## 3 Results and Discussion
### Active Freezing Period  
Firstly, the temperatures of the measuring points in area A1 of Figure 4 were analyzed. The area was located at the top of the tunnel section, which was just 5 to 6 m below the ground surface. Moreover, the longitudinal measuring face 02 was also near the east working shaft. Therefore, this area belonged to the weakest area of the entire frozen curtain where thermal disturbance was fairly strong. Figure 5 shows the temperature variation with time of the measuring points at measuring face 02 between the third and fourth pipe during the positive freezing period. It can be seen from the figure that, with the opening of the master freezing tubes in late February, the temperatures of the soil measuring points and the wall measuring points decreased, except for measuring point D04-02-S2 which was far from the freezing tube. When the enhancing freezing tubes were opened on 11 March, the temperature of each measuring point decreased significantly, until 12 May when the temperature of D04-02-S4, which was near the design boundary of the frozen soil curtain (see Figure 4, A1 area, enlarged view), dropped to the freezing point of the frozen soil. This indicated that, after about 75 days of freezing, the development of frozen soil between the third and fourth pipes basically met the design requirements, and the water resistance performance between the pipes was initially satisfied. Since then, due to the maintenance work of the refrigeration system, the temperature of the brine was raised, and the temperature drop of each measuring point slowed down or even started rising. After the salt water temperature was lowered again on 1 June, the temperature of each measuring point began decreasing again. Then, due to the influence of trial excavation (see the next section), the temperature of measuring points D04-02-S4 and D04-02-S5 had a relatively obvious rise, while the temperature of measuring point D04-02-B5 was basically unaffected because it was much closer to the enhancing tube than others. It is worth mentioning that, on 1 August, Hurricane Nicole struck; thus, the refrigeration system was forced to close, and the brine circulation stopped. The temperature of each measuring point then increased, especially the temperature of measuring point D04-02-B5, which obviously increased. When the hurricane was over and the brine circulation was restored, the temperature of the relevant measuring points dropped rapidly until official excavation began on 20 August. From the perspective of the whole process of the active freezing period, the frozen soil curtain developed to the design thickness after more than two months of refrigeration, despite being affected by the trial excavation and hurricane. The temperature of the relevant measuring points was basically kept below the freezing point temperature, and the thickness of the fro  
Figure 5: Temperature variation with time of measuring points at area A1 (see Figure 4). able to satisfy the design requirements. This section is an analysis of the temperature variation of the measuring points in area A2 of Figure 4. The area was located in the lower part of the tunnel section, and, in the longitudinal direction, the measuring face 17 was located in the middle of the tunnel. Therefore, the A2 area was less disturbed by external heat and the development condition of the frozen soil was good. Figure 6 shows the temperature variation with time of the measuring points at measuring face 17 between the 21 st and 22 nd pipes during the positive freezing period. The temperature change trend of each measuring point in the figure was similar to that of Figure 5 as a whole. The difference was that, despite measuring point D22-17-S1 being far away from freezing pipes, the temperature of the point decreased as the brine circulated and finally fell below freezing point.