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.

Understanding the relationship between soil moisture and vegetation is crucial for future projections of ecosystem and water resources. While their hysteresis loop relationship, which arises from their asynchrony in intra-annual variation, remains underexplored. This study used the hysteresis loop type and area (Ah) to characterize the relationship between root zone soil moisture (RZSM) and normalized difference vegetation index (NDVI) across China from 1986 to 2015, and examined its ecological implications. The results identified four types of hysteresis loops. The clockwise loop, with a delayed single peak of RZSM relative to NDVI, was primarily found in north China and the Qinghai-Tibet Plateau, indicating severe water limitation during early growth period. The counterclockwise loop, with an advanced single peak of RZSM relative to NDVI, was common in southeast China's forest, suggesting a shift towards energy limitation. The 8-shaped loop, resulting from double peaks in either RZSM or NDVI due to climate change (e.g., snowmelt) and human disturbance (e.g., irrigation and crop harvest), was observed in northwest China's glaciers and croplands in south and northeast China. The multicrossed loop, marked by multimodal intra-annual variations in both RZSM and NDVI, was predominantly found in northwest China's barren lands. Additionally, from 1986 to 2015, this study observed a shift from 8-shaped or multi-crossed loops to clockwise or counterclockwise loops in some regions like the Yellow River Basin, implying a trend of revegetation. Furthermore, a higher Ah generally indicated more severe water limitation or greater mismatch between RZSM and NDVI. Significant changes in Ah, such as increases in the Yellow River Basin, suggested intensified water limitations, while decreases in southeast and northwest China pointed to an earlier peak of the growing and rainy seasons. This study provides insights into the dynamic interactions between soil moisture and vegetation, offering valuable guidance for ecological management across diverse ecosystems.

Climate change has been a strong driving force impacting the distribution of global water resources over the past few decades, especially in cold regions at high latitudes. Hydrological models are essential to analyse complex changing cold region's processes, such as permafrost, seasonally frozen soil, and snow cover, which are prevalent across much of Canada and the pan-Arctic basins. Here, we utilize the Hydrological Predictions for the Environment (HYPE) model with seven discretized vertical soil layers to assess climate change response to different water balance portioning components and permafrost extent. The study also explores seasonal and interannual shifts, examining the implications of model uncertainty associated with streamflow generation for the Nelson Churchill River Basin (NCRB). The calibrated HYPE model is run with a suite of fourteen GCMs and two RCPs (RCP 4.5 and RCP 8.5) scenarios representing 87% of the variability of 154 climate scenarios to discern the relationship between climate projections and water balance components. Increasing precipitation and temperature are anticipated in the future, but reduced, or balanced runoff is projected due to the dominant impact of rising temperature on evapotranspiration from thawing soil layers. Under an extreme scenario (RCP 8.5) 82% reduction in permafrost degradation is projected by the mid-future period (2050s). In this study, the future projections of streamflow, soil moisture, permafrost projection, and interrelationships of water balance processes at a continental scale are presented to aid in large-scale planning and implementation of sustainable development principles and guidelines for decision-making in the NCRB. Le changement climatique a & eacute;t & eacute; une force motrice majeure influen & ccedil;ant la r & eacute;partition des ressources en eau & agrave; l'& eacute;chelle mondiale au cours des derni & egrave;res d & eacute;cennies, en particulier dans les r & eacute;gions froides des hautes latitudes. Les mod & egrave;les hydrologiques sont essentiels pour analyser les processus complexes en & eacute;volution dans les r & eacute;gions froides, tels que le perg & eacute;lisol, les sols gel & eacute;s de mani & egrave;re saisonni & egrave;re et le couvert neigeux, qui sont r & eacute;pandus dans une grande partie du Canada et des bassins pan-arctiques. Dans cette & eacute;tude, nous utilisons le mod & egrave;le Hydrological Predictions for the Environment (HYPE), qui comprend sept couches de sol verticales discr & eacute;tis & eacute;es, pour & eacute;valuer la r & eacute;ponse au changement climatique des composantes du bilan hydrique et de l'& eacute;tendue du perg & eacute;lisol. L'& eacute;tude explore & eacute;galement les variations saisonni & egrave;res et interannuelles, en examinant les implications de l'incertitude du mod & egrave;le associ & eacute;e & agrave; la g & eacute;n & eacute;ration des d & eacute;bits fluviaux dans le bassin de la rivi & egrave;re Nelson Churchill (NCRB). Le mod & egrave;le HYPE calibr & eacute; est ex & eacute;cut & eacute; avec une s & eacute;rie de quatorze mod & egrave;les climatiques globaux (GCM) et deux sc & eacute;narios RCP (RCP 4.5 et RCP 8.5), repr & eacute;sentant 87 % de la variabilit & eacute; de 154 sc & eacute;narios climatiques, afin d'analyser la relation entre les projections climatiques et les composantes du bilan hydrique. Une augmentation des pr & eacute;cipitations et des temp & eacute;ratures est anticip & eacute;e dans le futur, mais un ruissellement r & eacute;duit ou & eacute;quilibr & eacute; est projet & eacute; en raison de l'impact dominant de la hausse des temp & eacute;ratures sur l'& eacute;vapotranspiration provenant des couches de sol en d & eacute;gel. Dans un sc & eacute;nario extr & ecirc;me (RCP 8.5), une r & eacute;duction de 82 % de la d & eacute;gradation du perg & eacute;lisol est projet & eacute;e d'ici la p & eacute;riode du milieu du si & egrave;cle (ann & eacute;es 2050). Cette & eacute;tude pr & eacute;sente des projections futures du d & eacute;bit fluvial, de l'humidit & eacute; du sol, de la d & eacute;gradation du perg & eacute;lisol et des interrelations des processus du bilan hydrique & agrave; l'& eacute;chelle continentale afin de soutenir la planification & agrave; grande & eacute;chelle et la mise en oeuvre de principes de d & eacute;veloppement durable pour & eacute;clairer la prise de d & eacute;cision dans le NCRB.

To establish the hysteresis model of EPS particles amended light weight soil under multi-step cyclic loading, the dynamic deformation characteristics of light weight soil were studied by consolidated undrained dynamic triaxial tests. The results showed that the backbone curve of light weight soil is hyperbolic and has strain hardening characteristics. With the increase of dynamic stress, the hysteresis curve shape of light weight soil gradually transforms from spindle-shaped to crescent-shaped, showing nonlinearity, hysteresis and strain accumulation. Based on the Hardin-Drnevich model and Masing rules, a modified unloading and reloading rule for the hysteresis model of light weight soil is proposed. The maximum dynamic shear modulus correction coefficient k1 and dynamic shear modulus attenuation coefficient k2 are introduced to establish the modified hysteresis model of light weight soil. Based on the modified hysteresis model, the physical meanings of k1 and k2 are defined. The influence of k1 and k2 values on the shape of hysteresis curve is discussed, and the empirical formulas of k1 and k2 about the dynamic shear strain are obtained. Through the verified dynamic triaxial tests of light weight soil by changing stress state, it is found that the relative error between the predicted values of modified hysteresis model and measured values is between 3.19% and 19.41%, which indicates that the model can describe closely the mechanical response process of light weight soil under complex dynamic conditions. The modified hysteresis model can predict the complex mechanical response mechanism in the progressive evolution of structural soil from convex to concave-convex hysteresis loop.

The degradation of subarctic peatland ecosystems under climate change impacts surrounding landscapes, carbon balance, and biogeochemical cycles. To assess these ecosystems' responses to climate change, it is essential to consider not only the active-layer thickness but also its thermo-hydraulic conditions. Ground-penetrating radar is one of the leading methods for studying the active layer, and this paper proposes systematically investigating its potential to determine the thermal properties of the active layer. Collected experimental data confirm temperature hysteresis in peat linked to changes in water and ice content, which GPR may detect. Using palsa mires of the Kola Peninsula (NW Russia) as a case study, we analyze relationships between peat parameters in the active layer and search for thermal gradient responses in GPR signal attributes. The results reveal that frequency-dependent GPR attributes can delineate thermal intervals of +/- 1 degrees C through disperse waveguides. However, further verification is needed to clarify the conditions under which GPR can reliably detect temperature variations in peat, considering factors such as moisture content and peat structure. In conclusion, our study discusses the potential of GPR for remotely monitoring freeze-thaw processes and moisture distribution in frozen peatlands and its role as a valuable tool for studying peat thermal properties in terms of permafrost stability prediction.

Stress-strain behavior of two different soil specimens subjected to cyclic compressive loading are studied herein, the goal being to present a simple dynamic uniaxial mem-modeling approach that aids physical insight and enables system identification. In this paper, mem stands for memory, i.e., hysteresis. Mem-models are hysteresis models transferred from electrical engineering using physical analogies. Connected in series, a mem-dashpot and mem-spring are employed to model inter-cycle strain ratcheting and intra-cycle gradual densification of the two soil specimens. Measured time histories of stress and strain are first decomposed so that the two modeling components, mem-dashpot and mem-spring, can be identified separately. This paper focuses on the mem-dashpot, a nonlinear generalization of a linear viscous damper. A mem-spring model is also devised built on an extended Masing model. Nonlinear dynamic simulations (with inertia) employing the identified mem-dashpot and mem-spring demonstrate how well the identified mem-models reproduce the measured early-time data (first 200 cycles of loading). Choices of state variables inherited from bond graph theory, the root of mem-models, are introduced, while MATLAB time integrators (i.e., ode solvers) are used throughout this study to explore a range of contrasting damper and spring models. Stiff solver and the state event location algorithm are employed to solve the equations of motion involving piecewise-defined restoring forces (when applicable). Computational details and results are relegated to the appendices. This is the first study to use single-degree-of-freedom (SDOF) system dynamic simulations to explore the consistency of mem-models identified from real-world data.

This paper quantitatively analyses the macroscopic characteristics of soil hysteretic curves under dynamic loading and examines the elastic properties, viscosity, meso-damage degree and energy consumption of soil from a macroscopic perspective. Given the lack of research on the hysteresis characteristics of bioenzyme-modified silty soil, a series of dynamic triaxial tests were conducted under varying bioenzyme dosages, confining pressures, loading frequencies, and other conditions. The analysis focused on several parameters: the slope of the major axis of the hysteretic curve k, the ratio of the major to minor axes alpha, the distance between the central points of adjacent hysteretic curves d, and the area enclosed by the hysteretic curve S. These were used for quantitative analysis of the morphological characteristics, influencing factors, and changing patterns of the hysteresis curve in bioenzyme-modified silty soil. The results showed that the hysteresis curve of the bioenzyme-improved silty soil resembled an inclined ellipse. Under the influence of different bioenzymes dosages, confining pressures, and loading frequencies, k and alpha decreased as dynamic stress increased, while d and S increased exponentially with rising dynamic stress. When the bioenzyme dosage was 0.01%, the k value was largest, and alpha, d and S were smallest. With increasing confining pressure, k increased, while alpha, d, and S decreased. As the loading frequency increased, k, alpha, and d decreased, while S gradually increased. At a bioenzyme dosage of 0.01%, the bioenzyme had the greatest effect on improving the silty soil.

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.

Offshore wind turbines are subjected to long-term cyclic loads, and the seabed materials surrounding the foundation are susceptible to failure, which affects the safe construction and normal operation of offshore wind turbines. The existing studies of the cyclic mechanical properties of submarine soils focus on the accumulation strain and liquefaction, and few targeted studies are conducted on the hysteresis loop under cyclic loads. Therefore, 78 representative submarine soil samples from four offshore wind farms are tested in the study, and the cyclic behaviors under different confining pressures and CSR are investigated. The experiments reveal two unique development modes and specify the critical CSR of five submarine soil martials under different testing conductions. Based on the dynamic triaxial test results, the machine learning-based partition models for cyclic development mode were established, and the discrimination accuracy of the hysteresis loop were discussed. This study found that the RF model has a better generalization ability and higher accuracy than the GBDT model in discriminating the hysteresis loop of submarine soil, the RF model has achieved a prediction accuracy of 0.96 and a recall of 0.95 on the test dataset, which provides an important theoretical basis and technical support for the design and construction of offshore wind turbines.

Granite residual soil is widely used as a subgrade filler in highway construction. Dynamic loads induced by vehicles and earthquakes are complex and involve multidirectional loads, and the dynamic behavior of soil under multidirectional cyclic loading differs significantly from that under unidirectional cyclic loading. A series of horizontal cyclic direct shear tests under cyclic normal loading were conducted using a large-scale cyclic direct shear apparatus at different shear displacement amplitudes (1, 3, 6, and 9 mm) and normal stress amplitudes (0, 100, and 200 kPa). The test results indicate that under cyclic normal stress, the dynamic shear strength of granite residual soil increased during the forward shear process but decreased during the reverse shear process. The damping ratio increases with increasing shear displacement amplitude and normal stress amplitude. This behavior is associated with higher excess pore water pressure induced by greater normal stress amplitude and larger shear displacement, which drive the soil into the yielding phase. The Granite residual soil exhibited significant asymmetric hysteretic characteristics under bidirectional dynamic loading. However, no model has yet been found to describe the asymmetric hysteretic behavior of soil under bidirectional dynamic loading. To obtain the asymmetric hysteretic curve of granite residual soil under bidirectional cyclic loading conditions in the laboratory without the instruments for bidirectional cyclic direct shear tests, the Hardin-Drnevich model and the second Masing rule were extended to propose two asymmetric hysteretic curve models under bidirectional cyclic loading based on the tests. Both models fit with the test results well.