Field observations on temperature characteristics and insulation performance of an extra-long highway tunnel in high-altitude cold regions

Abstract

Temperature characteristics along tunnels in cold regions (CRTs) are the basis for insulation design and frost damage prevention. However, the temperature distribution patterns inside different CRTs generally differ significantly, particularly inside extra-long CRTs. This study conducts simultaneous field observations of air temperatures and wind data outside the tunnel portals, as well as air and lining temperatures along a 7527-m-long highway tunnel for over two years, thus exploring the driving effect of microclimates at the tunnel site on the interior temperature distributions. The results show that, due to the mountain climate effect, the air temperatures and wind data at the nearest municipal meteorological station, the tunnel entrance and exit differed considerably. Based on the interior air temperatures along the tunnel, the entire tunnel can be categorized into the entrance, middle and exit sections. At the entrance and exit sections, as the distance from the tunnel portals increased, the annual averages of the air temperatures rose at gradients of 0.06 and 0.07 °C per 100m, and the annual amplitudes decreased at gradients of 0.16 and 0.10 °C per 100m, respectively. However, the annual averages and annual amplitudes of the air temperatures in the middle section remained almost unchanged. Good insulation performance was observed within the insulated entrance (1000m long) and exit section (1200 m long), with the temperature differences between the two sides of the insulation layer ranging from 3.6 to 6.0 and from 4.2 to 3.2 °C, respectively. In the cold season, the frost profile along the tunnel was asymmetric. The freezing indices of the concrete lining within the entrance and middle sections were considerably larger than those within the exit section. Such as asymmetric frost profile along the tunnel is likely influenced by the chimney effect due to the air temperature difference between the entrance and exit, and the prevailing wind at the entrance during cold seasons. The field data and analyses will offer valuable case-study insights and could inform insulation design guidelines for the tunnel, pending further validation across varied geographies and tunnel configurations.