Abstract: Frost damage in tunnels located in cold regions poses a significant threat to the stability of engineering structures and operational safety. To address this challenge, a basic model for all-round passive insulation and anti-freezing is established using an off-wall thermal insulation method, fully considering the low thermal conductivity of air. Additionally, corresponding active heating measures are proposed, forming various anti-freezing methods that combine passive cold prevention with active heating. The involved multi-field coupling is theoretically analyzed. The energy-saving and thermal insulation performances of the passive and active coupling anti-freezing methods are compared and evaluated using COMSOL Multiphysics numerical simulation. Finally, an integrated anti-freezing method that combines the advantages of passive cold prevention and active heating is proposed, enabling their synergistic effect. The results show that: (1) Hot air with a velocity of 0.5 m/s and a temperature of 1 °C can prevent freezing damage in the upper lining and invert, and the positively accumulated temperature air in the middle of the tunnel can be directly utilized, resulting in an energy-saving and effective solution. (2) A heating cable with a line power of 1.1 W/m placed in the air layer of a hollow insulation structure pipe ensures that water inside the pipeline remains unfrozen, achieving 94.5% energy-saving efficiency compared with directly laying the heating cable, and preventing adverse effects from water on the heating cable. (3) The optimal spacing of the transverse heating cable laid between the invert lining and insulation layer is 2 m, and the longitudinal auxiliary heating cable set at the arch foot provides an effective anti-freezing effect.

Key words: tunnels in cold regions, off-wall thermal insulation method, hot air, hollow insulation structure pipe, numerical simulation, thermal insulation effect