Numerical Analysis of Thermo-Hydro-Mechanical Coupling of Diversion Tunnels in a Seasonally Frozen Region

Abstract

A thermo-hydro-mechanical (THM) coupling model that considers phase transition and frost heaving was developed for a diversion tunnel in a seasonally frozen region. Based on the partial differential equation defined by COMSOL software, a numerical simulation of the temperature field, the seepage field, and the stress field of the rock surrounding the tunnel was conducted. The distributions of temperature, pore-water pressure, and frost heave deformation were also analyzed. The results indicated that the temperature of the surrounding rock periodically changes with changes in the seasonal environmental temperature. Due to the superposition effect of the environment temperature in the tunnel and at the ground surface, the maximum radial frozen depth of the tunnel during the freezing period is the highest at the top of the tunnel, whereas it is at its lowest at the bottom of the tunnel. The maximum radial frozen depth of the tunnel side and bottom tends to be the same because the maximum negative temperature decreases, the specific heat capacity increases, or the heat conductivity decreases. Before freezing, the fissure water in the surrounding rock is smoothly discharged along the ground surface and in the tunnel, and the water supply and drainage of the surrounding rock are basically maintained in an equilibrium state. During freezing, the fissure water in the unfrozen zone is in a state of overpressure due to the failure of the drainage channel in the frozen zone, and the frozen zone changes from a saturated state to an unsaturated state. The lower the maximum negative temperature, the greater the negative pore-water pressure in the frozen zone. The maximum frost heave displacement of the ground surface occurs at the top of the tunnel; however, because the distance from the top of the tunnel increases, the frost heave effect of the tunnel gradually decreases.