# Key Issues in Using the Freezing Method in Power Tunnel Rehabilitation Projects
## 6 Conclusions  
In this study, the engineering background is tunnel collapse repair, where vertical freezing is employed to reinforce the soil body at the site of the collapse. The finite element software COMSOL was utilized to construct a three-dimensional water-heat coupling model, enabling detailed investigation of the temperature field within the collapsed soil body. The study concludes with the following findings. (1) In the absence of seepage, the permafrost curtain evolved stably and uniformly across the frozen zone. As evident from the isotherm chart, the permafrost curtains in the upstream and downstream regions exhibited instability and non-uniformity in the presence of seepage. The degree of non-uniformity in these curtains increased significantly with an escalating seepage rate. The temperature disparity between the upstream and downstream areas could be discerned through the temperatures of the symmetry points for each region. (2) The presence of permafrost curtains significantly altered the groundwater flow dynamics. Analysis of the cloud diagrams revealed that the introduction of permafrost curtains caused the groundwater to divert around these barriers, transitioning from a parallel flow pattern to one that bypassed the curtains. Once a closed permafrost curtain was established, groundwater flow was effectively blocked in the areas encircled by the curtains, leading to the development of a \"trailing tail\" downstream of the frozen zone. This phenomenon intensified as the groundwater carried cold from the upstream freezing pipes to the downstream regions, resulting in the formation of a \"trailing tail\" at the downstream edge of the frozen area. Moreover, as the groundwater seepage rate increased, the size of the \"trailing tail\" also expanded. Figure 16: Effect of different seepage rates on different observation points. Figure 17: Sensitivity analysis of different seepage rates to cooling at different observation points. (3) The intersection time of isotherms was a critical indicator for assessing the evolution of a permafrost curtain. Specifically, the \(-1\)\({}^{\circ}\)C and \(-10\)\({}^{\circ}\)C isotherms exhibited an increase in intersection time with a rise in seepage velocity. Notably, when the head difference surpassed 2 m, the intersection time of the \(-1\)\({}^{\circ}\)C and \(-10\)\({}^{\circ}\)C isotherms experienced \"explosive\" growth. Conversely, at a head difference of 3 m, the \(-1\)\({}^{\circ}\)C and \(-10\)\({}^{\circ}\)C isotherms did not intersect throughout the entire freezing cycle duration of 60 days. (4) Enhancement of seepage velocity led to a reduction in the thickness of the permafrost curtain, creating a vulnerable zone between the frozen pipes. The increase in groundwater flow rate exerted a more pronounced effect on the freezing process upstream, with the cooling curves of temperature measurement points upstream of the freezing area exhibiting heightened sensitivity to variations in groundwater flow rate as it rose. Conversely, the cooling curves of temperature measurement points downstream of the freezing area were nearly identical and showed less sensitivity to changes in groundwater flow rate. The influence of groundwater flow rate on the frozen soil curtain was observed to be greater upstream compared to downstream. As the artificial bottom freezing method is widely used in tunnel reinforcement work, some of the conclusions drawn in this study have some similarities with other studies, which further validates the correctness of the conclusions drawn in this paper. However, this study was based on the reinforcement and rehabilitation of collapsed tunnels and adopted a vertical freezing scheme, which is a special feature of this study and different from other studies. In the next study, the development of the frozen curtain will be further explored, mainly for field measurements and model tests. Conceptualization, [PERSON] and [PERSON]; methodology, [PERSON], [PERSON], [PERSON] and X.S.