Mining-Sequence Effects on Slope Deformation and Stress Redistribution During Cemented Backfill Mining of Multiple Extra-Thick Coal Seams Beneath an Open-Pit Highwall

Cemented backfill mining has become an effective approach for recovering coal resources beneath open-pit highwalls while maintaining slope stability. However, the repeated extraction of multiple extra-thick coal seams induces complex overburden responses, and the effects of mining sequence on slope deformation and stress evolution remain insufficiently understood. To address this issue, two physical similarity models representing upward and downward mining sequences were established. A non-contact digital image correlation (DIC) system, together with an embedded stress-monitoring system, was employed to monitor the evolution of displacement and stress fields within the overburden. The results show that the mining sequence governs slope deformation by controlling the propagation of mining-induced disturbances and the subsequent redistribution of stress within the interburden strata. Under the upward mining sequence, the disturbance generated by extraction of the lower seam propagated toward the slope and was further intensified by subsequent extraction of the upper seam, resulting in a maximum vertical displacement of 2.22 m and pronounced stress concentration in the middle and lower parts of the slope. In contrast, the downward mining sequence was associated with earlier stress redistribution toward the cemented backfill, which contributed to mitigating the disturbance caused by subsequent extraction of the lower extra-thick seam. Consequently, the maximum vertical displacement decreased to 1.76 m, representing a 20.5% reduction compared with the upward mining sequence, while cumulative overburden subsidence and stress concentration were effectively mitigated. These findings demonstrate that mining sequence plays a critical role in the mechanical response of open-pit slopes and provide mechanistic insights into the optimization of mining strategies for cemented backfill mining beneath open-pit highwalls.