Freezing Effect of Enhancing Tubes in a Freeze-Sealing Pipe Roof Method based on the Unsteady-State Conjugate Heat Transfer Model

The freeze-sealing pipe roof (FSPR) method was applied as an innovative construction technology to the Gongbei Tunnel of the Hong Kong–Zhuhai–Macau Bridge. A freezing scheme involving master freezing tubes, enhancing freezing tubes, and limiting freezing tubes is the key component of the freezing effect of the FSPR method during the construction process under various working conditions. This is related to whether the thickness and temperature of the frozen soil meet the design requirements under various complex working conditions, and it is also related to frost heave control and energy saving. Based on the unsteady-state conjugate heat transfer model, different freezing schemes of enhancing freezing tubes—that is, the shape, layout, operating duration, and heat preservation—were simulated to analyze the freezing effect, which can be measured by the thickness of frozen soil around the steel pipes and the average temperature of the frozen soil curtain. The results show that the greater the contact area between the enhancing tube and the inner wall of the steel pipe, the better the freezing effect, and that the semicircle enhancing freezing tube scheme is superior to the other three shapes of freezing tubes. The arrangement of enhancing freezing tubes far away from the excavation surface, without heat preservation measures, has a better freezing effect due to the function of the hollow pipe as a freezing pipe. Moreover, the enhancing freezing tube can be operated intermittently to control frost heave. Our research simulated the temperature fields of different media—such as steel pipes, frozen soil, and air—providing a design basis for similar projects, such as the combination of the pipe-roofing method and artificial freezing method.

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