## 3 Field Test of the Dege Tunnel  
The Dege tunnel is located in a high-altitude area in the west of Sichuan province in China. This tunnel was constructed with a length of 2943 m, while the surrounding rock of the tunnel is quartz rock. Detailed descriptions of the Dege tunnel are shown in its cross-section in Figure 2. The thickness of the secondary lining and the primary support are 40 cm and 20 cm, respectively. Detailed descriptions of the longitudinal section of the tunnel and the air-lining heat transfer coupling model are shown in Figure 3. The mileage at the tunnel entrance (near Chengdu) is set as zero, and the entrance is set as zero point, which is given the name of K0 + 000 m. In the Dege tunnel, from K0 + 000 m to K0 + 185 m is a shallow-buried section with an average buried depth of 35 m, while from K0 + 185 m to K2 + 943 m is a deep-buried section with an average buried depth of 250 m. The tunnel altitude is 3282 m, and the average annual temperatures measured at the entrance and the exit are \(-0.85\)\({}^{\circ}\)C and \(-0.45\)\({}^{\circ}\)C, respectively. As the tunnel is a single-arch tunnel with two traffic lanes, a mechanical ventilation system is operated from 07:00 to 19:00, with a ventilation velocity of 5 m/s according to the safety guidelines, while the natural ventilation starts from 19:00 to 07:00. In the Dege tunnel construction, a 4 cm heat-insulating layer made of FLOLIC material (an organic composite material made by Si Chuan Foolkoong Insulation Supplies Co. Ltd, China) was applied at the entrance and the exit, with a 700 m length and a 600 m length, respectively. Figure 4 shows the installation process of the heat-insulating layer in the Dege tunnel. A one-year field test (from January 2016 to December 2016) was carried out to investigate the anti-freezing effectiveness of the heat-insulating layer in the tunnel. In total, there were ten test sections locating at 15 m, 115 m, 215 m, 415 m and 715 m from the opening at the entrance and the exit of the tunnel. In order to measure temperatures of the tunnel lining and the surrounding rock, there were two measuring holes for each test section in the vault and the spandrel respectively. The temperature sensor used in the field test was resistance wire temperature sensor, which was customized with high sensitivity. Temperatures of different positions in the tunnel were obtained by measuring the resistance of the temperature sensors  
Figure 3: Longitudinal section and the air-lining heat transfer model of the Dege tunnel. (**a**) Longitudinal section of Dege tunnel (K0 + 000 m–K2 + 943 m) and (**b**) The air-lining heat transfer model of Dege tunnel (mechanical ventilation in the longitudinal direction). Figure 2: Cross-section of Dege tunnel (in cm). in the tunnel. It was found in the field test that there was no main flow direction under the natural ventilation condition, and the velocity of the natural ventilation changed significantly even within the same day. In order to save computational costs in the numerical analysis, the changes of the natural ventilation velocity are ignored in the proposed numerical model. The velocity of the natural ventilation is considered as a constant value of 2.5 m/s, which is the average velocity calculated in the field test. Besides, the natural ventilation direction in the proposed model is assumed to change every other day. This assumption was made because no dominant natural ventilation direction was found in the field test, and the two flow directions (from the entrance to exit and from the exit to entrance) each accounted for about half time in the tunnel operation. The numerical model is established by using the fluid dynamic software FLUENT (ANSYS Inc., Canonsburg, PA, US). Establishing a three-dimensional numerical model according to the actual buried depth of the Dege tunnel is very difficult because the computational cost is significant. Thus, it is necessary to propose an appropriate numerical model to ensure the numerical accuracy with reasonable simplifications.