1463 Figure 12 - Comparison of safety factors for all pillar size (stage II) From an extraction-efficiency standpoint (Figure 13), the 8×8 m configuration achieves the highest recovery (93%), followed by 5×6 m (90%) and 4×4 m (87%). However, when combined with the stability and deformation response, the 8×8 m option carries a higher operational risk in the final (liquidation) phase, while the 4×4 m option is limited by SF < 1 and associated instability. In contrast, 5×6 m pillars maintain SF ≈ 1.1 while still providing high recovery, and their deformation behaviour is more predictable—supporting this option as the optimal stability–recovery compromise for the analysed deep mining conditions. Figure 13 - Comparison of average value of safety factor and degree of extraction for three size of pillars 4. DISCUSSION Pillar design and optimisation in room-and-pillar mining is often based on empirical pillar strength formulations derived from case histories and back-analyses, where stability is assessed by comparing estimated pillar strength to average pillar stress (Salamon & Munro, 1967). However, empirical methods have limited ability to represent stage-dependent stress redistribution, 3D structural effects, and deformation modes that can control behaviour under deep, high-stress conditions. Therefore, numerical modelling is increasingly used to complement empirical design, particularly to interpret evolving mechanisms and stiffness changes during advance and liquidation (Potvin & Hadjigeorgiou, 2010). A key contribution of this study is the calibration of the FEM model using spatial underground convergence measurements, which improves realism relative to workflows relying only on laboratory intactrock parameters. This follows the established practice of updating predictions using field monitoring to reduce uncertainty, and the calibrated model reproduces excavation-level deformation with good agreement. The adopted approach is consistent with commonly used rock-mass parameterisation concepts that link intact-rock test results to rock-mass behaviour (Hoek & Brown, 1997).
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