Ice-water Phase Change Research Articles

Analysis on the evolution of frost heaving pressure of penetrating crack considering water content and migration

Frozen heaving failure of fractured rock mass is commonly encountered in engineering in cold regions, which is chiefly caused by the frost heaving pressure arising from the water–ice phase change in the crack. To explore the evolution of frost heaving pressure in penetrating elliptical crack considering water content and water migration, a new theoretical model embodying the frost heaving pressure evolutionary character was established by introducing freezing ratio function. The equivalent thermal expansion coefficient was used to analyze the evolution process of frost heaving pressure under the effect of water–ice phase change, which was then verified. It was found that the evolution process of frost heaving pressure can be divided into three stages: free expansion stage of water–ice phase change, rapid growth stage of frost heaving pressure, and stable stage of frost heaving pressure. Subsequently, the influences of rock thermal expansion effect, properties of rock and ice, and water content of crack on the frost heaving pressure were investigated. The results indicate that the impact of rock thermal expansion on frost heaving pressure is extremely slight, which is negligible. Comparing with the properties of rock, the properties of ice show significant effects on the frost heaving pressure, particularly the Poisson ratio of ice. In the case of identical water migration ratio, the peak frost heaving pressure increases linearly with the water content of crack.

Effect of different ice contents on heat transfer and mechanical properties of concrete

With annually decreasing shallow mineral resources, development and utilization of deep mineral resources are critical. However, heat hazards in deep mines pose hidden dangers to health and safe operation of equipment. Therefore, a suitable cooling method is an urgent concern in deep high-temperature mines. This study proposes a cooling method that introduces ice into concrete for phase change cooling and improved strength characteristics. It can be used in deep high-temperature mines, and is known as iced-concrete technology. Based on temperature monitoring, mechanical strength tests, and microstructure tests, this study explores the influence of different ice contents (0 %, 10 %, 20 %, 30 %, 40 %, 50 %, 60 %) on heat transfer and mechanical properties of concrete, and analyzes the mechanism of heat transfer and the mechanical properties of concrete under the action of ice particles. The results show that: (1) the temperature evolution law of concrete with different ice contents takes 18 h–26 h and has a temperature change inflection point interval that first increases and then decreases. With an increase in ice content, the lower the initial temperature, the greater the rate of temperature change. (2) Ice particles have an obvious positive effect on the uniaxial compressive strength and splitting strength of concrete; the final mechanical properties of concrete are positively correlated with the ice content with a constant water (ice)–cement ratio. (3) Compared with ordinary concrete, the initial low-temperature water storage capacity of iced concrete promotes formation of hydration products in the later stage to some extent, improving the content of hydration products in the curing stage, promoting the full filling of internal concrete pores, reducing the content of harmful macropores and mesopores, optimizing the pore structure, and improving the compactness and mechanical strength of the concrete. (4) The cooling efficiency of concrete was increased with an increase in ice content. The cooling efficiency of concrete with 60 % ice content was six times that of ordinary concrete. When the ice content exceeded 20 %, the cooling capacity generated by ice–water phase change was greater than the heat absorbed by the temperature change of the water.

Strategy optimization of proton exchange membrane fuel cell cold start

<p indent="0mm">Under the background of “carbon peaking and carbon neutrality” goals, the proton exchange membrane fuel cell (PEMFC) vehicle attracts extensive attention due to its high energy conversion efficiency, long range and zero emission. However, the challenge of cold start in low temperature environments becomes a large obstacle to its commercialization and application. The current cold start performance of typical commercial fuel cell vehicles cannot reach the optimal target yet. Several problems such as the lack of complete mechanical systems of water transport and phase change and the absence of simulation models for on-board fuel cells at system level still exist. Thus, researchers have carried out a great number of experiments and simulations to study the mechanism of degradation, water transport, phase change and heat transfer as well as start-up strategy optimization of PEMFC cold start. By measuring the output performance, it is found that the degradation of performance is attributed to the formation of ice which covers the reaction active region, blocks the gas channel and increases the electrical contact resistance. Researchers also believe that the reduction of durability is owing to the volume change of water-ice phase change which leads to the damage of the structure by characterizing the microstructure of the fuel cell. With the help of transparent cells, neutron imaging and simulations of multiphase PEMFC models, the distribution, transport and phase change of water and ice during cold start have been studied. The results indicate that the electrochemical product water first saturates the PEM and the catalyst layer (CL) as membrane water. Then the water remains liquid in the form of supercooled water at low temperatures, and finally flows out of the cell or freezes suddenly. By studying the heat transfer process both in plane and through plane during cold start, researchers find that at central regions of the single cell and in the middle part of the stack, the temperature rises fastest. They also find that the irreversible reaction heat generated by cathodic oxygen reduction reaction (ORR) is the main source of heat generation. Based on the cold start mechanism of PEMFCs, researchers have developed various self-start and auxiliary start strategies. The start-up mode and control strategies have been optimized. The internal structure and materials of each part have been improved. And a series of auxiliary startup strategies such as gas purging, heater heating, reaction heating, gas heating as well as coolant heating have been proposed. A large number of simulations and experiments are carried out to verify the advantages of these strategies. Through these excellent cold start strategies, the commercialization and promotion process of PEMFC vehicles can be accelerated.

Experimental study on deformation characteristics of chloride silty clay during freeze-thaw in an open system

Artificial ground freezing (AGF) is widely applied in coastal urban areas for underground transportation infrastructure construction. It is vital to understand deformation characteristics and the mechanisms of chloride soils during freeze-thaw (F-T) for the safe application of AGF. A series of unidirectional open-system F-t-tests were conducted to investigate the deformation characteristics of saturated clay with various NaCl content. Continuous measurements were performed on important parameters such as temperature, solution intake volume, frost heave, and thaw settlement during F-T. Besides, the pore size distributions were obtained via the Nuclear Magnetic Resonance relaxometry to help reveal the deformation mechanism. The results indicated that the freezing front propagated rapidly (Phase I) and then reached a plateau at the thermal equilibrium stage (Phase II). The location of the freezing front in Phase II was dependent on salt content. The amount of solution intake decreased with increased salt content. The rate of solution intake in two phases was also found to be salt content-dependent, while for a certain specimen the order of the rates in two phases varied with increasing salt content. The total frost heave for the same freeze time decreased with increasing salt content and the solution intake induced frost heave mainly contributed to the total frost heave. Total thaw settlement was relatively insensitive to the salt content of higher values. At the end of freezing and thawing processes, minimal frost heave ratio and thaw settlement coefficient were both identified in the specimen with 1% salt content. Moreover, the evolution of pore structure was found to be more prominent in specimens with lower salt content due to more water-ice phase change during freezing, which caused more significant frost heave.

Experimental and unified mathematical frameworks of water-ice phase change for cold thermal energy storage

Cold thermal energy storage (CTES) is a process that supplies cold thermal energy to a medium for storage and extracts it whenever is needed. The storage medium is phase change material (PCM), which makes great use of the large quantity of latent heat released during solidification or melting. However, some key fundamental and applied issues have not yet been resolved: how do we overcome the thermal interference in PCMs from unstable microscopic crystallization and external mechanical forces? Are there any unified and robust models to predict the whole time evolution of phase change accurately and rapidly? To fulfill these research gaps, we firstly establish a novel, well-controlled experimental system for PCMs that mitigates the thermal disturbance over a medium to large volume during solidification, capable of measuring transient temperature data and characterizing freezing stages at both macro- and micro-scale. We also develop in detail a unified semi-analytical mathematical framework to model the solidification of PCMs, consisting of five subsequent stages: liquid supercooling, nucleation, recalescence, equilibrium freezing, and solid subcooling. The modeling results yield a good agreement with our experimental data in several scenarios, particularly the nucleation temperature and time as well as the total freezing time are accurately predicted. Lastly, we extend our study to investigate the thermal effects of various radial positions, geometries, initial temperatures, heat transfer fluid temperature, and heat transfer coefficients in the context of CTES system.

Numerical analysis of cooling performance of a novel structure affected by aeolian sand clogging on expressway embankment in permafrost regions

As an efficient and economical engineering measure, the inverted T-shaped crushed-rock interlayer is used to protect the permafrost under expressway. Its cooling performance is threatened by the climate warming, strong heat absorption of asphalt pavement and aeolian sand on the Qinghai-Tibet Plateau. However, the influence effect of the aeolian sand is unclear. In this study, a hydro-thermo coupled model, including ice-water phase change, heat and water transfer, and air convection heat transfer in the crushed rock, is established to simulate and evaluate the cooling performance of the inverted T-shaped crushed-rock interlayer affected by aeolian sand clogging. A novel structure combining the L-shaped two-phase closed thermosyphons with the inverted T-shaped crushed-rock interlayer is proposed to satisfy the strict requirement of expressway. The numerical results show that the cooling effect of the inverted T-shaped crushed-rock interlayer on the underlying permafrost is influenced by aeolian sand to some extent, whose cooling effect is weak and concentrated at the embankment center. Permafrost beneath expressway was still degenerated, which is unfavorable for the long-term stability of expressway embankment. The novel structure can overcome the adverse effect of aeolian sand, produce a significant cooling performance on the permafrost under the embankment, and maintain the thermal stability of expressway embankment. Under the threat of aeolian sand, this novel structure is a candidate structure for the expressway embankment in permafrost regions.

Experimental Study on the Effect of Freezing and Thawing on the Shear Strength of the Frozen Soil in Qinghai-Tibet Railway Embankment

The direct shear tests of different dry density and moisture content samples at different temperatures of the frozen soil in the Qinghai-Tibet Railway embankment between Tanggula South and Anduo section were carried out to analyze the influence rules of each experimental factor on the mechanical properties of frozen soil during the freeze-thaw process. The results show the following. (1) When the frozen soil temperature is below 0°C and continues to drop during the freezing and thawing process, each sample shows the law of a significant increase in cohesion and a slight decrease in the internal friction angle. In the meantime, the cohesion obtained during the thawing process of the sample at the same temperature point is higher than that obtained during the freezing process. In contrast, the internal friction angles exhibit an opposite law, where the internal friction angle during the melting process is lower than the internal friction angle during the freezing process. After freezing-thawing action, it deserves to be mentioned that the cohesion increases slightly while the internal friction angles present a slight decrease trend compared to the initial state. (2) With the decrease in temperature and the gradual increase in cohesion, the temperature curve can be divided into a fast-growing section from 0 to −2°C, a slow-growing section from −2 to −8°C, and a second fast-growing section from −8 to −10°C owing to the combined effect of the pressure-thawing action and ice-water phase change. In addition, the rate of decrease in the internal friction angle also shows a similar pattern. (3) The cohesion and the internal friction angle of samples both tend to increase first and then decrease with the rise of the initial moisture content, and the critical initial moisture content is near the optimal moisture content of 15%. (4) Both the cohesion and the internal friction angle of the samples increase with dry density growth. The growth rate of cohesion will gradually increase as the temperature decreases. Moreover, the growth rate of cohesion of low dry density samples is more susceptible to temperature, while the internal friction angle growth rate is not affected by temperature.

Rayleigh-Bénard Convection and Freezing Propagation During Water-Ice Phase Change

Rayleigh-Bénard Convection and Freezing Propagation During Water-Ice Phase Change

Investigation of microscopic mechanisms for water-ice phase change propagation control

The influence of thermodynamic and structural conditions on water-ice phase change process was investigated with consideration of graphene-water surface interactions for fundamental understanding and effective control of freezing propagation. The phase change propagation as well as structural and energetic properties during the phase change were examined by analyzing atomic data from molecular dynamics simulations. The freezing propagation speed is affected by the competition between atomic mobility and thermodynamic driving force for phase change, which causes an optimal temperature for fast ice growth to appear at 252 K. In addition, the water-ice interfacial energy, which depends on the orientation of the water-contacting ice surface, changes interfacial structural stability and thus freezing propagation, i.e., a higher interfacial energy leads to a faster ice propagation. Therefore, we suggest that the water-ice phase change propagation can be controlled by adjusting the orientation of ice crystal. The surface interactions, or graphene-water interactions in this research, affect the energetics near the water-surface interface. The energetic change rearranges the ice crystal or surface structures to reach the most stable ice-surface configuration or minimum energy state, in which the basal plane of the ice crystal is parallel with the graphene surface. As ice propagation is the slowest perpendicular to the basal plane, ice can grow faster parallel to graphene surface. The findings from this study provides insights to ice propagation control mechanisms for versatile freeze casting and its broader applications.

Effects of freeze-thaw cycles on the erodibility and microstructure of soda-saline loessal soil in Northeastern China

Soda-saline loessal soils are distributed extensively in the Songnen Plain of Northeast China. Freeze-thaw effects on soil structure and mechanical properties are becoming more frequent and intense with global warming, which further affects soil erosion in cold regions. Quantifying the effects of freeze-thaw cycles (FTCs) on the erodibility of saline soils is challenging because of the abundant soluble salt content and complex soil mechanical responses. In this study, direct shear tests, disintegration tests, and scanning electron microscopy were conducted to investigate the effects of freeze-thaw action on the erodibility indicators (shear strength and disintegration velocity) of soda-saline loessal soils and their microscopic deterioration mechanisms. Four initial moisture contents (IMCs: 8%, 13%, 18%, 23%), three soluble salt contents (SSCs: 0.5%, 1.0%, 1.5%), and six FTCs (0, 1, 3, 6, 10, 15) were considered. The results showed that successive FTCs continued to degrade the erosion resistance of the soil until it stabilized after approximately 10 FTCs. After 15 FTCs, the shear strength and cohesion decreased by 8.6–19.8% and 24.1–59.2%, respectively, while the angle of internal friction was relatively unaffected. Meanwhile, the soil disintegration velocity increased by 52.3–208.8%. During the freeze-thaw process, the water-ice phase change and the crystallization of soluble salts both led to the continuous adjustment of the microstructure of soda-saline loessal soils. It was found that microcracks within the soil structural units increased significantly, cementation between the structural units weakened continuously, pore connectivity was enhanced, and pore size homogenization increased. Correlation analysis showed that IMC was the fundamental factor affecting the soil mechanical properties, whereas SSC mainly affected the soil's resistance to disintegration. The increase in IMC and SSC exacerbated the deterioration of the soil by freeze-thaw action. This paper presents a new understanding of the erosion characteristics and micromechanisms of soda-saline loessal soils in seasonally frozen regions under freeze-thaw action.

A first assessment of satellite and reanalysis estimates of surface and root-zone soil moisture over the permafrost region of Qinghai-Tibet Plateau

Long-term and high-quality surface soil moisture (SSM) and root-zone soil moisture (RZSM) data is crucial for understanding the land-atmosphere interactions of the Qinghai-Tibet Plateau (QTP). More than 40% of QTP is covered by permafrost, yet few studies have evaluated the accuracy of SSM and RZSM products derived from microwave satellite, land surface models (LSMs) and reanalysis over that region. This study tries to address this gap by evaluating a range of satellite and reanalysis estimates of SSM and RZSM in the thawed soil overlaying permafrost in the QTP, using in-situ measurements from sixteen stations. Here, seven SSM products were evaluated: Soil Moisture Active Passive L3 (SMAP-L3) and L4 (SMAP-L4), Soil Moisture and Ocean Salinity in version IC (SMOS-IC), Land Parameter Retrieval Model (LPRM) Advanced Microwave Scanning Radiometer 2 (AMSR2), European Space Agency Climate Change Initiative (ESA CCI), Advanced Scatterometer (ASCAT), and the fifth generation of the land component of the European Centre for Medium-Range Weather Forecasts atmospheric reanalysis (ERA5-Land). We also evaluated three RZSM products from SMAP-L4, ERA5-Land, and the Noah land surface model driven by Global Land Data Assimilation System (GLDAS-Noah). The assessment was conducted using five statistical metrics, i.e. Pearson correlation coefficient (R), bias, slope, Root Mean Square Error (RMSE), and unbiased RMSE (ubRMSE) between SSM or RZSM products and in-situ measurements. Our results showed that the ESA CCI, SMAP-L4 and SMOS-IC SSM products outperformed the other SSM products, indicated by higher correlation coefficients (R) (with a median R value of 0.63, 0.44 and 0.57, respectively) and lower ubRMSE (with a median ubRMSE value of 0.05, 0.04 and 0.07 m3/m3, respectively). Yet, SSM overestimation was found for all SSM products. This could be partly attributed to ancillary data used in the retrieval (e.g. overestimation of land surface temperature for SMAP-L3) and to the fact that the products (e.g. LPRM) more easily overestimate the in-situ SSM when the soil is very dry. As expected, SMAP-L3 SSM performed better in areas with sparse vegetation than with dense vegetation covers. For RZSM products, SMAP-L4 and GLDAS-Noah (R = 0.66 and 0.44, ubRMSE = 0.03 and 0.02 m3/m3, respectively) performed better than ERA5-Land (R = 0.46; ubRMSE = 0.03 m3/m3). It is also found that all RZSM products were unable to capture the variations of in-situ RZSM during the freezing/thawing period over the permafrost regions of QTP, due to large deviation for the ice-water phase change simulation and the lack of consideration for unfrozen-water migration during freezing processes in the LSMs.

Modeling of coupled transfer of water, heat and solute in saline loess considering sodium sulfate crystallization

The phase change of sodium sulfate significantly affects the salt weathering of loess slopes in seasonal frozen regions, which should be involved in the modeling of heat, water and solute transfer. The governing equation for heat transfer was established based on the Harlan model by incorporating ice-water phase change and crystallization of sodium sulfate and an empirical relationship for thermal conductivity of saline loess. Regarding the moisture field, a modified factor that characterizes the impedance of salt crystals and ice on water migration in frozen area was introduced into the Gardner model. The total suction as a function of water and salt contents was included in the unfrozen water migration equation. Considering the convection, diffusion and phase change of solutes, the equation for salinity field was established by Darcy's and Fick's laws, with a solubility function based on the solubility curve of sodium sulfate. The above models were numerically implemented on COMSOL Multiphysics through the user-defined partial differential equation module. In order to verify the rationality of the method, the profiles of temperature, water and salt contents of saline loess columns obtained by uniaxial freezing tests in closed system were compared with the numerical results. Comparisons show that the simulated profiles of temperature, water and salt contents at three cold-end temperatures (−5, −10 and − 20 °C) agree well with the measured data. According to the profile of the amount of crystalline salt, the concept of uncrystallized zone was proposed, which is found to be positively related to the cold-end temperature.

A novel method to study the energy conversion and utilization in artificial ground freezing

The artificial ground freezing (AGF) method is widely used for soil reinforcement in underground engineering. The AGF method has been confirmed very effective, however, it consumes a great deal of energy during construction. Therefore, it is of great significance to study its energy conversion and utilization, which is very important for better and more energy-efficient employment of this method. In this paper, a novel method, which could effectively calculate the energy utilization rate (EUR), is proposed to evaluate the energy conversion efficiency of a typical AGF project. First of all, the brine temperature and ground temperature are monitored and analyzed. Secondly, according to the monitored data and actual layout of freezing pipes, a sophisticated three-dimensional solid model is established, which considers the thermal-mechanical coupling and ice-water phase change. Based on the numerical simulation, the variation of the temperature field as well as the expansion process of the frozen soil curtain are analyzed, and the effectiveness of the numerical model is further validated by comparison between the measured and simulated results. Thirdly, variations of three key parameters including the output energy, elastic strain energy and heat energy during the active freezing period are calculated and analyzed. Finally, the EUR curve during the AGF process is acquired based on the calculated indicators. The trend of EUR can be divided into four stages, while the EUR is zero in the initial stage, reaches a peak of 0.79 in the rise stage, and then decreases slowly in the decline stage and stable stage. According to the EUR curve, constructive measures to improve energy conversion and utilization are proposed. It is concluded that this novel method is very effective for the assessment of energy utilization in AGF projects.

A fully coupled thermo-hydro-mechanical model with ice-water phase change for liquid nitrogen injection simulation

Cryogenic liquid nitrogen (LN2) fracturing or treatment in nearly impermeable reservoirs has attracted widespread attention due to its strong thermal shock and frost heave effects. We derive a fully coupled thermo-hydro-mechanical (THM) model considering the ice-water phase change to simulate the process of LN2 injection into shale gas reservoirs. The novel model accommodates the following dominant phenomena: (1) LN2 driving unfrozen water flow in porous media; (2) pore water freezing into ice below the freezing temperature; (3) fully coupled thermo-poroelastic effect incorporating frost heave pressure; (4) effective fluid permeability associated with unfrozen water content and phase saturation; (5) temperature and pressure-dependent fluid thermodynamic properties. The proposed model is verified against a classical thermo-poroelastic model and a coupled THM model for freezing rocks. Simulation results show that LN2 injection into water-bearing shale reservoirs will result in ultra-high injection pressure. A high injection rate is beneficial for the fracture initiation, but it is detrimental from phase solidification. Reducing the injection rate is not conducive to the subsequent penetration of LN2 into the reservoir. The evolution of effective permeability, as well as phase saturation, can be reasonably captured during LN2 injection. The comparative advantages between fluid infiltration and temperature propagation are different at high and low injection rates, which leads to significant differences in phase saturation and effective permeability changes. The multiple mechanisms, including pore pressure, frost heave pressure, and thermal stress that induce rock damage, can be efficiently revealed. At relatively low temperatures, the deformation of the reservoir at high and low injection rates is dominated by high-pressure expansion and cryogenic contraction, respectively. The research results can provide some insights for cryogenic non-aqueous fracturing.

Dual‐permeability modeling of preferential flow and snowmelt partitioning in frozen soils

AbstractThe infiltrability of frozen soils modulates the partitioning of snowmelt between infiltration and runoff in cold regions. Preferential flow in macropores may enhance infiltration, but flow dynamics in frozen soil are complicated by soil heat transfer processes. We developed a dual‐permeability model that considers the interacting effects of freeze–thaw and preferential flow on infiltration and runoff generation in structured soils. This formulation was incorporated into the fully integrated groundwater–surface water model HydroGeoSphere, to represent water–ice phase change in macropores such that porewater freezing is governed by macropore–matrix heat exchange. Model performance was evaluated against laboratory experiments and synthetic test cases designed to examine the effects of preferential flow on snowmelt partitioning between infiltration, runoff, and drainage. Simulations were able to reproduce experimental observations of rapid infiltration and drainage behavior due to macropores very well, and approximated soil thaw to an acceptable degree. Simulation of measured data highlighted the importance of macropore hydraulic conductivity, as well as macropore–matrix heat and water transfer, on controlling preferential flow dynamics. Test cases replicated a range of snowmelt partitioning behavior commonly observed in frozen soils, including subsurface conditions that produce rapid infiltration and deeper drainage, the contrast between limited vs. unlimited infiltration responses to snowmelt, and the temporal evolution of runoff generation. This study demonstrates the important influence that water freezing along preferential flowpaths can have on infiltrability and runoff characteristics in frozen soils and provides a physically based description of this mechanism that links infiltration behavior to hydraulic and thermal properties of structured soils.

The coupled moisture-heat process of a water-conveyance tunnel constructed by artificial ground freezing method

Artificial freezing ground method is widely applied in adverse geotechnical engineering conditions to improve the strength and reduce permeability of soil. It is a challenge to ascertain the progression of frozen wall and analyze the arrangement of freezing pipes due to the complexity of freezing process involving ice-water phase change and water migration. To analyze the progression of frozen wall and its influencing factors, a hydro-thermo coupling model is established, and a water-conveyance tunnel constructed by artificial freezing method is carried out. The results show that: (1) Water migration and ice-water phase transition have significant influences on temperature distribution, and their effects should not be neglected. (2) The thickness of frozen wall with temperature lower than −10 °C has attained the required design value at the end of active freezing. Meantime, under drive of temperature gradient, water migrates and redistributes. (3) The interval angle for two adjacent pipes is the main influencing factor on progression of frozen wall, followed by the freezing pipe diameter and horizontal spacing. Additionally, for coarse-grained soil, the influence of seepage on development of frozen wall is significant. This study will provide a theoretical reference for frozen wall design.

Using Latent Energy of Water-ice Phase Change to Reduce Energy Losses in Buildings in Cold Climate

Energy efficiency in buildings is a very important challenge that has to be faced in order to achieve the aims set by the new EU directive on Building energy efficiency encouraging nearly zero energy buildings. Unfortunately in countries with cold climate it is very hard to achieve this goal. The thickness of insulation needed to reach low energy consumption in cold climate is very big and in many cases it is not economically feasible. There is a need for new solutions for increasing building energy efficiency. In this paper a new solution for increasing building energy efficiency is proposed. It is proposed to use the latent energy of water-ice phase change to reduce heat conduction losses through building envelope. The latent energy is recovered by using low potential heat source. In this paper the validity of the proposed new solution is tested on a one dimensional scale – homogeneous infinite wall. The presented methodology is chosen to calculate systems operational efficiency throughout the whole year.

Study on Mechanical Properties and Energy Dissipation of Frozen Sandstone under Shock Loading

In order to understand the mechanical properties and energy dissipation law of frozen sandstone under impact loading, the cretaceous water-rich red sandstone was selected as the research object to conduct impact tests at different freezing temperatures (0°C, −10°C, −20°C, and −30°C). The test results suggested the following: (1) the peak stress and peak strain of frozen sandstone are positively correlated with strain rate and freezing temperature, and the strain rate strengthening effect and the low-temperature hardening effect are obvious. (2) The strain rate sensitivity of dynamic stress increase factor (DIF) is negatively correlated with temperature. Water-ice phase change and the difference in the cold shrinkage rate of rock matrix under strong impact loading will degrade the performance of rock together, so DIF is less than 1. (3) In the negative temperature range from −10°C to −30°C, DEIF is always greater than 1. The energy dissipation rate of red sandstone specimens fluctuated between 10% and 25% under the impact loading, and the data are discrete, showing obvious strain rate independence. The failure form changes from tensile failure to shear and particle crushing failure. (4) Combined with the micromechanism analysis, the difference in dynamic mechanical properties of red sandstone at different temperatures is caused by the water-ice phase change and the different cold shrinkage rates of the frozen rock medium. When the temperature drops from 0°C to −2°C, water migrates to the free space of the pore of frozen rock and freezes into ice crystal, resulting in frozen shrinkage. At −30°C, the expansion of ice dominates and the migration of water will stop, leading to frost heave.

Characterizing the pore size distribution of a chloride silt soil during freeze–thaw processes via nuclear magnetic resonance relaxometry

AbstractSoil pore size distribution (PSD) is typically used to predict the soil freezing characteristic curve and estimate the hydrological and mechanical properties during freeze–thaw cycles. However, direct measurements of frozen soil PSD remain a great challenge. This study proposed a method to determine the PSD of frozen soils based on nuclear magnetic resonance (NMR) relaxometry. Tests were performed on a saturated chloride silt soil at different salt contents (0.3, 1.0, 2.0, and 3.0%) and temperatures (between –30 and 0 °C) during a freeze–thaw cycle. The NMR‐detected PSD (only accounting for pores occupied by unfrozen water) varied with soil temperature, salt content, and freeze–thaw cycle. The sequence of the water–ice phase change and hysteresis were also identified in a freeze–thaw cycle. A regression analysis was performed on the cumulative NMR‐detected PSD via a variant van Genuchten model. The critical freezing pore radius and the thickness of unfrozen water film were computed and used to transform NMR‐detected PSD into the actual PSD of frozen soils on the basis of a pore radius transformation equation established in this paper. Notably, the actual PSD accounted for pores occupied by ice and unfrozen water. The actual PSD indicated that the water–ice phase change was more pronounced in macro‐ and mesopores, especially at lower temperatures and salt content. A comparison between the calculated average pore size and those presented in other studies showed that the proposed technique is a valuable alternative for the prediction of actual frozen soil PSD.

Study on the mechanism of crystallization deformation of sulfate saline soil during the unidirectional freezing process

AbstractThe freezing of sulfate saline soil involves the coupling effect of heat and mass transfer and crystallization deformation. The influence of salt content and temperature on the crystallization of ice and salt was analyzed by using a one‐side freezing experiment. Based on the effect of solute and temperature on water activity, a crystallization kinetics model of ice–water phase change in saline soil was established. The application of supersaturation in the crystallization kinetics model of solution–crystal phase change is improved with the Frezchem model. The calculated results are compared with experimental data to validate the effectiveness of the proposed model. the study shows that: (a) crystallization can be well illustrated by the proposed model based on the concepts of the formation factor of ice crystals and solution supersaturation; (b) convection and diffusion are the main means of salt redistribution, which cause the accumulation of salt above the freezing front and the formation of discontinuously distributed–layered salt crystal zones; and (c) the alternate crystallization of ice and salt increases variation i solute concentration and then increases crystallization pressure, which leads to the deformation of salt expansion.