## 6 Heat-Insulating Layer Design Considerations
### Mechanical Ventilation Velocities (Using FLOLIC Material)
Figure 16 shows that the average temperature in December of the tunnel secondary lining in the longitudinal direction after applying a 4 cm heat-insulating layer under the same ventilation conditions as in Figure 15. In the figure, the heat-insulating layer is considered with five different lengths (0 m, 500 m, 600 m, 700 m and 800 m). It can be found that the temperatures of the secondary lining surface without covering by the heat-insulating layer decrease by increasing the length of the heat-insulating layer in the entrance section and the exit section. It is found that the end of the heat-insulating layer for each length creates a new negative temperature region. This is because of the heat-insulating performance. From the practical point of view, the length of the heat-insulating layer should be gradually increased until the negative temperature region is eliminated. Thus, an extension of the heat-insulating layer in the entrance section and the exit section should be considered carefully according to the ventilation conditions in practical design procedures. Figure 16a shows that the length of the negative temperature region is different when applying the heat-insulating layer with varying lengths under the same ventilation conditions (mechanical ventilation velocity of 5 m/s). In order to eliminate the negative temperature region, the appropriate design length of the heat-insulating layer under 5 m/s mechanical ventilation condition is considered  
Figure 16: Average temperature in December of the surface of the tunnel secondary lining in the longitudinal direction (with different lengths of the heat-insulating layer): (**a**) under 5 m/s mechanical ventilation condition and (**b**) under mechanical 6 m/s ventilation condition. Figure 15: Average temperature in December of the surface of the tunnel secondary lining in the longitudinal direction (with different thicknesses of the heat-insulating layer): (**a**) under 5 m/s mechanical ventilation condition and (**b**) under 6 m/s mechanical ventilation condition. to be 700 m/600 m in the entrance/exit section. When the heat-insulating layer is not applied on the tunnel lining surface, the length of the negative temperature region in the entrance/exit section is 450 m/327 m. Accordingly, the design length of the heat-insulating layer in the entrance/exit section is 1.56/1.83 times longer than the length of the negative temperature region when the heat-insulating layer is not applied on the tunnel lining. In Figure 16b, it is found that the mechanical ventilation velocity has a great influence on the design length of the heat-insulating layer, especially in the entrance section. The design length in the entrance section is increased from 700 m to 800 m when the mechanical ventilation velocity is increased from 5 m/s to 6 m/s. On the other hand, the design length in the exit section is not significantly influenced by an increase in the velocity of mechanical ventilation. This is due to the fact that the temperature of exit section is mainly affected by the natural ventilation instead of mechanical ventilation.