Study on air temperature and soil hydrothermal distribution patterns in shallow-buried metro tunnels in cold regions

• Developed a coupled model of soil hydrothermal and tunnel air temperature. • Clarified the air temperature variation patterns in the tunnel. • Studied the hydrothermal distribution patterns in soil around the tunnel. • Analyzed seepage impacts and mechanisms on tunnel thermal environments. As a critical infrastructure of urban rail transit, the thermal environment of subway tunnels significantly impacts passenger comfort, energy efficiency, and operational safety. This study focuses on a shallow-buried subway tunnel in a cold region in northwest China, establishing a tunnel air temperature prediction model and formulating equations for soil hydrothermal transfer. The research reveals the tunnel air temperature and surrounding soil hydrothermal patterns in the tunnel during typical winter and summer days, over the course of the year, and in the long term under the influence of vertical seepage. Furthermore, it analyzes the impact of various factors on the thermal environment of the tunnel. The findings indicate that the tunnel air temperature exhibits periodic fluctuations in sync with outdoor temperatures, albeit with a noticeable lag. During summer, the tunnel air temperature is higher than the wall surface temperature, with significant daily fluctuations, while the opposite phenomenon occurs in winter. During the operation of the subway system, the average temperature inside the tunnel gradually increases over time. By the 10th year of operation, the annual average air temperature in the tunnel rises from 16.51 ℃ to 17.79 ℃, eventually reaching a stable state. Seepage has a significant impact on the thermal environment of the tunnel, followed by train frequency, while the effects of soil thermal conductivity and volumetric heat capacity are relatively minor.