## 3 Results and Discussion
### Excavation Period  
The excavation of the Gongbei tunnel is a continuous and long process considering its short length. The bench cut method was adopted because of the huge excavation section. A trial excavation was conducted from 19 June, and lasted about one month. The official excavation started from 20 August 2016 and ended on 10 April 2017. The excavation speed was relatively slow and the process lasted a long time. During the excavation, some frozen soil was dug out, and the \"thermal attack\" from the air at the excavation face of the frozen soil was much fiercer than before. It had a negative effect on the freezing effect of the FSPF. Moreover, various excavation steps such as welding and lining also resulted in a non-negligible thermal disturbance of the frozen soil. Two typical areas (A3 and A4 in Figure 4) were selected to study the temperature variation near the boundary of the excavation face. Figures 8 and 9 show how the temperature at the selected areas changed over time during the excavation period. Figure 8: Temperature variation with time of measuring points at area A3 (see Figure 4). Figure 7: Cloud charts of the temperature field of typical measuring faces: (**a**) measuring face 02; (**b**) measuring face 20. Figure 8 shows the temperature variation of the measuring points with time at measuring face 01 between the 34 th and 35 th pipes (see Figure 4, A3 area) during the trial excavation, and it can be seen that, since the trial excavation started from 19 June, the temperature of measuring point D35-01-B4 increased slightly from \(-\)12 \({}^{\circ}\)C. After shotcrete lining, its temperature rose to the highest value of about \(-\)8 \({}^{\circ}\)C around 28 June, then decreased steadily, and finally wobbled around \(-\)8 \({}^{\circ}\)C. The remaining measuring points were slightly affected by the trial excavation. From the enlarged view of A3 in Figure 4, the B4 and B5 measurement points were closer to the excavation surface than other measuring points. The largest temperature rise of point D34-01-B4 was about 8 \({}^{\circ}\)C. Their temperature was lower than the freezing point of the frozen soil, indicating that the frozen soil between the 34 th and 35 th pipes existed during the trial excavation. This is because the construction progress during trial excavation was slower than that of official excavation, and the cut face was small. Furthermore, the excavation construction control was strict, and there was basically no over-excavation; thus, the corresponding air thermal erosion and construction thermal disturbances were small. Figure 9 shows the temperature change of the measuring point with time between the 18 th and 19 th pipes (A4 area in Figure 4) during the official excavation. It can be seen that the temperature of measuring point D19-29-B4 was most affected by the excavation, and the maximum temperature rose to about 0 \({}^{\circ}\)C. Its total temperature increase was about 16 \({}^{\circ}\)C, 8 \({}^{\circ}\)C higher than point D34-01-B4 in Figure 8. The temperature change of D19-29-B5 was almost the same as that of B4; thus, it is not shown in the figure. Combined with the enlarged view of A4 area in Figure 4, it is known that B4 and B5 were located on the excavation boundary. Thus, the part of the pipe near these points will be exposed to the air if there exists over-excavation. This is why the temperature of the B4 and B5 measuring points rose sharply during the excavation, even exceeding the melting point of the frozen soil. The excavation had limited effects on the remaining measuring points. The temperature rise of measuring point B6 did not exceed 5 \({}^{\circ}\)C, and it could maintain \(-\)15 \({}^{\circ}\)C or below even at its highest value. The influence of the excavation on the frozen soil close to measuring point S3 was almost negligible.