A fully coupled thermo-hydro-mechanical model for fractured rock masses in cold regions

Freezing-thawing disasters are caused by the coupling effect of thermo-hydro-mechanical (THM) activity in rock masses at low temperatures, which seriously threatens the safety of rock mass engineering in cold regions. The accuracy of the coupled THM model is controlled by coupling parameters, such as the migration velocity of unfrozen water, equivalent thermal conductivity, and freezing point. The main difference in the THM coupling process between the frozen rock mass and frozen soil is the anisotropy caused by fractures, and the main difference between the frozen rock mass and normal rock mass is the existence of the ice-water phase transitions. In this paper, the governing equations for THM coupling of fractured rock masses at low temperatures are established, including the mechanical equilibrium equation, continuity equation and energy conservation equation. These equations include the effect of fractures, ice-water phase transitions, water migration and thermal expansion and contraction, which can also be degenerated to equations for rock masses without fractures. Then, the main parameters for the THM analysis are studied. The migration velocity of unfrozen water is closely related to the fractures, temperature, pressure, and phase transitions. Based on the energy conservation law, the expression of the equivalent thermal conductivity is proposed, which is a function of the geometric parameters of fractures, the thermal conductivity of each component, and the content of unfrozen water. Finally, the correctness of the proposed model are validated by Neaupane's experiment and an actual open-pit slope.