Evolution of temperature field and optimization of insulation length in cold-region tunnels for conventional-speed railways

Conventional-speed railway tunnels in cold regions are frequently subjected to frost damage caused by extremely low temperatures, posing a significant threat to tunnel operation. Studies focused on determining the appropriate insulation length under conventional-speed train operations remain limited. In this study, a full-scale, three-dimensional dynamic numerical model was developed using CFD dynamic mesh technology. Pearson correlation coefficients were employed to quantitatively evaluate the influences of both static environmental conditions and train-induced dynamic factors on the tunnel temperature field. The temperature field distribution pattern and the effectiveness of insulation layers under conventional-speed train operations were investigated. The key findings are as follows: (1) Tunnel frost damage is primarily governed by the surrounding rock temperature and portal temperature. Train speed is positively correlated with the degree of frost damage, whereas the train length exhibits only minor and statistically insignificant effects. (2) A theoretical formula for calculating the thermal insulation length in cold-region conventional-speed railway tunnels was proposed, with key parameters of surrounding rock ground temperature, portal temperature, and train speed taken into account. The rationality and accuracy of the formula were validated using existing theoretical frameworks and real-world engineering cases. (3) With a surrounding rock ground temperature of 5 °C and portal temperatures of -10 °C, -15 °C, and -20 °C, extruded polystyrene (XPS) demonstrated the best overall performance. A five cm-thick adhered XPS insulation layer could reduce frost depth by 60%-84% and shorten the negative-temperature length by 81.6%-100%.