The mining units are divided into ore rooms and ore pillars, and a two-step mining method is adopted: "first mine the ore rooms, then mine the ore pillars." The ore pillars consist of three types: roof pillars, intermediate pillars, and floor pillars.

The mined-out areas formed during the mining process remain open and are not yet treated; their stability is primarily maintained by the load-bearing capacity of the pillars and the inherent stability of the surrounding rock mass.

The structural dimensions of mining blocks (including the size parameters of ore rooms and ore pillars) and the mining sequence must be determined comprehensively based on ore characteristics, surrounding rock conditions, mining methods, and the equipment used. The primary goal is to optimize ground pressure management and ensure safe production.

After the mining room has been fully mined, it is essential to promptly commence pillar recovery operations and simultaneously address the previously created void areas. This ensures a closed-loop process for resource recovery and the elimination of safety hazards.

I. Division of Mining Units and Two-Step Mining Mode

 

The core organizational structure of the open-pit mining method clearly divides the mining area into two major extraction units: mining rooms and ore pillars, and strictly adheres to a fixed operational sequence—“first mine the mining rooms, then mine the ore pillars.” This pattern is the key distinguishing feature that sets it apart from filling mining methods and caving mining methods.

The mining room, as the primary area for ore recovery, is the core space of mining operations. Its layout must be comprehensively planned in conjunction with the strike, thickness, and dip angle of the ore body. Typically, the mining room extends along the strike of the ore body, while its width is determined by factors such as the operational range of mining equipment, ore transportation efficiency, and the load-bearing capacity of the surrounding rock. The mine pillars play a crucial role in maintaining the stability of the mining face. Based on their placement, they can be clearly categorized into three types: roof pillars, inter-pillars, and floor pillars. Each type of pillar has its own specific function and works in coordination with the others to bear the applied forces. Roof pillars are located at the top of the mining room and directly support the weight of the overlying rock strata, thereby preventing roof collapse. Inter-pillars are situated between adjacent mining rooms, serving to separate these rooms and transfer lateral stresses, thus averting the collapse of the mining room sidewalls. Floor pillars, positioned at the bottom of the mining room, not only bear part of the weight of the mining room itself and the overlying rock strata but also provide structural support for ore transportation and equipment passage. They are commonly found in mining areas equipped with transport drifts or ore chutes.

The core logic of the two-step mining approach is “utilize first, recover later”: In the first step, when mining the ore room, the pillars remain intact and fully capable of bearing loads, thereby providing a safe working environment for the ore room’s extraction. At this stage, resources can be efficiently concentrated on extracting the ore while minimizing the interference of the pillars on the mining operations. In the second step, once the ore room has been completely mined, the pillars must be recovered using appropriate techniques before the stability of the mined-out area significantly deteriorates, thus avoiding resource waste. This phased approach not only ensures the safety of the mining process but also maximizes resource recovery rates—making it particularly suitable for mining scenarios with thick ore bodies and good continuity.

II. Open Status of Gob Areas and Stability Maintenance Mechanisms

During the mining process in a stoping room, as ore is extracted, the space previously occupied by the ore body gradually forms an open stope. A distinctive feature of the open-stope mining method is that this stope remains in an open, unbackfilled state throughout the mining period. During extraction, no backfilling or rockfall control measures are implemented; instead, the stability of the stope is maintained solely by the load-bearing capacity of the pillars and the inherent stability of the surrounding rock mass.

The formation of open-pit mining voids is an inevitable outcome of stepwise mining. The spatial morphology of these voids is directly related to the structural dimensions of the ore rooms and typically takes the form of rectangular or nearly rectangular cavities. The height and width of these cavities correspond precisely to the mining parameters of the ore room, while their length extends along the strike of the ore body. This open-pit configuration provides ample operational space for mining operations, facilitating the entry and exit of large-scale mining equipment, ore transportation, and personnel activities. As a result, spatial constraints during the mining process are significantly reduced, greatly enhancing operational efficiency. However, at the same time, open-pit mining voids place stringent demands on the stability of the mining area. Once the strength of the pillars or surrounding rock is insufficient, it becomes highly likely that safety incidents such as roof falls and sidewall collapses will occur. Therefore, maintaining the stability of the mining voids is a key technical consideration in the application of the open-pit mining method.

The maintenance of stable mining areas relies primarily on two core factors: First, the load-bearing capacity of mine pillars. The dimensions, distribution density, and ore strength of these pillars directly determine their ultimate load-bearing capacity. During design, geological-mechanical calculations must be performed to ensure that the pillars can withstand the pressure exerted by the overlying rock strata and surrounding rock masses, thereby preventing compressive or shear failure of the pillars. Second, the inherent stability of the surrounding rock. Parameters such as the integrity, compressive strength, and resistance to weathering of the surrounding rock determine the self-stabilizing capability of the side walls and roof of the mined-out areas. If the surrounding rock exhibits well-developed joints and fractures and has low strength, even with the installation of mine pillars, phenomena such as spalling or falling rock blocks may still occur. Therefore, a detailed investigation and assessment of the surrounding rock stability must be conducted prior to mining operations.

In the actual mining process, it is necessary to dynamically monitor the stability of open and mined-out areas by deploying... Stress sensor Equipment such as displacement monitoring points enables real-time tracking of stress conditions in mine pillars and deformation data of surrounding rock. Once any abnormalities are detected, immediate emergency measures—such as reinforcing mine pillars and providing support to the surrounding rock—must be taken to prevent instability in the mined-out areas and avoid safety accidents. This “natural maintenance + dynamic monitoring” mechanism is a critical safeguard for safe operations in open-pit mined-out areas.

3. Principles for Determining the Dimensions of Ore Block Structures and the Sequence of Mining Operations

The effectiveness of the sublevel caving method largely depends on the rational design of ore block dimensions and the scientific arrangement of the mining sequence. Determining these two factors requires a comprehensive consideration of multiple factors, including ore characteristics, surrounding rock conditions, mining techniques, and the equipment used. The core objective is to optimize ground pressure management and ensure safe production.

The structural dimensions of mining blocks primarily include the length, width, and height of the mining rooms, as well as parameters such as the dimensions and spacing of ore pillars. Regarding ore characteristics, the compressive strength, toughness, and degree of jointing in the ore directly affect the maximum allowable span of the mining rooms. If the ore has high strength and good integrity, the width of the mining rooms can be appropriately increased; conversely, if the ore is more fractured and has lower strength, the width of the mining rooms must be reduced, the number of ore pillars increased, and the stability of the mining area enhanced. The surrounding rock conditions serve as the core basis for parameter design: highly stable surrounding rocks can provide better lateral support for the mined-out areas, allowing for larger mining room dimensions; whereas less stable surrounding rocks require smaller mining room dimensions and denser ore pillar arrangements to reduce the load-bearing pressure on the surrounding rock.

The mining recovery technology and equipment also significantly influence the size of ore blocks. When using large-scale mechanized mining equipment, it is essential to ensure that the width and height of the mining rooms meet the operational space requirements of the equipment. For example, when operating large loaders, the width of the mining room must at least accommodate the space needed for the equipment to turn and load/unload ore. If traditional manual mining or small-scale equipment is used, the size of the ore blocks can be appropriately reduced to enhance the stability of the mining area. Moreover, the size of ore blocks must also take into account the efficiency of ore transportation: excessively large blocks may result in longer transportation distances and higher transportation costs, while excessively small blocks could reduce mining efficiency. Therefore, it is crucial to strike a balance between safety and efficiency.

The determination of the mining sequence also follows the principles of ground pressure management and safe production. Common mining sequences include advancing mining along the strike of the ore body, retreating mining, as well as upward mining and downward mining perpendicular to the strike of the ore body. Advancing mining involves gradually advancing from one end of the ore body toward the other and is suitable for scenarios where the surrounding rock has good stability and the ore body exhibits strong continuity. Retreating mining, on the other hand, proceeds from the end of the ore body toward its starting point; during this process, previously mined stopes can be utilized as ventilation and transportation passages, and void areas can be promptly addressed. This method is appropriate for scenarios with moderate surrounding rock stability. Upward mining advances from the lower part of the ore body toward the upper part, while downward mining does the opposite. The specific choice of mining sequence should take into account factors such as the dip angle of the ore body and the contact relationship between the ore and the surrounding rock, so as to avoid concentrating ground pressure due to improper mining sequences and thereby preventing instability in the mining area.

IV. Closed-loop requirements for pillar mining and goaf treatment

After the mining of the ore rooms is completed, promptly reclaiming the pillars and simultaneously treating the mined-out areas constitute the final stage of the open-stope mining method. This stage is crucial for ensuring mining safety and improving resource recovery rates, thereby forming a complete closed loop of “mining—recovery—treatment.”

The core objective of pillar recovery is to extract the ore resources contained within the pillars, while preventing pillar failure due to instability in the mined-out areas and thereby avoiding resource waste. Pillar recovery must be carried out within a window period during which the stability of the mined-out areas in the room-and-pillar mining system can still be ensured. Typically, segmented recovery methods are employed. Layered mining or processes such as overall collapse—specific process selection must be determined based on factors including the size of the ore pillars, the strength of the ore, and the geometry of the mined-out areas. For example, for smaller ore pillars with lower strength, a staged blasting approach can be adopted for gradual recovery; whereas for larger ore pillars with higher strength, layered mining must be employed to gradually relieve the pressure in the ore pillars, thereby avoiding sudden collapse of the mined-out area caused by a single, rapid extraction.

During the mining pillar extraction process, special attention must be paid to changes in the stability of the mined-out areas. As the mining pillars are gradually extracted, their load-bearing capacity will progressively decline, disrupting the stable equilibrium of the mined-out areas. Therefore, it is essential to strengthen monitoring and promptly adjust the extraction rhythm. If necessary, some safety pillars can be retained to ensure the safety of the extraction operations. Meanwhile, the ore generated from mining pillar extraction should be promptly removed from the working face to prevent accumulation within the mined-out areas, which could compromise operational safety and complicate subsequent treatment of the mined-out zones.

The treatment of mined-out areas is a critical step in eliminating safety hazards. If mined-out areas remain open for extended periods, the surrounding rock will gradually weather and spall over time, and the pillars may also suffer damage due to prolonged stress. Eventually, this could trigger large-scale collapses, posing a serious threat not only to the safety of nearby mining operations but also potentially leading to geological disasters such as ground subsidence and sinkholes. Therefore, after pillar extraction, it is essential to promptly treat the mined-out areas. Common treatment methods include: Filling treatment There are three categories: collapse treatment and sealing treatment.

Filling treatment involves injecting materials such as sand, gravel, tailings, and cementitious binders into mined-out areas to fill the cavity space, support the surrounding rock mass, and prevent collapse. This method is particularly suitable for scenarios where there are buildings, bodies of water, or critical facilities on the surface, and surface subsidence is unacceptable. Collapse treatment By means of blasting or other methods, the surrounding rock above the mined-out area is caused to collapse and fill the void. The self-weight of the collapsed rock mass helps maintain the stability of the underlying rock mass. This method is suitable for scenarios where the surrounding rock has poor stability and surface settlement is permissible. Closed treatment It involves sealing off the entrances and exits of the mined-out area to isolate it from the external working zone. This approach is suitable for scenarios where the mined-out area is relatively small, located at a considerable depth, and has minimal impact on the surrounding environment.

The treatment of mined-out areas must be planned and implemented in synchronization with the extraction of ore pillars, ensuring that once the ore pillar extraction is completed, the mined-out areas can be promptly addressed, thereby forming a safe closed-loop system. After the treatment is completed, the effectiveness of the treatment must also be evaluated through methods such as ground-penetrating radar surveys and borehole sampling to confirm the compactness of the backfilling or the sealing effect in the mined-out areas, thus ensuring that no safety hazards remain.

In summary, the four core features of the open-pit mining method are interrelated and indispensable. From the division of extraction units to the treatment of mined-out areas, they form a comprehensive technical system. The key to applying this method lies in flexibly optimizing various parameters according to specific geological conditions, resource characteristics, and the level of available technological equipment, thereby achieving efficient resource recovery while ensuring safety.