205 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
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