## 3 Results and Discussion
### Active Freezing Phase  
The frozen soil between pipe-roofs is the key region for water sealing. Therefore, the temperature of this region can reflect the sealing effect directly. Figure 11 shows the temperature change curve of the monitoring point of C2-N-16, which represents three freezing schemes during the active freezing phase. The development and thickness of the frozen soil curtain can be shown in Figure 12[19,20].  
In the synergy freezing mode of the solid pipe and hollow pipe, soil temperature between the pipe roofs experienced a rapid cool down in a short time. Frozen soil was shaped in about 10 days, and the completed water-sealing curtain was formed in around 20 days around. At the completion of active freezing for 60 days, the frozen soil curtain developed into bulge-like shape between pipes, shown in Figure 12a. The maximum thickness of frozen soil between the steel pipes reached 2.9 m, which met the design value of 2 m. The soil freezing effect between pipes was strengthened because of the synergy effect. Such a non-uniform profile of frozen soil obviously improved the sealing performance between steel pipes.  
Compared with the synergy freezing mode, frozen soil was formed relatively slower in the single freezing mode of solid pipe. It took approximately 30 days for the monitoring points between steel pipes to reach the freezing point. After 45-50 days' active freezing, a completed water-sealing curtain was formed. Meanwhile, according to Figure 12b, it can be seen from the cloud chart that the air temperature in the hollow pipe and the upper part of the hollow pipe was relatively higher, only the temperature of soil between steel pipes was below freezing point. Moreover, no frozen soil was formed in the upper part of the hollow pipe after 60 days of active freezing.  
Through the comparison of the frozen soil development of the two above schemes after 60 days' active freezing, the important role of enhancing tubes inside the hollow pipe was highlighted. Enhancing tubes will not only strengthen the freezing effect of soil between steel pipes, but also ensure the formation of frozen soil above the hollow pipe.  
Many schemes were also carried out in the test. Principal freezing of the solid pipe with hollow pipe as a complement was simulated, which means the solid pipe as the main and the hollow pipe as the auxiliary. Figure 11 shows the time-temperature curve of soil between steel pipes with 39-day hysteresis opening of the enhancing freezing tubes. As is shown in this figure, after opening the enhancing  
Figure 10: Position of monitoring points.  
freezing tubes, the cooling rate of soil between pipes has been significantly improved. At the end of the active freezing, the temperature of this region is almost equal to the temperature in the synergy freezing scheme of the solid pipe and hollow pipe. The maximum thickness of frozen soil between steel pipes reached 2.5 m, which met the minimum design value of 2 m. Besides, the comparison in Figure 11 also illustrates that the scheme of enhancing freezing tubes hysteresis opening can finally achieve the same freezing effect as the synergy scheme finally. Therefore, such a principal freezing scheme of the solid pipe with hollow pipe as complement can not only ensure the safety of the frozen soil curtain, but also realize freezing with subsection, reducing construction cost and saving energy.  
Figure 11: The time-temperature curve of soil between steel pipes (C2-N-16).  
Figure 12: Cloud chart of temperature field of active freeze period after 60 days’ freeze: (**a**) Cloud chart of synergy freezing of the solid pipe and hollow pipe; (**b**) Cloud chart of single freezing of the solid pipe; (**c**) Cloud chart of principal freezing of the solid pipe with hollow pipe as complement.