Evaluation of Pillar Stability Across Different Extraction Depths Using FLAC3D Numerical Modeling: a case study on Kakatiya Khani No. 5 Incline located in the Godavari Valley Coalfield, Telangana

Coal pillar stability is one of the most critical factors governing the safety, productivity, and economic viability of underground coal mining operations employing the bord-and-pillar mining method. As mining activities progress to greater depths, the stress environment surrounding underground excavations becomes increasingly complex, resulting in higher pillar loading, stress concentration, and potential instability. The present study investigates the influence of extraction depth on coal pillar stability through three-dimensional numerical modeling using FLAC3D software. A detailed case study was conducted based on the geological and geotechnical conditions of Kakatiya Khani No. 5 Incline, operated by Singareni Collieries Company Limited (SCCL), Telangana, India. A three-dimensional numerical model representing the roof-pillar-floor system was developed using the Mohr–Coulomb constitutive model. Mine-specific geological parameters, material properties, pillar dimensions, and boundary conditions were incorporated into the simulation framework to replicate actual field conditions. Parametric analyses were performed for five different depths of working, namely 240 m, 280 m, 300 m, 330 m, and 360 m, while maintaining constant pillar geometry and gallery dimensions. The study evaluated stress distribution patterns, pillar strength characteristics, failure mechanisms, and factor of safety (FOS) values under varying overburden loading conditions. The simulation results revealed a significant increase in pillar stress with increasing depth of extraction. The average pillar stress increased from approximately 17.5 MPa at a depth of 240 m to about 21.1 MPa at 360 m depth. Stress concentration was predominantly observed along the pillar edges and corners, indicating these regions as the most vulnerable zones for failure initiation. Although pillar strength exhibited a marginal increase due to confinement effects, the rate of stress increase was substantially higher, resulting in a gradual reduction in the factor of safety with increasing depth. The numerical analysis further demonstrated that deeper workings are associated with higher risks of pillar instability, localized yielding, and potential roof control problems. Statistical evaluation of the simulation results established a strong positive correlation between extraction depth and induced pillar stress, confirming the significant influence of overburden loading on pillar performance. The findings emphasize the necessity of adopting depth-dependent pillar design methodologies rather than relying on conventional empirical approaches. The study demonstrates that FLAC3D-based numerical modeling provides a reliable and effective tool for evaluating pillar behavior, optimizing pillar dimensions, and improving mine safety in underground coal mines. The outcomes of this research contribute to the understanding of depth-related pillar behavior and provide valuable guidelines for mine planners, design engineers, and safety professionals in developing scientifically sound pillar design strategies for deep underground coal mining operations.