# Research Progress and Typical Case of Open-Pit to Underground Mining in China
## 2 Analysis of Key Technologies for the Transition from Open-Pit to Underground Mining
### Current Status of Slope Stability Research in Open-Pit Underground Mining
Design and Safety Balance of Crown Pillar Thickness  
The thickness of the crown pillar is crucial in balancing mine safety and resource utilization. If the crown pillar is too thick, although it enhances safety, it reduces resource recovery rates, thereby negatively affecting the economic efficiency of the mine. On the other hand, if the crown pillar is too thin, it may increase resource recovery but significantly raise the risk of safety incidents [53]. Thus, determining the optimal crown pillar thickness is a fundamental technical challenge in the transition from open-pit to underground mining. This process involves not only assessing the mechanical properties of the rock mass and mining conditions but also considering economic efficiency and safety requirements, ultimately achieving the best balance between safety and economics. 2.4.2. Application of Numerical Simulation and Computational Methods in Determining Crown Pillar Thickness  
A substantial body of research has been conducted by both domestic and international scholars to determine the appropriate thickness of crown pillars, resulting in the development of various theoretical and computational approaches. These methods primarily include semi-quantitative analysis, numerical analysis, model prediction, and elasticity analysis. Among these, numerical simulation techniques have been extensively utilized to investigate crown pillar stability. By employing discrete element methods or continuous element methods, these approaches enable precise simulations of jointed or massive rock masses, thereby facilitating the analysis of crown pillar stability under varying mining conditions. [PERSON] et al. [54] established a crown pillar thickness prediction model through multiple regression analysis, offering initial calculation guidelines for mine design. [PERSON] et al. [55] introduced an optimized design approach that combines empirical analysis, numerical simulation, and field monitoring data, significantly enhancing the accuracy of crown pillar thickness calculations and reducing field measurement errors. [PERSON] [56] utilized a discontinuous surface numerical model to investigate the peeling behavior of crown pillars in high-stress environments, providing critical insights into pillar stability under extreme mining conditions. [PERSON] and [PERSON] [57] applied an ultimate equilibrium model to simulate the degradation of hard rock mine pillars over time. [PERSON] [58] systematically investigated crown pillar stability using 240 nonlinear numerical models. [PERSON] [59], taking the Zuuntsagaan fluorite mine as a case study, employed FLAC3D 7.0 to simulate the stress distribution and failure mechanisms of the crown pillar during underground mining, thereby determining the optimal crown pillar thickness, span, and dip angle. These studies contribute theoretical foundations and computational tools for crown pillar design. 2.4.3. Technical Analysis and Challenge Evaluation  
Despite extensive research, determining the optimal crown pillar thickness during the mine transition process continues to present a series of technical risks and challenges. First, the complexity of geological conditions is the primary source of uncertainty, particularly in high-stress or structurally complex mining environments, where traditional calculation methods may not fully capture the true underground conditions. Second, variations in rock mechanics, especially the differences in stability between various types of rock masses, result in significant discrepancies in crown pillar performance during actual mining operations. Finally, throughout the mining process, the crown pillar may accumulate stress over time, leading to gradual changes in its stability. As a result, relying solely on a single theoretical calculation or numerical simulation method is insufficient to resolve these issues; a multi-method approach is necessary. The rational design of crown pillar thickness plays a crucial role in ensuring safe mining operations during the mine transition process.