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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 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.)

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]

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]

• The influence of stope roof structure movement on direct roof is expounded. • The mechanism of roof fall under repeated mining was studied. • Different instability conditions of cantilever beam structure are obtained. • Reasonable support working resistance is an important reason to maintain the stability of cantilever beam structure. In view of the problem that the broken stope roof under repeated mining is prone to end face roof leaks, this paper's research background is the close distance coal seams mining of a mine in Guizhou province, China. The impact of stope roof structure movement on the immediate roof was elucidated from the analysis of stope roof structure movement under repeated mining employing a combination method of field measurement, physical simulation, theoretical analysis, and numerical simulation. As a result, the mechanism of end face roof leaks under repeated mining is investigated, and the corresponding control measures are proposed. The results show that the cantilever beam structure instability of the stope roof under repeated mining is an important reason for the end face roof leaks and support crushing accident. The fracture forms of cantilever beam structure are divided into direct fracture and hinged structure fracture. The direct fracture is affected by the support working resistance. The cantilever beam hinged structure is divided into sliding and rotational instability. The periodic weighting interval is small and the fracture degree is large under repeated mining, which leads to the sliding instability. It is concluded that reasonable support working resistance is the important reason to keep the cantilever beam structure stability. Therefore, the stope roof leakage control countermeasures under repeated mining are proposed. The UDEC numerical simulation is used to verify the above analysis again, so as to provide the basis for the end face roof leaks prevention and control under repeated mining. [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. [ABSTRACT FROM AUTHOR]