Evolution of overlying strata and disaster-causing mechanisms under inclined thin coal seam group mining: insights...

To clarify the overburden evolution and asymmetric instability mechanism during bottom-up repeated mining of an inclined thin coal seam group, a 3DEC numerical model was established based on geological conditions in Guizhou Province. The caving behavior, stress redistribution, displacement response, and simulated fracture-trace complexity of the overlying strata were analyzed during the first mining (M1), second mining (M2), and third mining (M3) stages. The results show that M1 mining mainly induces local subsidence, goaf compaction, and limited fracture development. With the successive extraction of M2 and M3, interlayer disturbance is progressively superimposed, the overburden failure range expands upward, and the stress field evolves into a zoned pattern characterized by stress relief in the goaf and stress concentration near the working face and goaf boundaries. The displacement field also shows a cumulative response, with the high-displacement zone migrating upward and the subsidence trough becoming deeper and wider. Under the 30° inclined structure, overburden deformation and failure exhibit clear dip-direction asymmetry: the high side is dominated by separation, sliding, and fracture development, whereas the low side mainly shows compaction and constrained deformation. The apparent fractal dimension of simulated fracture traces increases from 1.023 at M1 to 1.096 at M2 and 1.144 at M3, indicating enhanced relative complexity under identical discretization conditions. The overburden hazard-causing mechanism is therefore interpreted as a coupled process of stress redistribution, uncoordinated subsidence, high-side fracture development, and asymmetric instability of the load-bearing structure. These findings provide guidance for roof stability assessment and hazard control in inclined thin coal seam group mining.