A hydro-thermal coupling model for artificial ground freezing considering dynamic phase-change and seepage effects: A case study of the freeze-sealing pipe-roof method

During the freezing period in the artificial ground freezing (AGF) method under seepage conditions, frozen curtain development confronts the coupling effect of phase change and seepage pathways. Traditional models often treat the phase change as static phenomena and set the permeability as a constant value. To capture the dynamic evolution of phase change and their impact on permeability, a temperature-dependent smooth step function is introduced as the phase change function, dynamically linked to water saturation. In this study, a 2D double-pipe model is established to validate the accuracy of the model. Considering phase change, the frozen curtain alters the seepage pathways, preventing forming excessively large and impractical freezing curtain. The results demonstrated that the phase change process under this model can be explicitly embedded within the hydro-thermal coupling framework, dynamically altering permeability and seepage pathways. Subsequently, a case study on Freeze-Sealing Pipe-Roof (FSPR) method was conducted using COMSOL Multiphysics. It was found that seepage induces asymmetric frozen curtain thickness at both upstream and downstream, the thickness of the upstream frozen curtain is generally less than that downstream, with the difference becoming more pronounced as seepage velocity increases. At seepage velocities of 0.2, 0.4 and 0.8 m/d, the required freezing time for the frozen curtain thickness achieving 1.5 m are 45 days, 53 days and 80 days respectively. The critical seepage velocity threshold for achieving 1.5 m within 60 days is 0.7 m/d. These findings highlight the necessity of considering coupled phase-change and seepage effects in simulations to ensure safety.