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Mining in an upper coal seam results in the redistribution of vertical stress within a lower coal seam until a new equilibrium state is attained. Close-multiple coal seams mining faces the occurrence of multi-goafs and a complex stress environment. Based on a case study of close-multiple coal seams with repeated mining in the Qianjiaying coal mine, a stress distribution model of the floor in the coal seam striking range was established by optimizing the load form of the abutment pressure. The floor stress state after primary and repeated mining of upper coal seams was analyzed in a visual and quantitative manner. The concentrations of vertical stress, horizontal stress, and shear stress are all located in the floor under the coal rib after primary and repeated mining. The curve of stresses gradually levels off as the depth of the floor increases, indicating a diminishing influence of mining-induced stress from the upper coal seam. Based on the stress analysis by numerical simulations, the overall lower coal seam remains in a state of large-scale stress relief. Stress relief occurred twice under repeated mining, which indicates that the lower coal seam has favorable mining feasibility. The results of the study can provide scientific guidance to prevent mining accidents. [ABSTRACT FROM AUTHOR]

The Double-Yield (D-Y) model is a widely used method for predicting the mechanical properties of caved rock. However, determining the numerical simulation parameters for this model typically requires extensive trial and error. In this paper, a new approach for parameter inversion of the D-Y model is proposed and applied to a real case study involving repeated mining in multiple coal seams. The proposed framework combines FLAC3D, a finite element numerical simulation software, with a new computational tool developed using MATLAB® App Designer. By applying this framework to the case study, the accuracy and efficiency of parameter determination in the mining area are improved. In addition, a 3D finite element model of multiple coal seams is generated based on the geological information of the case study, and the stress evolution is analyzed. The simulation results are then validated through field measurements, providing valuable reference information for safe and efficient production. Overall, this improved method for parameter inversion of the D-Y model, along with the combined numerical simulation and field measurement approach, enhances the understanding of stress evolution in multiple coal seams and contributes to safer and more efficient mining operations. Highlights: An in-depth analysis was conducted to examine how variations in Double-Yield parameters impact the numerical simulation curve. A breakthrough approach for parameter inversion of the Double-Yield (D-Y) model is introduced, and the computational tool is developed. A 3D finite element model is crafted to analyse the dynamic stress evolution during repeated mining of multiple coal seams. The accuracy of simulation outcomes is bolstered by validation against field measurements of mining-induced stress. [ABSTRACT FROM AUTHOR]

Purpose. The research aims to determine the parameters for the overlying stratum caving zones above the mined-out space during repeated mining of pillars at the Zhomart Mine of the Zhaman-Aybat field for the purpose of predicting hazardous zones of influence of mining operations. Methods. Research includes an analysis of the results of previous in-situ studies conducted using seismic surveys, as well as modeling with the Examine2D software to determine the stress-strain state of the overlying stratum mass. The elastic and strength characteristics of an anisotropic mass are used for modeling, taking into account the generalized Hoek-Brown criterion with the Geological Strength Index (GSI) based on a geomechanical description of the mass quality. Comparative analysis of modeling results with seismic survey results is conducted to confirm the effectiveness of the developed methodology. Findings. The research provides an opportunity to determine the overlying stratum caving parameters, such as the caving arch height and the condition for complete undermining of the overlying stratum at the Zhaman-Aybat field at different spans of the mined-out space (from 50 to 350 with a step of 50 m, reaching the maximum span of 370 m). Originality. It has been determined that the caving arch height depends on the outcrop span and increases exponentially (hcav = 16.473 e0008Le). In addition, the condition of the earth's surface complete undermining has been identified depending on the depth of the site to be gotten. Practical implications. The research results can be used to develop a normative document for calculating the earth's surface shear during the repeated mining of pillars at the Zhomart Mine of the Zhaman-Aybat field. The data obtained will also be useful in planning repeated mining and predicting the earth' s surface shear to avoid negative impacts of mining operations on surface structures. [ABSTRACT FROM AUTHOR]

Mining in an upper coal seam results in the redistribution of vertical stress within a lower coal seam until a new equilibrium state is attained. Close-multiple coal seams mining faces the occurrence of multi-goafs and a complex stress environment. Based on a case study of close-multiple coal seams with repeated mining in the Qianjiaying coal mine, a stress distribution model of the floor in the coal seam striking range was established by optimizing the load form of the abutment pressure. The floor stress state after primary and repeated mining of upper coal seams was analyzed in a visual and quantitative manner. The concentrations of vertical stress, horizontal stress, and shear stress are all located in the floor under the coal rib after primary and repeated mining. The curve of stresses gradually levels off as the depth of the floor increases, indicating a diminishing influence of mining-induced stress from the upper coal seam. Based on the stress analysis by numerical simulations, the overall lower coal seam remains in a state of large-scale stress relief. Stress relief occurred twice under repeated mining, which indicates that the lower coal seam has favorable mining feasibility. The results of the study can provide scientific guidance to prevent mining accidents.

Abstract Based on the mining conditions of shallow close coal seams in the Jurassic coalfield of northern Shaanxi, in order to explore the evolution and development height prediction of water-conducting fracture zone (WCFZ), combining with measured statistical analysis, physical simulation, numerical calculation, and theoretical analysis, the evolution of mining-induced fractures and the development characteristics of WCFZ are analyzed, and the development height predicting method of WCFZ in shallow close coal seams mining is proposed. The results show that in shallow single coal seam mining, during the stage of incritical mining, the height of the WCFZ increases in a “jumping” pattern with the main roof periodic caving. After critical mining, it is basically stable. Its development height is positively correlated with the mining height. The ratio of development height to mining height is generally 20–30, and the change is not significant with the mining height, overall, the distribution of the WCFZ is “saddle shaped”. Based on close distance coal seams repeated mining, the primary fractures act as activation development when the interburden broken completely, the development of WCFZ at the mining boundary is significantly intensified, and ratio of the height to mining height in repeated mining is obviously lower than that in single seam mining, which is generally 18–25. The traditional empirical formula cannot be applied to the development prediction in shallow close coal seams mining, with an error of 10–50%. Considering the mining height, interburden thickness, and roof caving characteristics, the development height predicting method of WCFZ in shallow close coal seams mining is proposed. Compared with measured results, the calculation error is basically within 15%, which has good prediction accuracy, The research results can provide a theoretical basis for predicting the height of WCFZ in shallow close coal seams mining in the Jurassic coalfield of northern Shaanxi.

A comprehensive study was undertaken at Jiaozi coal mine to investigate the development regularity of ground fissures in shallow buried coal seam mining with Karst landform, shedding light on the development type, geographical distribution, dynamic development process, and failure mechanism of these ground fissures by employing field monitoring, numerical simulation, and theoretical analysis. The findings demonstrate that ground fissure development has an obvious feature of subregion, and its geographical distribution is significantly affected by topography. Tensile type, open type, and stepped type are three different categories of ground fissure. Ground fissures emerge dynamically as the panel advances, and they typically develop with a distance of less than periodic weighting step distance in advance of panel advancing position. Ground fissures present the dynamic development feature, temporary fissure has the ability of self-healing. The dynamic development process of ground fissure with closed-distance coal seam repeated mining is expounded, and the development scale is a dynamic development stage of "closure → expansion → stabilized" on the basis of the original development scale. From the perspective of topsoil deformation, the computation model considering two points movement vectors towards two directions of the gob and the ground surface is established, the development criterion considering the critical deformation value of topsoil is obtained. The mechanical model of hinged structure of inclined body is proposed to clarify the ground fissure development, and the interaction between slope activity and ground fissure development is expounded. These research results fulfill the gap of ground fissures about development regularity and formation mechanism, and can contribute to ground fissure prevention and treatment with Karst landform. [ABSTRACT FROM AUTHOR]

The characteristics of surface subsidence under the condition of repeated mining in weakly cemented strata are of great significance to the safe and efficient mining and ecological restoration of coal resources in weakly cemented mining areas in western China. Theoretical analysis, similar simulation, numerical simulation and field monitoring are used to study the migration law of overlying strata and surface subsidence model under repeated mining conditions in weakly cemented strata, and the model is applied in engineering. The bulking characteristics of weakly cemented rock and the influence mechanism of repeated mining overburden strata movement on surface subsidence are discussed through theoretical analysis. The ‘maximum surface subsidence model under the condition of repeated mining in weakly cemented strata’ is established. There is a linear relationship between the bulking coefficient of weakly cemented rock, the mining thickness of lower coal and the maximum surface subsidence of weakly cemented strata. Through similar simulation and numerical simulation, the characteristics of repeated mining overburden and surface subsidence in weakly cemented strata are analyzed. The research results show that the development law of the separation height of the initial mining and repeated mining of the weakly cemented strata is basically the same, and both show a step-like rise. The surface subsidence curve of repeated mining is asymmetrically distributed, and the maximum subsidence value is biased towards the side of open cut. The maximum development height of overlying strata, the maximum surface subsidence value and the surface subsidence coefficient after initial mining and repeated mining are given. The established maximum surface subsidence model is used to predict the maximum surface subsidence value on site. The predicted value of the maximum surface subsidence model is similar to the measured value on site during the mining process of the working face, which verifies the rationality of the ' maximum surface subsidence model under the condition of repeated mining of weakly cemented strata '. At the same time, the predicted value of the maximum surface subsidence after the mining of the working face can provide a reference for the actual work on site.

Aiming at the fact that the water-conducting fissures in the overlying rock may cut through aquifers and induce water inrush disasters by repeated mining of shallow-buried and short-distance coal seams in Shennan Mining Area, this paper adopts the methods of similarity simulation, numerical simulation and field drilling measurement to study the short-distance 1-2 coal and 2-2 coal mining overlying rock water-conducting fracture zone development law. When the 1-2 coal is fully mined, the height of the overlying fissure zone is about 50 m, and it is not connected with the upper quaternary loose pore water and the Jurassic weathered bedrock confined water; when the 1-2 coal and 2-2 coal are in repeated mining, the theoretical prediction of the water-conducting fault zone is about 136 m, the development height of the overburden fault zone is about 140 m according to the physical and numerical simulation, and the height of the water-conducting fault zone measured by the field borehole is about 135 m. The three are very consistent, and the fault zone is connected with the confined water of Jurassic weathered bedrock. The measures of dredging confined water from mining roadway to weathered bedrock of Jurassic at 300 m in advance working face are proposed. The field practice shows that no accident of water inrush and sand collapse occurs in the process of coal mining.

This study focused on the stability control of roof in long wall stratified mining of large dip angle thick coal seam. Taking Panbei Coal Mine in Huainan as an example, the effect mechanism of water content, compression ratio and grain size grading on the compression characteristics of gangue is analyzed by comprehensive application of gangue lateral compression test, 3D imaging borehole detection test, physical simulation and numerical simulation method. In addition, the effect of these factors on the stability of the reclaimed roof were also analyzed, the structural characteristics and stress state of the reclaimed roof were obtained, the dynamic behavior of the reclaimed roof was revealed, the stability control measures of the reclaimed roof were formulated, and the control effect of the reclaimed roof stability was evaluated. The research results indicate that the clay minerals of goaf mudstone and sandy mudstone are beneficial for the secondary cementation of gangue. The contact state and stress state of the gangue during compression under load are constantly adjusted, manifested as the compression and bonding degree of the lower and upper gangue being higher than that of the middle, with the compression and bonding degree of the middle and upper gangue being the smallest, and the regenerated roof being more prone to damage. The particle size distribution of gangue is the main factor affecting the shear strength of recycled roof. As the particle size of the gangue increases, the regenerated roof exhibits ductile failure that slides along the shear line to the staggered and bulging of the gangue particles. That is, the degree of mining fracture in the lower part of the regenerated roof tends to be milder than in the middle and upper parts, and the middle and upper parts are the key prevention and control areas for the stability control of the regenerated roof. The deflection of the main stress can lead to the fracture of the regenerated roof, so a plan for the collapse of the regenerated roof and the grouting.

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.)

Aiming at the land subsidence problem caused by multiple coal seam mining in the Ehuobulake Coal Mine, this paper, considering the geological conditions of the first and fifth layers of coal, adopts the method of combining FLAC3D numerical simulation and SBAS-InSAR technology to analyze the dynamic evolution law of land subsidence amount and range under multiple coal seam repeated mining conditions. The reliability of the technology is verified by the field GPS monitoring data. The results show that, under the mining condition of multiple coal seams in the Ehuobulake Coal Mine, the land subsidence presents obvious asymmetry, and the size and range of the land subsidence in the mining area further increase due to the mining of lower layer coal. FLAC3D simulation results show that the maximum land subsidence is −211.8 mm. The results of SBAS-InSAR monitoring show that the maximum land subsidence is −225 mm, and the land subsidence obtained by the two methods has a high degree of fitting. The method of combining FLAC3D and InSAR technology can accurately and reliably monitor and analyze the land subsidence under the repeated mining of multiple coal seams in the mining area. It can provide effective guidance for the stability analysis of mined-out areas and the prediction of the influence of repeated mining on ground deformation. [ABSTRACT FROM AUTHOR]

The extraction of underground minerals in hilly regions is highly susceptible to landslides, which requires the application of InSAR techniques to monitor the surface displacement. However, repeated mining for multiple coal seams can cause a large displacement beyond the detectable gradient of the traditional InSAR technique, making it difficult to explore the relationship between landslides and subsurface excavations in both temporal and spatial domains. In this study, the Tengqing landslide in Shuicheng, Guizhou, China, was chosen as the study area. Firstly, the large-gradient surface displacement in the line of sight was obtained by the fusion of SAR offset tracking and interferometric phase. Subsequently, a multi-segment logistic model was proposed to simulate the temporal effect induced by repeated mining activities. Next, a simplified probability integral method (SPIM) was utilized to invert the geometry of the mining tunnel and separate the displacement of the mining subsidence and landslide. Finally, the subsurface mining parameters and in situ investigation were carried out to assess the impact of mining and precipitation on the kinematic process of Tengqing landslides. Results showed that the repeated mining activities in Tengqing can not only cause land subsidence and rock avalanches at the top of the mountain, but also accelerate the landslide displacement. The technical approach presented in this study can provide new insights for monitoring and modeling the effects of repeated mining-induced landslides in mountainous areas. [ABSTRACT FROM AUTHOR]

In order to explore the evolution law of mining-induced fractures in shallow single coal seam mining and repeated mining, based on the background of shallow-buried close coal seams mining in Huojitu of Daliuta Coal Mine, the activation development law of overburden fractures in single coal seam and repeated mining was revealed, under the effect of repeated mining, the evolution stages of fractures fractal dimension were proposed. The results show that after single coal seam mining, the soil layer will subsidence as a whole with the longwall face advances, and the growth rate of the fracture development height will accelerate. When it reaches critical mining, the development height of fractures does not change significantly with the face continue advances. After the first roof caving in lower seam mining, the interburden are completely broken, leading to the caving through the upper and lower goaf, and the development of original overburden fractures by single seam mining are activated development. After critical mining, the development height of the overburden fractures is 11 times of the composite mining height. The four stages of fractal dimension evolution of repeated mining fractures are proposed, fractal dimension slow-rising stage, fractal dimension fast-rising stage, fractal dimension stable stage and fractal dimension second-raising stage of repeated mining. Based the stages above, the development characteristics and fractal dimension evolution law were revealed. The research results can provide a theoretical basis for the study of the fracture evolution law in shallow-buried close seams repeated mining. [ABSTRACT FROM AUTHOR]

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.)

In view of the sudden change of mine pressure in 226 upper 02 fully mechanized caving face of Huangyuchuan Coal Mine, the mechanism and countermeasures of sudden change of mine pressure in fully mechanized caving face are studied by using the methods of theoretical analysis, numerical simulation and field measurement under the condition that the upper coal seam has been mined and affected by faults. The results show that 226 upper 02 fully mechanized caving face is located in the post mining stress influence area of the upper coal seam. During the mining process, the roof of the upper goaf is activated, the main key layer is secondary unstable, and the large-scale bench subsidence of overburden leads to the sudden change of mine pressure in 226 upper 02 fully mechanized caving working face. Therefore, the method of reducing mining height is used to control the occurrence of sudden change of mine pressure. After field application, the severity of ground pressure in the working face is significantly reduced, which ensures the safe and efficient mining of Huangyuchuan 226 upper 02 fully mechanized top caving face.

In view of the problem of surface subsidence and landslide caused by repeated mining in southwest China. Taking Anshun Coal Mine in Anshun City, Guizhou Province, as the engineering background, the surface subsidence is predicted by using the probability integral method, and then analyzed by using UDEC numerical simulation software and the physical similarity simulation experiment with XTDIC system. The results show that the maximum predicted subsidence value of single coal seam mining is 1.8 m, and the maximum predicted subsidence value under repeated mining is 3.78 m, which is similar to the results of similar simulation experiment and numerical simulation. Through similar simulation experiments, it is found that the initial weighting step of the main roof of Anshun Coal Mine is 32 m, the periodic weighting step is 12 m, and the key stratum caving step is 80 m. Under repeated mining, the first weighting step of the main roof is reduced to 20 m, the average periodic weighting step is reduced to 8 m, and the surface subsidence increases significantly. Through numerical simulation analysis, it is found that the surface subsidence under the original horizon of the key stratum is almost the same as that of the similar simulation experiment. The higher the key strata, the larger the collapse step of the key strata, and the smaller the subsidence caused by the surface. [ABSTRACT FROM AUTHOR]

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.)

Overburden separation layer filling technology for ground surface subsidence control is an important part in the system of coal mine green mining. In this study, by considering the overburden structure and ground surface subsidence law, we propose a method for predicting the horizontal position of overburden separation layer over repeated mining inclined panel according to the maximum subsidence angle. The method of using the underground existing grouting pipeline system, drilling sites in main roadway, and inclined boreholes to fill overburden separation layer is proposed. Five drilling sites are arranged for the panel; in each drilling site, there are three inclined boreholes pointing to the separation layer under the sub-key layer, the soft and hard rock alternating zone between the sub-key layer and the main key layer, and the main key layer, respectively. The end hole is located near the maximum subsidence line. Field monitoring result shows that the maximum subsidence is 0.3 m after filling, indicating the filling technology proposed in this study is effective. Usually, it is difficult to identify the accurate locations of the key layers and the target areas of the separation layer due to the uncertainty of rock occurrence, the complexity of rock mass structure, and the testing accuracy of rock’s physical and mechanical properties. Therefore, when underground borehole-based grouting and filling technology is carried out, reasonable filling targets should be explored by the test processes of alternating drilling and grouting. The study is of significant to provide a reference for other underground borehole-based filling design under similar engineering geological conditions.

[Display omitted] • The mechanical response characteristics of sandstone granules under complex stress paths are expounded. • The Acoustic emission evolution of granules deformation rupture is revealed. • The compaction and re-crushing mechanism of sandstone granules under multiple mining is proposed. Under the influence of multiple mining of coal seams, the granules structure formed by the mixing of different grain sizes exists in the collapse zone, and its compaction and re-crushing characteristics become a factor influencing the deformation and movement of the overlying rock layer. Therefore, the characteristics of sandstone granules with different gradations under cyclic loading were investigated in this manuscript. It is shown that the strain of sandstone granules increases with the increase of gradation index n , the dissipation energy of particle movement and crushing shows an increasing trend, and its porosity decreases with the increase of axial stress as a whole. At the early stage of stress loading, the high-gradation sandstone granules have high compression space and crushing potential due to larger size particles, the porosity declines the fastest, the compression modulus increases sharply, and the sandstone granules is compacted rapidly at this stage. When the stress exceeds a certain range, the energy density changes and the porosity reduction of the higher-gradation granules increases, and the larger size particles in the higher-gradations granules samples are broken down into small-size particles. At the same time, the amount of energy density changes and porosity attenuation of the high-gradations sandstone granules increases, the compressive modulus increases again at this stage, the position of the granules particles moves and the distribution is re-distributed, and the granules particles are more compact after the re-distribution, which corresponds to the higher re-distribution of the high-gradations granules samples. Under the external disturbance load, the sandstone granules show the characteristics of "three stages": pore compression period, elastic deformation period, and crushing and reorganization period. The results of this study can provide theoretical support for revealing the deformation and movement mechanism of the rock mass in the collapse zone under multiple mining. [ABSTRACT FROM AUTHOR]

Aiming at the land subsidence problem caused by multiple coal seam mining in the Ehuobulake Coal Mine, this paper, considering the geological conditions of the first and fifth layers of coal, adopts the method of combining FLAC3D numerical simulation and SBAS-InSAR technology to analyze the dynamic evolution law of land subsidence amount and range under multiple coal seam repeated mining conditions. The reliability of the technology is verified by the field GPS monitoring data. The results show that, under the mining condition of multiple coal seams in the Ehuobulake Coal Mine, the land subsidence presents obvious asymmetry, and the size and range of the land subsidence in the mining area further increase due to the mining of lower layer coal. FLAC3D simulation results show that the maximum land subsidence is −211.8 mm. The results of SBAS-InSAR monitoring show that the maximum land subsidence is −225 mm, and the land subsidence obtained by the two methods has a high degree of fitting. The method of combining FLAC3D and InSAR technology can accurately and reliably monitor and analyze the land subsidence under the repeated mining of multiple coal seams in the mining area. It can provide effective guidance for the stability analysis of mined-out areas and the prediction of the influence of repeated mining on ground deformation.

Underground mining induces the movement, deformation and failure of bedding rock slope in mountainous areas, which develops into geological disasters such as landslides and collapses. In this paper, based on UDEC discrete element method, this paper studies the influence of repeated mining of underground coal seam on the location of bedding rock slope. The results showed that: the subsidence affected by mining decreases with the decrease of buried depth, and the deformation of strata after mining is discontinuous. Compared with single-layer mining, repeated mining will lead to a significant increase in the horizontal deformation of slope. Repeated mining leads to the development of new fractures and fracture groups in the slope, which is more likely to lead to the destruction of the weak structural layer and further reduce the stability of the slope. The dynamic development process of “deformation accumulation-generation-expansion-closure” during single-layer mining will evolve into a dynamic development process of “deformation accumulation-development and expansion-development and stability” during repeated mining.

In this study, taking the Sheng'an coal mine as an engineering background, the failure characteristics of the surrounding rock of a roadway under repeated mining in a close-distance coal seam is comprehensively illustrated through field measurements (e.g., drilling imaging), theory analysis and numerical simulation (finite difference method (FDM)). The results show that although the return airway 10905 remains intact, the apparent failure of the roadway's roof and the coal pillar can be observed. In addition, the expression of floor failure depth caused by upper coal seam mining is obtained through elastic-plastic theory. Meanwhile, the deformation of the surrounding rock of the roadway increases with the increase of repeated mining times, especially for the horizontal displacement of the roadway on the coal pillar side. Moreover, the cracks' evolution of surrounding rock in the roadway can be observed as asymmetric characteristics. Finally, the stability control technology of "asymmetric anchor net cable + I-steel" is proposed to prevent potential mining disasters, and the feasibility of this support scheme is verified by numerical simulation and field practices. It can meet the requirement of safe mining and provide guidelines to effectively solve the failure of a roadway in close-distance coal seam mining. [ABSTRACT FROM AUTHOR]

The existing researches on face end roof falling are mostly focused on single coal seam, while the stability of the face end roof under short-distance coal seam group mining is different from that of a single coal seam. In the process of short-distance coal seam group mining, due to the small spacing between adjacent coal seams, the roof structure and stress environment of the lower coal seam mining area will change after the upper coal seam is mined. This easily lead to catastrophic problems such as face end roof falling, coal wall spalling and roof pressure frame. Taking the 17101 working face of a mine that has experienced 3 large-scale face end roof falling as the engineering background, based on the stability relationship between the coal wall, hydraulic support and the face end roof, the ‘face end roof-coal wall-support’ model is established under repeated mining of short-distance seam group. This paper introduces the main influencing factors of face end roof falling under repeated mining of short-distance seam group are the strength of the roof and surrounding rock, support working resistance, advancing speed and face end distance. The UDEC simulation software is used to simulate the influence of different influencing factors on the face end roof falling under repeated mining, and conclusions are summarized as follows. ① The face end roof and coal body are damaged during repeated mining, the strength of the face end roof and coal wall is reduced, and the face end roof at the front end of the support is prone to fall, causing disasters such as roof falling and coal wall spalling. As the strength of the roof and surrounding rock increases, the stability of the roof is better. ② The low working resistance of the hydraulic support is one of the important reasons for the face end roof falling of the working face. As the working resistance of hydraulic support increases, the stability of the face end roof is better. ③ The advancing speed of the working face has a significant effect on the stability of the face end roof. The slower the advancing speed of the working face is, the more serious is the roof subsidence phenomenon of the at the stope. ④ The tip-to-face distance is an important influencing factor of the face end roof falling. The face end roof falling is linearly related to the tip-to-face distance. The falling height increases with the increase of the tip-to-face distance. The smaller the tip-to-face distance is, the more stable is the face end roof. However, too short tip-to-face distance will also affect the normal mining of the working face. The tip-to-face distance should be determined according to the actual situation. In the context of the four main influencing factors, the prevention and control measures for face end roof falling under repeated mining are proposed. The measures include increasing the strength of the roof and surrounding rock, improving the working resistance of hydraulic support, controlling the advancing speed reasonably and reducing the tip-to-face distance. The above prevention and control measures can provide an effective solution for the face end roof falling under repeated mining of short-distance seam group.

In view of the influence of underground repeated mining on the bedding steep rock with weak interlayer, this paper uses the discrete element numerical software UDEC to simulate the upward mining in the gently inclined short-distance coal seam group under the steep rock, and analyzes the deformation and failure characteristics of the steep rock. The results show that the variation trend of subsidence curve, horizontal displacement curve, vertical stress curve and shear stress curve of basement rock is basically the same in the process of repeated mining when different coal seams are mined; the unloading crack of the steep rock slope shoulder will penetrate the weak interlayer downward and form a dangerous rock body at the slope shoulder after the mining of the upper coal seam. After the middle and lower coal seams are mined, the mining fissures will continue to extend from top to bottom into the slope body and near the basement rock layer, resulting in steep rock mountain fracture; with the increase of repeated mining times, the deformation and failure degree of steep rock will increase sharply.

The extraction of underground minerals in hilly regions is highly susceptible to landslides, which requires the application of InSAR techniques to monitor the surface displacement. However, repeated mining for multiple coal seams can cause a large displacement beyond the detectable gradient of the traditional InSAR technique, making it difficult to explore the relationship between landslides and subsurface excavations in both temporal and spatial domains. In this study, the Tengqing landslide in Shuicheng, Guizhou, China, was chosen as the study area. Firstly, the large-gradient surface displacement in the line of sight was obtained by the fusion of SAR offset tracking and interferometric phase. Subsequently, a multi-segment logistic model was proposed to simulate the temporal effect induced by repeated mining activities. Next, a simplified probability integral method (SPIM) was utilized to invert the geometry of the mining tunnel and separate the displacement of the mining subsidence and landslide. Finally, the subsurface mining parameters and in situ investigation were carried out to assess the impact of mining and precipitation on the kinematic process of Tengqing landslides. Results showed that the repeated mining activities in Tengqing can not only cause land subsidence and rock avalanches at the top of the mountain, but also accelerate the landslide displacement. The technical approach presented in this study can provide new insights for monitoring and modeling the effects of repeated mining-induced landslides in mountainous areas.

Upper protective seam mining has been widely applied in China, but the theory of long-distance multiple upper protective seam mining is not yet perfect. In order to investigate the overburden stress evolution law of repetitive mining of long-distance coal seam groups, an experimental study was conducted to simulate similar materials under repeated mining conditions in the long-distance double upper protective layer in the background of Pingmei Group 8th coal mine. By analyzing the roof-collapse structure and the stress evolution law of different layers of the floor during the superposition mining, the pressure-relief range of the protective layer after the mining of the double upper protective layer was determined. The study results showed that: the pressure relief of the protective layer in the long-distance upper protective layer mining was a dynamic process. After the mining of Group D coal seam, the maximum impact depth of the bottom plate could reach 182 m, and the pressure-relief angle of the upper side of Group E coal seam was 65°, and the pressure-relief angle of the lower side was 75°. The distance behind the vertical projection of the working face of Group D was 42 m. The overlapping back mining would affect the stress distribution of Group F coal seam. The pressure-relief angle of the upper side of Group F coal seam was 88°, and the pressure-relief angle of the lower side was greater than 78°. The distance behind the vertical projection of the working face of Group E was less than 61 m. The superposition and staggered mining of double protective layers could expand the protective layer. Through the verification of the measurement of gas parameters on site, it can be seen from the results that it has a certain protection effect. The research results can enrich the theory of long-distance multiple upper protective layer mining, and provide theoretical guidance for long-distance Coal Seam Group Mining in Pingmei coal-mine area. [ABSTRACT FROM AUTHOR]

Aiming at the problem that it is difficult to achieve accurate laying of model and precise excavation of roadways in special surrounding rock structure roadway according to conventional physical similarity simulation, which reduces the reliability of experimental results. An accurate laying of model and precise excavation of roadway method, named "labeling positioning and drawing line, presetting roadway model" (LPDLPRM), was proposed. The physical similarity simulation of deformation and failure characteristics of surrounding rock of coal-rock rise, under the influence of repeated mining in close distance coal seams, was carried out based on the method and infrared detection. The results show that the coal-rock rise in close distance coal seams was affected by repeated mining disturbances, and the surrounding rock of coal-rock rise was characterized by obvious asymmetric deformation, specific for the stress and strain near the coal pillar were higher than that of other parts, and cracks near the coal pillar were denser than other parts; when the coal seam is mined in which the coal-rock rise is located, the stress concentration of the surrounding rock near the rise was weakened by mining pressure relief in the upper coal seam; the stress concentration of the surrounding rock near the rise increases when the coal and the lower coal seam are mined, and the stress on the right side (coal pillar side) near the coal-rock rise was the most concentrated. Therefore, it is important to take measures to strengthen support near the coal pillar and to control asymmetric deformation when the coal-rock rise is influenced by repeated mining. [ABSTRACT FROM AUTHOR]

This study explores the influence of a repeated mining process on an upper coal pillar in a close coal seam group. The pillar's breaking and instability processes are emphasized, and the influence of fracture development on the oxidation and spontaneous combustion of coal pillars is revealed. A numerical simulation is used to simulate the dynamic evolution characteristics of stress, displacement of the upper coal pillar, and the numerical results elucidate that the mining of the lower adjacent coal seam is a pressure relief process for the upper coal pillar. The theoretical length of the fracture along the strike of the upper coal pillar is also obtained for the upper coal pillar. [ABSTRACT FROM AUTHOR]

In order to reveal the law of repeated mining of shallow coal seams under the peak cluster landform, the overlying strata fracture migration and peak cluster movement deformation laws induced by the mining face, Taking Guizhou typical peak cluster landform as the engineering background, with the help of similar simulation, UDEC numerical simulation and theoretical analysis, the overlying rock movement and surface movement and evolution law of working face are studied. The results showed that with the mining of coal seams, cracks appeared on the left and right sides of the mountain slope. As the working face advances, the cracks gradually shrink. Repeated mining has less damage to the rock mass structure on the left and right sides of the peak, but more serious damage to the internal rock mass structure. After the coal seam is mined, the UDEC model is used to analyze the development of the vertical stress and vertical displacement of the peak body. It is found that the vertical stress is mainly concentrated in the central part of the peak body, and the area with larger vertical displacement value is also the central part of the peak body. At the same time, after the coal seam mining is over, both the similar simulation model and the UDEC model have a certain degree of subsidence, and the area with the largest subsidence is the peak part of the peak cluster.

In order to study the deformation and failure process of rock slope induced by repeated mining under high and steep mountain body, a numerical calculation model of coal seam mining under high and steep terrain was established by using the method of discrete element numerical simulation. The progressive failure process of rock slope deformation induced by repeated mining was analyzed from horizontal and longitudinal displacement nephograms. It is concluded that there are three stages of slope collapse induced by repeated mining under high and steep mountains, namely, initial deformation stage of coal seam, uneven subsidence and cracking stage of slope caused by repeated mining, and instability and failure stage of slope. At the same time, it is concluded that the uneven deformation of the slope results in different forms of damage at different positions of the whole slope. The final instability forms of the slope surface are landslide and collapse, which are mainly determined by the original topographic gradient and stratigraphic lithology of the slope.

Underground coal mining in high relief terrain often causes problems such as ground collapse and mountain sliding. This produces disasters such as mountain collapse and landslides, which seriously endangers the ecological environment and human settlements in the coal mining area and causes a series of public security events. To improve and solve the related problems, it is necessary to master the impact law of underground mining on ground surface. Therefore, taking the working face 1310 of Jincheng coal industry as an example, this paper adopts a new and improved RTK measurement technology for field measurement and analyzes the surface disturbance law by using field data. The results show that under the action of repeated mining in high relief terrain areas, there is a sudden subsidence in the flat bottom area of the surface moving basin, the strike influence scope is small, the dip influence scope in the downhill direction is larger, the dip horizontal movement curve loses its antisymmetry, and the dip subsidence curve loses its left-right symmetry. Through the analysis, it is found that the high reverse slope weakens the original disturbance and reduces the influence range of surface mining. On the contrary, repeated mining enhances the original disturbance, expands the influence range of surface mining, and flattens the movement deformation curve. Research results can provide a technical reference for the formulation and implementation of environmental remediation and treatment technologies in mining areas under similar conditions. [ABSTRACT FROM AUTHOR]

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The roof usually collapses when exposed to the repeated underground mining; this will potentially lead to the instability of coal wall and even cause serious mining disasters. Based on this specific mining hazard, this work investigates the roof leak characteristics and corresponding control technology based on a coal mine in Guizhou province, China. The theoretical analysis, numerical simulation, and physical testing were carried out. By establishing a physical model, the roof structure and roof "pressure" law under repeated mining are studied, and the instable conditions of face-end roof are analyzed, so as to determine the influencing factors of face-end roof instability. By analyzing different roof structures, the working resistance of hydraulic support with stable face-end roof under repeated mining is determined. The leakage of face-end roof under different support working resistance, tip-to-face distance, and surrounding rock strength is simulated, and different methods are proposed to control face-end roof leaks. The results show that in the process of 17# coal seam mining, the roof collapse is serious, the roof pressure is frequent, and the broken roof instability would form the "granular arch" structure. Through the analysis of the roof structure, the tip-to-face distance, the strength of granular arch structure, and the working resistance of the hydraulic support affect the stability of the face-end roof. Using UDEC simulation software, it is obtained that the higher the strength of roof and surrounding rock, the smaller the tip-to-face distance, the more stable the face-end roof is. The prevention measures such as strengthening the broken face-end roof under repeated mining, improving the support strength of the working face hydraulic support and controlling the reasonable tip-to-face distance are put forward. Through field application, the face-end roof leaks have been effectively controlled, which has played a good reference for the prevention and control of face-end roof leaks under repeated mining. [ABSTRACT FROM AUTHOR]

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In this study, taking the Sheng’an coal mine as an engineering background, the failure characteristics of the surrounding rock of a roadway under repeated mining in a close-distance coal seam is comprehensively illustrated through field measurements (e.g., drilling imaging), theory analysis and numerical simulation (finite difference method (FDM)). The results show that although the return airway 10905 remains intact, the apparent failure of the roadway’s roof and the coal pillar can be observed. In addition, the expression of floor failure depth caused by upper coal seam mining is obtained through elastic-plastic theory. Meanwhile, the deformation of the surrounding rock of the roadway increases with the increase of repeated mining times, especially for the horizontal displacement of the roadway on the coal pillar side. Moreover, the cracks’ evolution of surrounding rock in the roadway can be observed as asymmetric characteristics. Finally, the stability control technology of “asymmetric anchor net cable + I-steel” is proposed to prevent potential mining disasters, and the feasibility of this support scheme is verified by numerical simulation and field practices. It can meet the requirement of safe mining and provide guidelines to effectively solve the failure of a roadway in close-distance coal seam mining.

Upper protective seam mining has been widely applied in China, but the theory of long-distance multiple upper protective seam mining is not yet perfect. In order to investigate the overburden stress evolution law of repetitive mining of long-distance coal seam groups, an experimental study was conducted to simulate similar materials under repeated mining conditions in the long-distance double upper protective layer in the background of Pingmei Group 8th coal mine. By analyzing the roof-collapse structure and the stress evolution law of different layers of the floor during the superposition mining, the pressure-relief range of the protective layer after the mining of the double upper protective layer was determined. The study results showed that: the pressure relief of the protective layer in the long-distance upper protective layer mining was a dynamic process. After the mining of Group D coal seam, the maximum impact depth of the bottom plate could reach 182 m, and the pressure-relief angle of the upper side of Group E coal seam was 65°, and the pressure-relief angle of the lower side was 75°. The distance behind the vertical projection of the working face of Group D was 42 m. The overlapping back mining would affect the stress distribution of Group F coal seam. The pressure-relief angle of the upper side of Group F coal seam was 88°, and the pressure-relief angle of the lower side was greater than 78°. The distance behind the vertical projection of the working face of Group E was less than 61 m. The superposition and staggered mining of double protective layers could expand the protective layer. Through the verification of the measurement of gas parameters on site, it can be seen from the results that it has a certain protection effect. The research results can enrich the theory of long-distance multiple upper protective layer mining, and provide theoretical guidance for long-distance Coal Seam Group Mining in Pingmei coal-mine area.

Aiming at the problem that it is difficult to achieve accurate laying of model and precise excavation of roadways in special surrounding rock structure roadway according to conventional physical similarity simulation, which reduces the reliability of experimental results. An accurate laying of model and precise excavation of roadway method, named “labeling positioning and drawing line, presetting roadway model” (LPDLPRM), was proposed. The physical similarity simulation of deformation and failure characteristics of surrounding rock of coal-rock rise, under the influence of repeated mining in close distance coal seams, was carried out based on the method and infrared detection. The results show that the coal-rock rise in close distance coal seams was affected by repeated mining disturbances, and the surrounding rock of coal-rock rise was characterized by obvious asymmetric deformation, specific for the stress and strain near the coal pillar were higher than that of other parts, and cracks near the coal pillar were denser than other parts; when the coal seam is mined in which the coal-rock rise is located, the stress concentration of the surrounding rock near the rise was weakened by mining pressure relief in the upper coal seam; the stress concentration of the surrounding rock near the rise increases when the coal and the lower coal seam are mined, and the stress on the right side (coal pillar side) near the coal-rock rise was the most concentrated. Therefore, it is important to take measures to strengthen support near the coal pillar and to control asymmetric deformation when the coal-rock rise is influenced by repeated mining.

Abutment pressure distribution is the decisive factor affecting the stability of coal pillars, which has an important significance in realizing safe and efficient mining. In recent years, many working faces adopting double-roadway arrangement have appeared in the western mining areas in China, which makes coal pillar abutment pressure complex under repeated mining. In order to reveal the distribution pattern of coal pillar abutment pressure under repeated mining, the evolution of a stress field of surrounding rock was analyzed by a numerical method based on the actual situation of a double-roadway arranged mining roadway in Longde Coal Mine, and the distribution characteristics of coal pillar abutment pressure were revealed. The reasonable width of the section coal pillar was given further. The research results showed that (1) the surrounding rock stress is transferred to the working face coal body and section coal pillar between roadways under the influence of mining advance the working face, while the stress is transferred to the section coal pillar between roadways under the mining influence behind the working face. The presence of goaf in the upper segment makes the stress distribution of surrounding rock more complicated. (2) The distribution of coal pillar abutment pressure shows a staged change. The influence of mining advance the working face on the peak abutment pressure is notable, which highlights the significance of temporary reinforcement support in the advanced section of the working face. (3) After the mining of the upper working face is finished, the yield damage width of the coal pillar on the side of the goaf was about 4.0 m. During the second mining period, the yield damage width of the coal pillar was totally about 7.0 m. (4) Taking into account factors such as safe production and economic rationality, it was finally determined that the reasonable section coal pillar width is 12.5 m for this mine. [ABSTRACT FROM AUTHOR]

Aimed at the problem that the mining roadway is seriously affected by the repeated mining under the up-down successive cooperative mining mode, this paper takes Qiuci Coal Mine as the research background. Based on analysis of mining influence stage of A502 haulage roadway, the stress evolution and deformation law of surrounding rock of roadway are analyzed through similar simulation, the principle that the ratio of main stress affects the stability of roadway under the influence of repeated mining is obtained based on the plastic failure theory of surrounding rock of roadway, the roadway stability control method by periods and stages is put forward based on the principle of roadway stability control, and the numerical simulation scheme is designed to determine the reasonable coal pillar width and support mode of roadway. The results show that the coal pillar with the width of 8 m can reduce roof stress of roadway by 38.4% and 64.3% during the tunneling period and period after formation of roadway, and the principal stress ratio is reduced to 2; the stability of surrounding rock during and after excavation can be guaranteed by adopting φ20 × 2400 bolt and advanced hydraulic support. Through field measurement, the maximum deformation of A502 haulage roadway is only 60 mm, and the cumulative output of coal during the period of mining influence is 1,833,700 tons, thus the safe mining of coal mine is realized, and enormous economic benefits are generated. [ABSTRACT FROM AUTHOR]

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In view of the fact that the roof breaking characteristics and overlying strata migration law of close coal seams under repeated mining conditions are not clear, which seriously affects the safety advancing of working face. A large-cutting-height panel in Panjiang of Guizhou Province is used as an example, the roof breaking characteristics and overlying strata migration law under repeated mining are studied using theoretical analysis, numerical simulation and field observation. The research results show that although the roof weighting is not strong in close coal seams mining, there are three kinds of roof weighting phenomena frequently in different overlying strata stratum and the coal face is easy to failure due to the repeated mining; the theoretical formula (5) for determining the support resistance is obtained by establishing the mechanical model and the calculated support resistance of 7200 kN can be determined; compared with the single coal seam mining, the initial weighting interval is smaller and the periodic weighting interval will not change much under the repeated mining; after theoretical analysis and numerical simulation, it can be known that it is necessary to strengthen the control of coal face stability and prevent safety accidents caused by coal face rib spalling in close seams group under repeated mining. The research results provide a theoretical basis for coal seam mining under similar conditions. [ABSTRACT FROM AUTHOR]

There is plentiful mineral under the water, and due to the limit of the natural environment, many difficult problems will appear in underwater mining. The paper aims at the problems in repeated mining of No.33lower01 working face, which is the close thick coal seam and under large surface water. Through field measurement, the thickness of bedrock and clay layer which above the working face was obtained. Then, using various methods to analyze the laws of overlying strata movement, and the height of the caving zone and water flowing fractured zone was measured, the heights of the two is 35.2 m and 107.6 m, respectively. At the same time, discussing the impact of working face mining on SF2 reverse fault, there is no obvious dislocation between the upper and lower plates of the fault, and the watertightness was well. Comprehensive analysis shows that repeated mining of No.33lower01 working face under the large surface water is possible. [ABSTRACT FROM AUTHOR]

The western region of China is rich in mineral resources. The vigorous development of mineral resources has exacerbated the environmental and safety problems in the region. One of the important links to solve this problem is to control the development laws and distribution characteristics of the overburdened cracks in the mining of this area. In this paper, the Xiashijie coal mine 3-2 coal seam and 4-2 coal seam are examples of repeated mining, and are examined as the background, through theoretical analysis to optimize the size of the coal pillars in the lower section, using the 3DEC numerical simulation experiment method and the rise of the cracks in the short-distance coal seam. Repeated mining monitoring and analysis of the development law are used to ascertain distribution characteristics of overburdened cracks caused by the repeated mining process of the working face. The results show that: (1) By establishing a mechanical model of the overlying strata structure under short-distance coal seam group mining, and carrying out the force analysis of the double section coal pillar under repeated mining, the reasonable size of a lower section coal pillar was determined to be 70 m. (2) As the development height of a fracture progresses with the working face, its expansion rate undergoes four obvious changes: fluctuations within a certain range, the expansion rate reaches the peak after the rock formation is concentrated and broken, the cyclical change gradually decreases, and the expansion rate is zero after complete mining. (3) The fracture zone height of 222 and 224 face under repeated mining in the 4-2 coal seam was 19.56–22.31 times and 22.38–24.54 times larger, respectively, and the post-mining fracture extension of the face with larger width and deeper burial under repeated mining was higher than that of the adjacent face. This study provides scientific guidance for the rational division of coal pillars and the solution of the problem of water conservation mining under repeated mining in the adjacent face of a short-distance coal seam.

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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.)