Dynamic optimization of retained-ditch height for turning-stage planning in inclined coal seam open-pit mines

Retained-ditch design is critical during the turning stage of inclined coal seam open-pit mines because it affects dumping-space utilization, stripping-flow organization, transportation cost, and post-turning secondary stripping. Existing studies have mainly treated retained-ditch height as a static parameter, which is insufficient for the dynamic production conditions of the turning period. In this study, a dynamic optimization method for retained-ditch height under internal dumping conditions is proposed. First, the maximum economically acceptable retained-ditch height is determined using a cost-compensation approach that balances coal-recovery benefit against additional internal haulage, external dumping, and post-turning secondary stripping costs. Then, a dynamic planning model is established to optimize the yearly retained-ditch sequence by minimizing the cumulative transportation cost during the turning period together with the post-turning secondary stripping cost. A case study was conducted for a seven-year turning period from 2023 to 2029. The optimized retained-ditch heights were 45.5, 69.5, 53.3, 58.7, 70.4, 78.8, and 94.0 m. Compared with the conventional constant-level scheme, the proposed fluctuating scheme reduced the cumulative total cost from 3.7766850 billion yuan to 3.6283658 billion yuan, saving 148.3192 million yuan. The optimized scheme also satisfied the adopted stability criterion, with a minimum factor of safety of 1.265. The proposed method provides a practical tool for turning-stage planning in inclined coal seam open-pit mines.