The hysteresis effect of unfrozen water during freeze-thaw cycles greatly influences the hydrothermal properties of soil. To better understand the hysteresis behavior of unfrozen water in the soil, this study utilized frequency domain reflectometry to measure the unfrozen water content variations in silty clay under both stepwise and rapid temperature change modes. The hysteresis effect of unfrozen water in soil was analyzed, also the underlying mechanism was revealed. The results indicate that unfrozen water content variations are consistent across the two temperature change modes, with hysteresis observed in both scenarios. This effect was more noticeable during the rapid temperature change mode, and soil samples with higher initial moisture content froze earlier and thawed more slowly in this mode. The hysteresis phenomena are mainly influenced by the ice crystal metastable nucleation, the blockage effect of pore ice crystallization, and the pore water pressure changes during phase transition. The main cause of unfrozen water hysteresis in soil during the initial freezing phase is the metastable nucleation process. In the later stages of freezing, the hysteresis effect is primarily driven by changes in capillary water curvature, induced by the blockage effect of pore ice crystallization, and shifts in pore water pressure during the ice-water phase transition. Also, a hysteresis model was proposed and validated against experimental data and existing models, demonstrating good performance and accurately predicting unfrozen water content under varying temperature conditions. This research enhances the understanding of the mechanism responsible for the hysteresis effect of unfrozen water content in frozen soil.

After the construction of the frozen wall of the vertical shaft is completed, it will undergo a long thawing process. Accumulation of damage under load may lead to the rupture of frozen walls and cause engineering accidents. The changes in mechanical properties during the thawing process of frozen rocks are key issues in controlling the stability of frozen walls. In view of the instability problem of the frozen wall of the vertical shaft, this article chooses the saturated sandstone of the Cretaceous system as the research object. Conduct triaxial compression tests under different temperature and confining pressure conditions. Obtain relevant parameters for analysis. And nuclear magnetic resonance technology was used to detect the changes in pore water content in saturated sandstone at different temperatures. The results indicate that: (1) At room temperature, pore water mainly exists in the form of free water, while at low temperatures, pore water mainly exists in the form of adsorbed water. (2) Compared with frozen soil, frozen rocks also exhibit significant supercooling phenomena. (3) According to the variation of unfrozen water content in saturated sandstone at different temperatures, it can be divided into three stages: freezing cessation (- 20 degrees C similar to - 6 degrees C), stable freezing (- 6 degrees C similar to - 2 degrees C), and rapid freezing (-2 degrees C similar to 20 degrees C). (4) As the temperature increases, the closure level of saturated sandstone gradually increases, while the initiation and expansion levels gradually decrease. (5) There is an exponential relationship between the unfrozen water content and the peak strength of saturated sandstone, with a good correlation. And show the same trend of change under different confining pressures. The research results can provide theoretical support and experimental basis for evaluating the instability and failure induced by thawing of frozen walls.

Engineering geological investigations indicate that confined water exists in the stratum during the warm season in permafrost regions and in underground engineering employing artificial ground freezing (AGF) to isolate groundwater, causing significant upward deformation of the stratum and frost damage to engineering structures. However, limited studies have explored the effect and mechanism of hydraulic pressure on ice growth during soil freezing upwards. Therefore, this study designs and conducts four groups of bottom-up freezing tests under various hydraulic pressures, and develops a model to investigate the mechanism of hydraulic pressure on ice growth, based on the theory that liquid water migrates towards the ice lens through an unfrozen water film. The experimental results, including thermal regime, frost heave, cryo-structure, and water redistribution are analyzed systematically, which show the frozen depth, frost heave increment, ice lens thickness, and the layered water content in the samples all increase with hydraulic pressure. The model is validated with experimental data, and the calculation results demonstrate that the ice growth rate increases with hydraulic pressure due to a higher pore water pressure (PWP) gradient in the unfrozen water film. Thus, the characteristics and mechanisms of ice growth in the stratum, accelerated by hydraulic pressure, are clarified. Finally, the applications and implications of this study to engineering geology are discussed, which contribute to a better understanding of ground ice formation in permafrost regions and frost damage prevention in underground engineering performing AGF.

Ice and water coexist in frozen soil, and their respective contents (ice content, theta i; unfrozen water content, theta u) are critical factors influencing the mechanical properties of frozen soil. Currently, these two parameters are measured separately. Existing measurement methods require specialized equipment, are time-consuming. To improve measurement efficiency, this paper proposes an inverse analysis surrogate model, which can simultaneously predict both theta i and theta u within one minute. The method process is as follows: 1. A three-dimensional numerical model is established to simulate the transient heat conduction in frozen soil under heat pulse. 2. Six parameters (theta i, theta u, rho s, lambda s, Cs, Gs) need to be determined for each simulation. Through Monte Carlo sampling of six parameters, thousands of numerical simulations are performed. Then, a dataset comprising thermal response curves (TRC) labeled with (theta i, theta u, rho s, lambda s, Cs, Gs) is established. 3. A machine learning algorithm is used, where TRC and soil property parameters serve as inputs, and (theta i, theta u) as outputs. 4. In the laboratory, soil property parameters are measured, and in the field, TRC within one minute of frozen soil is measured in real-time. By inputting soil property parameters and TRC into the machine learning model, (theta i, theta u) can be obtained in real-time.The method was validated through laboratory experiments. The results show that with TRC and rho s, lambda s, Cs as inputs, mean absolute errors (MAE) for theta i and theta u were 2.3 % and 3.1 %, respectively. The proposed method significantly improves measurement efficiency, allowing for the simultaneous measurement of theta i and theta u within one minute.

change of unfrozen water content in pores of rock during freeze-thaw process is one of the key factors affecting its mechanical properties. In this paper, the sandstone is taken as the research object, and the pore water content of rock during freeze-thaw process (20, 0, -2, -4, -6, -10, -15, -10, -6, -4, -2, 0, 20 degrees C) is monitored by low-field nuclear magnetic resonance system (NMR), and the evolution law of unfrozen water content with temperature is analyzed. The influence of the evolution of unfrozen water content on the mechanical properties of rock during freeze-thaw process is also discussed. The research findings show that the pore water in rocks during the freezing-thawing process is significantly influenced by temperature, passing through five stages: supercooling, rapid freezing, slow freezing, slow melting, and accelerated melting. A distinct hysteresis phenomenon is observed in the rock during thawing. At identical temperatures, the unfrozen water content during freezing is notably higher than during thawing. Consequently, the peak intensity and elastic modulus during thawing are significantly greater than during freezing. The relationship between uniaxial compressive strength, rock elastic modulus, and unfrozen water content in freeze-thaw process can be expressed by exponential function. At the beginning of freezing, the change of rock mechanical parameters is mainly affected by the increase of pore ice content and the cementation effect of pore ice on rock particles. With the further decrease of temperature, the thickness of adsorbed water film decreases, and the adsorption capacity increases, so that the integrity between pore ice and rock particles is enhanced, and rock mechanical parameters further change.

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.

Soluble salts significantly influence the freezing characteristic parameters of frozen soil. Previous studies have either insufficiently addressed the effect of sodium sulfate on matric suction or not comprehensively revealed the mechanism by which temperature affects matric suction at freezing temperature. In this study, the moisture and suction sensors were used to quantify the freezing temperature (FT), unfrozen water content (UWC), and matric suction (MS) of Ili loess with varying soluble salt contents. The impact of soluble salt content on three freezing characteristic parameters were investigated with the underlying mechanisms revealed. The results indicated that there was an initial decrease in both freezing and supercooling temperatures as the soluble salt content increased. Beyond a soluble salt content of 14 g/kg, an increase in both the freezing and supercooling temperatures was observed. Specimens with different soluble salt contents exhibited distinct UWC, which could be categorized into three stages based on temperature. A crystal precipitation stage was observed beyond the soluble salt content of 14 g/kg. Moreover, the proposed fitting model for UWC by incorporating the soluble salt content into the Gardner model demonstrated high accuracy. The MS can also be divided into three stages with temperature. Notably, specimens with soluble salt contents of 20 and 26 g/kg exhibited nonlinear increases in MS at temperatures of 5 degrees C and 10 degrees C due to crystal precipitation. Furthermore, theoretical calculations indicated the complete precipitation of sodium sulfate during the positive temperature stage.

Permafrost in marine sediments exhibits a lower freezing point and significant unfrozen water content. This paper investigates the role of the soil freezing characteristic curve (SFCC) in permafrost degradation. Three SFCCs, representing thawing-freezing characteristics of soils with varying clay content and salinity, were established based on experiments and existing data. These SFCCs were then applied in numerical analyses to simulate permafrost thawing under various warming scenarios, using measured ground temperatures and permafrost profiles for a site at Longyearbyen in Svalbard (Norway). It is shown that the ground temperature in non-saline permafrost soil increases more rapidly than saline permafrost, due to a greater downward net heat flux to the permafrost in the former case. Conversely, the thawing rate is more pronounced for saline permafrost soil, attributed to its lower freezing point and latent heat consumption. A more nonlinear ice-melting process is observed for permafrost soil with a lower salinity. The temperature rise follows three stages: a constant-rising, a damp-rising, and an accelerated-rising rates. The duration of the damp-rising rate becomes shorter for saline permafrost under a great warming condition. The study underscores the high significance of the soil-freezing characteristic curve for accurate estimations of permafrost degradation.

Permafrost carbon could produce a positive climate feedback. Until now, the ecosystem carbon budgets in the permafrost regions remain uncertain. Moreover, the frequently used models have some limitations especially regarding to the freeze-thaw process. Herein, we improved the IBIS model by incorporating an unfrozen water scheme and by specifying the parameters to estimate the present and future carbon budget of different land cover types (desert steppe, steppe, meadow, and wet meadow) in the permafrost regions. Incorporating an unfrozen water scheme reduced the mean errors in the soil temperature and soil water content by 25.2%, and the specifying leaf area parameters reduced the errors in the net primary productivity (NPP) by 79.9%. Further, the simulation results showed that steppes are carbon sources (39.16 gC/m(2)/a) and the meadows are carbon sinks (-63.42 gC/m(2)/a ). Under the climate warming scenarios of RCP 2.6, RCP 6.0, and RCP 8.5, the desert steppe and alpine steppe would assimilated more carbon, while the meadow and wet meadow were projected to shift from carbon sinks to carbon sources in 2071-2100, implying that the land cover type plays an important role in simulating the source/sink effects of permafrost ecosystem carbon in the IBIS model. The results highlight the importance of unfrozen water to the soil hydrothermal regime and specific leaf area for the growth of alpine vegetation, and present new insights on the difference of the responses of various permafrost ecosystems to climate warming.

The investigation into the complex mechanical properties of frozen calcareous clay under multi-factor interaction holds significant importance for the reliability and durability of engineering in cold regions. This study investigates the strength properties of frozen calcareous clay under different interaction levels by designing a four-factor, four-level orthogonal test that incorporates temperature, confining pressure, dry density, and water content. The study aimed to assess the sensitivity of each factor to failure stress, and establish an intrinsic model based on the Duncan-Chang model considering temperature, confining pressure, and water content. The results indicated that the stress-strain curves exhibit strain-hardening characteristics across various interaction levels. These curves can be divided into elastic and elastic-plastic phases, with the slope of the elastic phase and the stress value at the inflection point increasing with decreasing temperature and increasing confining pressure. When the confining pressure is maintained constant, the failure stress is negatively correlated with temperature. When the temperature is maintained constant, the failure stress is positively correlated with confining pressure. Sensitivity analysis shows that the influence of each factor on failure stress is as follows: temperature > confining pressure > dry density > water content. Additionally, the influence of temperature and confining pressure on failure stress is markedly greater than that of water content and dry density. The evolution of unfrozen water content follows three stages: sharp reduction, rapid reduction, and slow reduction. Verification against experimental data confirmed that the modified constitutive model effectively reflects the stress-strain relationship of frozen calcareous clay under the interaction of multiple factors.