Crown Pillar Thickness Optimization with Deformation Symmetry and Simulation Validation in Open Pit to Underground Mining Transition: A Kumusayi Li-Nb-Ta Case Study

Determining the safe thickness of a boundary crown pillar is critical during the transition from open-pit to underground mining, as it directly affects both mining safety and resource recovery. Crown pillar instability is commonly associated with asymmetric stress redistribution, nonuniform deformation, and progressive plastic failure. In this study, the Kumusayi Li-Nb-Ta mine in Xinjiang, China, was selected as an engineering case to optimize the boundary crown pillar thickness and evaluate its deformation characteristics. Four theoretical methods, namely the load transfer intersection method, span-to-thickness ratio method, simplified structural beam method, and Rubeneeite formula method, were first used to determine the feasible thickness range. The calculated thicknesses were 19.99, 14.00, 29.81, and 10.41 m, respectively, yielding an engineering design interval of 14.00–29.81 m. Based on this interval, four thickness schemes of 15, 20, 25, and 30 m were evaluated using FLAC3D simulations in terms of stress redistribution, displacement evolution, surface movement, plastic-zone development, and deformation symmetry. The results show that the 15 m pillar exhibits pronounced stress concentration, asymmetric deformation, and through-going plastic failure, indicating insufficient stability. Although the 20 m pillar improves the load-bearing capacity, a potential connected failure path remains. At 25 m, the high-stress zone becomes localized, the plastic zone no longer penetrates the pillar, and the maximum vertical displacement decreases by approximately 27.0% compared with the 15 m scheme. Increasing the thickness to 30 m provides limited additional improvement, with less than a 2% reduction in maximum vertical displacement compared with the 25 m scheme. Physical similarity model tests further confirm that a 20.8 cm model pillar, corresponding to a 25 m prototype pillar, effectively prevents through-going cracking and overall slope sliding. Therefore, a 25 m boundary crown pillar is recommended for the Kumusayi mine.