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.

The upper Nu-Salween River basin in the Tibetan Plateau is mainly covered with seasonal frozen soils. We used daily surface freeze-thaw states, detected from Special Sensor Microwave/Imager (SSM/I) daily brightness temperature data, to analyze the variations in surface freeze-thaw states and the relationship with air temperature. We also examined baseflow to explore the influences of interannual variations in the start time of soil freezing on hydrological processes. The results showed that (1) interannual air temperature fluctuations led to differences in the area and start time of surface freezing. When surface soil froze, flow was mainly dependent on existing groundwater storage. (2) The interannual variation in the surface freezing time directly affected the flow generation processes. When soil water froze and remained in the frozen layer, it was hard to generate surface flow, so flow mainly consisted of baseflow, causing the proportion of the baseflow in the total flow to gradually increase. (3) The surface freeze-thaw states obtained from the passive microwave remote sensing data may be applied to support further research on the hydrological impacts of freeze-thaw cycle variations in plateau mountain basins.