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Unsteady-State Conjugate Heat Transfer Model of Freeze-Sealing Pipe Roof Method and Sensitivity Analysis of Influencing Factors of Freezing Effect

The freeze-sealing pipe roof (FSPR) method has been applied to the Gongbei tunnel of the Hongkong–Zhuhai–Macau Bridge, which is the first application of this method in the world. The purpose of the ground-freezing method is soil waterproofing. Temperature is an important indicator for measuring the freezing effect; however, the FSPR method involves unsteady-state conjugate heat transfer between frozen soil, steel pipes, concrete, air, and other media. This paper proposes an unsteady-state conjugate heat transfer model and establishes a global solution algorithm of a strong coupling governing equation based on the virtual density method. Then, a calculation based on COMSOL software is realized and validated. The sensitivity of different factors such as initial formation temperature, different soil layers, and brine temperature on the freezing effect was studied by simulating the FSPR model. It is concluded that the brine temperature had a greater impact on the freezing effect, followed by the soil layer, whereas the formation temperature had the least impact. For muddy silty clay, if the brine temperature is −20 °C, it takes 44 days to meet the design requirements of 2 m. If the brine temperature is −30 °C, 27 days is enough. When the formation temperature is 20 °C, it takes 20 days for medium gravel sand to reach the thickness of the freezing curtain, and 32 days for muddy silty clay. Compared to other soil layers, the freezing effect of the medium gravel sand is relatively better. This research has a certain impetus to similar multimedia freezing heat transfer issues.