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

Coal mining, as a typical human-induced engineering disturbance, alters the original stress field of overlying strata, triggering rock collapse and forming mining-induced pores and stratum pores. This not only exacerbates the risk of mine water hazards and gas outbursts but also threatens the safety of ground-based buildings and structures. However, the development and utilisation of underground space in abandoned mine areas as a potential resource provides an innovative approach to their comprehensive management. This study takes the 1905 working face of the Longfeng Coal Mine as its research object, employing a combination of theoretical analysis, UDEC numerical simulation, and field monitoring to systematically investigate the collapse patterns of overlying strata and the development characteristics of pores in abandoned areas. By establishing a two-dimensional numerical model considering the physical and mechanical parameters of coal and rock layers, the study simulates the advancement process of the working face in stages (total excavation length of 300 m, divided into 30 steps), analyses the deformation and failure of overlying strata, displacement field, and stress field evolution characteristics, and combines the 'O'-ring theory to study the distribution of pore volume in the collapse zone. The research findings indicate: As the working face advances, the overburden forms a 'vertical three-zone' structure in the vertical direction, consisting of a collapse zone, a fracture zone, and a bending subsidence zone. The collapse zone and fracture zone develop upward in a trapezoidal shape, and the displacement of the overburden decreases with increasing rock layer height, forming a 'dynamic arch effect,' reflecting the coupled mechanism of stress diffusion due to mining and the self-supporting characteristics of fractured rock. The constructed 'dynamic stress arch evolution model' reveals that the vertical stress of the overlying rock is distributed in a symmetrical arch shape. During the initial stage of mining, a "V"-shaped stress relief zone is formed, which gradually evolves into a 'W'-shaped zone as the advance distance increases, ultimately restoring the central stress to the original rock level. Based on the 'O'-shaped ring theory and MATLAB modelling, it was found that the porosity of the collapse zone exhibits a non-uniform three-dimensional distribution with 'high edges and low centre,' with the porosity of the edge free accumulation zone reaching 0.4 and the central compacted zone decreasing to 0.2. The dynamic evolution model incorporating the fragmentation coefficient Kp quantifies the decay of the porosity of the collapsed rock mass. The findings of this study provide theoretical support for engineering practices such as grouting reinforcement of collapsed rock bodies, and lay the foundation for the development of intelligent and precise management of abandoned mine areas.