Development Characteristics of Mining-Induced Fractures and Surface Air Leakage Dynamics in Shallow Coalfields

Surface fissures induced by shallow coal seam mining create interconnected pathways for ambient air leakage, significantly aggravating coal spontaneous combustion (CSC) risks in goafs. However, the spatiotemporal evolution of these fractures and the quantitative dynamics of air leakage under repeated mining conditions remain poorly understood. This study investigates the evolutionary laws of mining-induced cracks and air leakage behaviors through laboratory physical similarity simulations and field tracer gas testing. The results demonstrate that during repeated extraction, vertical fractures in the goaf boundaries undergo an expansion-to-stabilization process with significantly increased widths, whereas fractures in the central region experience a process from expansion to closure and stabilization. Crucially, the fracturing of the inter-seam key stratum marks a vital milestone where the upper and lower goafs merge into a complex goaf, precipitating a sudden, sharp surge in air leakage volume. Field observations categorize surface cracks into graben type, collapse type, and tensile type. Graben-type and collapse-type cracks act as the principal pathways for surface air infiltration, collectively forming a rectangular distribution network across the goaf. These findings provide a critical theoretical framework and practical guidance for predicting and controlling surface air leakage disasters in close-distance shallow seam mining.