Researchers have pinpointed the optimal depth for transitioning from open-pit to underground mining at the Sijiaying Iron Mine in Hebei Province, North China. Published in Scientific Reports, the study presents an integrated approach that balances geological, technical, and financial factors to determine the most feasible and cost-effective transition point - offering a new method to optimize resource extraction while managing operational risks.
The Challenge of Transitioning Mining Depths
Open-pit mining is often the go-to method for extracting large ore volumes due to its cost efficiency and operational simplicity. However, as surface deposits are depleted and pits deepen, challenges such as rising costs, safety hazards, and environmental limitations increase. At a certain point, shifting to underground mining becomes practical and necessary.
Determining when to make that shift requires a comprehensive evaluation of site-specific conditions. Recent advances in numerical modeling and optimization - particularly mixed-integer linear programming (MILP) - now allow mining engineers to simulate various scenarios and identify transition points that maximize efficiency and safety.
How the Optimal Depth Was Determined
The Sijiaying Iron Mine has been in operation for over a decade and has reached a stage where further open-pit expansion is limited. Key constraints include fixed infrastructure, such as a provincial highway and the New Luan River.
Researchers developed a methodology for calculating the limiting depth for open-pit operations to address this. This method integrates field data, engineering models, and economic analysis.
Key parameters include ore body dip angles, maximum safe slope angles, and distances to permanent surface structures.
The team compared the costs of continuing with open-pit mining versus switching to underground methods, identifying the “economic crossover point” where the two approaches become financially equivalent. Technical constraints - especially those related to slope stability and proximity to infrastructure - were analyzed separately to ensure safety.
Stability modeling was used to determine the maximum safe slope configurations for the pit design, which is essential in defining the feasible depth for continued surface operations.
Key Findings
The analysis showed that the maximum technically feasible depth for open-pit mining at Sijiaying is 388 meters, mainly due to the proximity of the highway and river. Economically, the transition could have been delayed until 407 meters, but safety and technical concerns made 388 meters the more prudent limit.
This finding highlights the importance of not relying solely on economic models when physical and safety constraints are in play. The researchers developed a set of formulas that account for ore geometry, slope stability, and surface structure locations - offering a robust framework for similar assessments elsewhere.
Their approach supports timely transitions to underground mining, which helps preserve infrastructure and reduces surface disruption.
Real-World Implications for Mining Operations
This research offers a clear methodology for determining when and how to shift from open-pit to underground mining for mining companies operating in areas with surface development or environmental sensitivities. It reinforces the importance of proactive planning that balances economic returns with technical safety and long-term sustainability.
Combining field data, optimization modeling, and safety analysis creates a powerful toolset for mine planners. This framework can be adapted to various geological contexts, helping operations maximize resource recovery while minimizing environmental impact and risk.
The study suggests that ongoing monitoring and flexible mine planning - guided by real-time data - can further improve safety and efficiency over a project's lifetime.
Conclusion and Future Directions
This study provides a timely and practical contribution to mining engineering, especially as companies worldwide face increasing pressure to operate more sustainably and efficiently. The research empowers decision-makers to manage resources more effectively while safeguarding infrastructure and the environment by offering a detailed, systematic approach to determining transition depths.
Future research could refine these models with real-time analytics, explore the ecological and social impacts of transition decisions, and extend the framework to different mining conditions.
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.
Source:
Ma, C., et al. (2025). Investigation on the transition depth of open pit to underground mining in Sijiaying iron mine. Sci Rep 15, 37319. DOI: 10.1038/s41598-025-21311-z, https://www.nature.com/articles/s41598-025-21311-z