Numerical Study on Thermal Insulation of a Roadway Tunnel at Southeast Edge of the Qinghai-Tibet Plateau

Tunnels in seasonal frost regions are heavily affected by damage related to icing problem and cyclic freeze-thaw of surrounding rock. To mitigate frost related damage, thermal insulation is widely used at entrance and exit section of tunnels. In thermal insulation design, length of the insulated section and thickness of the thermal insulation layer (TIL) are two important parameters. In this study, a coupled mathematical model of heat and moisture transfer was built for tunnels in cold regions. Then, field observed air temperatures at entrance section of a roadway tunnel at Altun Mountain, southeast edge of the Qinghai-Tibet Plateau, were collected and analysed. Based on the mathematical model and field observed inside air temperatures, seasonal freeze-thaw of surrounding rocks of the tunnel entrance was numerically investigated with consideration of the thicknesses and thermal conductivities of insulation layer. The results showed that, within the tunnel, the air has its mean annual temperature increase and the its amplitude decrease with increase in the distance from the entrance. The significant variations mainly occurred within the section 375 m from the entrance. Without the TIL, the maximum seasonal freeze depth of surrounding rocks at the entrance can reach to 1.5 m. To ensure that the surrounding rocks do not experience freezing, the minimum thickness of the TIL with a thermal conductivity of 0.037 W/(m·°C) should be 7 cm. While when the thickness of the TIL is 5 cm, its thermal conductivity should be not greater than 0.014 W/(m·°C).