Research on the collapsing pattern of overburden rock and pore development characterization in the mining hollow area

Coal seam mining, as a typical man-made engineering disturbance, will change the original stress field of the overlying rock strata, resulting in the collapse of the rock strata and the formation of mining porosity and off-story porosity, which not only enhances the risk of mine disasters, but also affects the safety of ground structures, and the development and utilization of underground space in the air-sea zone as a potential resource provides a new way of thinking for the comprehensive management of air-sea zones. This paper takes 1905 working face of Longfeng Mine as the research object, uses UDEC numerical simulation software, combines theoretical analysis and field monitoring, and researches on the collapsing law of overburden rock and pore development characteristics of the mining hollow area. A two-dimensional numerical model considering the physical and mechanical parameters of coal and rock seams was established to simulate the advancing process of the working face through step-by-step excavation, analyze the deformation and damage of the overburden rock, the characteristics of the displacement field and the evolution of the stress field, and to study the distribution of porosity in the collapse zone based on the theory of the “O” circle. It is found that as the working face advances, the overburden rock forms a “vertical three-belt” structure vertically, and the collapse zone and fissure zone develops upward in a trapezoidal shape, and the displacement of overburden rock decreases with the increase of the height of the rock layer, forming a “dynamic arch effect”, which reflects the coupling of the stress diffusion of mining and the self-carrying characteristics of the crushed rock body. The coupling mechanism of the self-bearing characteristics of the rock mass. The established “dynamic stress arch evolution model” shows that the vertical stress of overburden rock is symmetrically distributed in the shape of an arch, forming a “V” shaped decompression zone at the early stage of mining, gradually evolving into a ‘W’ shaped zone with the increase of pushing distance, and finally the stress in the middle part of the rock is restored to a “W” shaped zone. “ shape at the beginning of mining, gradually evolve to ”W" shape with the increase of pushing distance, and finally the stress in the center recovered to the original rock level. Based on the “O” ring theory combined with MATLAB modeling, the porosity of the collapse zone shows a non-uniform three-dimensional distribution pattern of “high at the edge and low at the center”, the porosity of the edge free accumulation area reaches 0.4, and the compaction area in the middle decreases to 0.2, and the dynamic evolution model of the coefficient of expansion is introduced. The dynamic evolution model of Kp was introduced to quantify the porosity decay law of collapsed rock mass. The results of this research provide theoretical guidance for the practice of grouting and consolidation of collapsed rock bodies, and at the same time, lay the foundation for the development of the management of the hollow zone in the direction of intelligence and precision.