Pore Ice Research Articles

Permafrost slows Arctic riverbank erosion.

The rate of river migration affects the stability of Arctic infrastructure and communities1,2 and regulates the fluxes of carbon3,4, nutrients5 and sediment6,7 to the oceans. However, predicting how the pace of river migration will change in a warming Arctic8 has so far been stymied by conflicting observations about whether permafrost9 primarily acts to slow10,11 or accelerate12,13 river migration. Here we develop new computational methods that enable the detection of riverbank erosion at length scales 5-10times smaller than the pixel size in satellite imagery, an innovation that unlocks the ability to quantify erosion at the sub-monthly timescales when rivers undergo their largest variations in water temperature and flow. We use these high-frequency observations to constrain the extent to which erosion is limited by the thermal condition of melting the pore ice that cements bank sediment14, a requirement that will disappear when permafrost thaws, versus the mechanical condition of having sufficient flow to transport the sediment comprising the riverbanks, a condition experienced by all rivers15. Analysis of high-resolution data from the Koyukuk River, Alaska, shows that the presence of permafrost reduces erosion rates by 47%. Using our observations, we calibrate and validate a numerical model that can be applied to diverse Arctic rivers. The model predicts that full permafrost thaw may lead to a 30-100% increase in the migration rates of Arctic rivers.

Thickness and Structure of Permafrost in Oil and Gas Fields of the Yamal Peninsula: Evidence from Shallow Transient Electromagnetic (sTEM) Survey

The Yamal-Nenets Autonomous District, especially the Yamal Peninsula located in the permafrost zone, stores Russia’s largest oil and gas resources. However, development in the area is challenging because of its harsh climate and engineering–geological features. Drilling in oil and gas fields in permafrost faces problems that are fraught with serious accident risks: soil heaving leading to the collapse of wellheads and hole walls, deformation and breakage of casing strings, gas seeps or explosive emissions, etc. In this respect, knowledge of the permafrost’s structure is indispensable to ensure safe geological exploration and petroleum production in high-latitude regions. The extent and structure of permafrost in West Siberia, especially in its northern part (Yamal and Gydan Peninsulas), remain poorly studied. More insights into the permafrost’s structure have been obtained by a precise sTEM survey in the northern Yamal Peninsula. The sTEM soundings were performed in a large oil and gas field where permafrost is subject to natural and anthropogenic impacts, and its degradation, with freezing–thawing fluctuations and frost deformation, poses risks to exploration and development operations, as well as to production infrastructure. The results show that permafrost in the western part of the Yamal geocryological province is continuous laterally but encloses subriver and sublake unfrozen zones (taliks) and lenses of saline liquid material (cryopegs). The total thickness of perennially frozen rocks is 200 m. The rocks below 200 m have negative temperatures but are free from pore ice. Conductive features (<10 Ohm﮲m) traceable to the permafrost base may represent faults that act as pathways for water and gas fluids and, thus, can cause a geohazard in the oil and gas fields (explosion of frost mounds, gas blow during shallow drilling, etc.).

Direct tensile failures of concrete with various moisture contents and sizes at low temperatures via mesoscale simulations with ice explicit modelling

The enhancement of mechanical properties of concrete meso-components and the interaction caused by non-uniform deformation as well as phase change can cause significant changes in the macro-mechanical performances of concrete at low temperatures. Based on the action mechanism of the above low-temperature effect, this paper established a thermal-mechanical sequential coupled simulation method with explicit modelling of pore ice at the mesoscale level to quantitatively investigate the direct tensile failures and the corresponding size effect of concrete with four structural sizes (D75, D150, D225 and D300) and three moisture contents (2.0 %, 4.0 % and 6.0 %) at different temperatures (20, −30, −60 and −90°C), in term of failure mode, deformation curve, peak strength and residual strength. The numerical results show that the direct tensile peak strength performs an obvious low-temperature enhancement effect due to the more damaged aggregates and more areas being in a state of multi-axial stress caused by low-temperature non-uniform stress field. However, with the decreasing temperature, the residual strength shows a decrease trend and the trend slows down with the increasing moisture content. Besides, as the temperature drops from 20°C to −90°C, both the size effects on direct tensile peak strength and residual strength are strengthened (with the increase approaches nearly 200 % for peak strength while 33 % for residual strength). Finally, a modified size effect theoretical model was developed considering the quantitative coupling effects of low temperature and moisture content. The present research results can provide a reference for the performance evaluation and safe design of large-sized concrete exposed to low-temperature environments.

Pore structures and deterioration mechanism of concrete after cryogenic freeze-thaw cycles: Effects of moisture contents and aggregates

With the popularization of all-concrete liquefied natural gas (ACLNG) storage tanks, the performance of concrete after cryogenic exposure (−170 °C) has garnered significant interests from researchers. To investigate the damage mechanism of pore structures and the corresponding influencing factors, this study involved subjecting concrete with two moisture contents, namely air-dried and water-saturated to 10 cryogenic freeze-thaw cycles (CFTCs) to assess the variation laws in strengths. The mercury intrusion porosimetry (MIP), nitrogen adsorption and desorption (NAD), scanning electron microscopy (SEM), backscattered scanning electron (BSE) and low-temperature calorimeter (LTC) methods were employed to evaluate pore characteristics and the phase transition of pore water. Furthermore, pore structures were conducted using fractal dimensions based on the Zhang model and Frenkel-Halsey-Hill model. Findings indicate that there is a more pronounced reduction in strengths after 10 CFTCs in water-saturated state. Following cryogenic exposure, the porosity of paste increases from 4.97 % to 5.58 % and 7.04 %, and the porosity of mortar rises from 6.05 % to 7.20 % and 8.76 %, respectively. Aggregates in mortar contribute to more severe damage to pore structures in cryogenic circumstances, particularly capillary and macropores. The nucleation of pore water and ice contents are greatly influenced by moisture contents, and the frozen fraction is only 84.3 % in air-dried state. The destruction of gel pores is attributed to the continuous multi-order phase transition in water-saturated state. This work conducts a thorough cross-scale analysis of the pore structure in both paste and mortar, and it quantitatively evaluates the phase transition of pore water under cryogenic conditions using the LTC method. Ultimately, the influence of moisture contents and aggregates on the pore structures are evaluated, which provides deeper insights into the deterioration mechanism of concrete following cryogenic exposure.

Mechanism of pore water phase change, freezing expansion, and pore enlargement of coal samples under low-temperature liquid nitrogen action

Liquid nitrogen fracturing, a technology for enhancing coal seam permeability without water, garnering considerable attention due to the intricate network of fractures induced by the low-temperature impact of LN2 on coal reservoirs. The phase transition freezing expansion of water-ice plays a pivotal role in determining the efficacy of coal damage. As direct measurement of water signal change during freezing is impractical, studying the variation in unfrozen water content during coal sample re-warming subsequent to cold shock is beneficial. This approach aids in elucidating the phase transition expansion and pore expansion mechanisms at different pore scales. The findings reveal the sequence of pore ice melting in coal samples frozen by LN2: micropores exhibit initial water appearance, followed by medium-sized pores, with micropores reaching peak levels first. Subsequently, water migrates from micropores to medium-sized pores, leading to their peak levels, before macropores show water presence. The temperature gradient from the coal sample center to the coal wall, owing to thermal resistance, results in the part melting first also freezing first. Analysis indicates that the porosity signal of lignite and bituminous coal increased by 203 % and 467 % during medium-sized pore melting, emphasizing the prominence of the medium-sized pore melting stage. It is further inferred that the phase transition freezing expansion and pore expansion of coal sample water induced by LN2 primarily occur in medium-sized pores.

Mechanism analysis on tensile fracture properties of heterogeneous BFLAC at cryogenic temperature: Experimental investigation and mesoscale simulation

This study aims to investigate the tensile failure behaviours and corresponding fracture mechanisms of basalt fiber reinforced lightweight-aggregate concrete (BFLAC) at various temperatures via a comprehensive cryogenic tests and mesoscale simulations, with a special focus on the quantitative effects of cryogenic temperature and fiber volume fraction. Firstly, Macro- and micro-scale split-tensile tests of BFLAC with fiber volume fractions of 0.0–0.3 % at 20∼-90 °C were conducted. Secondly, a two-steps sequentially thermo-mechanical coupled meso-scale analysis approach with explicit modelling of fibers and pore ice was developed to simulate the corresponding direct-tensile failures of BFLAC with more fiber volume fractions. The results show that as the temperature falls from 20 °C to −90 °C, the dominant action mechanism of basalt fibers changes from Mode-1 (pull-out of fibers) to Mode-2 (rupture of fibers) due to the ice formation and the interactions between meso-components. Tensile strengths of BFLAC present a significant low-temperature enhancing effect, with a maximum increase of 90 % for direct-tensile strength while 104 % for split-tensile strength. Besides, as the temperature drops, although a larger proportion of fibers are in a low bridging stress state and a smaller proportion reach yield stress for rupture, the average fiber stress increases and the utilization degree of fibers with more stresses transferred improves, which results in that the fiber reinforcement effect is strengthened. Finally, based on experimental and numerical results, the quantitative relationships between split-tensile and direct-tensile strengths at different cryogenic temperatures were given. The present research results can better understand the cryogenic mechanical properties of BFLAC, which have important reference value for its extensive promotions and applications in engineering structures exposed to extreme low-temperature environments.

Freezing‐Thawing Hysteretic Behavior of Soils

AbstractThe soil freezing characteristic curve (SFCC) plays a crucial role in investigating the soil freezing‐thawing process. Due to the challenges associated with measuring the SFCC, there is a shortage of high‐quality or rigorous test results with sufficient metadata to be effectively used for applications. Current researchers typically conduct freezing tests to measure the SFCC and assume a singular SFCC when studying the freezing‐thawing process of soils, although limited studies indicated that there is a hysteresis during the freezing and thawing process. In this paper, a series of freezing‐thawing tests were performed to assess the SFCC, utilizing a precise nuclear magnetic resonance apparatus. The test results reveal a hysteresis between the SFCC obtained from the freezing process and that from the thawing process. Through analyzing the test results, the hysteresis mechanism of the SFCC is attributed to supercooling. Supercooling inhibits initial pore ice formation during freezing, causing a drastic liquid water‐ice phase change once supercooling ends. Despite being considered closely related, the hysteresis of the SFCC differs from the soil water characteristic curve (SWCC), and the models used to simulate the hysteresis of SWCC cannot directly be used. To address the impact of supercooling on soil freezing‐thawing hysteresis, a novel theoretical model is proposed. Comparisons between the measured and predicted results affirm the validity of the proposed model.

Thermal-mechanical modelling on cumulative freeze-thaw deformation of porous rock in elastoplastic state based on Drucker-Prager criterion considering dilatancy

The comprehensive understanding of freeze-thaw (F-T) deterioration and deformation properties of rock is a critical basis for frost damage prevention on cold regional rock engineering. In freezing process, plastic strain generates in rock when the stress induced by pore ice pressure (PIP) becomes higher than yield strength, and unrecoverable deformation accumulates in rock under multiple F-T cycles. Therefore, this study presents a thermal-mechanical modelling on cumulative F-T deformation of rock under cyclic F-T action. In the modelling of mechanical action, the microporomechanics theory is applied with a spherical element model in the derivation of equilibrium equations of rock, and the Drucker-Prager criterion is utilized with dilatancy effect of rock considered to describe unrecoverable plastic deformation cumulated in rock under multiple F-T cycles. The thermal process is modelled with unfrozen water content and latent heat accounted. The proposed thermal-mechanical modelling method is validated based on experiments and numerical simulations of F-T deformation of sandstone, and it is able to simulate the temperature and F-T deformation of rock in acceptable precision. The numerical distributions of temperature, PIP, maximum principal strain and displacement are exhibited and show that the plastic region begins to develop in rock matrix with PIP exceeding the elastoplastic critical pressure when temperature drops to about −2 °C and grows rapidly when temperature drops from −2 to −6 °C. Several F-T cycles later, large unrecoverable plastic strain remains in sandstone though PIP dissipates after thawing. The cumulative F-T deformation, porosity increment and maximum of plastic volume ratio are much greater under condition with freezing temperature of −20 °C, compared with condition −10 °C. Besides, as cycle number increases, the plastic volume ratio becomes smaller and its decrease rate attenuates.

Dynamic mechanical behaviours of frozen rock under sub-zero temperatures and dynamic loads

Dynamic mechanical behaviours of frozen rocks are essential for dealing with the design and stability of various engineering structures in cold regions. A dynamic-cryogenic testing system integrated with high-speed Digital Image Correlation (DIC) is adopted to investigate the dynamic mechanical behaviours of rock subjected to ambient room and sub-zero temperatures, along with high-rate loading. Dynamic compression, tensile and Mode I dynamic fracture tests were performed at 20, -20, −50 and −70 °C using dry and saturated sandstone from Yulong Copper Mine at high-altitude cold region. A DIC-based approach using pixel-based virtual extensometers is proposed to accurately capture the real-time cracking speed with high precision. The proportion of ice and unfrozen water in rock pores across various temperatures is quantified using the nuclear magnetic resonance (NMR) method. The results indicate significant temperature and rate dependencies in dynamic fracture toughness, crack propagation velocity, dynamic tensile strength, dynamic compressive strength, and failure modes; These parameters exhibit an increase with decreasing ambient temperature as unfrozen water transitions into ice, demonstrating the frozen effect of ice filling pores, supporting the rock skeleton, and bonding ice-rock interfaces. With decreasing temperature, crack branching becomes more pronounced in dynamic notched semi-circular bending (NSCB) tests. Additionally, dynamically compressed frozen rocks tend to transition from pulverization (Type II) to splitting (Type I) with multiple elongations, attributed to the strengthening effect. The findings elucidate the failure mechanisms observed in a higher proportion of larger blast blocks and hold valuable implications for blasting designs and dynamic disaster prevention in cold regions.

Effect of steel fiber content on failure strength and toughness of UHPC-CA at low temperature: An experimental investigation

As a potential and sustainable construction material, ultra-high performance concrete containing coarse aggregate (UHPC-CA) finds promising applications in engineering constructions, particularly in severe cold regions. This study conducts an experimental study to investigate the influence of fiber content (ranging from 0.0 % to 3.0 %) on the uniaxial compression and splitting-tension failures of UHPC-CA across a temperature range of −90 °C∼20 °C. Through microstructure analysis, the underlying mechanisms and quantitative analysis behind the low-temperature enhancement effect and fiber reinforcement effect on the nominal strength and toughness of UHPC-CA were elucidated. The test results show that as the temperature drops from 20 °C to −90 °C, nominal strengths of UHPC-CA monotonically increase (with a maximum increase 88.1 % for splitting-tension strength while 72.6 % for compressive strength), but the compressive toughness reduces with a maximum decrease 54.6 %. Additionally, the incorporation of steel fibers can enhance the nominal strengths and compressive toughness, exhibiting the fiber reinforcement and toughening effects. Compared to the UHPC matrix, all the increases for nominal strengths and compressive toughness of UHPC-CA with 2.0 % steel fibers at room temperature are higher than those at low temperatures, demonstrating that the decreasing temperature can weaken the fiber reinforcement and toughening effects, which may attribute to fact that the formation of microcracks (due to the expansion of phase change of pore ice) in the surface between steel fiber and UHPC matrix at low temperatures, as observed in the microscopic test. However, as the fiber content exceeds 2.0 %, the splitting-tension strength decreases, thereby it is recommended that the optimal steel fiber content is around 2.0 % for UHPC-CA at low temperatures. Finally, considering the quantitative effects of steel fiber characteristics and temperature, the empirical prediction models for cryogenic nominal strengths were proposed and verified, which can well predict compressive and splitting-tension strengths of UHPC-CA with various steel fiber contents at low temperatures. This study can provide an experimental data reference for the safety design of UHPC structures and contribute to promoting the application of UHPC-CA in the engineering construction of cold regions.

Water accumulation unrelated to ice segregation near the freezing front during soil freezing: Implications for frost heave in high-speed railway embankments and ground ice formation

Investigating water migration in frozen ground is crucial for understanding the hydrothermal dynamics of frozen soil, which is central to frozen soil engineering (e.g., frost heave) and permafrost environments (e.g., ground ice formation). Previous studies have mostly focused on water migration related to ice segregation in frost-susceptible soils, but less on water migration that is independent of ice segregation in non-frost-susceptible soils. Using a novel layer-scanning method based on the nuclear magnetic resonance (NMR) techniques, the water migration dynamic was observed during the freezing of non-frost-susceptible loess under different drainage conditions (with or without water drainage). Results showed an apparent phenomenon of water migration to the freezing front (TFF) (layer L7) (i.e., water accumulation that is independent of ice segregation) occurred in the early freezing stage of non-frost-susceptible soils under no draining water conditions. The traditional water migration mode is related to ice segregation, a new water migration (including upward and downward) mechanism was proposed to reveal this water accumulation unrelated to ice segregation at TFF. The water accumulation at the freezing front is independent of ice segregation, and is attributed to 9% volume expansion due to pore ice formation in the frozen zone, causing water migration downward TFF; while the decrease of the pore volume of the unfrozen zone (being compressed) accounts for the water migration upward TFF. Investigation into whether ice segregation exists and different modes of water migrations (depending on ice segregation or not) will be greatly helpful for exploring the mechanisms of micro frost heave in the high-speed railway embankments and the thick-layered ground ice formation.

On water freezing in slag-blended cementitious materials at early ages

Concretes casted in cold climate can be threatened by frost action from the initial stages of maturing. Hence, to secure their long-term functionality, it is essential to be acquainted with the material's early response to sub-zero temperatures. Addressing this challenge, the article concerned the resistance of young slag-blended cementitious materials against the early frost attack. An extensive experimental analysis covering both macro- and microscales was designed for this purpose. At first, development of the pore size distribution and ice content profile of the 100 % Portland cement and slag-blended pastes were evaluated from the early ages until maturing via mercury intrusion porosimetry and differential scanning calorimetry. It was reported that the distinction between gel pore and capillary pore is created after 3rd day for OPC and after 21st day for slag-blended paste. Next, the destructive impacts of a single freezing-thawing cycle on the mechanical properties of the young Portland cement and slag-blended mortars were measured. Subsequently, the strength development of the Portland cement and slag-blended concretes that experienced the frost exposure were evaluated and compared with each other. It was observed that the early freezing strengthen (up to 15 %) the OPC based concrete and weaken (up to 13 %) the slag-blended concrete. Additionally, the analysis was proposed to simulate the winter conditions at the building sites. The strength development of the Portland cement and slag-blended concretes that were cured at low temperatures and experienced an early freezing episode were assessed. The decrease of curing temperature caused the reduction of compressive strength of OPC-based concrete around 25 % and slag-blended concrete around 50 %. As a result, micro- and macrostructural requirements were proposed to secure the proper resistance of the slag-blended cementitious materials against the possible early frost damage.

Estimation of unfrozen water content of saturated sandstones using nuclear magnetic resonance, mercury intrusion porosimetry, and ultrasonic tests

The unfrozen water content (UWC) is a crucial parameter that affects the strength and thermal properties of rocks in relation to engineering construction and geological disasters in cold regions. In this study, three different methods were employed to test and estimate the UWC of saturated sandstones, including nuclear magnetic resonance (NMR), mercury intrusion porosimetry (MIP), and ultrasonic methods. The NMR method enabled the direct measurement of the UWC of sandstones using the free induction decay (FID). The MIP method was used to analyze the pore structures of sandstones, with the UWC subsequently calculated based on pore ice crystallization. Therefore, the MIP test constituted an indirect measurement method. Furthermore, a correlation was established between the P-wave velocity and the UWC of these sandstones based on the mixture theory, which could be employed to estimate the UWC as an empirical method. All methods demonstrated that the UWC initially exhibited a rapid decrease from 0 °C to −5 °C and then generally became constant beyond −20 °C. However, these test methods had different characteristics. The NMR method was used to directly and accurately calculate the UWC in the laboratory. However, the cost and complexity of NMR equipment have precluded its use in the field. The UWC can be effectively estimated by the MIP test, but the estimation accuracy is influenced by the ice crystallization process and the pore size distribution. The P-wave velocity has been demonstrated to be a straightforward and practical empirical parameter and was utilized to estimate the UWC based on the mixture theory. This method may be more suitable in the field. All methods confirmed the existence of a hysteresis phenomenon in the freezing-thawing process. The average hysteresis coefficient was approximately 0.538, thus validating the Gibbs–Thomson equation. This study not only presents alternative methodologies for estimating the UWC of saturated sandstones but also contribute to our understanding of the freezing-thawing process of pore water.

The U-net-based ice pore parameter extraction method for establishing the SAW icing sensing mechanism

The porous ice layer formed upon the SAW (surface acoustic wave) propagation path will produce transient acoustic attenuation, and a new icing sensing method can be constructed based on this feature. Obtaining the pore characteristics of an ice layer is a prerequisite and basis for deriving the sensing mechanism of SAW icing. Employing the U-net network, a segmentation model of frozen pores is developed by collecting frozen images and calibrating sample data sets for training in this work. The MIOU and the MPA of the model reach 92.16 % and 95.15 % respectively. Then, the watershed algorithm and post-processing were used to accurately obtain the pore characteristic parameters (porosity, pore number, and average pore area) of the ice layer. On this basis, applying the Biot's theory for two-phase porous media, a theoretical model of the SAW icing sensing mechanism was established, and the acoustic sensing response under different icing types was theoretically simulated. To prove the accuracy of the theoretical simulation, a SAW sensor using a waveguide structure of SU-8/ST-90 °X quartz operating at 80 MHz was constructed and evaluated experimentally. The results demonstrate that the acoustic sensing responses of different ice types are completely consistent with the theoretical results. This fully verifies the accuracy of the ice pore parameters extracted in this work and also confirms the feasibility of using pore microscopic information to achieve ice detection and ice type discrimination.

Analytical solution for one‐dimensional thaw consolidation model with double moving boundaries

AbstractA one‐dimensional thaw consolidation model considering the density change from pore ice to pore water is established, and the model describes a special type of moving boundary problem with double moving boundaries. An analytical solution for the model under a time‐varying external load is developed using certain form of superposition principle and the similarity type of general solution. Some known solutions in literature can be recovered as special cases of the analytical solution once the density change from pore ice to pore water is neglected. If the thawing front was to cease at certain time, consolidation of thawed soil in a fixed region is then encountered, and an analytical solution for the post‐thawing consolidation problem is developed using the Green's function method. Computational examples of the analytical solutions are presented. First, comparison between our model and classical model of Morgenstern and Nixon (MN model) is conducted, showing the error caused by neglecting the density change from pore ice to pore water. The MN model overestimates the excessive pore water pressure at locations near the soil surface, while underestimates it at locations near the thawing front; the error caused by neglecting the density change becomes more pronounced with increasing ice content of frozen soil. Second, comparison between the thaw consolidation process under an instant load and that under a corresponding exponential load is made. The difference in thaw consolidation behavior between the two situations mainly displays during the early stage, when there is obvious discrepancy between the two external loads; the degree of consolidation and thaw consolidation settlement are more sensitive to the discrepancy in external load than the excessive pore water pressure is.

Direct Analysis of Marine Dissolved Organic Matter Using LC-FT-ICR MS.

Marine dissolved organic matter (DOM) is an important component of the global carbon cycle, yet its intricate composition and the sea salt matrix pose major challenges for chemical analysis. We introduce a direct injection, reversed-phase liquid chromatography ultrahigh resolution mass spectrometry approach to analyze marine DOM without the need for solid-phase extraction. Effective separation of salt and DOM is achieved with a large chromatographic column and an extended isocratic aqueous step. Postcolumn dilution of the sample flow with buffer-free solvents and implementing a counter gradient reduced salt buildup in the ion source and resulted in excellent repeatability. With this method, over 5,500 unique molecular formulas were detected from just 5.5 nmol carbon in 100 μL of filtered Arctic Ocean seawater. We observed a highly linear detector response for variable sample carbon concentrations and a high robustness against the salt matrix. Compared to solid-phase extracted DOM, our direct injection method demonstrated superior sensitivity for heteroatom-containing DOM. The direct analysis of seawater offers fast and simple sample preparation and avoids fractionation introduced by extraction. The method facilitates studies in environments, where only minimal sample volume is available e.g. in marine sediment pore water, ice cores, or permafrost soil solution. The small volume requirement also supports higher spatial (e.g., in soils) or temporal sample resolution (e.g., in culture experiments). Chromatographic separation adds further chemical information to molecular formulas, enhancing our understanding of marine biogeochemistry, chemodiversity, and ecological processes.

Pore‐scale freezing of a sandy saline soil visualized with micro‐computed tomography

AbstractSandy saline soils are widely distributed and commonly experience seasonal or long‐term freezing, yet the freezing process in these soils is rarely studied. This research utilized in situ X‐ray computed tomography (CT) to visualize pore‐scale freezing processes in sandy saline soils under various initial water and salt contents. Micron‐resolution observations of pore ice and unfrozen water produced new insights into the preferential orientation and grouping of pore ice crystals, intersections between different pore ice crystal groups causing anisotropic behavior, decreasing pore ice crystal size under faster freezing rates, and the formation of interconnected networks of both pore ice and unfrozen water upon freezing. From a thermodynamic perspective, the salt content in the unfrozen liquid is dependent on the local temperature as described by water–salt phase diagram. Furthermore, the local volume ratio between unfrozen water and pore ice reflects the initial salt content and salt mass transfer occurring due to both diffusion and fluid flow processes. This work improves the understanding of complex freezing phenomena in sandy saline soils through high‐resolution evidence of crystallization patterns, transformation mechanisms, and coupled heat‐mass transfer.

Environmental controls of winter soil carbon dioxide fluxes in boreal and tundra environments

Abstract. The carbon cycle in Arctic–boreal regions (ABRs) is an important component of the planetary carbon balance, with growing concerns about the consequences of ABR warming for the global climate system. The greatest uncertainty in annual carbon dioxide (CO2) budgets exists during winter, primarily due to challenges with data availability and limited spatial coverage in measurements. The goal of this study was to determine the main environmental controls of winter CO2 fluxes in ABRs over a latitudinal gradient (45∘ to 69∘ N) featuring four different ecosystem types: closed-crown coniferous boreal forest, open-crown coniferous boreal forest, erect-shrub tundra, and prostrate-shrub tundra. CO2 fluxes calculated using a snowpack diffusion gradient method (n=560) ranged from 0 to 1.05 g C m2 d−1. To assess the dominant environmental controls governing CO2 fluxes, a random forest machine learning approach was used. We identified soil temperature as the main control of winter CO2 fluxes with 68 % of relative model importance, except when soil liquid water occurred during 0 ∘C curtain conditions (i.e., Tsoil≈0 ∘C and liquid water coexist with ice in soil pores). Under zero-curtain conditions, liquid water content became the main control of CO2 fluxes with 87 % of relative model importance. We observed exponential regressions between CO2 fluxes and soil temperature in fully frozen soils (RMSE=0.024 gCm-2d-1; 70.3 % of mean FCO2) and soils around the freezing point (RMSE=0.286 gCm-2d-1; 112.4 % of mean FCO2). FCO2 increases more rapidly with Tsoil around the freezing point than at Tsoil<5 ∘C. In zero-curtain conditions, the strongest regression was found with soil liquid water content (RMSE=0.137 gCm-2d-1; 49.1 % of mean FCO2). This study shows the role of several variables in the spatio-temporal variability in CO2 fluxes in ABRs during winter and highlights that the complex vegetation–snow–soil interactions in northern environments must be considered when studying what drives the spatial variability in soil carbon emissions during winter.

A coupled thermomechanical peridynamic correspondence model for damage prediction in a freezing rock

This study presents a coupled thermomechanical (TM) bond-associated non-ordinary state-based peridynamic (BA-NOSB PD) (correspondence) model for simulating the thermal behavior and crack propagation in a saturated rock due to freeze. Considering the change in thermal property and the latent water–ice phase transition at low-temperature conditions, the PD form of the nonlinear thermal conduction equation is derived by using the PD differential operator (PDDO). The deformation response is obtained in the framework of BA-NOSB PD model considering the pore ice pressure and thermal expansion. The accuracy of the TM BA-NOSB PD model is verified by simulating the thermal behavior of tuff and sandstone and crack propagation in specimens with pre-existing cracks at low-temperature conditions. The PD predictions of temperature and frost heaving strain are in close agreement with the experimental and previous numerical results. The present model also accurately predicts crack propagation and coalescence in saturated rocks due to the freezing frost.

Investigations on the freezing characteristic and flexural mechanical properties of frozen loess

The exploitation of deep resources increases with the implementation of the western development strategy. The artificial freezing technique is used for shaft construction when facing complex geological conditions. Due to the large deformation of frozen soil wall, disasters often occur during shaft excavation, such as freezing pipe deflection, and rupture. The flexural mechanical property of frozen loess is important for the design of shaft construction in the western region. In this study, the nuclear magnetic resonance (NMR) tests of frozen loess were carried out under different freezing temperatures, and the evolution law of unfrozen water content in frozen loess was investigated. The results show that the pore ice content increases rapidly when the temperature decreases from −1 °C to −7 °C; then the phase transition of residual unfrozen water becomes difficult as the further decrease in temperature. The flexural tests of frozen loess with different water content (10%, 15%, 20%, 25%) were carried out at different freezing temperatures (−5 °C, −10 °C, −15 °C, −20 °C). The flexural strength of frozen loess increases with the decrease of unfrozen water content, and it increases first and then decreases as the initial water content increase. The flexural toughness gradually increases at −5 °C and − 10 °C with the increase of water content, while increasing first and then decreasing at −15 °C and − 20 °C. The roughness of the fracture surface increases with the decrease in temperature.