Pre-existing concealed goafs underneath open-pit slopes (PCO-goafs) pose a serious threat to the stability of open-pit slopes (OP-slopes), which is a common problem worldwide. In this paper, the variable weight-target approaching method, equilibrium beam theory, Pratt’s arch theory, and numerical simulation are used to analyze the management solutions and stability of five PCO-goaf groups in the Nannihu molybdenum mine located in Luoyang City, Henan Province, China. The five PCO-goaf groups, numbered 1#, 2#, 3#, 4#, and 5#, are divided into four hazard classes, ranging from extremely poor to good stability. The stability of 1#, 2#, and 4# is poor and must be managed by filling, and the design strength of backfill is 1.2 MPa; caving is used to treat 3# and 5#, and the safe thickness of the overlying roof is calculated to be 10.5–41 m. After treatment, the safety coefficient of the slope is greater than 1.2, indicating that the slope is stable. This study provides insight and guidance for the safe operation of open-pit mines threatened by the existence of PCO-goafs.

In underground mining, the stage pillar (SP) is essential for maintaining stope stability, acting as a load-bearing structure between stages. Determining the minimum safe SP thickness is critical to balancing mineral recovery and operational safety. This study investigates the design and mechanical stability of SP under substantial backfill loads, using a representative Iron ore mine as a case study. Based on the geometry of the overlying backfill and core sampling data, extreme loading conditions were identified, with the stope measuring 85 m in height, 72 m in length, and 18 m in width. A mathematical model incorporating the pressure arch effect and triangular pillar geometry was developed to estimate the backfill-induced load. Safety factors for various SP thicknesses were calculated using thin plate and elastic beam theories. Considering sequential excavation of the first- and second-step stopes, the minimum safe SP thickness was determined to be 6.0 m. This design was evaluated using FLAC^3D numerical simulations. The results reveal that during the first step, stress concentrations occurred mainly at the pillar base, with a maximum displacement of approximately 2.0 cm and peak tensile stress of 0.36 MPa—both within acceptable limits. These findings support improved pillar design for safe, efficient ore recovery in underground metal mining.