Block Caving Characteristics and Spatiotemporal Evolution of a Hard-Rock Orebody During the Transition from Open-Pit to Underground Mining

Block caving is a promising method for large-scale underground extraction of low-grade orebodies, but its application requires reliable prediction of ore rock caving and associated ground response. This study develops a three-dimensional cavability assessment framework that integrates rock mass quality evaluation, analysis of surrounding rock failure mechanisms, Monte Carlo simulation of joint networks, and geostatistical modeling. The framework was applied to a gold mine, where fragmentation characteristics, caving body evolution, and surface ground movement over the mining life cycle were evaluated using fragmentation prediction and numerical simulation. The results show marked lithology-dependent differences in mechanical behavior and failure mode: limestone and biotite schist have relatively high strength, whereas andalusite schist is comparatively weak. Ore rock caving is jointly governed by intact rock strength, structural discontinuities, and post-peak failure behavior. Three-dimensional mining rock mass rating (MRMR) indicates that the orebody is mainly Class III, corresponding to moderate cavability, with the 1216 m, 1066 m, and 956 m levels identified as favorable initial undercut horizons. Although the deposit satisfies the basic conditions for block caving, continuous caving requires an initial undercut area of about 15,000 m2, and fragmentation varies significantly with elevation. Numerical results indicate that the caving body evolves from a cone to a frustum-like geometry and reaches the ground surface in the ninth year. The predicted collapse and rock-movement areas are approximately 0.42 km2 and 0.57 km2, respectively, highlighting nonlinear, stage-wise, and spatially heterogeneous ground deformation.