Abstract: The frost heave of subgrade has an important effect on the operation of high-speed railway in cold regions, while the ice-water phase transition is the key to understanding the mechanism of frost heave. The lattice Boltzmann method is applied in this study, which is a mesoscale numerical method. The modified freezing temperature algorithm of pore water is combined with the enthalpy-based lattice Boltzmann phase transition model. Two freezing processes including the freezing of suspended droplets and the formation of pore water into ice in frozen soil are investigated, which aim to reveal the mesoscopic mechanism of the ice-water phase transition in free state and pore-bound state, respectively. The numerical results show that the process of ice crystals growing from the inside to the outside in the pores is completely opposite to the freezing process of droplets suspended in the air, and the pore water has a lower freezing temperature when it is closer to the surface of the soil particles. The soil freezing characteristic curves (SFCCs) differ obviously for the particles with the same size but in different particle arrangements. Meanwhile, the morphology of SFCC becomes steeper with increasing soil particle size, and the residual water content gradually decreases. The numerical results of the ice-water phase transition process are validated by measured data in the literature, which indicate that the lattice Boltzmann method can provide a new tool to study the water-gas migration and phase transformation process in porous media in mesoscale.

Key words: lattice Boltzmann method, droplet freezing, frozen soil, pore ice formation, numerical simulation