Geotechnical Stability Analysis and Numerical Modelling for Optimization of Pit-to-Dump Distance Between an External Overburden Dump and an Opencast Coal Mine: A Case Study of the Godavari Valley Coalfields

Abstract

The safe placement of large overburden (OB) dumps adjacent to opencast coal-mine pit slopes is an important geotechnical and mine-planning challenge, particularly with the progressive deepening of opencast pits, increasing dump elevations, constrained dumping land and the need to maintain adequate stability margins. The present study evaluates the influence of dump surcharge, dump height and pit-to-dump setback distance on slope stability by integrating field investigation, laboratory geotechnical characterization, limit-equilibrium analysis (LEM) and three-dimensional distinct-element numerical modelling. selected opencast coal mines in the Godavari Valley Coalfields of Singareni Collieries Company Limited (SCCL), Telangana as the case study. Twenty-seven representative OB samples from Mine A, II and III were characterized using sieve analysis, moisture-content determination, Standard Proctor compaction, specific gravity and direct shear testing. The measured cohesion ranged from 3 to 13.5 kPa and the internal friction angle from 26.9° to 52°, while specific gravity varied from 1.46 to 2.51. Bishop’s Simplified Method and the Swedish slip-circle (Fellenius) approach were applied for dump heights of 30, 60, 90 and 120 m and pit-to-dump distances of 30, 60, 90 and 120 m. In addition, 3DEC sensitivity modelling examined overall slope angles of 25°–45°, pit-to-dump distances of 25–100 m, homogeneous and weak-layer foundation conditions, and a critical dump height of 120 m. The LEM results show a systematic increase in Factor of Safety (FoS) with increasing setback and a reduction in FoS with increasing dump height. Under the investigated dry-condition assumptions, pit-to-dump distances of 60, 90 and 120 m corresponded to FoS above 1.3 for dump heights up to 60, 90 and 120 m, respectively, across the three case mines. The numerical results independently demonstrate the sensitivity of the pit–dump system to slope geometry and weak geological horizons: for the homogeneous 25 m setback sequence, FoS decreased from 2.390 at 25° to 1.477 at 45°, whereas the mean weak-layer FoS increased from 1.242 at 25 m setback to 1.382 at 100 m. A regression relationship incorporating friction angle, cohesion, unit weight, dump height and setback distance achieved R² = 0.742. The integrated assessment indicates that pit-to-dump distance should be established as a mine-specific geotechnical design parameter governed jointly by dump geometry, material strength, foundation competence, slope configuration and geological structure rather than by adoption of a single universal distance.