Advances in indicators for defining cold levels within tunnels: Current state and future perspectives

Abstract

This study aims to identify key indicators influencing cold levels within tunnels and to explore methods for their precise definition. It provides a comprehensive review of various indicators currently used to characterise cold levels in cold-region tunnels and their application in freeze-proof design. Most existing studies rely on the mean air temperature of the coldest month (T_z) and the freezing depth of surrounding rock to define cold levels within tunnels. However, these approaches do not account for the cumulative freezing effects resulting from the difference between the mean annual air temperature (T_a) and original rock temperature (T_r) or the influence of time-dependent ventilation airflow velocities. Current methods for defining cold levels have not fully identified the fundamental indicators governing these levels. A systematic and integrated approach that combines meteorological indicators—such as T_z, T_a, T_r, and time-dependent ventilation airflow velocities—is necessary for a precise definition. This study introduces a novel conceptual approach that integrates these meteorological indicators, emphasising cumulative freezing effects and the impact of time-dependent ventilation airflow. It specifically proposes the use of the equivalent mean air temperature of the coldest month, the equivalent mean annual air temperature, and the difference between the equivalent mean annual air temperature and T_r as key parameters for defining cold levels. Furthermore, it explores the application of this approach in determining the optimal freeze-proof axis, designing insulation layers, and implementing active ventilation in cold-region tunnels with time-dependent ventilation airflow. This study provides a theoretical foundation for enhancing the operational safety of cold-region tunnels.