I. Core Functions and Layout Principles of Underground Mining Return Roadways

(1) Analysis of the Core Function of Return Airway巷道

As a critical component of mine ventilation systems, the return airway plays an essential role in exhausting stale air, balancing air pressure, and regulating the underground climate. Its core functions include: effectively separating contaminated air from fresh airflow through rational layout planning, thereby ensuring high-quality air at working faces; optimizing ventilation resistance via the network structure of the roadway system, thus reducing energy consumption of main ventilation fans; and providing a stable ventilation environment that supports safe mining operations. Particularly when multiple levels are being simultaneously mined, it is crucial to prevent contaminated air from flowing backward and compromising safety on upper and intermediate levels. For instance, in some large-scale metal mines with extensive extraction areas and numerous working faces, improper placement of return airways can lead to chaotic circulation of polluted air underground. This not only exposes workers to harmful, stagnant air, jeopardizing their health, but also increases the risk of hazardous conditions such as methane accumulation.

(II) Layout Principles for Different Mining Scales

Single-segment production mines adhere to the principle of "resource reuse," prioritizing the repurposing of upper-stage transport roadways as lower-stage return airways to minimize redundant excavation work. Key considerations include thoroughly assessing the surrounding rock stability of existing roadways, verifying whether their cross-sectional dimensions meet ventilation requirements, and ensuring seamless integration with facilities such as return air shafts and air doors. For instance, in a small-scale lead-zinc mine, during the next phase of extraction, the upper-stage transport roadway was reinforced and widened to accommodate return airflow needs. At the same time, the mine ingeniously leveraged its original ventilation shafts, enabling the ventilation system to operate efficiently while significantly reducing mining costs. Multi-segment simultaneous production mines follow the principle of "zone-specific independent ventilation." Whether or not to establish dedicated return airways depends on whether the flow direction of contaminated air from the lower stage could potentially impact production in the upper and middle segments. If there’s a risk of airflow crossover between upper, middle, and lower working areas, it’s essential to construct an independent return airway system, ensuring that each segment’s ventilation network remains stable and isolated. For example, in several large coal mines where multiple segments operate simultaneously at considerable depths, specialized return airways have been meticulously designed to prevent contaminated air from affecting other segments. Each segment is equipped with its own independent ventilation system, effectively safeguarding the safety of underground operations.

II. Single-Middle-Section Mine Return Airway Layout Mode and Engineering Case Studies

(1) Key Technical Points for Reusing Transport Roadways from the Previous Stage

Applicability Assessment System: The integrity inspection of roadway surrounding rock is the primary checkpoint for reuse. This process employs acoustic wave detection, where sound waves are emitted and reflected waves are received. By analyzing changes in wave velocity, it becomes possible to identify internal defects such as cracks or fractured zones within the surrounding rock. Meanwhile, ventilation resistance simulation calculations are conducted using Ventsim software to build a roadway ventilation model. Users input parameters like tunnel dimensions, surface roughness, and airflow rates to simulate how ventilation resistance varies under different operational conditions, thereby predicting whether the ventilation resistance of the reused roadway falls within acceptable limits. For fire and moisture protection performance evaluation, existing roadways are thoroughly examined for the integrity of their fire-resistant coatings and moisture-proof layers. Key indicators such as wall permeability and humidity levels are tested to assess the long-term effectiveness of these protective measures in the return-air environment. For instance, before reusing the 12905 working face return airway at Tao Yi Mine, the roof was reinforced with anchor cable support systems to enhance overall stability. As a result, roadway deformation was successfully controlled within 100 mm, ensuring compliance with the requirements for safe and efficient use of the return airway.

Key construction techniques for the renovation project: Follow the "Support first, then renovate" process. For locally damaged areas in the existing tunnels, implement a combined support system of anchor bolts, mesh, and shotcrete. First, spray concrete to seal the surrounding rock surface; next, lay down the steel reinforcement mesh; and finally, install the anchor bolts to significantly enhance the overall structural integrity of the tunnel. Additionally, optimize the layout of facilities within the tunnel by removing transportation equipment to create more space for airflow. At the same time, integrate advanced airflow guidance devices—such as deflectors and flow straighteners—to ensure uniform air velocity across at least 85% of the tunnel cross-section, thereby minimizing turbulence and reducing resistance losses.

(II) Typical Case: Practice of Roadway Retaining along Goaf at Taoyi Mine

In its single-level mining operations, Taoyi Mine has boldly embraced innovation by retaining the transport roadway from the previous stage along the goaf area, repurposing it as the return airway for the next phase. During the technical implementation, numerical simulations were used to determine that the optimal anchor cable inclination angle is 10°. This carefully chosen angle ingeniously leverages mechanical principles, effectively preventing the roof strata from rotating toward the goaf side and ensuring the stability of the roadway roof. During construction, the mine adopted the "one excavation, one exploration" method, conducting advance drilling every 5 meters to precisely assess the geological conditions ahead, thereby guaranteeing that the alignment error during roadway breakthrough remained below 200 mm. From a practical standpoint, this innovative approach has proven highly effective: it reduced roadway excavation volume by 30%, significantly cutting both excavation costs and time; meanwhile, ventilation energy consumption was slashed by 15%, optimizing the entire ventilation system and boosting energy efficiency. As a result, the mine has achieved a remarkable win-win outcome—enhancing both economic and safety performance.

III. Design and System Construction of Dedicated Return Airways for Multi-Level Mines

(1) Conditions for Setting Up Dedicated Return Air Ducts and Structural Design

Trigger Condition Determination: During multi-level mining operations, when both the upper and lower levels are simultaneously active, if the dust concentration in the contaminated airflow from the lower level exceeds 1.5 times the allowable limit of the upper level, or if the gas concentration difference between the two levels surpasses 0.5%, a dedicated return airway must be installed. For instance, in the southern section of the Tianxing Iron Mine, during simultaneous mining at two different levels, frequent blasting activities in the lower section generated dust levels far exceeding the permissible range of the upper level. Professional testing confirmed that the dust concentration in the lower-level contaminated airflow had already reached twice the upper level's allowable threshold. Therefore, to safeguard the health and operational safety of workers on the upper level, an additional dedicated return airway measuring 4.5 meters wide and 3.8 meters high was constructed. This design effectively prevents contamination of the upper level by the lower section's polluted air, ensuring safe production across all mining levels.

The system's key components: The dedicated return airway system consists of several critical elements. Among these, the independent return shaft serves as the core of the entire system, with a diameter of at least 3 meters to ensure adequate ventilation capacity. The return air shaft well acts as a crucial link connecting each intermediate section to the return shaft, with spacing no greater than 50 meters—this arrangement allows contaminated air to efficiently converge toward the return shaft in a timely manner. Equally important is the installation of damper groups: an automatic interlocking damper system is set up every 50 meters. When airflow passes through, the dampers automatically open; once the airflow has moved past, they promptly close and lock in place, preventing air from taking shortcuts. Finally, the dedicated return airway is connected to the stage transportation roadway via connecting tunnels, which are equipped with air velocity sensors. These sensors continuously monitor airflow conditions in real time, and if any abnormality in airspeed is detected, they immediately trigger an alert, enabling operators to make prompt adjustments.

(II) Application of the Southern Section Project at Tianxing Iron Mine

The southern section of the Tianxing Iron Mine employs a "double drifts on the hanging wall plus connecting tunnels" layout during multi-level mining operations. In areas with stable surrounding rock, dedicated return airways are excavated—zones where the rock is particularly hard and exhibits excellent stability, ensuring the long-term reliability of these critical ventilation pathways. During construction, advanced medium-to-deep hole smooth blasting techniques are utilized. This method allows precise control over the blast area, minimizing disturbance to the surrounding rock while achieving a borehole utilization rate exceeding 90%, thereby significantly boosting construction efficiency. Additionally, ahead-of-time exploration drilling is conducted just 5 meters before breakthrough, with each probe reaching 10 meters deep. This proactive approach enables miners to gain early insights into the geological conditions ahead, ensuring breakthrough accuracy remains within ±100mm. Since being put into operation, the dedicated return airway has delivered remarkable results: dust concentrations in the upper and middle mining levels have dropped by 40%, while the stability of the ventilation system has improved by 60%. These advancements provide robust support for the mine’s safe production and efficient extraction operations.

IV. Key Technologies and Safety Controls for Return Airway Construction

(1) Integrated Construction Safety Technical Measures

Measurement and Control System

During the through-driving construction of the return airway, measurement accuracy directly determines whether the two roadways can be precisely aligned. To achieve this, a combined measurement approach is employed: "total station + gyro-theodolite." The total station utilizes advanced optical-electronic distance measurement and electronic angle-measuring technologies, enabling rapid and highly accurate measurements of horizontal angles, vertical angles, and distances—providing essential planar positioning data for roadway construction. Meanwhile, the gyro-theodolite leverages a gyroscope to determine true north, offering unparalleled precision in measuring the roadway's azimuth angle while remaining unaffected by geomagnetic interference. This ensures reliable directional guidance even for long-distance tunneling projects. Throughout the construction process, every 5 meters of excavation requires the use of these two instruments to verify the roadway's centerline and waistline. The centerline serves as a critical reference point, precisely controlling the roadway's planar position, while the waistline governs its slope. By meticulously calibrating these lines, the errors are tightly managed within a range of ±50 mm, guaranteeing that the roadway advances exactly according to the designed direction and gradient. In the crucial final 10-meter stage before breakthrough, when the two roadways are about to connect, any measurement inaccuracies could significantly amplify their impact. Therefore, the frequency of measurements is intensified, with coordinate checks conducted three times daily to provide robust assurance for a smooth and successful connection. For instance, during the long-distance through-driving project of the lower coal return airway at the Sanjiaoh River Mine, the innovative "three-station method" using a total station and gyro-theodolite-based orientation technology was successfully implemented. This approach effectively minimized measurement errors caused by challenging terrain and geological conditions, ultimately achieving precise and seamless roadway alignment.

Ventilation Adjustment Strategy

The stability of the ventilation system is a critical factor in ensuring the safety of tunnel construction, especially before and after breakthroughs, when changes in the ventilation system can lead to safety issues such as gas accumulation and poor airflow. Twenty-four hours prior to the breakthrough, the chief team leader takes charge of preparing the ventilation system by installing a temporary fan in the connecting passage ahead of time. This temporary fan is set to deliver an air volume of 200 m³/min, capable of supplying fresh airflow during the breakthrough and diluting any potentially accumulated hazardous gases like methane. During the breakthrough itself, a "gradual, small-scale blasting" method is employed, with each blast using no more than 2 kg of explosives. This approach effectively controls the intensity of the shock waves generated by the blasts, preventing damage to ventilation equipment such as air ducts and dampers—thereby maintaining the integrity and stability of the entire ventilation system. For instance, during the breakthrough construction between the return air inclined shaft and the intake ventilation connecting passage at a certain mine, meticulous adjustments were made to the ventilation system, and blasting parameters were strictly controlled, ultimately enabling a safe and successful breakthrough operation.

(II) Surrounding Rock Support and Disaster Prevention

Dynamic Support Technology

In return air roadways under different geological conditions, the surrounding rock characteristics vary significantly. Particularly in soft-rock roadways, the surrounding rock exhibits low strength, substantial deformation, and poor self-stabilizing ability, placing higher demands on support technologies. For soft-rock roadways, a combined support technique—using "anchor mesh cables plus shotcrete"—is employed. Anchors are arranged at intervals of 1.2m × 1.2m, penetrating deep into the surrounding rock to consolidate fragmented rock masses through anchoring forces, thereby forming a robust load-bearing structure. Meanwhile, anchor cables, with a length of 7 meters and pre-tightening forces no less than 150 kN, provide even greater anchoring capacity and suspension effects, further enhancing the stability of the roadway roof. Every 10 meters of excavation, rock displacement is monitored by embedding displacement monitoring points and measuring changes using instruments such as total stations or convergence meters. If the deformation rate exceeds 5 mm/day, it indicates that the surrounding rock has entered an unstable state. In such cases, steel arches are promptly added to reinforce the support system. Steel arches boast superior load-bearing capacity, effectively controlling rock deformation and preventing roadway collapse. At Qianqiu Coal Mine, during the expansion and renovation work on the 21st district's track downhill section, a combined support technique integrating "anchor mesh spray + anchor cable trusses + post-grouting behind walls" was successfully implemented, effectively addressing the challenging support issues associated with deep-seated soft-rock roadways.

Fire and Gas Prevention and Control

Ventilation roadways contain flammable and explosive materials such as gas and coal dust, and electrical equipment may also pose potential ignition sources. Therefore, fire and gas prevention measures are of paramount importance. To ensure early detection, CO sensors and gas sensors are installed every 50 meters along the ventilation roadway. The CO sensor is set to trigger an alarm at concentrations ≥24 ppm—indicating a potential fire hazard that requires immediate investigation and resolution. Meanwhile, the gas sensor is calibrated to activate an alarm when gas levels exceed 1%. If the gas concentration surpasses this threshold, the system promptly issues a warning, prompting staff to take swift action, such as enhancing ventilation or halting operations. Additionally, an automatic misting system for dust suppression is installed as a complementary measure. When either the gas or dust sensors detect abnormal concentrations, the misting system automatically activates, spraying fine droplets into the air to settle suspended particles and significantly reduce the risk of gas explosions. Finally, laying power cables within the roadway is strictly prohibited, as operational power cables could generate electric sparks, potentially igniting gas or triggering fires. If lighting cables are absolutely necessary, only intrinsically safe mining-grade cables should be used, accompanied by protective conduit installation. Mining-grade intrinsically safe cables are inherently designed to prevent ignition under fault conditions, ensuring they cannot produce enough energy to ignite gas. Coupled with the added safeguard of conduit protection, these measures further enhance the overall safety of the electrical system in the roadway.

V. Optimized Design of Return Airway and Future Development Trends

(1) Multi-Dimensional Optimization Strategy

Improved ventilation efficiency

In the design and optimization of return air roadways, enhancing ventilation efficiency is crucial. By leveraging CFD simulation software, we conducted an in-depth analysis and optimization of the roadway's cross-sectional shape. For instance, at a certain mine, the original semi-circular arch roadway section was redesigned into a three-center arch configuration. Simulation results revealed that this modification led to a significant 12% reduction in the wind resistance coefficient. Lower wind resistance directly translates to reduced ventilation resistance, meaning the fan encounters less opposition during operation—thus cutting down on energy consumption and boosting overall ventilation efficiency. Additionally, installing guide vanes at the entrance of the return air shaft proves to be an effective measure. These guide vanes help channel and direct airflow more efficiently, ensuring smoother convergence into the return air shaft and improving airflow convergence efficiency by up to 20%. As a result, stale air can be expelled from the mine more rapidly, creating a fresher and healthier working environment for underground operations.

Intelligent Monitoring

With the rapid advancement of IoT technology, deploying an IoT monitoring system in return airways has become a crucial approach to enhancing the reliability of ventilation systems. This system integrates real-time data collection for 12 key parameters, including wind speed, wind pressure, and dust concentration, and uses sensors to swiftly transmit this information to the monitoring center. More importantly, by leveraging AI algorithms, the system can perform in-depth analysis of the collected data, enabling it to predict potential abnormalities in the ventilation system well in advance. When an anomaly occurs, the system responds within 10 seconds or less, instantly triggering alerts to notify staff so they can take immediate action, thereby preventing safety incidents caused by ventilation system failures. For instance, if the system detects a sudden drop in wind speed in a particular area, the AI algorithm can analyze historical data alongside real-time parameters to determine whether the issue stems from factors such as duct blockages or fan malfunctions—and promptly provide actionable recommendations for addressing the problem.

(II) Integration of Green Mining and Low-Carbon Technologies

Waste Heat Recovery and Utilization

Against the backdrop of promoting green mining and energy conservation with reduced emissions, recovering and reusing the waste heat from polluted air discharged in return-air roadways is highly significant. Installing heat-exchange devices at the outlets of return-air shafts can effectively extract the thermal energy contained in the contaminated airflow. This recovered heat can then be utilized for ground-based heating systems, enabling a second round of energy utilization. For instance, at a large-scale mine, the installation of heat-exchange equipment has resulted in annual savings of approximately 200 tons of standard coal, significantly reducing the mine's reliance on conventional energy sources, cutting carbon emissions, and simultaneously lowering heating costs—thus achieving a win-win scenario that benefits both the economy and the environment.

Innovation in Tunnel Support Materials

Innovation in roadway support materials is also a key direction for the future development of return air roadways. Promoting the use of high-toughness fiber-reinforced concrete shotcrete—compared to traditional concrete shotcrete—this material boasts significantly greater toughness. When meeting the same support strength requirements, its thickness can be reduced by 20%, not only cutting material costs but also easing the load on the roadway. Moreover, the service life of high-toughness fiber-reinforced concrete shotcrete is extended by 30%, reducing both the frequency and cost of roadway maintenance. Additionally, actively exploring new polymer-based air door materials is another crucial step. These innovative polymer air doors deliver excellent sealing performance, effectively lowering air leakage rates to below 5%, thereby enhancing the efficiency of ventilation systems and minimizing energy waste—providing strong support for low-carbon, high-efficiency mining operations.

Conclusion

The design and construction of underground mining return airways must closely align with the scale of mine operations, geological conditions, and safety requirements. By adopting a differentiated strategy of "single-section reuse + multi-section independence," it is possible to achieve an optimal balance between ventilation efficiency and engineering costs. In single-section production mines, rational reuse of roadways can significantly reduce costs; whereas in multi-section mining scenarios, dedicated return airways are essential for ensuring safe ventilation. Throughout the construction process, rigorous through-measurement and ventilation control, coupled with scientifically sound surrounding rock support and robust disaster prevention measures, lay the foundation for roadway safety.