## 7 Conclusions  
Based on the proposed three-dimensional numerical model, a comprehensive parametric study is carried out to examine the heat preservation and anti-freezing effectiveness of the heat-insulating layer in tunnel, taking into account the influence of tunnel ventilation conditions. Furthermore, the influence of different mechanical ventilation velocities as well as different heat-insulating materials on the design parameters of heat-insulating layer is carefully discussed. Conclusions are surmised as follows:  
1. According to the field test results, the proposed design parameters of the heat-insulating layer for the Dege tunnel can meet the anti-freezing requirement. And the accuracy of the proposed numerical model is verified by using the field test data, with the numerical results showing consistency with the field test results in terms of the curve trend and the data magnitude. 2. The monthly average temperature contours of the airflow in the tunnel are parabolic curves, with the temperature gradient being larger in the near-wall regions while smaller in the tunnel center. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Material Type**} & \multirow{2}{*}{**Covering Section**} & \multicolumn{2}{c}{**Different Lengths (m) for**} & \multicolumn{2}{c}{**Lining Length in Negative**} & \multirow{2}{*}{**Reasonable**} \\  & & **Insulating Layer with** & **Temperature at the End of** & & **Lengths (m)** \\  & & **Certain Thickness** & **Insulating Layer (m)** & & \\ \hline \multirow{4}{*}{FLOLIC} & \multirow{2}{*}{Entrance} & 500 (4 cm thickness) & 74 & \multirow{2}{*}{700} \\  & & 600 (4 cm thickness) & 25 & & \\  & & 700 (4 cm thickness) & 0 & & \\ \cline{2-5}  & \multirow{2}{*}{Exit} & 500 (4 cm thickness) & 52 & \multirow{2}{*}{600} \\  & & 600 (4 cm thickness) & 0 & & \\  & & 700 (4 cm thickness) & 0 & & \\ \hline \multirow{4}{*}{Polyurethane} & \multirow{2}{*}{Entrance} & 600 (6 cm thickness) & 85 & \multirow{2}{*}{800} \\  & & 700 (6 cm thickness) & 37 & & \\ \cline{1-1}  & & 800 (6 cm thickness) & 0 & \\ \cline{1-1} \cline{2-5}  & \multirow{2}{*}{Exit} & 600 (6 cm thickness) & 48 & \multirow{2}{*}{700} \\  & & 700 (6 cm thickness) & 0 & & \\ \cline{1-1}  & & 800 (6 cm thickness) & 0 & & \\ \hline \multirow{4}{*}{Stem method aluminosilicate sheet} & \multirow{2}{*}{Entrance} & 600 (6 cm thickness) & 33 & \multirow{2}{*}{700} \\  & & 700 (6 cm thickness) & 0 & & \\ \cline{1-1}  & & 800 (6 cm thickness) & 0 & & \\ \cline{1-1} \cline{2-5}  & \multirow{2}{*}{Exit} & 600 (6 cm thickness) & 17 & \multirow{2}{*}{600} \\  & & 700 (6 cm thickness) & 0 & & \\ \cline{1-1} \cline{2-5}  & & 800 (6 cm thickness) & 0 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Optimization of design length for different heat-insulating materials. The airflow outside the tunnel requires a longer time to reach the same airflow temperature after applying the heat-insulating layer, when compared with the case having no heat-insulating layer. Furthermore, the heat-insulating layer prevents the heat transferring from the surrounding rock to the airflow in cold months (from January to March and from October to December), which plays a role of heat preservation. On the other hand, the heat-insulating layer prevents the heat transfer from the airflow to the surrounding rock in warm months (from April to September), which plays a role of thermal insulating. 3. After applying the insulating layer made of the FLOLIC material, the monthly average temperatures above the lane plate are always positive even in the coldest month, and the temperature gradient becomes smaller. Attaching the insulating layer to the lining wall would not protect the lane plate and the tunnel springing from damages from frost. The surface temperatures at different locations in the same tunnel cross-section from the lowest to highest points are at the center of lane plate, the tunnel vault, the tunnel spandrel and the tunnel springing, respectively. 4.