## 5 Conclusions  
This study is based on the construction of a geometric model of the pipe curtain freezing method in the Gongbei Tunnel. The empty top pipe in the pipe curtain structure is replaced by a freezing pipe, and a new type of pipe curtain freezing method is formed by the alternate combination of the pipe curtain and the freezing pipe. A three-dimensional numerical calculation model of the new pipe curtain freezing method under seepage conditions was established according to the actual material parameters of the Gongbei Tunnel. Firstly, the three-dimensional model was cut at different positions to analyze the temperature field cloud map and isotherms of the new pipe curtain freezing method under seepage conditions. Secondly, the cooling curve of symmetric points upstream and downstream of the freezing pipe was analyzed, and the influence of the groundwater flow on the freezing temperature field was studied. Finally, in order to investigate the effects of different factors on the new pipe curtain freezing method under seepage conditions, calculation models with different seepage velocities, different initial ground temperatures, and different salt content were established, and the results were analyzed through single-factor analysis. This study mainly draws the following conclusions. (1) An increase in the groundwater flow rate will affect the development of frozen soil curtains, prolonging the formation time of frozen soil curtains. When the seepage velocity reaches 4.27 m/d, the \"tail\" formed by the groundwater flow at the downstream position of the new pipe curtain freezing structure increases, and the encirclement time of the frozen soil curtain shows \"explosive\" growth. When the seepage velocity reaches 5.27 m/d, the frozen soil curtain does not close, and with the increase in the seepage velocity, a certain number of \"frozen gaps\" are formed. (2) An increase in the initial ground temperature will lead to an increase in the time taken for the formation of frozen soil curtains during the freezing process. The temperature of the frozen soil curtain formed in the end will slightly rise with the increase in the initial ground temperature. For every 5 \({}^{\circ}\)C increase in the initial ground temperature, the temperature of the frozen soil curtain increases by 1.6 \({}^{\circ}\)C. When the initial temperature is greater than 35 \({}^{\circ}\)C, it significantly affects the formation of the frozen soil structure under seepage conditions, necessitating the enhanced monitoring of the original ground temperature in practical freezing engineering projects. (3) With an increase in the groundwater salinity, different temperature measurement points undergo varying changes. The temperature at the temperature measurement point located upstream of the freezing pipe increases with an increase in salinity, while the temperature at the temperature measurement point downstream of the freezing pipe decreases with an increase in salinity. The average temperature of the frozen soil curtain also increases with an increase in the groundwater salinity. The salinity of the groundwater has an impact on the development of the freezing temperature field in frozen engineering, especially in coastal areas; thus, attention needs to be paid to the groundwater and its salinity during construction using the freezing method in underground engineering. Conceptualization, J.H. and Y.L. ; methodology, Y.L., S.Z. and T.W. ; validation, Y.L. and X.L. ; writing--original draft preparation, Y.L. and S.Z. ; writing--review and editing, X.L. and Y.L. ; funding acquisition, J.H. and X.L. All authors have read and agreed to the published version of the manuscript. This research was funded by the State Key Laboratory for GeoMechanics and Deep Underground Engineering, China University of Mining and Technology (SKLGDUEK2204); the Hainan University Collaborative Innovation Center Project (Grant No. XTCX2022 STB09); the High Technology Direction Project of the Key Research and Development Science and Technology of Hainan Province, China (Grant No.