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Article

Research Progress and Typical Case of Open-Pit to Underground Mining in China

by
Shuai Li
,
Wencong Su
,
Tubing Yin
*,
Zhenyu Dan
* and
Kang Peng
School of Resource and Safety Engineering, Central South University, Changsha 410083, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(15), 8530; https://doi.org/10.3390/app15158530
Submission received: 11 May 2025 / Revised: 10 July 2025 / Accepted: 29 July 2025 / Published: 31 July 2025
(This article belongs to the Topic New Advances in Mining Technology)
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Abstract

As Chinese open-pit mines progressively transition to deeper operations, challenges such as rising stripping ratios, declining slope stability, and environmental degradation have become increasingly pronounced. The sustainability of traditional open-pit mining models faces substantial challenges. Underground mining, offering higher resource recovery rates and minimal environmental disruption, is emerging as a pivotal technological pathway for the green transformation of mining. Consequently, the transition from open-pit to underground mining has emerged as a central research focus within mining engineering. This paper provides a comprehensive review of key technological advancements in this transition, emphasizing core issues such as mine development system selection, mining method choices, slope stability control, and crown pillar design. A typical case study of the Anhui Xinqiao Iron Mine is presented to analyze its engineering approaches and practical experiences in joint development, backfilling mining, and ecological restoration. The findings indicate that the mine has achieved multi-objective optimization of resource utilization, environmental coordination, and operational capacity while ensuring safety and recovery efficiency. This offers a replicable and scalable technological demonstration for the green transformation of similar mines around the world.

1. Introduction

After decades of intensive mining, most open-pit mines in China have transitioned into deep open-pit mining, facing a series of significant challenges. Firstly, as the mining area expands horizontally, available working space decreases, the width of working platforms narrows, and ore transportation distances increase. This reduces transportation efficiency, resulting in a decline in mining capacity and an increase in operational costs [1]. Secondly, as shallow resources deplete, open-pit mining continues to expand both vertically and horizontally, creating deep pits with steep slopes (as shown in Figure 1a,b). This not only increases technical complexity but also substantially reduces economic returns [2]. Thirdly, solid waste generated by open-pit mining accounts for more than 90% of the total stripping volume [3], leading to environmental pollution in the mining area (as shown in Figure 1c,d). This damages surface ecosystems, degrades working conditions, and results in a significant decline in labor productivity. In this context, the advantages of traditional open-pit mining, such as rapid production, low infrastructure investment, high safety factors, and minimal dilution losses, have gradually diminished [4]. Consequently, the traditional open-pit mining model is no longer sustainable, necessitating the transformation and upgrading of mining technologies. In this process, underground mining is emerging as a key approach for mineral resource development thanks to its superior resource recovery capacity and ecological adaptability [5]. Underground mining, characterized by precise development techniques and efficient extraction methods, can significantly reduce resource loss and enhance both the economic performance and safety of the mining system. From a policy perspective, the introduction of the “Green Mining Standards for the Non-ferrous Metals Industry” in 2018 marked the establishment of China’s first national-level green mining construction standards [6]. These policies emphasize the adoption of environmentally friendly and efficient mining technologies to control surface disturbances and ecological damage. Underground mining avoids large-scale surface excavation and aligns with the core principles of green mining and sustainable development [7].
It should be particularly emphasized that the transition from open-pit to underground mining is not merely a change in production methods but a complex systems engineering process that demands comprehensive planning [8]. Open-pit and underground mining should be viewed as an integrated system, with safe and efficient operations achieved through scientific planning and coordination. Several key technical challenges must be addressed during this transformation: how to achieve spatial integration and functional transition between open-pit and underground systems, how to restructure the development system and select appropriate mining methods, how to control the stability of slopes and surrounding rock, and how to design crown pillars effectively to ensure operational safety and maximize resource recovery rates. These challenges are highly interconnected and mutually restrictive, placing higher demands on the overall planning of mining projects, technical integration, and intelligent management.
In recent years, several domestic and international mines have successfully completed the transition from open-pit to underground mining, such as the Chuquicamata copper mine in Chile, the Palabora mine in South Africa, and the Xinqiao iron mine in China. Engineering practices show that designing a scientific development system and selecting appropriate mining methods directly affect resource recovery efficiency and cost control as well as influencing the overall safety and sustainability of the mining system. Furthermore, the spatial overlap, stress disturbance, and hydrogeological coupling within the open-pit system are prone to triggering slope instability, crown pillar failure, and other disasters, becoming key risk factors that limit the success of the transition [9]. Therefore, systematically studying key technologies for the transition from open-pit to underground mining is not only a practical necessity for mining safety but also essential for developing a green mining technology system.
Based on the background outlined above, this paper focuses on the core technical issues involved in the transition from open-pit to underground mining. It systematically analyzes the types of development systems and their applicable conditions, reviews the technical pathways and application boundaries of mainstream mining methods, and explores the research progress and engineering strategies for slope stability and crown pillar design. Building on this foundation, the Anhui Xinqiao Iron Mine is selected as a typical case study. This paper provides an in-depth analysis of the technical pathways and engineering experiences related to mining method selection, system coordination layout, and crown pillar safety design, intending to offer systematic theoretical support and practical insights for the green, efficient, and intelligent development of deep resources in China.

2. Analysis of Key Technologies for the Transition from Open-Pit to Underground Mining

2.1. Analysis of Mine Development Systems in the Transition from Open-Pit to Underground Mining

The mine development system serves as the core support structure for mining operations and plays a critical role in the transition from open-pit to underground mining, effectively bridging the two methods [10]. Its design directly affects the selection of mining methods, safety, and transportation efficiency while also imposing overarching constraints on slope stability control, resource recovery rates, and environmental management [11]. Therefore, the scientific selection of an appropriate development system is a fundamental prerequisite for ensuring safe and efficient mining operations [10]. Depending on how underground and open-pit development technologies are integrated, engineering practices commonly categorize the mine development systems for the transition from open-pit to underground mining into three types: independent development systems, partially integrated development systems, and fully integrated development systems [12]. To gain a deeper understanding of the applicability and advantages of these systems, the following sections will examine the characteristics, implementation conditions, and practical applications of each system in detail.

2.1.1. Independent Development System

The independent development system fully separates underground and open-pit operations, ensuring no interference between the two in terms of working areas, transportation routes, and auxiliary facilities. This system is particularly suited for mines with deep ore bodies, significant disturbances from open-pit mining, or marked differences in the operational conditions of the two methods. Examples of mines employing this system include China’s Yishui Iron Mine and Yinchangou Copper Mine as well as international mines such as Chuquicamata [13], Geita [14], and Tropicana [15]. The primary advantage of this system is its high safety, as it effectively prevents open-pit blasting from affecting underground ventilation, drainage, and equipment operation. However, this system typically involves higher infrastructure costs, as it requires the construction of independent shafts, transportation, and ventilation facilities. As a result, it demands a longer economic return period.

2.1.2. Partially Integrated Development System

The partially integrated development system integrates operations by sharing certain resources, such as transportation routes, ventilation facilities, or connecting tunnels, between open-pit and underground operations. This system is particularly suitable for mines with complex geological conditions and extended transition periods. Mines such as Palabora [16], Ekati [17], and Grasberg [18], both in China and abroad, implement this model. By sharing resources, the partially integrated development system effectively balances construction costs with operational efficiency, making it adaptable to the transformation requirements of different geological conditions.

2.1.3. Fully Integrated Development System

The fully integrated development system focuses on operational integration, where open-pit and underground operations share transportation, ventilation, and drainage facilities. This system is widely used in mines with moderately shallow ore bodies, providing benefits such as reduced infrastructure investment and enhanced operational efficiency. The Hasselmi mine in Finland is a typical example, where efficient integration of open-pit and underground operations is achieved by sharing the lower slope tunnel. However, this model imposes stricter requirements on tunnel layout precision, ventilation safety, and traffic management, particularly during high-intensity integrated operations, where stringent control of operational interference and risk propagation is crucial.
The three mining system types each offer distinct advantages in terms of engineering adaptability, construction cost, operational safety, and efficiency. The selection of an appropriate mining system requires a well-planned approach that accounts for factors such as the depth of the ore body, geological structure, existing infrastructure, and ecological constraints. In practical applications, independent systems are most suitable for mines in the early stages of transformation, where minimizing operational disruptions is critical. Combined systems are better suited for mines characterized by strong continuity and high synergy between surface and underground resources, while certain hybrid systems provide a flexible transitional model, particularly for complex mines undergoing extended transition periods. Xinqiao Iron Mine, a representative example of China’s transition from open-pit to underground mining, has innovatively adopted a combined mining system utilizing both the main and auxiliary shafts on the upper bench side along with inclined ramps, tailored to the ore body’s burial characteristics, resource distribution, and ecological protection requirements. The mining system layout at Xinqiao Mine is illustrated in Figure 2. This approach has not only increased mining efficiency by 25% but also reduced surface disturbance by 40%, thus complying with green mining construction standards. The successful implementation of the Xinqiao Mine model demonstrates that, through modular design and thoughtful system selection, mining efficiency can be enhanced while significantly minimizing environmental impact, offering valuable insights for the transformation of other mining operations.

2.2. Selection of Mining Methods and Technical Adaptability Analysis

In open-pit to underground mining operations, the scientific selection of mining methods is critical for ensuring the safe and efficient transformation of mines as well as achieving optimal resource recovery and sustainable development [19]. Given the significant differences between open-pit and underground mining in terms of geological conditions, spatial structure, stress distribution, and operational methods, the technical adaptability of mining methods directly impacts resource recovery rates, mining efficiency, operational safety, and surface ecological effects [20]. Consequently, the systematic evaluation of method applicability, potential risks, and mitigation strategies holds substantial practical significance. Currently, the primary methods employed in open-pit to underground mining include the stope method (Figure 3a), caving method (Figure 3b,c), and backfilling method [21]. To further investigate the specific applications, advantages, and limitations of each mining method, a detailed analysis of the stope method, caving method, and backfilling method will be presented in the following sections.

2.2.1. Stope Mining Method

The stope mining method relies on the self-supporting ability of the surrounding rock for excavation, eliminating the need for artificial support structures [23]. It is primarily suitable for mining areas with regular orebody shapes, strong roof stability, and simple geological conditions. This method features a straightforward process, low initial investment, and strong economic adaptability. For instance, during the transformation of the Shiren Gou Iron Mine in Hebei Iron and Steel Group, a segmented stope method was adopted based on the orebody geometry and roof structure conditions. This approach significantly enhanced unit capacity and reduced operational disturbances [24]. However, the method is highly dependent on roof stability and presents considerable safety risks in deep high-stress zones, weak surrounding rock formations, or tectonically fractured zones. Moreover, prolonged exposure of the stope may lead to void collapse, extension of surrounding rock fissures, and far-field disturbance effects, which can negatively impact the continuity and safety of subsequent operations [25]. As a result, this method is typically applied to shallow ore bodies or utilized as a transitional technology for pre-mining trials prior to backfilling.

2.2.2. Caving Mining Method

The caving mining method leverages the self-weight of the ore or rock mass to induce natural collapse for recovery [26]. It is suitable for mining scenarios where the ore is loose, the orebody is inclined, the roof is unstable, or surface subsidence constraints are minimal. This method is characterized by high efficiency and a high recovery rate, making it widely applicable in large-scale deep metal mines. For instance, when the Shilu Iron Mine transitioned to underground mining in 2018, it employed the sublevel caving method without crown pillars [27]. This approach effectively utilized the orebody’s dip angle and joint surface distribution characteristics to enhance unit production capacity. However, the method involves considerable uncertainties and irreversibility, particularly in mines with surface structures, water-sensitive zones, or strict slope control requirements [28]. The large-scale ground pressure disturbances, subsidence, or instability risks induced by the method are significant and cannot be overlooked [29]. Improper control of the caving range may lead to the expansion of large-scale voids and structural disasters, placing high demands on engineering controllability and risk management capabilities.

2.2.3. Backfilling Mining Method

The backfilling mining method improves the stability of surrounding rock and controls surface subsidence by filling mined-out areas with cemented tailings, slurry, or solid waste. This method ensures both the safety and environmental sustainability of mining operations [30]. With the advancing green mining policies, the use of backfilling methods has become more widespread in both metallic and non-metallic mines. Various forms have emerged, such as cemented tailings backfilling, slurry backfilling, and strip-style backfilling, to accommodate diverse mining requirements [31]. For example, the Jinfeng Gold Mine [32] utilizes an upward access cemented backfilling method, which significantly increases ore recovery and reduces surface disturbance and collapse risks, highlighting the dual benefits of this approach in enhancing mining efficiency and protecting the environment.
In the context of green mine development, numerous scholars have explored backfilling mining technologies. Hou J. et al. [31] examined the application of green backfilling technologies and analyzed their effectiveness in a coal mine working face. Chen F. et al. [33] argued that backfilling is an effective solution to environmental and safety challenges in mining and serves as a key technology for green mining. H. Yu et al. [34] further asserted that backfilling mining not only maximizes mineral recovery but also protects underground and surface environments, aligning with sustainable development goals. In terms of engineering practice, D. Hou [35] proposed a composite backfilling scheme for gently inclined phosphate ores—“pseudo-inclined segmented strip backfilling + upward horizontal layered backfilling”—which has demonstrated stability across varying depths and structural conditions. This further confirms the potential of backfilling methods in mining applications.
Importantly, recent policy documents, such as the “Green Mining Guidelines for Non-Coal Mines” and the “14th Five-Year Plan for Mining Safety Production,” explicitly state that “new mines should prioritize the use of backfilling methods,” thereby promoting the standardization and institutionalization of this technology [36]. The backfilling mining method’s unique advantages in reducing tailings discharge, optimizing waste utilization, and protecting surface ecology position it as a critical technological enabler of green, safe, and efficient mining practices.
A comparative analysis reveals that the backfilling method is the most effective in most open-pit to underground mining operations, demonstrating high safety, high resource recovery rates, and minimal ecological impact. Consequently, it is emerging as the preferred choice driven by policy. The stope method is suitable for rapid development in stable areas and offers economic advantages. The caving method is effective for releasing large-scale resources; however, its application is restricted, and it carries significant risks. Mining methods should be systematically assessed based on factors such as orebody geometry, rock mass structure, hydrogeological conditions, and slope stability. These factors should be integrated with construction schedules, economic feasibility, and environmental objectives to develop a zoned, phased strategy. It is recommended that 3D modeling, GIS [37], and intelligent decision-support systems be incorporated to enhance the scientific basis of method selection and improve dynamic adjustment capabilities [38]. Overall, the selection of an appropriate mining method not only influences resource utilization efficiency and operational safety but also plays a crucial role in determining the sustainability of the transition. In the context of green development, the backfilling method, with its high adaptability and environmental benefits, is increasingly becoming the dominant technology for transitioning from open-pit to underground mining, providing a replicable and scalable model for the mining industry’s transformation.

2.3. Current Status of Slope Stability Research in Open-Pit Underground Mining

Slope stability in open-pit to underground mining is a critical safety concern throughout the entire lifecycle of the mine [39]. The interaction between open-pit and underground mining, which differs significantly in orebody occurrence conditions, stress redistribution patterns, and rock mass disturbance mechanisms, forms a dynamic composite system. This interaction leads to highly complex and uncertain slope stability challenges. During the transition period, underground mining-induced rock loosening, stress concentration, and structural deformation frequently compromise the original stability of open-pit slopes, significantly increasing the risk of geological hazards such as landslides and collapses. Statistics indicate that approximately 65% of safety incidents in typical open-pit to underground mines in China are directly related to slope stability issues, underscoring the urgency and importance of further research in this area.
In the investigation of instability mechanisms, T.L. Baskari [40] introduced the progressive slope instability theory, which outlines the evolution of slope failure from local initiation to overall instability. Sultan et al. [41] constructed a multi-factor interaction model for slope instability by coupling external triggering mechanisms with the physical and mechanical properties of the soil and rock mass. Stead et al. [42], based on the theory of complexity classification, systematically discussed the application range and technical advantages of numerical simulation methods in slope stability analysis. These theories provide a crucial theoretical foundation for a deeper understanding of slope instability mechanisms.
In the field of technical methodology research, numerical simulation and physical similarity experiments have become the predominant approaches, offering substantial application value in understanding slope stability mechanisms. Initially, Nguyen, P.M.V. [43] employed the finite difference method to evaluate slope stability in the Cao Shan open-pit mining area in Vietnam. Luo et al. [44] applied the stochastic finite element method to analyze the probability of stability for slopes and underground mining rock masses, presenting a novel perspective on how underground mining affects slope stability. Li et al. [45], through centrifugal modeling and numerical simulation, identified that underground mining induced overturning failures in overlying rock masses, providing experimental evidence for further studies on the effects of underground mining disturbances on rock mass stability. Scholtes and Donze [46] utilized the three-dimensional discrete element method to simulate the crack propagation process in fractured rock masses, offering vital technical support for analyzing failure mechanisms in such formations. Ding, Q.L. [47] developed a numerical analysis model based on the geological conditions of the Anjialing open-pit mine to investigate the evolution of slope stability during bench mining. Johari and Lari [48] categorized rock slope failure modes into four types and created a direct model for evaluating the probability of slope instability.
With advancements in computational technology, three-dimensional numerical analysis has emerged as the mainstream approach, enabling more accurate simulations of complex geological conditions and stress distributions. For example, Zhao et al. [32] developed a three-dimensional finite difference model for the Jinfeng gold mine, systematically assessing the effects of mining disturbances on shaft stability and surface deformation, thereby validating the practical utility of three-dimensional models in mine stability assessments. Shen and Karakus [49] proposed a FLAC3D strength reduction method, based on the Hoek–Brown criterion, which successfully analyzed the stability of three-dimensional rock slopes and provided a scientific basis for slope stability design in engineering practice. Additionally, Gischig et al. [50] employed a three-dimensional difference element code to investigate the role of the movement release mechanism in planar transitional slope failure, emphasizing how the geometry of different discontinuity sets impacts the three-dimensional block shape and volume of unstable rock masses, thus delivering more precise simulation data for slope stability studies. Li S. [51] used the Shizhuyuan nonferrous metal mine as a case study and applied the true three-dimensional safety analysis method to evaluate the safety and stability of mine slopes. The model’s overall results reveal only minor tensile damage (Figure 4a). The compressive and tensile stresses experienced by the open-pit mine remain within the safe range (Figure 4b); the Z-direction displacement shows that the high point of the model settles while the low point rebounds, with no penetration of the plastic zone (Figure 4c), and the model monitoring simulation shows that displacement at monitoring points fluctuates initially before stabilizing (Figure 4d), indicating that the slope ultimately reaches a stable state, thus confirming the effectiveness of the slope stability control plan.
Despite significant advancements in both theoretical and technical aspects of slope stability research, several challenges persist. On the one hand, a unified theoretical framework for slope instability mechanisms in open-pit and underground combined mining remains to be established, especially in complex geological conditions such as high-stress zones, multiple fault zones, and weak interlayers, where research on the response characteristics of rock masses remains underdeveloped. On the other hand, although existing numerical models demonstrate high theoretical accuracy, they face limitations in param sensitivity and generalizability, and their applicability to engineering practice requires further enhancement. Future research should progress along three collaborative paths: first, the development of a multi-field coupling analysis model that integrates stress disturbances, hydrogeological variations, and structural evolution; second, the advancement of slope instability prediction methods using machine learning and graph neural networks; and third, the creation of a dynamic feedback system that combines real-time monitoring with model simulations to improve the responsiveness and adaptability of early warning systems. Additionally, in line with the principles of green mining, efforts should focus on the synergistic optimization of slope protection and resource recovery to achieve an integrated approach to safety, efficiency, and ecology.

2.4. Research and Technical Risk Analysis of Crown Pillar Thickness in the Transition from Open-Pit to Underground Mining

As the depth of open-pit mining increases, the mining challenges and economic-technical constraints become more pronounced. To ensure the continuous operation of the mine, it is generally necessary to transition to underground mining once the critical depth is reached. However, the slopes formed during open-pit mining, when subjected to the synergistic effects of underground mining, may create significant safety hazards. Specifically, if these hazards are not properly addressed, they could lead to major incidents such as open-pit slope collapse, instability in underground mining areas, or flooding of underground workings caused by surface water accumulation [52]. Consequently, a certain thickness of crown pillars is typically left between the open-pit and underground mining areas to ensure safe and coordinated operations.

2.4.1. 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. Hemant et al. [54] established a crown pillar thickness prediction model through multiple regression analysis, offering initial calculation guidelines for mine design. Xu 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. Lavoie [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. Wessels and Malan [57] applied an ultimate equilibrium model to simulate the degradation of hard rock mine pillars over time. Guggari [58] systematically investigated crown pillar stability using 240 nonlinear numerical models. Dintwe [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. By combining numerical simulations, theoretical calculations, and field monitoring, mining engineers can more accurately assess crown pillar stability, providing reliable technical support for the transition from open-pit to underground mining. With ongoing technological advancements and the accumulation of practical experience, future research on crown pillar thickness will continue to deepen, offering more scientifically grounded and reliable solutions to support safe mining operations.

3. Typical Case: Integrated Demonstration of Xinqiao Iron Mine

3.1. Overview of Xinqiao Mine

Xinqiao Mine has a proven geological reserve of 170 million tons and an industrial reserve of 110 million tons, covering an area of 3.529 km2. The current mining scale is 1.5 Mt per year (900,000 tons from open-pit mining and 600,000 tons from underground mining). All the elevations mentioned in this paper, including those at −106 m, −156 m, and others, are relative to the reference horizon (horizon 0). The geological resource reserve in the open-pit to underground transition zone below the −156 m level between lines 1 and 21 amounts to 48.057 million tons. The open-pit to underground mining transition at Xinqiao Mine involves the area below the −156 m level and to the east of line 21, with the primary extraction target being the eastern wing of ore body No. 1. The design follows a downward staged mining sequence. The ore bodies below −156 m are classified by thickness as follows: Class A (thickness < 5 m, 4.4%), Class B (thickness 5–15 m, 11.8%), Class C (thickness 15–50 m, 79.4%), and Class D (thickness > 50 m, 4.4%). As illustrated in Figure 5, the three-dimensional geological model of Xinqiao Mine shows the classification of ore body thicknesses and their spatial distribution, providing a solid foundation for the design of the subsequent underground mining plan.
In terms of capacity transition, the mine currently operates at a total capacity of 1.5 Mt per year, with future plans to increase this to 1.8 million tons per annum. Given that the remaining service life of the western underground mining area is similar to the eastern open-pit to underground transition phase, consideration must be given to the capacity transition after the western mining phase concludes. The second phase of the open-pit to underground transition is scheduled for implementation in 2028. Upon completion of the western mining phase, the additional 900,000 tons per year capacity from the second phase will be achieved by constructing a blind inclined shaft or a blind vertical shaft in a suitable location in the western area, with ore transported to the surface via the main shaft to maintain the total capacity of 1.8 million tons per year. The capacity transition diagram for Xinqiao Mine is presented in Figure 6.

3.2. Measures for the Transition from Open-Pit to Underground Mining at Xinqiao Mine

As outlined above, the transition from open-pit to underground mining at Xinqiao Mine involves a large-scale operation, high-capacity integration demands, and the extraction of deep ore bodies. The open-pit mining operation at Xinqiao Mine is illustrated in Figure 7a. To ensure a safe, efficient, and smooth transition between open-pit and underground mining, and to resolve the series of challenges associated with the closure of the open pit, detailed connection measures and technical solutions must be developed.
  • Post-closure treatment: Following the closure of the open pit, the pit floor is promptly treated to prevent seepage. Following the open-pit waste rock management plan, the pit is then backfilled to the −106 m level. This procedure ensures the stability of the mining area while meeting environmental protection standards.
  • Safety isolation layer: The ore body above the 180 m level, with a thickness of 24 m, serves as a safety isolation layer between surface and underground mining. This layer acts as a buffer, preventing potential safety risks between the surface and underground operations. The underground mining site is illustrated in Figure 7b.
  • Open-pit slope monitoring system: An open-pit slope monitoring system is established to closely monitor the impact of underground mining activities on the stability of the open-pit slopes. This system provides real-time data to assess and manage risks during the transition period.
  • Comprehensive drainage system planning: During the transition period, water from the open-pit excavation is discharged through the existing open-pit drainage facilities. Once the open pit is closed and backfilled to the 106 m level, a collection pit (10 m × 6 m × 6 m) is constructed at the lowest point. Water is then directed through the existing drainage infrastructure to a permanent pumping station located at the 48 m level.

3.3. Selection of Mining Methods for the Transition from Open-Pit to Underground Mining at Xinqiao Mine

In the mine transition process, the selection of mining methods directly influences resource recovery rates, safety, and environmental impact, making it a key factor in the transition from open-pit to underground mining at Xinqiao Mine. The choice of transition method varies significantly among mines, depending on factors such as ore body characteristics, geological conditions, and environmental policies. At Xinqiao Mine, the proportion of C-class medium-thick ore body resources reaches 79.4%. The ore body characteristics, including moderate thickness and stable strata, require the mine to prioritize the use of backfilling mining methods suitable for medium-thick ore bodies. This decision takes into account multiple factors, such as ground pressure control, resource recovery, and environmental protection. The selection of mining methods depends not only on the fundamental characteristics of the ore body but also on factors such as the coordination between open-pit and underground operations, the complexity of the ore body’s structure, production capacity requirements, and the protection of surface infrastructure. Therefore, the choice of mining method in the transition process reflects the diverse strategies adopted by different mines in addressing similar challenges. For instance, globally, mines choose mining methods based on factors such as ore body dip, geological complexity, and economic conditions, highlighting the technical diversity in mining transitions.
After evaluating several options, Xinqiao Mine identified two potential methods: mechanized upward horizontal layered backfill mining and segmented stope post-backfilling. The mechanized upward horizontal layered backfill method provides advantages such as simple processing, strong adaptability to the ore body, and effective ground pressure control. It uses horizontal deep-hole blasting combined with trackless equipment, significantly reducing loss and dilution rates. Although its drilling efficiency is relatively low, which may affect production capacity, its proven technology and higher safety levels make it a viable option. In contrast, the segmented stope post-backfilling method offers high recovery rates and good safety, making it suitable for multi-working face coordination. However, it demands complex construction techniques, causes substantial pillar resource loss, and results in higher secondary crushing costs due to uneven ore particle sizes, which negatively impacts its economic feasibility.
After evaluating technical maturity, adaptability, and economic feasibility, Xinqiao Mine selects mechanized upward horizontal layered backfill mining as the preferred method. This method ensures safety and recovery efficiency while offering significant potential for broader application. It provides robust technical support for Xinqiao Mine’s successful transition and serves as a valuable reference for the transformation of similar ore bodies.

3.4. Determining the Crown Pillar Thickness at Xinqiao Mine

The stability of the crown pillar plays a crucial role in ensuring the safe transition from open-pit to underground mining at Xinqiao Mine. The ore body characteristics below the −156 m level combined with complex hydrogeological conditions pose significant challenges to crown pillar stability. To address this, a combination of theoretical calculations and numerical simulations was employed to assess the stability of both the original crown pillar and the composite pillar under varying mining span conditions. This approach provides both the theoretical foundation and engineering support necessary for determining the optimal crown pillar thickness.
Theoretical calculations employ methods such as the thickness-to-span ratio, load transfer line intersection, and structural mechanics to analyze the safety thickness of the original crown pillar. The results show that as the pillar thickness increases, the stability of the original crown pillar improves progressively. However, when the mining span exceeds 40 m, the pillar’s stability reaches a critical threshold. Therefore, the theoretical analysis recommends that during the transition from open-pit to underground mining, the mining span should be kept under 40 m to ensure the stability and safety of the crown pillar.
To further assess the stability of the crown pillar, numerical simulations were conducted, with the results for the composite crown pillar thickness shown in Figure 8. The simulation results show that, with a fixed mining span, the maximum tensile stress in the original crown pillar decreases as the pillar thickness increases, while the tensile stress safety factor increases with the thickness. For the composite pillar, the application of a 1 m thick reinforced concrete layer results in a more even distribution of stress, effectively reducing tensile stress and enhancing the pillar’s safety and stability. This indicates that the composite pillar provides clear stability advantages in high-stress environments.
Based on the results of both theoretical calculations and numerical simulations, the research team recommends a 24 m thick original rock crown pillar for the ore body above the −180 m level, ensuring that the tensile stress safety factor remains at or above 1.2. This design not only satisfies safety requirements but also meets the technical demands for ground pressure control and resource recovery during the transition from open-pit to underground mining. Furthermore, this design considers both the economic viability and practical feasibility of operations, providing a valuable reference for pillar design in similar mining projects.

3.5. Implementation of Mining Methods

  • Block Layout and Structural Params:
The ore chambers and pillars are alternately arranged along the strike of the ore body, with their lengths matching the horizontal thickness of the ore body. The ore chambers measure 14 m in width, while the pillars are 10 m wide. The bottom pillar height is 5 m, the top pillar height is 2 m, and the segment height is 9.9 m. Each segment is responsible for three layers, with each layer having a height of 3.3 m. During the mining process, the minimum roof management height is 3 m, and the maximum height is 6.3 m.
2.
Mining Development Layout:
The layout of the mining development is shown in Figure 9. It primarily consists of essential passageways for ore recovery, including the ramp, segmented haulage drifts, layered haulage routes, ore unloading crosscuts, chute shafts, and backfilling ventilation shafts.
3.
Mining Process:
Rock drilling and blasting operations utilize the Boomer 281 drilling rig (as shown in Figure 10a), with a hole spacing of 1.3 m and a blast hole spacing of 1.1 m. The side-hole spacing is appropriately reduced, with the distance from the contour line of the mining area ranging from 0.8 m to 1.0 m, and the drill hole diameter is 42 mm.
4.
Ore Removal:
After each blasting and exhausting the fumes through adequate ventilation, the collapsed ore is transported by the diesel front loader through the haulage routes and segmented connecting drifts to the ore chute (as shown in Figure 10b). The loader’s estimated capacity is 406.7 tons per shift. Given that the segmented ore volume is 11,400 tons, the net ore removal time is 29 shifts.
5.
Backfilling Process:
The first-stage backfilling of the ore pillars, the first layer of ore chambers and pillars at the bottom, the crown backfilling, and the backfilling of each layer of ore chambers and pillars are performed using a binder material with a cement/fly ash/tailings mass ratio of 1:2:6, along with a binder concentration of approximately 70%. The second-stage backfilling utilizes a cement/fly ash/tailings mass ratio of 1:2:15, along with a binder concentration of approximately 70%. For crown backfilling, a sectional pressure-reduction method is applied to transport the backfilling material, where partition walls are constructed at the crown backfilling layer. The material is initially filled one to two times, and after the material settles and shrinks, additional backfilling is done to improve the crown backfilling ratio. The on-site backfilling process is illustrated in Figure 10c,d.
6.
Key Technical and Economic Indicators:
Following the implementation of the upward horizontal layered cemented backfilling method, Xinqiao Mine achieves a recovery rate of 84.66% and a dilution rate of 3.41%. To further improve the recovery rate and reduce the dilution rate, measures such as enhancing the quality of the backfilling, ensuring timely and stable backfilling maintenance, eliminating waste rock layers, and optimizing the mining process are recommended.
Through the implementation of the mechanized upward horizontal layered cemented backfilling mining method, Xinqiao Mine effectively ensures the safety, efficiency, and relatively low dilution loss of underground mining operations. As open-pit mining concludes and the pit is closed, achieving ecological restoration of the open-pit area and the resource utilization of solid waste has become the final phase and a key demonstration of the green mining concept.

3.6. Tailings Resource Utilization and Open-Pit Ecological Restoration Practices

As mineral resource extraction continues to intensify, the mining industry generates vast amounts of waste rock and tailings each year. If improperly managed, these by-products can cause significant environmental and ecological damage [60]. For example, in Papua New Guinea, waste rock and low-grade ores are commonly stored in waste piles or tailings ponds, leading to severe land degradation and ecological decline [61]. In China, the total amount of solid waste from mining has exceeded 25 billion tons, with the number and size of tailings ponds continuously increasing. Traditional methods of tailings storage not only occupy large areas of land but also exacerbate environmental risks [62].
In this context, the adoption of “green mining” practices, including the resource utilization and harmless treatment of tailings, has become a core issue for the sustainable development of the mining industry. Green mining initiatives focus on utilizing advanced mining technologies to achieve efficient resource recovery and environmentally friendly disposal of solid waste [63]. Among these approaches, tailings backfilling technology has emerged as a significant area of research in recent years.
Tailings backfilling technology, an environmentally friendly method of tailings disposal, utilizes tailings as underground backfilling material, promoting the recycling of waste and significantly reducing the surface environmental burden [64]. L. Yang [65] highlighted the notable advantages of cemented tailings sand backfilling technology in terms of environmental protection, technical implementation, and economic benefits. At the Laisvall lead-zinc mine in Sweden, coarse-grained tailings produced by flotation were used to create a cement-based slurry for underground mine backfilling, leading to effective ecological restoration [66]. Similarly, at the Pinto Valley Copper Mine in the United States, tailings were successfully used for open-pit backfilling, offering valuable technical insights for ecological restoration efforts in China.
Xinqiao Mine is a representative polymetallic mine that employs a combined open-pit and underground mining method. The bottom of the pit reaches a depth of −156 m, forming steep and high slopes, with a reserve of 43.6 million cubic meters, resulting in significant slope stability and safety risks. In response to the national requirements for green mining, Xinqiao Mine has launched an open-pit ecological restoration project. Due to its low tailings sand output, which cannot fully meet the needs of underground backfilling, Xinqiao Mine has innovatively used tailings from Dongguashan Copper Mine to solidify and backfill the open pit. The schematic of the restoration project is shown in Figure 11a, and the tailings transportation pipeline layout is shown in Figure 11b. This solidification backfilling process not only solves the tailings disposal issue at Dongguashan Copper Mine but also eliminates safety hazards at Xinqiao Mine’s open pit, facilitating regional mining resource synergy. The ecological restoration outcomes are illustrated in Figure 11c.
In conclusion, the open-pit ecological restoration project not only effectively mitigates safety risks during the mine transition process but also plays a crucial role in supporting green mining construction and the sustainable development of the mine’s environment. Future research should prioritize optimizing the performance of tailings backfill materials, innovating backfilling processes, and conducting long-term monitoring of ecological restoration outcomes. These efforts will contribute to the development of a standardized technical framework that can be adapted to various mining conditions, providing robust technical support for the green transformation of China’s mining industry.

4. Conclusions

As shallow resources are depleted and stripping ratios continue to rise, the transition from open-pit to underground mining has become a critical strategy for ensuring the sustainable development of large-scale metal mining operations. This paper systematically reviews the current state of research on key technologies involved in the transition from open-pit to underground mining, and, based on the engineering practices at Xinqiao Mine, presents representative technical pathways and optimization strategies. The findings indicate that the following factors are essential for a successful transition:
  • The transition from open-pit to underground mining necessitates a mine-specific, phased implementation approach. It is essential to choose an appropriate development system—whether independent, partially integrated, or fully integrated—based on geological conditions, resource depth, and production scale. The joint development system at Xinqiao Mine, which integrates a main and auxiliary shaft with a ramp, has successfully increased mining efficiency by 25% and reduced surface disturbance by 40%, confirming the model’s effectiveness and environmental advantages under specific conditions.
  • The selection of mining methods directly impacts the success of the transition and the efficiency of resource recovery. Compared to stope mining and caving mining methods, backfilling mining offers distinct advantages in stabilizing surrounding rock structures, controlling surface subsidence, and enhancing resource recovery rates. As backfilling materials and techniques continue to evolve, their application in green mining construction becomes increasingly significant. Xinqiao Mine has adopted the upward horizontal layered backfilling method, achieving a recovery rate of 84.66% and a low dilution rate of 3.41%, while significantly reducing environmental disturbances. This demonstrates the comprehensive advantages of the method.
  • Slope stability control is essential for ensuring the safe operation of a mine throughout its lifecycle. During the transition from open-pit to underground mining, the original slope stability system faces rebalancing challenges due to stress redistribution and disturbances from mining activities. Existing research has developed several theories and models, including limit equilibrium analysis, numerical simulations, and physical modeling, providing a scientific basis for predicting slope behavior and disaster risks. However, the mechanisms of slope instability under multi-factor coupling conditions remain highly uncertain, necessitating further studies in multi-field coupling, time evolution, and system dynamics.
  • National-level green mining policies, especially the “Zero Waste Mine” initiative, have driven significant technological innovations in the mining sector, particularly in tailings backfilling and solid waste resource utilization. Tailings backfilling not only reduces tailings discharge but also facilitates the ecological restoration of closed pit land. Additionally, it enhances resource utilization efficiency and lowers energy consumption per unit of production. For example, Xinqiao Mine has successfully integrated solid waste resource utilization with environmental protection by utilizing tailings from the Dongguashan Copper Mine for open-pit backfilling. This approach demonstrates both the feasibility and the exemplary role of green mining transformation.
Overall, the transition from open-pit to underground mining is a highly complex system engineering process, with key technical challenges that are highly interconnected and interdependent. These challenges must be addressed through coordinated efforts in comprehensive planning and dynamic adjustments. The engineering practices at Xinqiao Mine not only validate the feasibility of the proposed technical pathways but also offer a reference model for the green transformation of similar mining operations. Future research should focus on optimizing the performance of backfilling materials, innovating mining processes, and developing a standardized technical framework, providing solid technical support for the green transformation of the global mining industry.

Author Contributions

Conceptualization, S.L., W.S., T.Y., and Z.D.; Data curation, S.L. and W.S.; Formal analysis, S.L., T.Y., and K.P.; Funding acquisition, S.L.; Investigation, S.L., T.Y., W.S., Z.D., and K.P.; Methodology, S.L., T.Y., and W.S.; Project administration, K.P., W.S., and Z.D.; Resources, S.L., W.S., and T.Y.; Software, T.Y., W.S., and Z.D.; Supervision, S.L., T.Y., Z.D., and K.P.; Validation, S.L., T.Y., Z.D., and K.P.; Visualization, T.Y. and W.S.; Writing—original draft, W.S.; Writing–review and editing, S.L., W.S., and Z.D. All authors have read and agreed to the published version of the manuscript.

Funding

The authors thank the Major National Science and Technology Project for Deep Earth: Theory and Technology of Green backfilling of Solid Waste with Strong Acid and Alkali (Grant No. 2024ZD1003808) and the Science and Technology Innovation Program of Hunan Province (Grant No. 2023RC3035) for their financial support.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Environmental impacts of open-pit mining: (a) open deep concave pit; (b) open slopes and subsidence areas; (c,d) environmental pollution from quarries. Source: (a,b) adapted from [http://www.lyy.com.cn/] (accessed on 9 July 2025); (c) adapted from [https://www.hangzhou.com.cn/index.htm] (accessed on 9 July 2025); (d) adapted from [http://www.gyepchina.com/557/0/208] (accessed on 9 July 2025).
Figure 1. Environmental impacts of open-pit mining: (a) open deep concave pit; (b) open slopes and subsidence areas; (c,d) environmental pollution from quarries. Source: (a,b) adapted from [http://www.lyy.com.cn/] (accessed on 9 July 2025); (c) adapted from [https://www.hangzhou.com.cn/index.htm] (accessed on 9 July 2025); (d) adapted from [http://www.gyepchina.com/557/0/208] (accessed on 9 July 2025).
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Figure 2. Site diagram of the Xinqiao Mine development system. Source: Figure 2 adapted from [http://www.xqmcl.com/] (accessed on 8 July 2025).
Figure 2. Site diagram of the Xinqiao Mine development system. Source: Figure 2 adapted from [http://www.xqmcl.com/] (accessed on 8 July 2025).
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Figure 3. Schematic diagrams of common mining methods (adapted from Shuai Li [22]): (a) room and pillar method; (b) stope mining method; (c) caving mining method.
Figure 3. Schematic diagrams of common mining methods (adapted from Shuai Li [22]): (a) room and pillar method; (b) stope mining method; (c) caving mining method.
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Figure 4. Three-dimensional model and stress analysis of open-pit mining (adapted from Shuai Li [51]): (a) three-dimensional model of the final realm of open-pit mining; (b) maximum principal stress diagram; (c) Z-directional displacement diagram of the overall model; (d) the locations of the open-pit boundary slope displacement monitoring points.
Figure 4. Three-dimensional model and stress analysis of open-pit mining (adapted from Shuai Li [51]): (a) three-dimensional model of the final realm of open-pit mining; (b) maximum principal stress diagram; (c) Z-directional displacement diagram of the overall model; (d) the locations of the open-pit boundary slope displacement monitoring points.
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Figure 5. Three-dimensional geological model of Xinqiao Mine. Source: Figure 5 adapted from [http://www.xqmcl.com/] (accessed on 8 July 2025).
Figure 5. Three-dimensional geological model of Xinqiao Mine. Source: Figure 5 adapted from [http://www.xqmcl.com/] (accessed on 8 July 2025).
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Figure 6. Capacity transition diagram of Xinqiao Mine.
Figure 6. Capacity transition diagram of Xinqiao Mine.
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Figure 7. Overview of mining operations at Xinqiao Mine: (a) overview of open-pit mining operations at Xinqiao Mine; (b) underground mining site at Xinqiao Mine. Source: Photographs by Shuai Li.
Figure 7. Overview of mining operations at Xinqiao Mine: (a) overview of open-pit mining operations at Xinqiao Mine; (b) underground mining site at Xinqiao Mine. Source: Photographs by Shuai Li.
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Figure 8. Numerical simulation model of the thickness of the top column of the composite realm. Generated by ANSYS (ANSYS, Inc., Canonsburg, PA, USA), version 2022 R1. Available online: https://www.ansys.com/ (accessed on 28 July 2025).
Figure 8. Numerical simulation model of the thickness of the top column of the composite realm. Generated by ANSYS (ANSYS, Inc., Canonsburg, PA, USA), version 2022 R1. Available online: https://www.ansys.com/ (accessed on 28 July 2025).
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Figure 9. Schematic illustration of the upward horizontal layered backfilling mining method (modified from Shuai Li [23]): I-I shows the front view of the extraction method, II-II shows the left side view of the extraction method, and III-III shows the top view of the extraction method.
Figure 9. Schematic illustration of the upward horizontal layered backfilling mining method (modified from Shuai Li [23]): I-I shows the front view of the extraction method, II-II shows the left side view of the extraction method, and III-III shows the top view of the extraction method.
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Figure 10. Mining equipment and backfilling system at Xinqiao Mine: (a) boomer 281 rock drill dolly; (b) diesel front loader; (c,d) o-site layout of the Xinqiao Mine backfilling system. Source: Photographs by Shuai Li.
Figure 10. Mining equipment and backfilling system at Xinqiao Mine: (a) boomer 281 rock drill dolly; (b) diesel front loader; (c,d) o-site layout of the Xinqiao Mine backfilling system. Source: Photographs by Shuai Li.
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Figure 11. Tailings management and ecological restoration at Dongguashan Copper Mine: (a) schematic diagram of the tailings solidification backfilling project for the Dongguashan Copper Mine open pit; (b) site layout of the tailings transportation pipeline; (c) site view of the open-pit ecological restoration project. Source. All images adapted from [http://www.tlys.cn/index.aspx] (accessed on 9 July 2025).
Figure 11. Tailings management and ecological restoration at Dongguashan Copper Mine: (a) schematic diagram of the tailings solidification backfilling project for the Dongguashan Copper Mine open pit; (b) site layout of the tailings transportation pipeline; (c) site view of the open-pit ecological restoration project. Source. All images adapted from [http://www.tlys.cn/index.aspx] (accessed on 9 July 2025).
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MDPI and ACS Style

Li, S.; Su, W.; Yin, T.; Dan, Z.; Peng, K. Research Progress and Typical Case of Open-Pit to Underground Mining in China. Appl. Sci. 2025, 15, 8530. https://doi.org/10.3390/app15158530

AMA Style

Li S, Su W, Yin T, Dan Z, Peng K. Research Progress and Typical Case of Open-Pit to Underground Mining in China. Applied Sciences. 2025; 15(15):8530. https://doi.org/10.3390/app15158530

Chicago/Turabian Style

Li, Shuai, Wencong Su, Tubing Yin, Zhenyu Dan, and Kang Peng. 2025. "Research Progress and Typical Case of Open-Pit to Underground Mining in China" Applied Sciences 15, no. 15: 8530. https://doi.org/10.3390/app15158530

APA Style

Li, S., Su, W., Yin, T., Dan, Z., & Peng, K. (2025). Research Progress and Typical Case of Open-Pit to Underground Mining in China. Applied Sciences, 15(15), 8530. https://doi.org/10.3390/app15158530

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