Sulfate Saline Soil Research Articles

Effects of increased rainfall on heat and mass transfer and deformation of sulfate saline soil: An experimental investigation

To study the effect of increased rainfall on the heat and mass transfer and deformation characteristics of sulfate saline soil, a geometric similarity ratio model (1:6) of the natural site was created inside the self-developed indoor baseplate-atmospheric dual-temperature control model box. For the first time, combined with the characteristics of the surface energy change, the characteristics of water-heat-salt-mechanical coupling changes within sulfate saline soil under normal rainfall and twice the increase in rainfall were studied. The results show that the increased rainfall leads to a more significant decrease in upward shortwave radiation and downward longwave radiation, as well as a more significant increase in the surface net radiation and surface evaporation rate. Additionally, the increase in rainfall leads to an obvious cooling trend in the surface temperature. Compared with normal rainfall, an increase in rainfall leads to a significant increase in soil water content and conductivity, while soil heat flux and temperature significantly decrease. The increased rainfall caused a temperature drop of 1.6 °C at 5 cm of saline soil. Moreover, the increased rainfall leads to an increase in the heat release time of sulfate saline soil. Meanwhile, the impact of increased rainfall on the soil water content, conductivity, and temperature gradually weakens with increasing depth. The increased rainfall can exacerbate thawing settlement deformation and alleviate salt frost heave deformation. Compared with normal rainfall, twice the increase in rainfall results in a 0.9 mm increase in thawing settlement deformation and a 2.5 mm decrease in salt frost heave deformation.

Study of Heat–Mass Transfer and Salt–Frost Expansion Mechanism of Sulfate Saline Soil during the Unidirectional Freezing Process

Study of Heat–Mass Transfer and Salt–Frost Expansion Mechanism of Sulfate Saline Soil during the Unidirectional Freezing Process

Research on Solidification Methods and Stabilization Mechanisms of Sulfate Saline Soils

In cold regions, saline soils can cause dissolution, settlement, and salt expansion of the roadbed under the influence of freeze–thaw cycles, so they need to be stabilized during road construction. In this study, lime, fly ash (FA), and polyacrylamide (PAM) were used to stabilize sulfate saline soils, and the stabilized saline soils were subjected to the unconfined compressive strength test (UCS), splitting test, and freeze–thaw cycle tests (FTs). The stabilization mechanism of the three materials on saline soils was also studied via scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), and X-ray photoelectron spectroscopy (XPS). The test results showed that the addition of lime, FA, and PAM to saline soils can improve the mechanical properties and frost resistance of saline soils. After 28 d of curing, the UCS of FA-, PAM-, and lime-stabilized saline soils increased by at least 55%, 23%, and 1068%, respectively, and the splitting strength increased by at least 161%, 75%, and 2720%, respectively. After five freeze–thaw cycles, the residual strength ratios (BDRs) of the UCS of L2 (lime 8%), F2 (FA 11%), and P2 (PAM 1%) stabilized soils and saline soils were 71.78%, 56.42%, 39.05%, and 17.95%, respectively, and the decreasing trend tended to be stable. The saline soils stabilized by lime and FA were chemically stabilized, and their mechanical properties and frost resistance were better than the physical stabilization of PAM.

Study on the hydro-thermal-salt-mechanical coupling characteristics of sulfate saline soil under freeze-thaw cycles

The theoretical and experimental studies have been carried out on the water and salt migration and deformation characteristics of sulfate saline soil during freeze-thaw cycles. Based on the theory of unsaturated soil mechanics and the thermoelastic continuum and considering the influence of phase change within the pore on thermodynamic and hydrodynamic parameters, the multi-physical fields coupled model of hydro-thermal-salt-mechanical in unsaturated sulfate saline soil has been established. The variation processes of the temperature field, water field, salt field, and stress field of the soil during freeze-thaw cycles were analyzed, and the validity of the theoretical model was verified by indoor experiments. The results show that there are significant attenuation and hysteresis effects when heat is transferred in the soil during freeze-thaw cycles. The water content of migration in the soil increases with the height of the soil column, while the increment of migration water content decreases with the number of freeze-thaw cycles. The formation and dissolution of salt crystals from top to bottom and the sudden increase in the salt crystallization rate are mainly caused by variations in the solubility of the salt solutions due to temperature changes. The formation and dissolution of ice and salt crystals in the soil induce expansion and contraction, and the freeze-thaw cycle conditions have a significant effect on the expansion and residual deformation of the soil.

Water–salt migration and deformation characteristics in gravelly sulfate saline soil under the effect of localized fine sand accumulation

The water-salt migration law and deformation characteristics of coarse-grained saline soils have been extensively studied and illustrated. However, owing to the influence of the chemical composition and physical properties of the soils, coarse-grained soils are prone to localized soil absorption during mixing and compaction. This type of working condition of the existing localized fine sand accumulation layers is seldom discussed in the literature. In this study, water-salt migration and deformation of natural gradation specimens and specimens with localized fine sand accumulation layers in natural gradation were monitored and detected for the field fill conditions in an airport embankment project using self-designed test equipment based on nine freeze–thaw cycle physical simulation tests at environmental temperatures ranging from −30 °C to 25 °C. Under the freeze–thaw cycle, compared with the natural gradation, the specimens with localized fine sand accumulation layers had a higher influence on water and salt migration, which indicates that the depth range of drastic changes in water and salt increased by 80% and 84%, respectively. The cumulative deformation curves under the effects of natural gradation and localized fine sand accumulation exhibited similar trends. The difference between the deformation of the natural samples and samples with localized fine sand accumulation layers was 16% when the salt content of the upper part of the roadbed was 0.3%. In addition, the cumulative vertical settlement deformation of the specimens decreased with an increase in the salt content of the upper part of the roadbed and gradually transformed into vertical uplift deformation. The results of this study provide a basis for the selection of materials for airport roadbed backfill and their application in construction in seasonally frozen areas.

Effect of Polyethylene Microplastics on the Microbial Community of Saline Soils

Mulching to conserve moisture has become an important agronomic practice in saline soil cultivation, and the effects of the dual stress of salinity and microplastics on soil microbes are receiving increasing attention. In order to investigate the effect of polyethylene microplastics on the microbial community of salinized soils, this study investigated the effects of different types (chloride and sulphate) and concentrations (weak, medium, and strong) of polyethylene (PE) microplastics (1% and 4% of the dry weight mass of the soil sample) on the soil microbial community by simulating microplastic contamination in salinized soil environments indoors. The results showed that:PE microplastics reduced the diversity and abundance of microbial communities in salinized soils and were more strongly affected by sulphate saline soil treatments. The relative abundance of each group of bacteria was more strongly changed in the sulphate saline soil treatment than in the chloride saline soil treatment. At the phylum level, the relative abundance of Proteobacteria was positively correlated with the abundance of fugitive PE microplastics, whereas the relative abundances of Bacteroidota, Actinobacteriota, and Acidobacteria were negatively correlated with the abundance of fugitive PE microplastics. At the family level, the relative abundances of Flavobacteriaceae, Alcanivoracaceae, Halomonadaceae, and Sphingomonasceae increased with increasing abundance of PE microplastics. The KEGG metabolic pathway prediction showed that the relative abundance of microbial metabolism and genetic information functions were reduced by the presence of PE microplastics, and the inhibition of metabolic functions was stronger in sulphate saline soils than in chloride saline soils, whereas the inhibition of genetic information functions was weaker than that in chloride saline soils. The secondary metabolic pathways of amino acid metabolism, carbohydrate metabolism, and energy metabolism were inhibited. It was hypothesized that the reduction in metabolic functions may have been caused by the reduced relative abundance of the above-mentioned secondary metabolic pathways. This study may provide a theoretical basis for the study of the effects of microplastics and salinization on the soil environment under the dual pollution conditions.

Mechanical properties of sulfate saline soil stabilized by coal gangue-slag composite geopolymers

Geopolymers, which are composed of solid waste, are eco-friendly binders characterized by rapid hardening and high strength. Using geopolymers to stabilize saline soils presents a sustainable solution for soil improvement. This study investigated two geopolymers, HG and SG, composed of a coal gangue-slag composite, to improve saline soils. The stabilized soils were prepared by blending geopolymers with saline soils in varying proportions of 10, 20, 30, and 40% by weight. The strength of the stabilized soils was evaluated based on the geopolymer dosage. Moreover, we examined the strength of the stabilized soils with curing time and elucidated the underlying mechanisms using microscopic analyses including XRD, FTIR, and SEM. This study revealed that: 1) the SG geopolymer outperformed the HG geopolymer in soil strength enhancement; 2) cohesion of the stabilized soil increased as the dosage and curing time increased, while the internal friction angle showed no clear trend; 3) the geopolymer stabilized soil mainly contained quartz, C-A-S-H, N-A-S-H, and C-N-A-S-H gels, and these gels were distributed on the soil particle surface and within the interstitial voids, improving the soil strength, and 4) in saline soil engineering, HG or SG geopolymer dosages should be above 30% and 20%, respectively.

Characterization and mechanism study of sulfate saline soil solidification in seasonal frozen regions using ternary solid waste-cement synergy

Infrastructures built on sulfate saline soil foundations in seasonal frozen regions are highly susceptible to salt-frost heave damage and salt corrosion. To address this issue, a method has been proposed utilizing a ternary blend of industrial solid waste materials - fly ash (FA), silica fume (SF), and brick powder (BP) - in conjunction with Portland cement (PC) for the solidification of sulfate saline soil. The feasibility of this solidification technique has been validated through a series of tests, including unconfined compressive strength tests, freeze-thaw cycle tests, salt leaching tests, and microstructural analysis. The results showed that: The unconfined compressive strength of the solidified saline soil at 56 days increased by up to 61.27 times compared to untreated saline soil. During the freeze-thaw cycles, the volume of salt-frost heave in the solidified saline soil was only 10.75% of that in untreated soil, with a reduction in salt-frost heave force by up to 90.94%. Furthermore, during the salt leaching process, the rate of salt migration in the solidified saline soil could be slowed by up to 4.15 times, while the total amount of salt leached was only 31.34% of that in untreated saline soil. Additionally, through the combined use of X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray Spectroscopy (EDS), it was discovered that the interactive synergistic effect of the solidifying agents in a SO42- rich environment facilitated the dissolution of Si-O and Al-O micro-lattices on the surface of the solidifying agent particles. This led to the extensive formation of C-(A)-S-H gels and AFt products, resulting in the transformation of the soil structure from dispersed to flocculated. The comprehensive test results indicate that the mechanical properties, frost resistance, and salt corrosion resistance of the solidified saline soil have significantly improved, with an optimal solidifying agent mixture ratio of 3% PC, 5% FA, 5% SF, and 6% BP. These findings can provide a reference for the solidification treatment of sulfate saline soil foundations in seasonal frozen regions.

Study on the coupling mechanism of water-heat-vapor-salt-mechanics in unsaturated freezing sulfate saline soil

Understanding the internal regularity of unsaturated freezing sulfate saline soil is of utmost importance concerning the durability and stability of cold region engineering in sulfate saline soil areas. One-sided freezing experiments were performed on unsaturated sulfate saline soils with varying salt and moisture contents and temperature gradients to investigate the interplay of water-vapor-salt migration, heat transfer, ice-water and water-vapor phase transformations, salt crystallization, and salt-frost heave. This study presented a novel water-heat-vapor-salt-mechanics coupling model that considers water-vapor-salt migration and its feedback on soil deformation. The results signify that the water accumulation at the top surface of unsaturated freezing sulfate saline soil is primarily due to water vapor migration. Water-vapor-salt migration, ice-water phase transition, and salt crystallization form more ice and salt crystals, filling the air-filled pores and causing significant salt-frost heave. Water vapor transfer and accumulation further exacerbate the soil deformation for unsaturated freezing sulfate saline soil. The calculating results agree with the test data and demonstrate the efficacy of the proposed five-field coupling model. These findings hold significant implications for developing practical solutions for mitigating the negative impacts of salt-frost heave induced by water-vapor-salt transfer in unsaturated freezing sulfate saline soil on construction projects.

Investigation of the liquid water content in composite saline soil containing chloride and sulfate ions

Different salt types have different effects on the liquid water content of saline soil, resulting in differences in the physico-mechanical properties and water/salt migration process of saline soil. In order to investigate the phase transition process and the change of the liquid water content in composite saline soil, saline soils with the same total salt content and different ratios of sodium chloride and sodium sulfate were taken as the objects. The results indicated that two phase transitions occur in the saline soil with single salt type, while three phase transitions can be found in the composite saline soil with two salt types. Mirabilite crystallization contributes to the 1st phase transition, mirabilite and ice precipitate together in the 2nd phase transition process, and mirabilite, ice, and hydrohalite precipitate simultaneously in the 3rd phase transition process. The liquid water is reduced in the phase transition process during cooling, and the pore characteristic has been changed significantly. The change of the liquid water content reflects the processes of salt crystallization and ice formation in saline soil, then the amounts of ice and hydrated salt were calculated at different temperatures, and the mechanism of inhibiting the deformation of sulfate saline soil was examined by adding sodium chloride. The results have reference value for those seeking understanding of the deformation in natural composite saline soil, and these findings can provide theoretical basis for the phase transition mechanism of saline soil in cold regions.

Experimental and Numerical Modeling of the Freeze–thaw Response of U-Shaped Channel Lining on Sulfate Saline Soil Foundation

Under the influence of seasonal freeze-thaw cycles, the concrete lining of channels situated on saline soil foundations is highly vulnerable to an array of deleterious phenomena, such as frost heave, salt expansion, and corrosion. To investigate the synergistic interplay among saline soil substrates, U-shaped channel lining structures, and ambient temperatures during freeze-thaw cycles, a novel Hydraulic-Thermal-Salt-Mechanical (HTSM) model is established in this study. The HTSM model elucidates the migration of moisture and salt solute, ice-water phase transitions, and sodium sulfate crystallization within unsaturated saline soils throughout the freeze-thaw cycles. It considers the variations in volume and pore solution volume before and after the phase transition of sodium sulfate. Temperature and salt frost heaving force measurements obtained from outdoor freeze-thaw cycles are compared with numerical simulation results to validate the efficacy of the HTSM model and investigate the distribution patterns of salt frost heaving force (frost swelling force) along the channel. The results showed that the frost swelling force and depth exhibit a great linear relationship and the maximum stress of the U-shaped channel appears at the bottom of the channel, highlighting the good arching force characteristics of the U-shaped structure which prove that bottom of the channel is also the most prone to fracture. Significantly, the initial water content significantly impacts the frost swelling force—it increases by 4% while the frost swelling force increases by 76%. Comparing the numerical simulation results of temperature variations, moisture changes, and salt-frost heaving forces with the outdoor experimental data, it is evident that the proposed HTSM model effectively simulates the salt-frost heaving patterns observed in U-shaped channel linings.

Study on Mechanical Properties of Sulfate Saline Soil Improved by CLI-Type Polymer Active Agent

Large amounts of soluble salts in a soil enhance the soil sensitivity to changes in its properties induced by changes in environmental conditions, such as easy dissolution in water and easy occurrences of salt heaving in low-temperature environments, which make the soil volume swell rapidly, leading to a series of engineering disasters. Moreover, the growth and development of surface vegetation will be inhibited due to excessive salinity, resulting in a gradual decline in the ecological functionality of the area. A polymer active agent (CLI) was selected for the ecological improvement of sulfuric acid saline soils. Triaxial compression tests and a test on the soluble salt content of the treated soil were carried out to investigate the effects of polymer active agent content and maintenance time on the mechanical properties and soluble salt content of sulfate saline soils. The results showed that the addition of CLI can improve the soil strength by increasing the cohesion of the specimen, and the improvement increases significantly with the content of CLI and the curing age. Meanwhile, the calcium ions in CLI can react with sulfate ions in sulfate-salted soils to produce calcium sulfate precipitation to alleviate soil salinization. The scanning electron microscopy (SEM) images indicated that an appropriate content of CLI (about 8%) can strengthen the soil structure through an excellent chelating ability, enhancing the strength of the soil.

Macro-micro characteristics of geopolymer-stabilized saline soil in seasonal frozen soil region

Chemical stabilization is an effective approach to address various engineering problems related to saline soil, including salt heaving, frost heaving, dissolution, and corrosion. Through unconfined compression strength (UCS) test, x-ray diffraction (XRD) test, scanning electron microscope (SEM) tests and mercury intrusion porosimetry (MIP), this research investigates the macroscopic strength characteristics, microscopic mineral composition, and pore structure variation of quicklime, fly ash and sodium silicate joint solidifying sulfate saline soil under various freeze-thaw cycles, and discusses its solidification mechanism. The results indicate that when the contents of fly ash and sodium silicate are held constant, the unconfined compressive strength of the solidified soil increases first and then decreases with the increase of quicklime content. Additionally, the peak strength occurred at a quicklime content of 3 %; The strength of all the solidified soils decreased with the increase of the freeze-thaw cycles and the solidified soils with the optimum quicklime content has a relatively strong ability to resist freezing and thawing; The influence of quicklime content on solidification is reflected in the formation of hydrate gels and the variation of pore structure; The influence of freeze-thaw cycle on solidification is reflected in the destruction of solidified pore structure.

Understanding unsaturated sulfate saline soil in cold regions: A comprehensive hydraulic–thermal–air–salt–mechanical model with experimental research

The destruction of infrastructure built on saline soil in cold regions is closely related to water and salt migration, and soil deformation in foundations. Consequently, it is imperative to understand the mechanisms of heat and mass transfer, along with deformation in saline soil. Based on the unsaturated soil mechanics and porous media theory, a hydraulic–thermal–air–salt–mechanical (HTASM) coupled mathematical model for frozen unsaturated sulfate saline soil is proposed. The model incorporates the impact of ice-water phase change, sodium sulfate crystallization, and vapor flow on heat and mass transfer during the freezing process. Two parameters—salt crystallization pressure, and ice pressure—are introduced into the mechanical control equation to capture the effects of phase change on deformation. In addition, the relationship between the density of soil particles, water, ice and gas, and temperature and pressure is considered to reflect the dynamic change of density during freezing. Finally, the numerical simulation using the HTASM model is conducted and its efficacy is validated by comparing the numerical results with those obtained from indoor unidirectional freezing tests. The coupling effect of the HTASM model is revealed by analyzing the dynamic changes of temperature, water content, salt content, vapor flux and crystallization pressure in the soil column during freezing. The results show that the proposed HTASM model can well simulate the heat and mass transfer behavior in freezing unsaturated sulfate saline soil.

An improved heat-water–vapor-salt based salt swelling model for unsaturated sulfate saline soil under cooling

The salt swelling destruction in arid sulfate saline soil areas represents a severe problem in road, railway, and foundation engineering. It is imperative to explore a potential solution to solve the salt swelling for the sulfate saline soil. However, vaporous water has a large effect on the forms of sodium sulfate present in the soil, which has not been taken into account by existing models. This work proposes an improved heat-water-vapour-salt based salt swelling model by considering the vaporous water effect. Experiments on unsaturated saline soils were conducted to validate the accuracy of the model. Besides, the effects of temperature, water content, relative humidity, and solute concentration on salt swelling were reasonably analyzed. The results show that the salt swelling rate increases to 64% when the relative humidity reaches 60–90%; once the relative humidity exceeds 90%, the swelling rate will decline to 0%. Moreover, the salt swelling rate follows an upward convex curve along the relative humidity and decreases with increasing liquid water content. When the liquid water content is above 10%, the salt swelling rate is stable at around 0.3. Overall, the improved model is more accurate in predicting salt swelling than the existing model, which shows exceptional promise in providing useful insight into understanding the mechanism of salt swelling and the prevention of salt swelling in saline soils.

Numerical study on the spatial–temporal distribution of solute and salt accumulation in saturated sulfate saline soil during freezing–thawing processes: Mechanism and feedback

Knowledge of freezing–thawing (F–T) cycles of sulfate saline soil is crucial regarding the spatial–temporal distribution of solutes in the soil, soil salinization, and cold region engineering in sulfate saline soil areas. F–T tests of saturated sulfate saline soils with different salt contents, water supply conditions, and the number of F–T cycles were conducted. The coupling mechanism of water and salt migration, ice–water phase change, salt crystallization–dissolution, and deformation caused by salt expansion and frost heave of saturated sulfate saline soil during F–T processes were analyzed. A water–heat–salt–mechanics coupling mathematical model for saturated sulfate saline soil under F–T cycles is proposed using the modified salt crystallization–dissolution kinetics model. The results show that the calculated profiles of temperature, water content, concentration, and soil displacement are in good agreement with the experimental data, demonstrating that the proposed coupling model can describe the coupled water–heat–salt–mechanics process of sulfate saline soil during F–T processes. Additionally, the modified salt crystallization–dissolution kinetics model employing the FREZCHEM model can theoretically reveal the processes of salt crystal growth and dissolution. Also, the water and temperature potential gradients are the main driving force for water migrating toward the freezing front. Convection and diffusion are the dominant factors for the spatial–temporal distribution of solutes of sulfate saline soil subjected to F–T cycles, causing salt accumulation and precipitation and forming subflorescence in the frozen and unfrozen layers near the freezing front and efflorescence in the soil surface–layer at a certain number of F–T cycles. Due to the blocking effect of salt accumulation and precipitation on water and salt migration, salt expansion during freezing and residual deformation during thawing gradually stabilize with the increase of F-T cycles.

Crystallization deformation and phase transitions of coarse-grained sulfate saline soils upon cooling

The volumetric expansion of saline soils under cooling could pose a threat to infrastructure constructed on these soils. This study examines the crystallization deformation characteristics of coarse-grained sulfate saline soils subject to stepwise cooling from 30 °C to −15 °C. Soil columns are prepared in plexiglass buckets with different fine contents, percent compaction, moisture contents, and sodium sulfate content. Cumulative heave and temperature evolution are measured when the specimens are maintained in a temperature-controlled environmental chamber (freezing in three dimensions). Upon cooling, salt and ice crystals grow in soil pores, leading to volumetric expansion. A model is proposed to describe the precipitation-induced deformation varying with temperature based on random forest (RF). The RF-based model reveals that the initial salt-to-water ratio exerts the most significant influence on the accumulated deformation, followed by moisture content, soil salinity, percent compaction, and soil gradation. The initial precipitation and freezing points (response variables) are expressed as two separate functions of soil properties (predictor variables) using the multivariate adaptive regression splines (MARS). The MARS-based models demonstrate that the precipitation points are mainly affected by the initial salt-to-water ratio and soil salinity; all factors contribute to the freezing point prediction, given the combined action of salt and ice crystallization. A threshold soil salinity of 33.33% existed in the evolution of precipitation points, while a threshold fine content of 35% and a threshold soil salinity of 39.68% were observed in freezing point predictions. The interaction mechanism of salt and ice crystallizations led to incredible difficulty capturing freezing points rather than precipitation points and predicting the sample heave. Although the evaluation metrics (RMSE, MAPE, and R2) confirm that the RF- and MARS-based models achieve the regression tasks well, more efforts are needed toward the second phase transition phenomenon and its implications for explaining thermally induced saline soil response. The findings may guide the utilization and management of land resources in saline areas.

Study on the changing pattern of the salt-freeze swelling force of sulfate saline soil containing sodium chloride under variable temperature environment

The salt-freeze swelling forces generated by sulfate saline soils when experiencing changes in ambient temperature can cause a large degree of damage to building foundations, endangering the safety of people's lives and property. We performed an experimental study on the change of salt-freeze swelling force under different conditions of salt content, cooling rate, and chlorine-sulfur ratio by using the self-developed salt-freeze swelling force test device in combination with the methods of indoor tests and mathematical statistics. The findings revealed that: The growth rate of the salt-freeze swelling force was positively correlated with the salt content. The salt-freeze swelling force of soils increased first and then decreased as the temperature decreased, and the maximum reduction reached 71.7%, which could be divided into three stages: rapid growth, slow growth, and fallback. The rate of temperature reduction had a significant impact on the deviation of the salt-freezing swelling force, and the two were positively correlated. The addition of sodium chloride could effectively inhibit the occurrence of salt-freeze swelling. Considering the economy and applicability, 0.3 is recommended to be considered as the chlorine-to-sulfur ratio in soils with CSR≥0.3; in soils with CSR<0.3, 0.3 and 0.1 are recommended to be considered for soils with high salt content (S>2%) and low salt content (S ≤ 2%), respectively. The final salt-freeze swelling force of sulfate-saline soils and sulfate-saline soils containing sodium chloride was predicted using a specific model, and its accuracy was confirmed by comparing it with previous studies.

Experimental study on the water retention properties of sulfate saline soils during the cooling process

The water retention properties of sulfate saline soils during cooling were investigated by theoretical derivation as well as by nuclear magnetic resonance (NMR) tests and matrix suction determination tests by the contact filter paper method. The equations for calculating the free water content of sulfate saline soils during the cooling process and the amount of salt crystallization were derived based on the law of conservation of mass. The free water content of sulfate saline soil during the cooling process was determined by the NMR test. The reliability of the derived calculation equation was verified, which laid the foundation for the subsequent processing of the test data for matrix suction determination. The study showed that when the soil pore solution is saturated with sodium sulfate, the free water content in the soil decreases as the temperature decreases, accompanied by the crystallization process, and that the matrix suction of the soil increases as the temperature decreases. For silty clay without salt, the effect of temperature on its matrix suction is small. Use of either the serial filter paper method or the parallel filter paper method to measure the matrix suction has no effect on the matrix suction soil-water characteristic curve. The parallel filter paper method can exponentially reduce the test time compared with the serial filter paper method. Salt crystallization has no effect on the matrix suction soil-water characteristic curves (SWCCs) of sulfate saline soils. The matrix suction SWCCs of silty clay soils without salt can be employed to calculate the matrix suction of sulfate saline soils during the cooling process.

Impact of sodium chloride on shear behavior of saline soil with high sulfate during cooling

Saline soils are widespread in the northwest and northeast inland regions of China, among which chloride saline soils and sulfate saline soils are prevalent in the northwest region, subgrade engineering activities have increased. To find sodium chloride (NaCl) impact on shear behavior of saline soil subgrade fillers in cold season, a fine-grained saline soil was sampled along highway from Qarhan to Golmud on the Qinghai-Tibet plateau and were added NaCl varying from 2% to 8% to conduct laboratory direct shear testing in a cool environment. The tests of volumetric change and direct shear were performed at set key temperatures (20 °C, −10 °C, and −20 °C) to obtain the NaCl impact on shear behavior of saline soil with high sulfate during cooling. The test results demonstrate the following properties: (1) there is a critical value on the proportion of NaCl content ranging from 4% to 6% for shear behaviors of saline soil; (2) NaCl can effectively inhibit volumetric expansion when added NaCl vary from 0% to 4%, but volumetric expansion can't be suppressed at low temperature when NaCl content was added more than 6%; (3) the shear stress-strain relationships of saline soils with NaCl content of 2%–8% show strain softening behavior at set key temperatures (20 °C, −10 °C, and −20 °C); (4) during the cooling process, the cohesion of saline soil with different NaCl content increases in varying degrees, and when the sodium chloride content is greater than 4%, the increase of cohesion is more obvious. These conclusions show that the improvement of soil shear strength parameters during cooling are conducive to highway engineering, but NaCl content also has an adverse direct impact on the saline soil engineering, which is related to temperature, initial salt content, water content and different proportions of sulfate and chloride.