# Key Issues in Using the Freezing Method in Power Tunnel Rehabilitation Projects
## 1 Introduction  
Since the 20 th century, the rapid economic development of cities, coupled with the gradual expansion of urbanization and the resulting population pressure and insufficient space above ground, has led to the full development and utilization of underground space as a preferred method to address these issues in many countries. Underground tunnels, which are engineering structures widely used in the ground, represent an efficient use of underground space. However, when constructing urban tunnels through saturated, water-rich, soft ground, the density of urban buildings necessitates the prevention of groundwater seepage during tunnel construction without affecting surrounding buildings, as well as the prevention of collapse accidents during the excavation of underground tunnels. Thus, meeting the high requirements of urban tunnel construction presents a challenging problem that needs to be solved [1, 2, 3, 4]. Artificial ground freezing (AGF) technology is a method that uses freezing pipes to lower the temperature of the soilaround the area to be excavated, gradually reducing the natural soil temperature to become permafrost, forming a permafrost body with a certain strength [5; 6; 7; 8; 9]. This technology, after a certain period of active freezing, can form a stable and solid frozen soil curtain, which can prevent the flow of groundwater. Under its support, the construction of underground projects can be completed safely and efficiently. Therefore, the artificial ground freezing method (AGF) has become one of the important engineering techniques for underground engineering research, and its reinforcement of the soil to be excavated and excellent water sealing properties have important engineering value [10; 11; 12; 13; 14; 15; 16]. In recent years, scholars at home and abroad have reported relevant achievements in the study of artificial ground freezing in underground restoration projects. [PERSON] et al. [17], in conjunction with the repair project of Shanghai Metro Line 4, used the horizontal freezing method to repair collapsed tunnels and analyzed the impact of each construction process on the temperature of the permafrost wall. Through on-site measurement and analysis, the results showed that temperature is an important index to judge the characteristics of the permafrost wall. By reasonably arranging temperature monitoring points and adopting informationized temperature monitoring, the development of the freezing wall can be grasped, ensuring the safety of freezing method-based construction. [PERSON] et al. [18], in the context of submarine fault shield repair construction, considered the adverse effect of the salt content of seawater on freezing, such as reducing the thickness of the permafrost curtain and decreasing its strength. The freezing program was specifically designed with measurement of the freezing and construction process in mind. Through calculation and analysis of permafrost temperature monitoring data, it was shown that the hood-shaped permafrost curtain achieved good results in submarine shield repair, verifying the feasibility of this freezing scheme in submarine shield tunnel repair. [PERSON] and [PERSON] [19], for shield tunnel construction under the river, after the occurrence of sudden water and sand accidents, used the filling and freezing method to form a temporary freezing blocking wall and then discharged the water inside the tunnel to construct a permanent blocking wall for the next step of the tunnel project, providing a guarantee for the safety of the construction of the tunnel project and the smooth progress of the project. Practice shows that use of the filling and freezing method for the construction of temporary blocking walls in complex geological conditions can effectively prevent the occurrence of sudden water and sand accidents, meaning personnel do not need to enter the water tunnel under construction, which results in high controllability and safety, as we; as the popularity and widespread application of this approach. [PERSON] et al.