This study investigates the freezing process and mechanical impact behavior of saturated soil to provide new insights into soil thermodynamic and improve its comprehensive investigation under a cryogenic engineering environment. The unfrozen water content is a major focus of study during soil freezing. Many studies have proposed models for calculating the unfrozen water content in frozen and unfrozen pores. However, they lack uniformity and consistency on a physical basis and mathematical derivation. An unified theoretical model was derived based on the principle of thermodynamic equilibrium. The main theoretical results indicated that the dimensionless total volume of the unfrozen water membrane in the frozen pores first increased and then decreased with increasing temperature, revealing the temperature effect on the unfrozen water content in frozen pores. By combining the theoretical model with the distinct element method (DEM), water freezing into ice in saturated soil was numerically simulated using two modes of particle expansion. One of the two modes proposed by the authors was to change the coefficient of expansion during saturated soil freezing to further consider the non-linear variation in unfrozen water content. Subsequently, the effects of the two modes on crack generation during saturated soil freezing were compared and analyzed. Finally, based on the dissipation energy produced in particle contacts, a method for calculating the rises in impact temperature in different particles was proposed for revealing the local and discrete changes in frozen saturated soil under impact loading. The main numerical results indicated that the proportion of the number of particles for different temperature rise ranges followed a Weibull distribution, and the average temperature rise of the particles near the incident end was higher than that of the particles near the transmission end.
