Numerical simulation study on the cooperative movement of overburden and fracture healing mechanisms in shallow-buried coal seams

During the mining of shallow-buried coal seams, overburden movement leads to the formation of water-conducting pathways, resulting in surface subsidence and ground fissures, which in turn pose serious threats to shallow water systems and surface ecological environments. The development and stage-wise healing behavior of fractures in thick loose layers play a significant regulatory role in the transmission of mining-induced damage, yet these processes are challenging to quantitatively characterize due to their concealed nature. To address this, this study takes the 12,403 working face of the Ulanmulun Coal Mine in the Shendong Mining Area as its engineering background. A numerical simulation model for the mining process was established and validated through field measurements. By extracting key characteristic data such as the area of the plastic zone and fracture density, the evolution of mining-induced damage in both the bedrock layer and the loose layer was quantitatively analyzed, revealing the fracture healing mechanism in the loose layer and the interactive response pattern among the bedrock layer, loose layer, and surface. The research demonstrates that: (1) The area of the plastic failure zone in the overburden increases nonlinearly with working face advance, which can be divided into three stages: slow growth, rapid expansion, and stable linear development. The overall failure mode is dominated by shear failure, while tensile and tensile-shear failures are primarily concentrated at lithological interfaces and the soil–rock contact zone. (2) There are significant differences in fracture development behavior between the bedrock layer and the loose layer. Under the combined influence of fracture propagation and compaction-induced healing, the fracture density in the bedrock layer first increases, then decreases, and eventually stabilizes. In contrast, the fracture density in the loose layer exhibits logarithmic growth accompanied by periodic fluctuations. (3) Following the collapse of the key stratum, a temporary load-bearing fracture arch structure forms within the loose layer, delaying surface subsidence. As mining-induced damage accumulates and goaf compaction progresses, the fracture arch gradually destabilizes and closes, leading to accelerated surface subsidence and the development of tensile ground fissures at the edges of the subsidence basin. These findings deepen the understanding of cooperative deformation and fracture healing mechanisms in the overburden of shallow-buried coal seams, providing a theoretical basis for predicting surface damage and protecting shallow water resources.