by the integrated prevention and control measures such as strengthening the support of tunneling roadway, grouting in full section, grouting in upper adjacent goaf and gas drainage drilling.

Taking the grouting filling treatment project in the goaf of 8712 working face of Huoshizui Coal Mine as the background, based on the theory of mining subsidence and the method of combining borehole wave velocity test and pressure water test, the characteristics of fracture development in overburden strata after fully mechanized caving are expounded. The results show that during the overall movement of the overlying strata after fully mechanized caving mining, the integrity of the rock mass was damaged to varying degrees, and a tensile gap with a horizontal width of about 60 m was formed at the edge of the goaf. The comprehensive treatment technology of “seepage-proofing at the bottom of the reservoir, full-clay filling and grouting in the goaf, and monitoring the movement and deformation of the affected area” has achieved good governance effects.

In view of the objective conditions of high sulfur content and local enrichment, large amount of residual coal and closing to upper goaf of coal seams in the 8104 fully mechanized coal mining face of Xinwei Coal Mine, the distribution law of coal spontaneous combustion danger zone in goaf is studied. In the implementation process, we used beam tube monitoring system to test the gas field distribution in the goaf in real time. On this basis, “three zones” of the goaf of the 8104 fully mechanized mining face are divided by taking O2 concentration change as the main index and the temperature change as the auxiliary index, and the numerical simulation method is used to compare and analyze the field test result. The result shows that the measured results are consistent with the numerical simulation results. Finally, the range of spontaneous combustion zone in the goaf of the fully mechanized mining face was determined: 40.5 m to 95.5 m on the inlet side and 15.3 m to 59.7 m on the return side.

On May 18, 2006, a disastrous groundwater inrush occurred at the Xinjing coal mine in Datong, Shanxi province, China. Great effort was taken during the post-accident rescue. However, triggered by the accumulated water rushed from the nearby abandoned tunnels and goafs of the Yanzishan coal mine, which had been closed for tens of years, it caused great damage, including 56 deaths and direct economic loss of over 53 million yuan. The outburst groundwater was from the abandoned goafs in the No.14-1 coal seam of the neighbour Yanzishan coal mine. The passage formed in northeast part of the mining area in the No. 14-1 coal seam. The average inflow rate was 23,333.3 m3/h. Unidentified spatial distribution and water-filled degree of the abandoned tunnels and goafs of the Yanzishan coal mine are critical contributions to the accident. Illegal mining in the No.14-1 coal seam is an anthropogenic contribution. That mandatory regulations for excessive groundwater exploration and release were not carried out in the mining is the third fatal cause leading to the accident. Finally, the poor awareness on water inrush recognition and control of the miners also induced the accident to a disastrous extent.

A number of longwall mines suffer significant gas delays due to high methane gas concentration levels (i.e. above 1.0% statutory limit for methane gas level) near the tailgate (TG) motor area of the longwall face. Therefore, in addition to development of effective goaf gas drainage and management strategies for highly gassy mines, development of face control strategies to minimize methane gas concentration levels near the tailgate motor of the longwall face is highly critical to achieve high production rates in high capacity longwall mines. To address this issue, detailed CFD models of longwall face were developed and extensive modelling simulations were carried out to investigate the effectiveness of various control options on gas concentration distribution near the tailgate motor area. To investigate the effect of mining and ventilation parameters on effectiveness of various control strategies, simulations were carried out with CFD models of Mine A. The details and results of all these studies are presented in this paper.

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., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2024 The Authors. Published by Elsevier Ltd.)

The risk evaluation indexes of goaf are multi-source and have complex mutual internal correlations, and there are great differences in the risk identification of goaf from different mines among the various influencing factors. This paper mainly focuses on principal component analysis (PCA) and the differential evolution algorithm (DE), while a multi-classification support vector machine (SVM) is adopted to classify the risks of goaf. Then, the K-fold cross-validation method is used to prevent the overfitting of selection in the model. After the analysis, nine factors affecting the risk identification of goaf in a certain area of East China were determined as the primary influencing factors, and 120 measured goafs were taken as examples for classifying the risks. More specifically, the classification results show that: (1) SVM has the useful ability of generalization, especially when solving the problems of overfitting, and it is easy to fall into the local minima under the conditions of small samples; (2) PCA is employed to realize the intelligent dimensionality reduction and denoising of multi-source impact indicators for goaf risk identification, which immensely improves the prediction accuracy and classification efficiency of the model; (3) after using the DE, the optimal solutions of the problems to be optimized are automatically obtained through the global optimization search mechanism, namely, the kernel function parameter, ‘γ’, and the penalty factor, ‘C’, of the SVM, which further verifies that the characteristics of clear logic, strong convergence, and good robustness can be found in the DE. As demonstrated, this method has the advantages of guiding significance and application value for goaf risk identification.