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Article

Experimental Study on Water and Salt Migration and the Aggregate Insulating Effect in Coarse-Grained Saline Soil Subgrade under Freeze–Thaw Cycles

by
Haoyuan Yang
1,2,3,
Bingbing Lei
2,3,
Liangfu Xie
1,2,3,
Changtao Hu
2,3,* and
Jie Liu
1,2,3,4,*
1
College of Architectural Engineering, Xinjiang University, Urumqi 830047, China
2
Xinjiang Key Laboratory for Safety and Health of Transportation Infrastructure in Alpine and High-Altitude Mountainous Areas, Urumqi 830006, China
3
Xinjiang Transportation Planning Survey and Design Institute Co., Ltd., Urumqi 830006, China
4
China Gezhouba Group Municipal Engineering Co., Ltd., Yichang 443000, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2024, 14(19), 8970; https://doi.org/10.3390/app14198970 (registering DOI)
Submission received: 3 September 2024 / Revised: 29 September 2024 / Accepted: 3 October 2024 / Published: 5 October 2024
Browse Figures
Versions Notes

Abstract

:
Understanding multiphase transformations and the migration of heat, water, vapor, and salt in coarse-grained saline soil under groundwater recharge and environmental freeze—thaw cycles is crucial for ensuring the stability of highway infrastructures. To clarify the water, heat, vapor, and salt migration patterns in coarse-grained saline soil, as well as the salt-insulating effect of the aggregate insulating layer, an experimental study was conducted in a soil column model under pressureless water replenishment with fluorescein-labeled liquid water under freeze—thaw cycles. The results showed that the temperature in the saline soil columns periodically changed and that hysteresis effects occurred during temperature transfer. External water replenishment and the content of liquid water inside the soil exhibited nonlinear changes with environmental temperatures. After multiple freeze—thaw cycles, two water and salt accumulation zones formed within the coarse-grained saline soil subgrade. The migration of liquid water resulted in a water and salt accumulation zone in the nonfrozen zone, whereas the migration of water vapor yielded a water and salt accumulation zone in the frozen zone. To prevent water and salt migration, a 20 cm thick gravel insulating layer could be laid at a distance of 10 cm from the bottom of the roadbed, which could provide a satisfactory salt-insulating effect. The research results provide a theoretical basis and guidance for regulating the stability of subgrades in saline soil areas.

1. Introduction

Saline soil is widely distributed in arid and semiarid seasonally frozen soil areas all over the world, especially in Northwest China [1,2,3,4,5]. As a unique type of soil, frozen saline soil is a porous medium, as shown in Figure 1, which comprises soil particles, dissolved salt ions, ice crystals, salt crystals, and gas. Saline soil provides effective communication channels and storage space for the migration of matter. Unsaturated frozen sulfate saline soil in arid and semiarid seasonally frozen soil areas involves the interaction among water—vapor—salt migration, heat transfer, the phase transition of ice, liquid water and water vapor, salt crystallization, and other processes under the action of seasonal temperature variations, leading to a series of geological and environmental problems such as soil salinization, deterioration of biological properties, and desertification [6,7]. Research has indicated that there is a mutual feedback effect between the geological environment and engineering construction. Significant changes in the environmental temperature can cause water and salt migration in saline soil, leading to the accumulation of water and salt in the soil interior or surface, resulting in diseases such as frost heave, salt heave, subsidence, and corrosion [8,9,10,11,12,13,14,15,16], which poses a serious threat to the service performance of roadbeds in saline soil areas [17,18], and the aforementioned saline soil problem is relevant for all types of road surfaces. According to the Specifications for the Design of Highway Subgrades (JTG D30—2015) [19], coarse-grained soil is commonly used as roadbed filling material in the design and construction of highway subgrades in saline soil areas. Therefore, clarifying the water and salt transport characteristics of coarse-grained saline soil subgrades under freeze—thaw cycles is particularly important for preventing diseases, such as salt expansion, and the subsidence of coarse-grained saline soil subgrades in seasonally frozen soil areas, as well as improving the service performance of coarse-grained saline soil subgrades.
The main driving force for salt migration in soil is the water gradient, and water migration is affected by temperature. Under the alternating temperature changes in seasonally frozen soil areas, water freezes and migrates, driving salt migration and accumulation, resulting in a series of diseases in roadbeds. Numerous scholars have explored the heat and mass transfer characteristics of saline soil under freeze—thaw cycles and have achieved certain results. Zhang et al. [20] reported that, under freezing conditions, water and salt in saline soil migrate from the unfrozen layer to the frozen layer, causing soil salinization. Stahli and Stadler [21] reported that salt migrates with water driven by two mechanisms: a convective flow of salt toward the freezing front and diffusion in the opposite direction owing to concentration gradients. Melaku et al. [22] and Sun et al. [23,24] reported that the dynamic response of saline soil to seasonal temperature changes leads to the formation of temperature gradients within the soil, causing the migration of water and salt. Yuan et al. [25] indicated that the trend in unfrozen moisture content changes during temperature increases and decreases varies, implying that there is a hysteresis effect. Sarsembayeva and Collins [26] reported that the direction and redistribution of water are controlled by the matrix potential, while temperature and matrix potential gradients serve as the main driving forces for water migration. Lai et al. [10] and Zhou et al. [27] reported that the freezing front gradually diffuses underground in the freezing process, causing salt to migrate with water to the freezing zone, resulting in changes in the distributions of water and saltwater in both the frozen and unfrozen zones. Watanabe et al. [28], through F-T tests in a close system, revealed that the amount of water migration from the unfrozen zone to the freezing front depends on the initial solute concentration. Teng et al. [29] revealed that the water in the pores of unsaturated frozen soil includes liquid water, water vapor, and ice. The freezing process in unsaturated soil is a liquid water—heat—vapor coupling process, and, in low-saturation soils, vapor migration may be the main mechanism of water transfer. Bai et al. [30] reported that temperature gradients in unsaturated frozen soil leads to the transfer of liquid water and water vapor to the cold end. De Vries et al. [31] considered the combined effect of the temperature gradient, moisture gradient, and gravity potential on water heat vapor migration in frozen soil. Huang and Rudolph [32] experimentally revealed the interactions among water vapor migration, heat transfer, ice–water and water–vapor phase transitions, as well as the deformation in unsaturated frozen soil. He et al. [33] analyzed the water and heat transfer properties of unsaturated frozen soil on the basis of previous research. To block flow channels of upward water and salt migration in saline soil, a geotextile partition layer can be installed during roadbed construction, and geotextile materials are generally selected. However, Mao et al. [34] analyzed the salt content distribution in a roadbed with a geotextile partition layer in the saline soil area of Xinjiang under natural environmental conditions and reported that the impermeability of the geotextile partition layer led to salt accumulation underneath it, resulting in a covering effect. At present, there are also new geotechnical materials, such as wicking geotextiles. Wang et al. [35] validated the effectiveness of using wicking geotextiles in aggregates through indoor model experiments. Guo et al. [36] studied the water migration mechanism of wicking geotextiles in compacted aggregates through indoor soil column tests and analyzed the effective working distance. However, new materials have not yet been developed for practical engineering applications. In saline soil areas, such as Xinjiang and Qinghai in China, engineering practices have revealed that aggregate insulating layers impose a certain effect on separating saline soil.
In summary, scholars have investigated the migration of water and salt in saline soil under freeze—thaw cycles and have focused on this migration process by mixing Na2SO4 or NaCl into plain soil. There is relatively little research on natural coarse-grained sulfate saline soils. There are few analysis studies of the mechanisms of the liquid—vapor—ice phase transition of water and the phase transition of the crystallization solution under freeze—thaw cycles, and it is difficult to distinguish between liquid and water vapor migration. In addition, many freeze—thaw cycle tests have been conducted in closed systems, but, owing to the significant impact of groundwater on water and salt migration in soil, closed systems cannot fully capture the heat and mass transfer processes in saline soil under the action of groundwater. Moreover, the actual construction of saltwater migration-insulating layers has not been investigated in depth, and the use of materials such as geotextiles is mostly ineffective. The selection and design parameters of the location of the aggregate insulating layer are also unclear.
Therefore, in this study, a self-designed water–salt migration test device was adopted to conduct freeze—thaw cycle tests of unsaturated sulfate saline soil under pressureless water replenishment conditions. With the use of fluorescein to label liquid water, a CS655 temperature—humidity—conductivity soil three-parameter sensor, Markov bottle, and camera were employed to monitor the changes in temperature, moisture, conductivity, external water replenishment, and fluorescein-labeled liquid water levels in saline soil in real time, revealing the water—vapor—salt migration trend in coarse-grained saline soil subgrades under multiple freeze—thaw cycles. Finally, the effectiveness of establishing an aggregate insulating layer to prevent water and salt migration was evaluated. Notably, aggregate insulating layers with thicknesses of 10, 20, and 30 cm were laid at a distance of 10 cm from the bottom of the roadbed model to determine the optimal thickness, providing the most economical and feasible reference for laying aggregate insulating layers during actual road construction in saline soil areas.

2. Materials and Methods

2.1. Test Soil Sample

The natural saline soil used in this study was obtained from the vicinity of the K6 + 100 section of the National Highway G314 in Kashgar, Xinjiang, approximately 14 km from the center of the city of Kashgar. In accordance with the Test Methods of Soils for Highway Engineering (JTG E40-2007) [37], particle size distribution tests and soluble salt content analyses of this natural saline soil were conducted, and the test results are shown in Figure 2 and Table 1, respectively. In Table 1, c(Cl−) and c(SO42−) denote the molar concentrations of chloride and sulfate ions, respectively, in 100 g of soil (mmol/100 g). On the basis of the c(Cl−)/2c(SO42−) ratio and reference [38], the saline soil type was identified as weakly sulfate saline soil.

2.2. Experimental Equipment and Experimental Scheme

2.2.1. Test System

The experimental device used in this experiment consists of custom-made freeze—thaw cycle equipment, which mainly encompasses a temperature control system, a data acquisition system, and a water replenishment system. The temperature control system comprises two high- and low-temperature constant-temperature baths (with the accuracy of bath 1 reaching ±0.01 °C and that of bath 2 reaching ±0.1 °C) and upper and lower guide plates. The No. 1 constant-temperature bath is connected to the upper guide plate through a circulating liquid conduit, and the No. 2 constant-temperature bath is connected to the lower guide plate. The upper and lower guide plates are placed above the soil sample and below the water replenishment tank, respectively. Different temperatures are attained to simulate the temperature difference between the upper and lower soil parts in the actual roadbed construction process. The water replenishment system comprises a Markov bottle, a water guide pipe, and a water replenishment tank. The external water replenishment amount is determined by the water level change in the Markov bottle. The data acquisition system relies on a CS655 soil three-parameter sensor, with a temperature accuracy of ±0.5 °C, a volumetric moisture content accuracy of ±3% VWC, and a conductivity accuracy of 0.05 dS/m. The data collector is a CR1000X device and is set to automatically collect data at 30 min intervals. A diagram of the overall device structure of the equipment is shown in Figure 3.

2.2.2. Experimental Scheme

After drying the collected natural coarse-grained saline soil, a coarse-grained saline soil sample with a moisture content of 6% was prepared onsite on the basis of the initial moisture content, which was then sealed for 24 h. The sample was loaded into an organic glass cylinder with a height of 85 cm and a diameter of 50 cm in 16 batches, and the compaction degree was controlled to 0.95 by unidirectional compaction. The thickness of the organic glass cylinder was customized to 5 cm to achieve visualization of the internal soil while minimizing heat exchange with the outside environment. The test soil sample was divided into 9 sections, with a total of 8 three-parameter sensors arranged every 9 cm to monitor the temperature, humidity, and conductivity of the saline soil in real time. After sample preparation, a layer of plastic film was placed on the top surface for sealing to prevent moisture evaporation during the subsequent freeze—thaw cycles. The temperature control strategy during the freeze—thaw cycles was based on the annual temperature data of the city of Kashgar. The upper guide plate exhibited a freeze—thaw cycle duration of 96 h, with 48 h of freezing and 48 h of thawing. The specific cycle temperatures were set to 15, 10, 5, 0, −5, −10, and −15 °C.

2.2.3. Tracer-Labeled Liquid Water Migration and Visualization Verification

In the traditional one-dimensional moisture migration test, only moisture migration is monitored through sensor readings. Owing to the height difference in the arrangement of the sensor devices, it is difficult to determine the height of moisture migration on the basis of sensor readings when moisture migrates to the middle position between two sensors. Therefore, this method lacks visibility. Fluorescein (chemical formula: C20H12O5) is commonly used as a tracer for sewage discharge detection. It is soluble in water and appears yellow—green under purple light irradiation [39], allowing for real-time tracking of the migration and redistribution spatiotemporal characteristics of liquid water by fluorescein doping. Therefore, the use of fluorescein as a tracer to label liquid water is the key to visualizing the migration of liquid water, which can more intuitively reflect the characteristics of water vapor migration in coarse-grained saline soil subgrades.
On the basis of the research results of Wang et al. [40] and Zhang et al. [41], a fluorescence solution evaporation condensation test was conducted, as shown in Figure 4a. The soil sample was mixed with fluorescein in an evaporating dish and covered with a glass plate for heating. Research has revealed that small water droplets that condense on glass cover plates do not exhibit color under purple light irradiation, indicating that fluorescein only migrates with liquid water and not with water vapor. Therefore, under purple light irradiation, the addition of fluorescein to replenished external water can be used to track the change process of the migration height of liquid water. The actual effect is shown in Figure 4b.

2.2.4. Data Monitoring

During the experiment, the data acquisition system was used to monitor the temperature at different heights inside and at the top of the saline soil column in real time, the conductivity at different heights inside the soil column, changes in external water replenishment, and height changes during the migration of fluorescein-labeled liquid water. Because the volumetric moisture content and conductivity are measured, it is necessary to calibrate and convert the data collected by the CS655 sensor to obtain the moisture and salt contents in the soil at different heights.

2.3. Calibration of Sensors

2.3.1. Moisture Calibration

Five sets of natural saline soil samples with mass moisture contents of 3%, 6%, 9%, 12%, and 15% were prepared by washing and drying. The samples were sealed and stored for 24 h to ensure an even distribution of moisture. The prepared samples were placed in a beaker. The CS655-1—CS655-5 sensor probes were inserted into the saline soil in sequence to obtain 3 readings, and the average value was determined. The mass moisture content measured by the drying method was used as the vertical axis, and the volumetric moisture content measured by the sensor was used as the horizontal axis to fit the test results, as shown in Figure 5. There was a satisfactory linear relationship between the mass and volumetric moisture contents, with a correlation coefficient greater than 0.98.

2.3.2. Salt Calibration

Five sets of samples with a moisture content of 6% and salt contents of 0.0%, 0.5%, 1.0%, 1.5%, and 2.0% were prepared by washing the natural coarse-grained saline soil. The samples were sealed and stored for 24 h to ensure a uniform distribution of salt and moisture and then dried in an oven. A total of 20 g of dried sample from each group was weighed, 100 g of distilled water was added, and a saline soil solution was prepared at a soil–water ratio of 1:5. After thoroughly stirring and shaking with a glass rod to completely dissolve the salt in the saline soil, the CS655-1—CS655-5 sensor probes were sequentially inserted into the saline soil solution for performing three conductivity tests, and the average value was calculated. The test results were plotted, with the horizontal axis representing the salt content and the vertical axis representing the conductivity, as shown in Figure 6. There was a favorable linear relationship between the conductivity and salt content, with a correlation coefficient greater than 0.99.

3. Results and Analysis

3.1. Analysis of the Heat Transfer Results for Saline Soil under Freeze—Thaw Cycles

The temperature curves for the different heights within the saline soil column under different numbers of freeze—thaw cycles are shown in Figure 7. The temperature inside the sample periodically changes, and the greater the distance from the cold end position the temperature is, the smaller the amplitude of the temperature change. The temperature change near the cold end in saline soil occurs 8 h earlier than that far away, indicating a significant hysteresis effect. Research has shown that the thermal conductivity and equivalent specific heat capacity of saline soil vary with the composition and content of substances in the pores, resulting in a significant hysteresis effect on the internal temperature transfer of saline soil samples.
Figure 8 shows the distribution of isotherms in the saline soil under different numbers of freeze-thaw cycles. The blue part in the figure denotes the freezing stage and the red part denotes the thawing stage. At the freezing stage, the internal heat of the saline soil is transferred from bottom to top, whereas, at the thawing stage, the internal heat of the saline soil is transferred from top to bottom. The black dashed line in Figure 8 indicates the freezing temperature isotherm. The freezing temperature of the saline soil was −2.14 °C. The freezing temperature isotherm divides the saline soil column into frozen and nonfrozen zones. The dark blue area in the figure denotes the frozen zone, and the red and light blue areas denote the nonfrozen zone. The position of the freezing temperature isotherm is the freezing depth of the saline soil column. The maximum freezing depth values of consecutive cycles are connected by a green curve. With the increasing number of freeze–thaw cycles, the freezing depth continuously decreases at the early stage and gradually stabilizes at the later stage, with a stable freezing depth of 40.5 cm.

3.2. Analysis of the Water and Salt Migration Results for Saline Soil under Freeze Thaw Cycles

3.2.1. Changes in the External Water Replenishment Amount

In the freeze—thaw cycle process, a pressureless water replenishment method was used for the saline soil column where the liquid water was labeled with a fluorescent tracer. The variation curve of external water replenishment with a freeze—thaw cycle duration is shown in Figure 9. The external water replenishment amount under freeze—thaw cycles exhibited an increasing trend over time, and the rate of increase gradually decreased. The maximum replenishment amount was 15.20 L. The external water replenishment curve also showed a certain periodic pattern with freeze—thaw temperature changes. With decreasing temperatures, the external water replenishment amount increased, and with increasing temperature, the external water replenishment amount decreased. The process of external water replenishment lagged behind the temperature change.
The analysis revealed that a decrease in the temperature led to the formation of phase change crystals in the pores of saline soil, which in turn drove the migration of water and saltwater, resulting in an increase in external water replenishment. However, after heating, the crystallization–dissolution thawing process reduced the vacuum suction. Owing to the periodic changes in external water replenishment under freeze—thaw cycles, the rising height of fluorescein-labeled liquid water also changed accordingly. The liquid level heights at different times are shown in Figure 10.
The figure shows that the overall height of the fluorescein liquid surface generally increased, but it may also decrease. The increase in the liquid water surface height occurred mainly at the heating stage, whereas the decrease in the liquid surface height occurred largely at the cooling stage. This is opposite to the change in the water replenishment amount with temperature, mainly because, although the amount of water replenishment increases during cooling, the increase in the freezing depth causes the liquid water surface to decrease, and the height of the fluorescein liquid surface decreases accordingly. Although the amount of water replenishment decreases in the heating process, the external water replenishment entering the soil in the previous cooling process increases with the increasing freezing front, causing the height of fluorescein-labeled liquid water to rise. However, the height of the liquid level gradually decreases after each decrease, which is related to the gradual decrease in the rate of increase in external water replenishment.

3.2.2. Analysis of Water—Salt Changes

After five freeze—thaw cycles, the final moisture and salt contents inside the soil column at different heights were compared with the initial values. The experimental results are shown in Figure 11, and the height of the fluorescein-labeled liquid water is shown in Figure 12. There is a significant accumulation of water (see Figure 11a) and salt (see Figure 11b) in the saline soil column at distances of approximately 28.2 and 65.8 cm from the bottom. For H = 28.2 cm, the final water content reached 20.1%, an increase of 14.1% over the initial water content of 6% (see Figure 11a), and the salt content reached 7.5%, an increase of 5.9% over the initial salt content (see Figure 11b). For H = 65.8 cm, the final moisture content reached 10.7%, an increase of 4.7% compared with the initial moisture content of 6% (see Figure 11a), and the salt content reached 3.2%, an increase of 1.6% compared with the initial salt content (see Figure 11b).
However, as shown in Figure 10 and Figure 12, the maximum migration height of fluorescein labeled liquid water was approximately 52 cm, which was greater than the height of water–salt aggregation zone 1 (H = 28.2 cm) but lower than the height of water–salt aggregation zone 2 (H = 65.8 cm). The reason for this finding is that the natural coarse-grained saline soil itself is unsaturated. Although the H = 28.2 cm area is a nonfrozen zone, the content of external replenishment water mixed with fluorescein rapidly increases under capillary action and soil matrix suction, and water mainly accumulates at 28.2 cm. During the upward migration of water, the bottom salt is also moved to a depth of 28.2 cm and more salt in the surrounding soil is dissolved, forming water–salt aggregation zone 1. After 480 h, the liquid water in the aggregation zone migrates to the cold end under the action of upper soil matrix suction and a temperature gradient, resulting in the migration height of fluorescein-labeled liquid water reaching 52 cm. H = 65.8 cm is located in the frozen zone, and its height is much greater than the migration height of fluorescein-labeled liquid water because only liquid water can be labeled, not water vapor. There is no fluorescence color at H = 65.8 cm in Figure 12, so the changes in the water and salt contents can only be caused by the migration of water vapor. Although the larger pores of coarse-grained saline soil are not conducive to the migration of liquid water, they provide favorable conditions for the migration of water vapor.
During multiple freeze—thaw cycles, water vapor migrates from bottom to top under the action of the temperature gradient. Owing to the sealing effect of the top film, water vapor accumulates at the top of the saline soil sample, and a series of freezing—condensation—crystallization processes occur as the temperature is reduced, resulting in an increase in the top moisture and salt contents. Water vapor condenses and forms liquid water on the upper part of the heat transfer plate. The condensed liquid water moves downwards under the action of gravity and dissolves more salt in the soil, forming water–salt accumulation zone 2.
The variations in the liquid water content and temperature in the frozen zone (H = 65.8 cm) and nonfrozen zone (H = 28.2 cm) of the saline soil column with freeze–thaw time were analyzed under freeze—thaw cycles, as shown in Figure 13. Under freeze—thaw cycles, the liquid water content in the frozen and unfrozen zones of the same part within the saline soil column exhibited a periodic variation pattern with temperature, and there was essentially no hysteresis phenomenon. In the freezing zone at H = 65.8 cm (Figure 13a), when the temperature dropped below the freezing temperature, the water began to freeze and salt crystallized. In the crystallization process, a large amount of liquid water was consumed, resulting in a corresponding decrease in the liquid water content. When the freeze—thaw cycle temperature increased above the freezing temperature, the ice crystals in the soil pores gradually thawed, the salt crystals dissolved, and the liquid water content increased. However, multiple freeze—thaw cycles led to an increasing trend in the liquid water content, confirming the existence of water vapor in the coarse-grained soil. After migrating to the cold end, the water vapor exhibited a vapor—liquid phase transition, resulting in an increase in the liquid water content. In the nonfrozen zone at H = 28.2 cm (Figure 13b), the temperature remained higher than the freezing temperature, and no ice crystals were produced during the freeze—thaw cycles. The liquid water content reached its maximum within the first 80 h and then stabilized, which occurred synchronously with the changes in external water replenishment. The increase in the liquid water content during the first 80 h was mostly due to the unsaturated characteristics of the coarse-grained saline soil. The high matric suction of the soil caused the migration of a large amount of water to H = 28.2 cm, and the fluctuation in the liquid water content during the following 400 h could be attributed to the consumption–release of liquid water due to salt crystallization–dissolution caused by temperature changes, as well as the increase–decrease in matric suction caused by temperature variations, leading to changes in the liquid water content at H = 28.2 cm in the nonfrozen zone.

4. Analysis of the Salt-Insulating Effect of the Aggregate Insulating Layer

Engineering practices have shown that the aggregate insulating layer affects the separation of water and salt migration processes in saline soil, and, as a bearing layer, the aggregate layer can also improve the overall strength of the foundation and reduce the uneven settlement. To separate the water and salt migration processes in coarse-grained saline soil subgrades and prevent the occurrence of subgrade damage, in this experiment, an aggregate insulating layer was laid to analyze the water- and salt-insulating effects. The specific laying position and thickness are shown in Figure 14. According to the analysis of the water and salt migration results for saline soil under freeze—thaw cycles, water and salt mainly accumulated at a distance of H = 28.2 cm from the bottom. Therefore, an aggregate insulating layer was placed at H = 10.0 cm to prevent the upward migration of external water replenishment.
To determine a reasonable thickness of the aggregate insulating layer and provide the most economical and feasible reference for actual construction, aggregate insulating layers were laid with thicknesses of 10 cm (Figure 14a), 20 cm (Figure 14b), and 30 cm (Figure 14c). No sensors were buried within the aggregate layer, namely, no sensors were buried at H = 18.8 cm when the aggregate insulating layer thickness was 10 cm. Moreover, no sensors were buried at H = 18.8 and 28.2 cm when the aggregate insulating layer thickness was 20 cm, and no sensors were buried at H = 18.8, 28.2, and 37.6 cm when the aggregate insulating layer thickness was 30 cm. The remaining experimental settings and test conditions remained unchanged. The actual liquid level height after five freeze—thaw cycles is shown in Figure 14. When the thickness of the aggregate insulating layer was 10 cm, the liquid level height exceeded that of the aggregate insulating layer (Figure 15a). When the thickness of the aggregate insulating layer was 20 cm, the liquid level occurred only approximately 5 cm from the top of the aggregate insulating layer (Figure 15b). When the thickness of the aggregate insulating layer was 30 cm, the liquid level occurred approximately 20 cm from the top of the aggregate insulating layer (Figure 15c).
The reason for these findings is the continuous filling of the aggregate insulating layer with coarse-grained saline soil, in which the gaps between the aggregate insulating layers are naturally filled, resulting in the presence of soil particles in the insulating layer and the formation of a notable matrix suction force, causing the water and salt in the lower soil to move upwards. The 10 cm aggregate insulating layer is relatively thin, and liquid water directly submerged the insulating layer under the action of the matrix suction force of the soil, continuing to migrate upwards. The 20 cm and 30 cm layers are relatively thick, and there are fewer soil particles in the aggregate gaps. Although liquid water moved upwards under the action of the matrix suction force, it did not submerge the insulating layer.
The final moisture and salt contents at the different heights inside the soil column after five freeze—thaw cycles were compared with the initial values, and the experimental results are shown in Figure 16. Under the condition of an aggregate insulating layer thickness of 10 cm, water and salt still yielded two water and salt aggregation zones at H = 28.2 and 65.8 cm, similar to the water and salt aggregation effect without an aggregate insulating layer. When the thicknesses of the aggregate insulating layer were 20 and 30 cm, because the rising height of the liquid water did not exceed the aggregate insulating layer, the soil did not yield a salt accumulation zone at H = 28.2 cm, and only salt accumulation occurred at H = 65.8 cm.

5. Discussion

The objective of this study was to clarify the water, heat, vapor and salt migration patterns in coarse-grained saline soil, as well as the salt-insulating effect of the aggregate insulating layer. The results of the final moisture and salt contents without the aggregate insulating layer reveal that, after multiple freeze—thaw cycles, two water and salt accumulation zones form inside the coarse-grained saline soil subgrade. The water and salt accumulation zone in the nonfrozen zone is due mainly to external replenishment, namely, the rapid rise of liquid water under capillary action and soil matrix suction. This conclusion is consistent with the conclusion of Melaku et al. [22] and Sun et al. [23,24] The water–salt accumulation zone in the frozen zone is due to the vapor—liquid phase transition of water vapor at the cold end, which condenses to form liquid water. Teng et al. [29] and Bai et al. [30] have come to similar conclusions. The water and salt contents in the saline soil increase with time in the frozen zone, whereas they remain basically unchanged in the unfrozen zone.
A comparison of the final moisture and salt contents of the aggregate insulating layer with the test results without the aggregate insulating layer revealed that, when the thickness of the insulating layer reached 10 cm, the final moisture content at H = 28.2 cm was 19.0%, an increase of 13.0% over the initial moisture content. The final salt content reached 6.8%, an increase of 5.3% over the initial salt content (without an aggregate insulating layer, the moisture content increased by 14.1% and the salt content increased by 5.9%). The increases in the moisture and salt contents were smaller than those when an aggregate insulating layer was not emplaced, indicating that the 10 cm aggregate insulating layer imposed a certain inhibitory effect on water and salt migration. However, owing to its small thickness, the insulating effect was not significant. When an insulating aggregate layer was laid at H = 65.8 cm, water and salt accumulated in the 10 cm, 20 cm, and 30 cm aggregate insulating layers. The final moisture and salt contents in the 10 cm thick aggregate insulating layer increased by 5.7% and 2.5%, respectively, at H = 65.8 cm. The final moisture and salt contents in the 20 cm thick aggregate insulating layer increased by 6.4% and 3.0%, respectively, at H = 65.8 cm. The final moisture and salt contents in the 30 cm thick aggregate insulating layer increased by 7.6% and 3.3%, respectively, at H = 65.8 cm.
When an aggregate insulating layer was not used, the moisture content increased by 4.7% and the salt content increased by 1.6%. The increases in the moisture and salt contents were greater than those when an aggregate insulating layer was employed, and with the increasing thickness of the aggregate insulating layer, the increase in the moisture and salt contents accordingly increased. The reason for this phenomenon is that, after replacing a portion of the original coarse-grained saline soil with an aggregate insulating layer, the aggregate gaps are much larger than the soil particle pores are, and a large amount of water vapor migrates upwards through the aggregate insulating layer. At the cold end, water vapor condenses and exhibits a phase change [41], resulting in higher final moisture and salt contents in the frozen zone of H = 65.8 cm in the experiment with the aggregate insulating layer. Moreover, the thicker the aggregate insulating layer is, the larger the number of aggregate gaps and the greater the migration of water vapor [40], which leads to an increase in the moisture and salt contents at H = 65.8 cm with the increasing thickness of the aggregate insulating layer.
Laying an aggregate insulating layer could effectively block the migration of water and salt in the soil, prevent the accumulation of water and salt, and induce salt swelling. This method mainly prevents the rise of liquid water but, to a certain extent, facilitates the migration of water vapor. To achieve favorable water- and salt-insulating effects in practical engineering and minimize project costs, during construction in seasonally frozen soil areas, the laying of a 20 cm thick aggregate insulating layer at a distance of H = 10 cm from the roadbed base should be considered to prevent the formation of large water and salt accumulation areas due to liquid water migration. Moreover, a drainage layer should be established 19.2 cm below the roadbed cover layer to prevent water vapor from condensing at the cold end under multiple freeze—thaw cycles, causing notable water and salt accumulation below the roadbed cover layer and resulting in diseases within saline soil roadbeds.
In actual construction, according to different types of saline soil and grading sizes, it should also be noted that the increase in the surface of the pavement of a road structure above the level of ground or surface water in slightly and moderately saline soils should be increased by 20% (for loams and clays by 30%) and from 40% to 60% in highly saline soils.
There are other measures to improve the stability of the subgrade, including appropriate elevation of the pavement surface above the calculated water level, taking the appropriate value of the soil compaction coefficients of the embankment body, lowering the groundwater level and using imported non-saline or slightly saline soils, which could be used as a research direction for the stability of the saline soil subgrade.

6. Conclusions

The water and salt migration patterns in coarse-grained saline soil under freeze—thaw cycles and the insulating effect of the aggregate layers under different design parameters were investigated through periodic soil column model tests with no-pressure water replenishment, i.e., fluorescein-labeled liquid water migration and aggregate partition layer tests were performed under freeze—thaw cycles. The research results provide the following conclusions:
(1)
In the freeze—thaw cycle process, the temperature inside the saline soil column periodically changes, and there are depth and hysteresis effects in terms of temperature transfer. The farther away from the cold end, the smaller the temperature change amplitude is. Moreover, the temperature change near the cold end occurs 8 h earlier than the far away change. With an increasing number of freeze—thaw cycles, the freezing depth gradually decreases at the early stage and stabilizes at the later stage, with a freezing depth of 40.5 cm at the stable stage.
(2)
Under freeze—thaw cycles, the amount of external water replenishment increases with decreasing temperature and decreases with increasing temperature, and the external water replenishment variations lag behind the temperature changes. The liquid water content inside the soil increases with temperature and decreases with temperature.
(3)
After multiple freeze—thaw cycles, two water and salt accumulation zones form inside the coarse-grained saline soil subgrade. The water and salt accumulation zone in the nonfrozen zone is due mainly to external replenishment, namely, the rapid rise of liquid water under capillary action and soil matrix suction. The water–salt accumulation zone in the frozen zone is due to the vapor—liquid phase transition of water vapor at the cold end, which condenses to form liquid water. The water and salt contents in the saline soil increase with time in the frozen zone, whereas they remain basically unchanged in the unfrozen zone.
(4)
During construction in seasonally frozen soil areas, the laying of a 20 cm thick aggregate insulating layer at a distance of H = 10 cm from the roadbed base should be considered to prevent the formation of large water–salt accumulation areas due to liquid water migration in saline soil roadbeds. Moreover, a drainage layer should be installed 19.2 cm below the roadbed cover layer (as shown in Figure 17) to prevent the formation of water–salt accumulation areas due to water vapor migration during condensation and liquid water generation at the cold end under multiple freeze—thaw cycles.

Author Contributions

Conceptualization, H.Y. and J.L.; methodology, H.Y.; investigation, B.L.; resources, J.L.; data curation, J.L.; writing—original draft preparation, H.Y.; writing—review and editing, J.L.; supervision, L.X.; project administration, C.H.; funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Xinjiang Transportation Technology Project (KY2022080901), the Xinjiang Transportation Planning Survey and Design Institute Co., Ltd., and the Science and Technology Research and Development Project (KY2022042503).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding authors.

Conflicts of Interest

Author Yang, H.; Lei, B.; Xie, L.; Hu, C. was employed by the Xinjiang Transportation Planning Survey and Design Institute Co., Ltd. Author Liu, J. was employed by the Xinjiang Transportation Planning Survey and Design Institute Co., Ltd. and China Gezhouba Group Municipal Engineering Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Multiphase compositions of frozen saline soil.
Figure 1. Multiphase compositions of frozen saline soil.
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Figure 2. Particle size accumulation curve of the experimental saline soil.
Figure 2. Particle size accumulation curve of the experimental saline soil.
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Figure 3. Diagram of the experimental liquid and vapor coupling migration device.
Figure 3. Diagram of the experimental liquid and vapor coupling migration device.
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Figure 4. Tracer-labeled liquid water migration and visualization validation.
Figure 4. Tracer-labeled liquid water migration and visualization validation.
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Figure 5. CS655 sensor water content calibration function.
Figure 5. CS655 sensor water content calibration function.
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Figure 6. CS655 sensor salt content calibration function.
Figure 6. CS655 sensor salt content calibration function.
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Figure 7. Temperature variation curves of saline soil at the different heights under different numbers of freeze–thaw cycles.
Figure 7. Temperature variation curves of saline soil at the different heights under different numbers of freeze–thaw cycles.
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Figure 8. Isothermal distribution in saline soil under freeze—thaw cycles.
Figure 8. Isothermal distribution in saline soil under freeze—thaw cycles.
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Figure 9. Variation curve of external water replenishment with freeze—thaw cycle duration.
Figure 9. Variation curve of external water replenishment with freeze—thaw cycle duration.
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Figure 10. Fluorescein liquid level height change.
Figure 10. Fluorescein liquid level height change.
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Figure 11. 5 Variation curves of the water and salt contents along the height of the saline soil column under freeze—thaw cycles.
Figure 11. 5 Variation curves of the water and salt contents along the height of the saline soil column under freeze—thaw cycles.
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Figure 12. Migration height of fluorescein-labeled liquid water.
Figure 12. Migration height of fluorescein-labeled liquid water.
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Figure 13. Variation curves of the liquid water content and temperature with respect to the freeze—thaw time.
Figure 13. Variation curves of the liquid water content and temperature with respect to the freeze—thaw time.
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Figure 14. Location and thickness of the aggregate insulating layer.
Figure 14. Location and thickness of the aggregate insulating layer.
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Figure 15. Actual liquid level height of water.
Figure 15. Actual liquid level height of water.
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Figure 16. Variation curves of the water and salt contents along the height of the saline soil column after the installation of an aggregate insulating layer.
Figure 16. Variation curves of the water and salt contents along the height of the saline soil column after the installation of an aggregate insulating layer.
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Figure 17. Schematic diagram of road structure layer.
Figure 17. Schematic diagram of road structure layer.
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Table 1. Ion contents of the saline soil samples.
Table 1. Ion contents of the saline soil samples.
SectionSampling LocationSO42−
/%		Cl−/%CO32−/%HCO3−/%Mg2+/%Ca2+/%Na+/%Soluble Salt/%Type of Saline Soil
Near K6 + 100 of National Highway G31410.7620.1140.0230.0190.0320.3570.5001.250Weakly sulfate saline soil
20.5630.0790.0170.0160.0270.2130.3450.818
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Yang, H.; Lei, B.; Xie, L.; Hu, C.; Liu, J. Experimental Study on Water and Salt Migration and the Aggregate Insulating Effect in Coarse-Grained Saline Soil Subgrade under Freeze–Thaw Cycles. Appl. Sci. 2024, 14, 8970. https://doi.org/10.3390/app14198970

AMA Style

Yang H, Lei B, Xie L, Hu C, Liu J. Experimental Study on Water and Salt Migration and the Aggregate Insulating Effect in Coarse-Grained Saline Soil Subgrade under Freeze–Thaw Cycles. Applied Sciences. 2024; 14(19):8970. https://doi.org/10.3390/app14198970

Chicago/Turabian Style

Yang, Haoyuan, Bingbing Lei, Liangfu Xie, Changtao Hu, and Jie Liu. 2024. "Experimental Study on Water and Salt Migration and the Aggregate Insulating Effect in Coarse-Grained Saline Soil Subgrade under Freeze–Thaw Cycles" Applied Sciences 14, no. 19: 8970. https://doi.org/10.3390/app14198970

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