Study on Overburden Fracture Patterns and Support Load Mechanism in Shallow Coal Seam Mining Under Gully Terrain

Shallow-buried coal seams in western China are commonly overlain by deeply incised gully terrain, where mining is often accompanied by coal-wall spalling and abnormal increases in support resistance, which affect safe and efficient production. To investigate overburden failure during shallow-buried coal seam mining under gully terrain and to clarify the support–resistance mechanism, a typical working face was selected as the engineering background. Physical similarity simulation, 3DEC numerical simulation, and theoretical analysis were used to analyze overburden failure characteristics and the coupled evolution of the stress, displacement, and fracture fields. Mechanical models of key-stratum fracture and a support–resistance estimation model were established to reveal the influence of overburden-thickness variation on key-stratum fracture and support resistance. The results show that overburden failure in gully areas exhibits pronounced stage-dependent and asymmetric characteristics. In the similarity simulation, the initial fracture intervals of the key stratum in the downhill section were 32 m and 36 m, indicating an asymmetric fracture pattern with a shorter span on the left side and a longer span on the right side. In the uphill section, the periodic fracture interval of the key stratum decreased from 30 m to 24 m as the overburden thickness increased. During overburden failure in gully areas, the three fields exhibited a coupled relationship: stress concentration at the working face caused overburden failure and subsidence, which promoted fracture propagation, whereas stress redistribution in the goaf compacted the fractured overburden and promoted fracture closure. The overburden failure characteristics differed significantly between mining stages. During downhill mining, the key stratum behaved as a fixed-ended beam with a relatively large fracture interval, whereas during uphill mining, it formed a cantilever beam, and its fracture interval decreased with increasing overburden thickness. The loading mechanism of support resistance was shown to be jointly controlled by variations in gully overburden thickness and key-stratum fracture. During downhill mining, support loading increased gradually under the support of the fixed-ended beam key stratum. During uphill mining, support loading exhibited periodic abrupt increases under the combined effects of increasing overburden thickness and periodic fracture of the cantilever-beam key stratum. These findings provide a theoretical basis for strata pressure control at working faces in gully areas.