Abstract
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.
Highlights
Mining operations are gradually deepening due to the reduction in more accessible coal resources.At the same time, more and more attention has been paid to the laws of fracture development in the process of mining under various complex conditions
For the deformation of overlying rock caused by underground mining, many domestic and foreign scholars have used simulation experiments to study the laws of overlying rock migration [1,2,3,4,5], which effectively guide the safety of mine operations in different geological environments
Iwanec et al used numerical methods to accurately predict the subsidence above single-layer and multi-layer long-wall panels, and combined the constitutive relationship to properly describe the mechanical behavior of the coal-measure formation [7]
Summary
More and more attention has been paid to the laws of fracture development in the process of mining under various complex conditions. The law of overlying rock movement under mining conditions has Energies 2020, 13, 6064; doi:10.3390/en13226064 www.mdpi.com/journal/energies. For the deformation of overlying rock caused by underground mining, many domestic and foreign scholars have used simulation experiments to study the laws of overlying rock migration [1,2,3,4,5], which effectively guide the safety of mine operations in different geological environments. In order to optimize the gas pre-drainage scheme of dangerous outbursts from coal seams, Cheng et al studied the stress–fracture distribution and evolution characteristics of surrounding rock under the influence of double mining of the protective and protected layer [8]. Liu et al based on fractal theory, using the Brazilian splitting test and a field peep statistical test as research means, discussing the influence of rock mass fragmentation scale and uniformity on fractural characteristics [10]
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