The effect of freeze-thaw cycles on the frost resistance of energy tunnels in cold regions

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Seasonal temperature fluctuations in cold regions induce freeze–thaw cycles that can significantly impact the structural safety and maintenance of tunnels. This study uses the Daban Mountain Tunnel as a case study to develop a full-scale numerical model for analyzing the long-term heat transfer behavior of energy tunnel portal sections under freeze–thaw conditions. The numerical model's reliability was verified by comparing with laboratory-scale experiments. Based on this validated model, the study investigates the long-term evolution of temperature fields within the tunnel lining and surrounding rock, as well as the development of freezing depth under freeze–thaw cycles. The results show that under freeze–thaw cycles, the heat exchange efficiency at the tunnel portal can decrease by up to 77%, negatively related to seasonal variations in ambient temperature. After one year of continuous heat pump operation, the freezing depth along the tunnel lining surface decreases by 50–96% compared with unheated conditions. The freezing depth also exhibits a downward gradient across the tunnel cross-section, with the greatest depth at the invert, where no heat-exchange pipes are installed. These findings highlight the importance of implementing supplementary frost protection measures at the invert to ensure comprehensive freeze prevention and long-term stability of energy tunnels in cold regions.