## 2 Project Overview  
The traffic network of the first phase of Hohhot Metro Line 2 is an \"L\"-shaped backbone line running north-south, with a total length of 27.33 km and an average station distance of 1.166 km, including 24 stations that are underground. Among them, the right line of the shield tunnel starts and ends at DK15 + 497.653 to DK16 + 986.740, with a total length of 1490.226 m, including a long chain of 1.139 m. The left line starts and ends at DK15 + 497.653 to DK16 + 986.740, including a long chain of 1.047 m and a short chain of 2.005 m, with a total length of 14.33 km. There are two liaison tunnels constructed using the mining method. The No. 6 liaison tunnels are located at the mileage of DK16 + 500/left DK16 + 495.535, with an overburden of about 16.26 m at the top of the vault and a buried depth of about 21.91 m at the bottom of the water collection pit; the structure is located in the 3-4 chalk and 3-9 round sand layers. The distance between the two tunnel centers is 10 m, the tunnel radius is 2.75 m, and the thickness of the pipe sheet is 0.35 m. The structure has two layers of chalk and rounded sand. Due to its special geological structure, the surface water at the proposed site is slightly corrosive to the concrete structure and the reinforcing steel in the reinforced concrete structure.  
The area runs along the west lane of the Meteorological Bureau through Princess House Street and the Zadagai River to the north of the Genghis Khan Primary School in the new city. The terrain is relatively gentle, with the ground elevation ranging from 1062.776 to 1067.348 m, and the geomorphological unit is a pre-hill alluvial sloping plain. According to the borehole data and indoor geotechnical test results, the soil layers in the exploration area of the project are classified into the Quaternary Holocene Artificial Fill Layer (Q4 ml), the Quaternary Holocene to Upper Pleistocene Alluvium Layer (Q3-4 al + l), and the Quaternary Pleistocene Lake Layer (Q21) according to the deposition age and genesis type of the strata. According to the drilling survey, the groundwater at the site is submerged, and the depth of the stable water table measured by the borehole is 8.5-12.2 m, corresponding to an elevation of 1054.211-1055.166 m, with an elevation difference of 0.955 m and annual water level change of 1.5-3.0 m. The aquifers in this area are mostly strongly permeable, and the lower water barrier is mostly discontinuous and incomplete. This offers a major construction challenge. Figure 1 shows the schematic diagram of the geological stratigraphy, freeze tube construction around the contact channel, and the arrangement of the frozen tube on both sides of the contact channel.  
Figure 1: Cont.  
Figure 1: Freeze tube and left and right line tunnel borehole layout of the No. 2 contact channel. (**a**) Geological stratigraphy, (**b**) section of the freeze hole arrangement, (**c**) plan of the freezing hole on the side of the freezing station, and (**d**) plan of the freezing hole on the opposite side of the freezing station.  
The No. 2 contact channel consisted of 55 freeze holes, 8 temperature measurement holes, and 4 pressure relief holes. Of these, 42 freezing holes, 2 temperature measurement holes, and 2 pressure relief holes were arranged on the side of the machine room; 13 freezing holes, 6 temperature measurement holes, and 2 pressure relief holes were set on the opposite side. The specific layout characteristics of the temperature measurement tubes are shown in Table 1. In order to keep abreast of the changes in soil temperature with the freezing time, three to five measurement points were generally set up in each temperature measurement hole to monitor the soil temperature in real-time. The temperature measurement tubes were installed at the left and right ends of the connection channel, and the points were set at 0.5, 1.25, and 2 m, respectively, to monitor the development of temperature changes.