Research Information System
Geotechnical and Geological Engineering, vol.44, no.1, 2026 (ESCI, Scopus)
This study presents an integrated approach to assess challenges related to maintaining rock mass stability during the design and extraction of a crown pillar (CP) in a Tanzanian mine transitioning from open pit to underground operations. The analysis combines empirical and numerical methods. The scaled span method is used to estimate the required CP thickness, and the result is evaluated using FLAC3D simulations that model ore extraction and stress-induced deformations. The simulation reveals a potential failure caused by an underestimated CP thickness, with a discrepancy of 4 m. This underestimation leads to stress concentrations in the hanging wall and deformations recorded by on-site multipoint borehole extensometers (MPBXs). The resulting instability requires additional ground support and complicates extraction of high-grade ore within the CP. Further analysis highlights significant deformation around a weak zone intersecting the mine, worsened by concurrent ore removal near the pit slope. To account for structural discontinuities, a 3DEC model is used, and its deformation results closely match field observations following CP extraction. The findings confirm the effectiveness of the scaled span method when applied with updated empirical guidelines and highlight the value of combining continuum and discontinuum models. This approach offers a practical framework for assessing CP stability and supporting design decisions in similar geotechnical environments.