In the context of global climate change, changes in unfrozen water content in permafrost significantly impact regional terrestrial plant ecology and engineering stability. Through Differential Scanning Calorimetry (DSC) experiments, this study analyzed the thermal characteristic indicators, including supercooling temperature, freezing temperature, thawing temperature, critical temperature, and phase-transition temperature ranges, for silt loam with varying starting moisture levels throughout the freezing and thawing cycles. With varying starting moisture levels throughout the freezing and thawing cycles, a model describing the connection between soil temperature and variations in unfrozen water content during freeze-thaw cycles was established and corroborated with experimental data. The findings suggest that while freezing, the freezing and supercooling temperatures of unsaturated clay increased with the soil's starting moisture level, while those of saturated clay were less affected by water content. During thawing, the initial thawing temperature of clay was generally below 0 degrees C, and the thawing temperature exhibited a power function relationship with total water content. Model analysis revealed hysteresis effects in the unfrozen water content curve during freeze-thaw cycles. Both the phase-transition temperature range and model parameters were sensitive to temperature changes, indicating that the processes of permafrost freezing and thawing are mainly controlled by ambient temperature changes. The study highlights the stability of the difference between freezing temperature and supercooling temperature in clay during freezing. These results offer a conceptual framework for comprehending the thawing mechanisms of permafrost and analyzing the variations in mechanical properties and terrestrial ecosystems caused by temperature-dependent moisture changes in permafrost.

Due to the presence of ice and unfrozen water in pores of frozen rock, the rock fracture behaviors are susceptible to temperature. In this study, the potential thawing-induced softening effects on the fracture behaviors of frozen rock is evaluated by testing the tension fracture toughness ( K IC ) of frozen rock at different temperatures (i.e. - 20 degrees C, - 15 degrees C, - 12 degrees C, - 10 degrees C, - 8 degrees C, - 6 degrees C, - 4 degrees C, - 2 degrees C, and 0 degrees C). Acoustic emission (AE) and digital image correlation (DIC) methods are utilized to analyze the microcrack propagation during fracturing. The melting of pore ice is measured using nuclear magnetic resonance (NMR) method. The results indicate that: (1) The K IC of frozen rock decreases moderately between - 20 degrees C and - 4 degrees C, and rapidly between - 4 degrees C and 0 degrees C. (2) At - 20 degrees C to - 4 degrees C, the fracturing process, deduced from the DIC results at the notch tip, exhibits three stages: elastic deformation, microcrack propagation and microcrack coalescence. However, at - 4 degrees C-0 degrees C, only the latter two stages are observed. (3) At - 4 degrees C-0 degrees C, the AE activities during fracturing are less than that at - 20 degrees C to - 4 degrees C, while more small events are reported. (4) The NMR results demonstrate a reverse variation trend in pore ice content with increasing temperature, that is, a moderate decrease is followed by a sharp decrease and - 4 degrees C is exactly the critical temperature. Next, we interpret the thawing-induced softening effect by linking the evolution in microscopic structure of frozen rock with its macroscopic fracture behaviors as follow: from - 20 degrees C to - 4 degrees C, the thickening of the unfrozen water film diminishes the cementation strength between ice and rock skeleton, leading to the decrease in fracture parameters. From - 4 degrees C to 0 degrees C, the cementation effect of ice almost vanishes, and the filling effect of pore ice is reduced signi ficantly, which facilitates microcrack propagation and thus the easier fracture of frozen rocks. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).