Heat transfer analysis in artificial ground freezing for subway cross passage under seepage flow

Abstract

Artificial ground freezing is an environmentally friendly and reliable method for ground improvement during subway cross passage construction under variable hydrological conditions. However, natural or induced seepage flow influences significantly the closure of frozen wall, as it supplies a substantial source of heat. In this study, a physical model test was conducted based on the freezing project of a subway cross passage in Beijing Subway Line 19. A series of thermistors were installed in some representative locations to measure the temperature distribution and further calculate power demand. Besides, a hydro-thermal coupling model of freezing process under seepage conditions was derived to consider the variations of physical and thermodynamic parameters of the porous media and the latent heat of ice/water phase transition. The numerical approach was validated by the experimental results. Furthermore, four crucial indicators affecting the closure time during AGF process were quantitatively evaluated. The results showed that the closure time at seepage velocities of 0, 25.6 and 51.2 m/d were respectively 60, 110 and 441 min. Nevertheless, it was impossible to form continuous frozen bodies at 76.8 m/d. Among these indicators, the seepage velocity had the greatest effect on the thickness and shape of frozen body, while seepage temperature had the least impact.