Research on Tee Position Selection for Tunnel Construction Ventilation Air Distribution

Literature Overview

This paper, authored by Gou Hongsong, Wu Yuanjin, Li Yongsheng, and Zhang Wenxin from China Railway Tunnel Survey and Design Institute Co., Ltd., was published in "Tunnel Construction" in 2016 (Vol. 36, No. 4, pp. 384-389). The study addresses a practical engineering challenge in tunnel construction ventilation: the optimal positioning of air-distribution tees (branch tees) in parallel ventilation duct systems that supply two excavation faces simultaneously. The research was funded by a major project from China Railway Tunnel Group Co., Ltd. (Project No. 2013-01). The keywords are tunnel, construction ventilation, and air-distribution tee.

Core Technical Approach

In tunnel construction, ventilation is essential for removing diesel exhaust from drilling equipment, controlling dust levels, and ensuring adequate oxygen supply for workers. When two excavation faces are advanced simultaneously using parallel ventilation ducts, the position of the tee (branch connection) that splits the main airflow between the two faces significantly affects the airflow distribution and the face velocities achieved at each excavation site.

The authors developed a theoretical framework based on the principles of fluid mechanics and ventilation engineering. The key insight is that the ratio of the main duct diameter to the branch duct diameter is the critical parameter governing the effect of tee position on the face airflow. This ratio determines the relative resistance of the main and branch ducts, which in turn controls the airflow splitting ratio at the tee.

The theoretical analysis was validated through three engineering calculation examples and one actual project case, demonstrating the practical applicability of the proposed method.

Key Technical Parameters and Results

The theoretical analysis reveals that when the diameter ratio D/d is less than a critical value, the tee position has a significant effect on the airflow distribution between the two faces. When D/d exceeds this critical value, the tee position has minimal influence on the airflow splitting. This finding provides a clear design criterion for tunnel ventilation engineers.

Theoretical Analysis and Validation

The airflow splitting at the tee is governed by the resistance balance between the two branch ducts. The resistance of each branch is composed of:

1. Frictional resistance: Proportional to the duct length, inversely proportional to the fifth power of the diameter (Darcy-Weisbach equation).
2. Local resistance: Associated with the tee fitting itself, bends, and other fittings in the branch duct.
3. Face resistance: The resistance at the excavation face, which depends on the face geometry and the ventilation hood configuration.

The tee position affects the frictional resistance of each branch duct differently. When the tee is closer to one face, the branch duct to that face is shorter, resulting in lower frictional resistance and higher airflow. However, the main duct segment between the fan and the tee also changes in length, affecting the total system resistance.

The three calculation examples and one project case demonstrated that:

• Optimal tee positioning can increase the minimum face airflow by 15-30% compared to poorly positioned tees.
• The diameter ratio D/d is the primary factor; when D/d > 1.3, the tee position effect is negligible.
• When the two faces are at different distances from the tee, the tee should be positioned closer to the farther face to equalize the airflow.

Engineering Practice Implications

For tunnel construction projects using parallel ventilation systems, the following practical guidelines should be followed:

1. Diameter ratio selection: Design the main and branch duct diameters such that D/d is within the range of 1.0-1.3 to allow effective control of airflow distribution through tee positioning.
2. Tee position optimization: For unequal face distances, position the tee closer to the farther face. The optimal position can be calculated using the resistance balance equation derived in the paper.
3. System balancing: If the two faces have different ventilation requirements (e.g., different equipment loads), the tee position should be adjusted accordingly to meet the more demanding face's requirements.
4. Flexible tee placement: In practice, the tee should be installed in a location that allows easy repositioning as the excavation faces advance, to maintain optimal airflow distribution throughout the project duration.
5. Monitoring and adjustment: Regular measurement of face airflows should be conducted to verify that the ventilation system is performing as designed, and the tee position should be adjusted if necessary.

Study Insights and Reflections

This paper addresses a practical and often overlooked aspect of tunnel ventilation design. The systematic theoretical analysis, combined with engineering validation, provides a reliable method for optimizing tee position in parallel ventilation systems. The finding that the diameter ratio is the key parameter simplifies the design process significantly. However, the analysis assumes steady-state flow conditions and ideal duct geometries. In practice, factors such as duct flexibility, connection leakage, and variable face conditions can affect the actual airflow distribution. Future research could incorporate transient flow effects and the influence of duct material properties (flexibility, leakage rate) on the ventilation performance.

Reference Value and Outlook

The methodology presented in this paper is directly applicable to tunnel construction projects that use parallel ventilation systems with tee fittings. The simple and practical approach to tee position optimization can be implemented with minimal additional cost or complexity. As tunnel construction techniques continue to advance, with increasing emphasis on simultaneous multi-face advancement, the importance of optimizing ventilation system design will only grow. This research contributes to safer and more efficient tunnel construction practices.