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Article

Experimental Study on the Influences of the Fines Contents and Initial Moisture on the Water and Salt Migration of Coarse-Grained Saline Soil Subgrades

by
Haoyuan Yang
1,2,3,
Jie Liu
1,2,3,*,
Jianyong Ma
2,3,*,
Yong Wang
4,
Bo Wang
4 and
Jiangpeng Zhang
2,3
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.
Sustainability 2024, 16(24), 11280; https://doi.org/10.3390/su162411280
Submission received: 28 October 2024 / Revised: 6 December 2024 / Accepted: 21 December 2024 / Published: 23 December 2024
Browse Figures
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Abstract

:
The construction of roads in saline soil areas usually involves using coarse-grained soil as roadbed fill material; studying the water–vapor–salt migration mechanism in coarse-grained saline soil subgrades is crucial for ensuring the stability of highway infrastructure. In order to clarify the influence of fines content and initial moisture on the water–salt migration and to clarify the water–vapor–salt migration patterns in coarse-grained saline soil, a model test of coarse-grained saline soil was conducted to study the response patterns of external water replenishment, final moisture content, final salt content, and liquid level height of coarse-grained saline soil. The results indicated that the water vapor migration amount only causes a change in the final moisture content, albeit not enough to cause salt redistribution. With increasing initial moisture content in coarse-grained saline soil, the migration characteristics of water vapor are weakened, and it imposes a significant inhibitory effect on liquid water migration at the same time. Increasing fines content in coarse-grained soil significantly inhibits water vapor migration, whereas liquid water migration is promoted. Water and salt accumulate in the liquid and vapor coupling migration mode at different heights. Based on the mechanisms of water vapor and salt transport characteristics, this study proposes a novel roadbed structure, which is vital for ensuring the long-term service performance of coarse-grained saline soil roadbeds 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]. The distribution of saline soil in the Xinjiang region is extensive, rendering it the largest saline soil area in China. As a unique type of soil, the physical and mechanical properties of saline soil change upon phase transition, such as when soil salt components come into contact with water [5]. This impact varies according to the composition of the soil particles, salt content and type, moisture content, and changes in climate and environmental factors [6,7,8,9]. 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 [10,11,12], which poses a serious threat to the service performance of roadbeds in saline soil areas [13,14], and the aforementioned saline soil problem is relevant for all types of road surfaces. With the increasing implementation of the Belt and Road Initiative and the Western Development Strategy in China, the construction of transportation infrastructure in the western region of China inevitably involves sections of saline soil [15]. According to the Specifications for the Design of Highway Subgrades (JTG D30—2015) [16], coarse-grained soil is usually employed as a subgrade fill material in saline soil areas for highway subgrade design and construction. Therefore, it is particularly important to clarify the water and salt migration characteristics of coarse-grained saline soil subgrades to prevent salt swelling and dissolution subsidence and to improve the service performance of coarse-grained saline soil subgrades.
Most coarse-grained saline soils in arid desert areas exhibit an unsaturated state. The large pores in unsaturated soils do not facilitate the capillary rise of liquid water but create favorable conditions for water vapor migration [17]. Li et al. [18] and Luo et al. [19] found that the impermeable layer at the ground surface could limit the evaporation effect and cause water vapor accumulation below the impermeable cover layer, forming a pot effect, in their study of uneven pavement settlement at a certain airport in the western region of China. Zhang et al. [20] and Yao et al. [21] conducted numerical simulation and experimental studies of the pot effect of water migration in fine-grained soils, suggesting that water vapor will migrate and freeze at the top and at the freezing front, respectively, causing an increase in the moisture content in these areas. Gao et al. [22] reported that water accumulation below the impermeable cover is caused by water vapor migration in the soil pores, and based on this mechanism, Bai et al. [23] experimentally studied liquid and vapor coupling migration of coarse-grained soil fill for high-speed rail construction, determining that the fine particle and initial moisture contents exerted the greatest impact on the experimental results; moreover, an increase in initial moisture content will decrease matrix suction, while an increase in fine particle content will increase matrix suction, and the increase in the temperature gradient promoted water vapor migration. Dobchuk et al. [24] proposed that the main factors influencing water vapor migration are the bulk density, porosity, and diffusion coefficient. De Vries et al. [25] considered the combined effect of the temperature gradient, moisture gradient, and gravity potential on water heat vapor migration in frozen soil. Huang and Rudolph [26] experimentally revealed the interactions among water vapor migration, heat transfer, ice–water and water vapor phase transitions, and deformation in unsaturated frozen soil. At present, there is a certain experimental understanding of the water vapor migration mechanism and scenarios.
The commonly accepted viewpoint is that salt follows water, which suggests salt redistribution in soil with water migration. Guo et al. [27] investigated water–salt migration in saline soil samples with different initial moisture contents and bulk densities after saltwater infiltration through indoor experiments, and the results indicated that the surface soil moisture and salt contents increased with increasing initial moisture content and bulk density. Liu et al. [28] experimentally investigated water–salt migration in saline soil samples with different salt contents and demonstrated that salt migration is influenced by both convection and diffusion, while water migration is mainly driven by convection. Water and salt migration do not occur simultaneously, but ultimately, salt moves with water migration. Tian et al. [29] investigated the water–salt migration mechanism of saline soil under temperature changes and illustrated that the salt content and type also influence water–salt migration. Sarsembayeva and Collins [30] 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. Yang et al. [31] explored the water–salt migration process in saline soil roadbed fill under evaporation through indoor experiments and found that the higher the fine particle content, the smaller the water–salt migration height. Stahli and Stadler [32] 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. Bing et al. [33] found that the size of soil pores significantly impacts water–salt migration. Hou et al. [34] clarified the diffusion mechanism of soil water and salt by controlling factors such as the initial moisture content and constructed a relevant function. Zhang et al. [35] studied water–salt migration within roadbeds in desert areas with notable temperature differences by controlling the compaction level and initial moisture content. Overall, the above studies only focused on the liquid water migration patterns in saline soil. However, there is limited research on the occurrence and characteristics of water vapor migration in coarse-grained saline soil. In addition, these studies revealed that factors such as the fines content and the initial moisture content significantly impact liquid water migration in soil, but the impact of fines content and the initial moisture content on the migration of water vapor and liquid water in coarse-grained saline soil is not clear. Moreover, while most studies concentrate on mixing Na2SO4 or NaCl into virgin soil for water–salt migration research, research on natural coarse saline soils is relatively scarce.
In order to clarify the influence of fines content and initial moisture on the water salt migration and to clarify the water–vapor–salt migration patterns in coarse-grained saline soil, in this study, model tests of naturally coarse-grained saline soil from the Kashgar region of Xinjiang were conducted. Water vapor migration and liquid and vapor coupling migration were achieved through the use of porous plates, and liquid water migration was investigated by the fluorescent tracer method. This study aimed to clarify the influences of the initial moisture and fines’ contents on the external water intake amount, final moisture content, final salt content, and liquid water migration height of coarse-grained saline soil. This research has certain reference significance for enhancing the understanding of the water–vapor–salt transport characteristics of coarse-grained saline soil subgrades.

2. Materials and Methods

2.1. Experimental Soil Samples

The natural saline soil used in this study was obtained from near the K6+100 section of National Highway G314 in Kashgar, Xinjiang, approximately 14 km from the center of Kashgar city. According to the Test Methods of Soils for Highway Engineering (JTG E40-2007) [36], particle size distribution tests and soluble salt content tests of this natural saline soil were conducted, and the test results are provided in Table 1 and Table 2, respectively. In Table 2, c(Cl−) and c(SO42−) denote the molar concentrations of chloride and sulfate ions, respectively, in 100 g of soil (mmol/100 g). Based on the c(Cl−)/2c(SO42−) ratio and reference [37], the saline soil type was determined as weakly saline sulfate soil.

2.2. Experimental Design

2.2.1. Fluorescein Tracer Key Technology Validation

In the traditional one-dimensional test method, only water migration is monitored through sensor readings. Due to the height difference in sensor placement, it is difficult to determine the water migration height between two sensors based on their readings. Therefore, this method lacks visualization ability. A fluorescein tracer (chemical formula: C20H12O5) is commonly used for sewage discharge detection. This tracer is soluble in water and appears yellow–green under ultraviolet light [38]. By adding fluorescein to water, the spatiotemporal characteristics of liquid water migration and redistribution can be tracked in real time. Therefore, the addition of a fluorescein tracer to liquid water for marking purposes is key to visualizing liquid water migration, which can more intuitively reflect the liquid and vapor coupling migration characteristics of coarse-grained saline soil subgrades.
Based on the research results of Wang and Zhang [39,40], a fluorescein solution evaporation–condensation test was conducted, as shown in Figure 1. Soil samples containing fluorescein were placed on an evaporating dish and covered with a glass plate for heating. Notably, the small water droplets condensed on the glass cover plate did not exhibit color under ultraviolet light, indicating that fluorescein only migrates with liquid water, not with water vapor. Therefore, fluorescein can be added to the external water intake to track the changes in the height of external liquid water migration under ultraviolet light irradiation.

2.2.2. Test Apparatus

The experimental liquid and vapor coupling migration device is shown in Figure 2, which mainly includes a temperature control system, liquid and vapor migration system, and data monitoring system. The soil test cylinder is an organic glass cylinder with an inner diameter of 50 cm, an outer diameter of 60 cm, a height of 80 cm, and a wall thickness of 5 cm, which provides the advantages of high strength, favorable friction resistance, and high transparency. It is convenient for real-time tracking of the external water intake migration height and image acquisition and for optimally reducing heat exchange between the soil sample and the external environment while achieving visualization.
The temperature control system includes two constant-temperature baths (with precisions of ±0.01 °C for bath 1 and ±0.1 °C for bath 2) and upper- and lower-temperature guide plates. Constant-temperature bath 1 is connected to the upper-temperature guide plate through a circulating liquid pipe, while constant-temperature bath 2 is connected to the lower-temperature guide plate. The upper- and lower-temperature guide plates are arranged above the soil sample and below the water replenishment tank, respectively, to achieve different temperatures for simulating the temperature difference between the upper and lower parts of the soil subgrade in the actual road construction process.
The external water intake system comprises a Mariotte bottle and a porous plate, as shown in Figure 3. The Mariotte bottle is connected to the bottom water tank of the sample cylinder to replenish water from the external environment. A porous plate is used to separate liquid water. When the liquid level of the external water intake is below the porous plate, only water vapor migration occurs. When the liquid level of the external water intake exceeds the porous plate and contacts the soil sample, liquid and vapor coupling migration is achieved. The external water intake amount can be calculated by the scale of the Mariotte bottle as the liquid level decreases.
In the image monitoring system, a combination of ultraviolet light and a camera is utilized to track the migration of the fluorescent tracer in the soil sample in real time. A glass plate was employed to cover the evaporating dish in the fluorescent dye solution evaporation–condensation test. The results showed that the color of the small water droplets on the glass cover plate did not change under ultraviolet light irradiation, indicating that the fluorescent dye only migrates with liquid water, not with water vapor. Therefore, with the assistance of ultraviolet light, a fluorescent dye can be added to the Mariotte bottle for external water intake to track the variation in the migration height of external liquid water entering the soil sample. In this study, the concentration of the fluorescent dye tracer in the Mariotte bottle was 5 g/L.
A CS655 soil three-parameter sensor and CR1000X data logger were used to monitor the real-time changes in the internal temperature (with an accuracy of ±0.5 °C), moisture content (with an accuracy of ±3%), and electrical conductivity (with an accuracy of 0.05 dS/m) of the soil samples.

2.2.3. Experimental Plan

This experiment aimed to clarify the water and salt migration characteristics of coarse-grained saline soil subgrades. By adjusting the inlet valve of the Mariotte bottle to control the water level in the tank, different modes of water migration could be achieved. Due to the high evaporation rate at the surface in the Kashgar region, the moisture content of subgrade soil is generally lower than the optimal moisture content. To better observe the wetting effect of water vapor on the soil samples, the initial moisture content was increased from 0% to a maximum value of 6% in increments of 1.5% during the experiment, which better conforms with the initial moisture content of the field soil samples. The fines content (a particle size less than 0.075 mm) of the undisturbed soil is 14.05%. Therefore, in the experiment, the fines content was increased from 5% to 15% in increments of 2.5%. The experimental conditions are detailed in Table 3 [40]. Water vapor migration tests and liquid and vapor coupling migration tests of coarse-grained saline soil were conducted under different conditions. In the liquid and vapor migration tests, fluorescent dye was added to the liquid water, facilitating the observation of the liquid water migration height in the soil sample.
Notably, salt-affected soil samples were prepared considering different experimental conditions and allowed to sit for 24 h. Then, the soil was divided into 16 portions and placed in an 80 cm high, 50 cm diameter organic glass tube. The soil was unidirectionally compacted to a degree of 0.95, and it was allowed to settle for another 24 h before the experiment. The soil samples were divided into nine sections, with a soil three-parameter sensor placed approximately every 9 cm, totaling 8 sensors. The temperature, humidity, and electrical conductivity of the saline soil samples were monitored in real time.
The obtained meteorological data for Kashgar City indicate that the average temperature does not decrease below zero from March to October. The soil in this area remains essentially unfrozen. Therefore, in this study, temperature variations at positive temperatures were considered to obtain the water and salt migration characteristics of coarse-grained saline soil in this region. To simulate the phenomenon of notable temperature differences between day and night at the actual construction site containing unfrozen soil, the temperature of the upper heat transfer plate was adjusted to 5 °C, 10 °C, 15 °C, 10 °C, and 5 °C for 4.8 h each, totaling 24 h. The temperature of the lower heat transfer plate was maintained at 20 °C throughout the experiment. Each temperature cycle was repeated 4 times, totaling 96 h.

3. Results and Analysis

3.1. Study on Water Vapor Migration with Different Initial Moisture Contents

The experimental results and analysis of water vapor migration in coarse-grained saline soil with different initial moisture contents are provided in Table 4 as a function of the external water intake amount, and the final moisture content and salt distribution are shown in Figure 4. With increasing initial moisture content, the soil salt content gradually increased to approximately 4.60%. This occurred because the increase in the initial moisture content led to a higher internal moisture content, resulting in the dissolution of more salt in the saline soil. The external water intake amount gradually decreased, with the largest external water intake amount under test condition 1 reaching 1564 mL. The moisture content of the soil at 9, 18, and 27 cm above the bottom increased by 2.80%, 2.25%, and 1.30%, respectively, while the salt content increased by 0.20%, 0.18%, and 0.11%, respectively. The lower part of the soil was moistened by water vapor, resulting in notable darkening of the soil color, as shown in Figure 5. This phenomenon further confirmed the occurrence of water vapor migration in the coarse-grained saline soil.
Due to a certain initial moisture content under test conditions 2–5, the effect of soil moistening by water vapor was difficult to capture with a camera, and the changes in sensor readings at the different heights were used to monitor the moisture content variations. According to Figure 4, under test conditions 2–5, the sensor 9 cm above the bottom showed moisture content increases of 1.8%, 1.0%, 0.80%, and 0.60%, and salt content increases of 0.13%, 0.10%, 0.08%, and 0.04%, respectively. The changes in sensor readings under the different test conditions were mainly concentrated at the three sensors located 9–27 cm above the bottom, with lower moisture and salt content variations observed at greater heights. The moisture content changes at 9 cm above the bottom of the soil gradually decreased under test conditions 1–5, indicating that the increase in the initial moisture content imposed an inhibitory effect on water vapor migration. When the initial moisture content reached 3% or higher, the replenishment amount for water vapor migration was less than 700 mL, and the moisture content variations at the different heights were all less than 1%. The inhibitory effect of the initial moisture content on water vapor migration was notable.
According to the above analysis, water vapor migrates in coarse-grained saline soil, and the large and connected pores in coarse-grained soil provide favorable conditions for water vapor migration. Notably, water vapor migration will lead to changes in the final soil moisture content, but as the initial moisture content is increased, the larger soil pores become blocked, the channels of upward water vapor migration decrease, and the external water intake amount accordingly decreases. The magnitude of the change in the moisture content gradually decreases, and the moisture content change under the different test conditions is less than 2.80%. In addition, the range of influence of water vapor migration is small, within 27 cm from the bottom of the soil. Moreover, water vapor migration does not cause significant water redistribution in the soil, whereas the soil salt exhibits obvious migration. Hence, the impact of water vapor migration on salt content changes can be neglected. This conclusion is consistent with the conclusions of Gao et al. [22] and Bai et al. [23].

3.2. Study on Water Vapor Migration with Different Fines Contents

The external water intake amount, final moisture content, and salt content distribution in coarse-grained saline soil with different fines contents in the water vapor migration experiment are provided in Table 5 and Figure 6. According to Table 5, when the initial moisture content was maintained at 6%, the initial soil salt content remained at approximately 4.50%. As the fines content was gradually increased, the external water intake amount decreased. Under test condition 6, the maximum external water intake reached 848 mL. The soil moisture content increased by 1.80%, 0.93%, 0.52%, and 0.34% at 9, 18, 27, and 36 cm, respectively. The salt content increased by 0.10%, 0.06%, 0.03%, and 0.02%, respectively, at the same locations. Under test conditions 7–10, the final moisture content at 9 cm from the bottom of the soil increased by 1.50%, 1.22%, 0.89%, and 0.60%, respectively. Moreover, the salt content increased by 0.11%, 0.07%, 0.05%, and 0.06%, respectively. With increasing distance from the bottom of the soil, the change in the moisture content decreased, and the change in the salt content could be neglected. However, there were significant changes in the readings of the four sensors located 9–36 cm from the bottom of the soil under the different test conditions, with the height affected by water vapor reaching 36 cm. The change in the moisture content at 9 cm from the bottom of the soil under test conditions 6–10 gradually decreased, indicating that the increase in the fines content also imposed an inhibitory effect on water vapor migration. When the fines content reached 10% or greater, the change in the final soil moisture content at the different heights was less than 1.3%, indicating a significant inhibitory effect on water vapor migration.
The comparison of Table 4 and Table 5 reveals that the increase in both the fines and initial moisture contents inhibited water vapor migration. The analysis showed that the increase in the fines and initial moisture contents led to a reduction in the size and number of pores in the coarse-grained soil, thereby blocking the migration pathways of water vapor and inhibiting its movement [23]. The external water intake amount under the conditions of a 15% fines content and an initial moisture content ranging from 0% to 6% was greater than that under the conditions of a 6% initial moisture content and a fines content ranging from 5% to 15%. This indicates that an initial moisture content of 6% exerted a greater inhibitory effect on water vapor than did a fines content of 15%, suggesting that in coarse-grained soil, the initial moisture content more notably affects pore blockage than does the fines content. Comparing Figure 4 and Figure 6, when the initial moisture content reaches 6% and the fines content varies, the changes in the sensor readings are concentrated at 9–36 cm from the bottom of the soil, indicating a wider range of influence of water vapor. However, because the initial moisture content is 6%, which exerts a significant inhibitory effect on water vapor, the variation in the fines content does not cause significant water redistribution. The changes in the final moisture content under the different test conditions are all less than 1.80%, which is less than the impact of the variation in the initial moisture content.
Regardless of the variation in the initial moisture or fines content, water vapor migration does not lead to significant soil moisture redistribution or notable salt content changes. In other words, water vapor migration does not cause changes in the salt content. However, when the fines content is varied, the size and number of the internal pores of coarse-grained saline soil increase, and the influence of water vapor can extend up to 36 cm from the bottom.

3.3. Study on Liquid and Vapor Coupling Migration with Different Initial Moisture Contents

Curves of the external water intake for the liquid and vapor migration tests with different initial moisture contents are shown in Figure 7. A comparison of the total external water intake amount between the water vapor migration tests and the liquid and vapor coupling migration tests with different initial moisture contents is provided in Table 6. Figure 7 shows that the change patterns of the replenishment curves under the different test conditions are basically similar. Specifically, for test condition 1, the growth rate is 1.13 L/h for the first 16 h, 0.81 L/h for the period from 16 to 48 h, and 0.04 L/h from 48 to 96 h. Consequently, the process can be divided into three stages: a rapid growth stage encompassing the first 16 h, a slow growth stage from 16 to 48 h, and a basically stable stage from 48 to 96 h. In addition, with increasing initial moisture content of the soil, the external water intake amount gradually decreases, with test condition 5 yielding the smallest external water intake amount at the highest initial moisture content. Considering that under experimental conditions 1–5, except for the water migration mode, the soil, and external environment are consistent, we can assume that the water vapor migration amounts in the liquid and vapor coupling migration and water vapor migration tests are the same. This allows us to calculate the percentage of water vapor migration in liquid and vapor coupling migration. As indicated in Table 6, the percentage of water vapor migration is relatively low, below 7%. Furthermore, with increasing initial moisture content, the percentage of water vapor migration in the total liquid and vapor coupling migration gradually decreases from 6.67% to 2.49%, indicating that the change in the final moisture content is mainly caused by liquid water and that an increase in the initial moisture content inhibits water vapor migration. Based on this pattern and in conjunction with the sensor readings, the final moisture content and salt content distribution in the liquid and vapor coupling migration tests with different initial moisture contents can be obtained. The experimental results are shown in Figure 8.
Figure 8 shows that the initial salt content at 9 cm from the bottom under the different test conditions is greater than the final salt content. This occurs because regardless of the initial moisture content, there is a clear aggregation phenomenon of the final moisture content and salt content. Salt accumulates in the lower soil part as moisture migrates upward, resulting in a lower salt content at the bottom than the initial state. This phenomenon not only reflects the objective process of salt following water but also confirms the occurrence of the pot effect. Under test condition 1, the final salt content reaches 22.55%, significantly higher than the levels observed under test conditions 2 through 5. This disparity is attributed to the uneven distribution of salt within the natural saline soil. During the excavation of the soil samples for testing, a concentration of salt forms at specific locations within the soil. Despite meticulous mixing and compaction of the soil samples prior to filling, the initial salt content under test condition 1 was already at 5.22%, which is considerably higher compared to the other test groups. With the increase and decrease in the temperature of the upper-temperature plate, the moisture content in the surface soil layer changes first, leading to a decrease in the soil water potential in the upper soil layer. This results in a hydraulic gradient between the upper and lower soil layers, causing moisture to migrate upward. However, the water and salt aggregation height varies under different experimental conditions involving varying initial moisture contents. Test conditions 1–5 exhibited water and salt aggregation phenomena at approximately 45, 40, 36, 30, and 27 cm. The final moisture content increased by 22.38%, 19.51%, 19.44%, 16.38%, and 15.50%, respectively, compared to the initial moisture content. Notably, the increase in the initial moisture content not only inhibited water vapor migration but also imposed an inhibitory effect on liquid water migration. The analysis suggests that the increase in the initial moisture content not only results in blockage of the pores for upward water vapor migration but also results in blockage of the capillary rise pathways of liquid water. This leads to a decrease in soil matrix suction, thereby restricting the migration volume and height of liquid water. The inhibitory effect of the initial moisture content on liquid water migration can be further illustrated by the migration height of liquid water containing fluorescent dye.
Fluorescein was added to the water for external water intake in the liquid and vapor coupling migration experiments under the different test conditions, and the liquid water migration height was recorded using a camera under ultraviolet light irradiation, as shown in Figure 9. The final fluorescein liquid levels under test conditions 1–5 were 58, 56.5, 54.8, 53, and 50 cm. The gradual decrease in the liquid water migration height could be attributed to the higher initial moisture content, which caused a reduction in the capillary rise pathways within the soil, resulting in smaller final migration heights of liquid water. However, the final migration heights of fluorescein-containing liquid under the different test conditions were always greater than the water–salt aggregation height within the soil samples. This occurs because even after water–salt aggregation within the soil, water can still migrate upward under soil matrix suction, leading to a height difference between the two layers. Combining Figure 7 and Figure 9, during the rapid growth phase of the first 16 h of external water intake, the fluorescein liquid level had already reached a relatively high position, and the rate of liquid water migration essentially remained consistent with the rate of external water intake. This conclusion is consistent with the conclusion of Zhang et al. [40] and Wang et al. [39].

3.4. Study on Liquid and Vapor Coupling Migration with Different Fine Contents

Curves of the external water intake in the liquid and vapor coupling migration experiments with different fines contents are shown in Figure 10. The external water intake under test conditions 6–10 can also be divided into three stages: rapid growth, slow growth, and basic stability. However, compared to those under conditions 1–5, the changes in the external water intake volume under the various fines contents reached a basically stable state approximately 8 h earlier. The total external water intake amounts in the water vapor migration experiments and liquid and vapor coupling migration experiments with the different fines contents are provided in Table 7. At a fines content of only 5%, the migration percentage of water vapor was the highest, at 14.75%. In contrast, at a fines content of 15%, the migration percentage of water vapor decreased to 2.49%. The comparison of Table 7 and Table 6 reveals that the total external water intake in the liquid and vapor coupling migration experiments with an initial moisture content of 6% and a fines content ranging from 5–15% is lower than that at a fines content of 15% and an initial moisture content ranging from 0–6%. This indicates that the inhibitory effect of a 6% initial moisture content on liquid water migration is greater than that of a 15% fines content. Considering that test conditions 6–10 exhibit consistent soil and external environmental conditions apart from the water migration mode, it can be assumed that when the initial soil moisture content remains the same, the migration of water vapor in the water vapor migration tests is equal to that in the liquid and vapor coupling migration tests, allowing for the calculation of the percentage of water vapor in liquid and vapor coupling migration. The external water intake volume during liquid and vapor coupling migration under test conditions 6–10 is smaller than that under test conditions 1–5, but the percentage of water vapor is higher, indicating that water vapor migration in coarse-grained saline soil is more notably influenced by the fines content. Based on this pattern and in conjunction with the sensor readings, the final moisture content and salt content distribution in the liquid and vapor coupling migration experiments with the different fines content can be obtained, as shown in Figure 11.
Figure 11 shows that test condition 10 exhibited the highest final moisture content, with a final moisture content of 21.69% at a distance of 27 cm from the bottom of the soil. This represents an increase of 15.69% compared to the initial moisture content of 6%. Subsequently, as the fines content was gradually reduced to 5%, the maximum final moisture content under test conditions 6–9 decreased from 21.15% to 19.75%. This occurred because the decrease in the fines content, while beneficial for water vapor migration, is outweighed by the influence of liquid water migration in the liquid and vapor coupling migration process, which plays a decisive role in the change in the final moisture content. The reduction in the fines content leads to a decrease in capillary channels within the soil, a reduction in the soil matrix suction, a decrease in liquid water migration, and a subsequent decrease in the final moisture content. However, among test conditions 6–10, test condition 8 exhibited the highest final salt content, with a final salt content of 17.12% at a distance of 24 cm from the bottom of the soil. This could be attributed to the use of naturally saline soil in preparing the samples for the different test conditions and ensuring uniform soil sample mixing. Nevertheless, there were still significant differences in the initial salt content among the different test conditions, leading to variations in the final salt content. Additionally, observation of the salt content changes at the soil sensors within the top part revealed that the final salt content under the different test conditions was slightly greater than the initial salt content. This could be attributed to the increase in the water vapor migration height resulting from the decrease in the fines content and the covering effect of the heat transfer plate. Water vapor condenses into liquid water near the heat transfer plate after reaching the upper part of the soil, dissolving salt in the surrounding soil. However, due to the small amount of water vapor migration, the numerical change in the salt content is also small.
Under test conditions 6–10, both the final moisture content and the final salt content demonstrated significant water–salt accumulation phenomena at approximately 18, 22, 24, 26, and 27 cm. This phenomenon indicates that with constant initial moisture content, an increase in the fines content enhances the soil matrix suction, facilitating liquid water migration and leading to an increase in the water–salt accumulation height. However, compared to those under test conditions 1–5, although the height gradually increased, the accumulation height decreased. This suggests that while the fines content promotes liquid water migration, the inhibitory effect of a 6% initial moisture content on liquid water migration is greater than that of a 15% fines content.
Referring to the migration height of liquid water containing fluorescent dye, as shown in Figure 12, the final migration heights of fluorescein were 26, 30, 36, 43, and 50 cm. With increasing fines content, the migration height of fluorescein gradually increased. This occurred because the presence of fine soil particles yields more capillary water ascent channels, facilitating liquid water migration and highlighting the promoting effect of the fines content on this process.

4. Discussion

The objective of this study was to clarify the influences of the fines contents and initial moisture on the water and salt migration, as well as the water, vapor, and salt migration patterns in coarse-grained saline soil. The results of the final moisture and salt contents reveal that in the liquid and vapor coupling migration mode, water and salt will accumulate at a certain height inside the coarse-grained saline soil subgrade. The water and salt accumulation 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. [41] and Sun et al. [42].
Salt concentration significantly affects osmotic suction. The migration of salt is driven by the concentration gradient, moving with water through convection towards the freezing front and diffusion in the opposite direction due to concentration gradients. An increase in salt concentration leads to increased migration of both water and salt, with less migration occurring away from the freezing front. Moreover, the presence of salts can significantly lower the freezing point of pore water, resulting in a substantial amount of unfrozen water content even at temperatures below freezing, which affects osmotic suction. An increase in salt concentration may lead to a decrease in osmotic suction because the increased salt content could reduce the unfrozen water content in the soil, thereby affecting osmotic suction [43].
Water migration significantly affects matric suction. Matric suction is inversely proportional to temperature, meaning that the higher the soil temperature, the lower the matric suction. Water migration alters the soil’s pore structure and matric suction, thereby affecting the soil’s water retention capacity and water migration pathways. Changes in matric suction due to water migration subsequently affect the soil’s water distribution and migration behavior. Therefore, water migration influences the soil’s moisture state and migration characteristics by altering matric suction [44].
Changes in the migration of water and salt significantly affect total suction. The migration of water and salt alters the soil’s pore structure and its compressibility, which is a major cause of deformation after freeze–thaw cycles. The redistribution of water and salt shows an increase in water content in the frozen zone and a decrease in the unfrozen zone, due to temperature gradients and matric potential gradients. Additionally, the ion content in the unfrozen zone increases as the distance from the freezing front decreases and remains almost constant in the frozen zone. Therefore, water and salt migration affect total suction by altering the distribution of water and ions in the soil [28,45].
Water migration may affect the collapse potential of soil. Increased water and salt migration at the freezing front can affect soil stability. Water migration leading to increased water content in the frozen zone may increase soil deformation, thereby affecting soil stability and collapse potential. Furthermore, water migration may also affect the soil’s collapse potential by altering the soil’s pore structure and matric suction. Thus, water migration has the potential to increase soil collapse by influencing the soil’s moisture state and structure [46].
The decrease in the fines content not only affects the compaction quality of coarse-grained saline soil subgrades in actual engineering but also leads to an increase in the proportion of water vapor in liquid and vapor coupling migration. Water vapor may condense and moisture may accumulate above the subgrade through migration via the soil pores under the influence of high-temperature gradients, leading to frost heave in cold regions or areas with notable temperature differences. Therefore, it is necessary to strictly control the fines content of coarse aggregate filler materials during actual construction. When the fines content is controlled at 10%, it imposes a significant inhibitory effect on water vapor migration and does not cause an excessive increase in liquid water. This can serve as a reference for determining the fines content during subgrade construction in seasonally frozen soil areas.
The experimental results indicated that the external water intake amount and fluorescein migration height in liquid and vapor coupling migration with the different initial moisture contents are greater than those with different fines contents. Notably, liquid water migration is mainly influenced by the initial moisture content. Salt will migrate along with liquid water. As shown in Figure 8 and Figure 11, under the different test conditions, due to the coverage of the heat conduction plate, water will accumulate, and this will inevitably lead to salt accumulation. The pattern of salt following water also applies to water–salt transport in coarse-grained saline soil. The accumulation of water and salt can cause damage to the subgrades, such as salt swelling and dissolution subsidence. For sulfate saline soil subgrade, during the cement hydration process of the roadbed cement stabilization layer, expansive substances such as ettringite are formed due to the participation of accumulated sulfates. The content of sulfate in the roadbed exceeds the standard, and temperature changes cause the formation of expansive sodium sulfate crystals mainly composed of sodium sulfate decahydrate.
Therefore, to prevent potential damage during the construction of coarse-grained saline soil subgrades in actual engineering, a drainage layer can be set in the lower subgrade layer according to the actual construction conditions. At high temperatures, the channels for upward movement of water vapor due to transpiration can be blocked. In contrast, at low temperatures, the channels for groundwater collection toward the freezing front can be blocked. When the water content within this layer reaches a certain level, pore water can be collected and discharged into a drainage ditch (pipe) under the action of a water pressure difference [47], thereby blocking further upward migration of water and salt. But if the drainage layer is impermeable, it will form a new “pot effect”, water vapor and liquid water will accumulate under the drainage layer, causing damage to the roadbed. So a new material barrier layer can be installed to intercept or alter the water and salt migration paths within the subgrades [48,49] (as shown in Figure 13). To achieve favorable water and salt insulating effects in practical engineering, an aggregate insulating layer can be set in the lower subgrade layer to prevent the formation of large water and salt accumulation areas due to liquid water migration. Moreover, due to the permeability of the aggregate insulating layer, it can separate the migration of liquid water but cannot block the migration of water vapor; a drainage layer should be established below the roadbed cover layer to prevent water vapor from condensing at the cold end, causing notable water and salt accumulation below the roadbed cover layer and resulting in diseases within saline soil roadbeds.

5. Conclusions

By using a self-designed liquid and vapor coupling migration device, two distinct modes of water migration, namely, water vapor migration and liquid and vapor coupling migration, were achieved. The migration and redistribution characteristics of liquid water were tracked in real time using the fluorescein tracer method. By analyzing the effects of the initial moisture and fines contents on the external water intake amount, final moisture content, final salt content, and liquid water migration height in coarse-grained saline soil, the water and salt transport characteristics of subgrades were studied, yielding the following conclusions:
(1)
Water vapor migration occurs in coarse-grained saline soil, but its influence is limited. Increases in both the initial moisture and fines contents inhibit water vapor migration. Water vapor migration affects the final moisture content of the soil sample, but this change is not sufficient to cause salt redistribution. When the initial moisture content is greater than 3% or the fines content is greater than 10%, the inhibitory effect on water vapor migration is significant.
(2)
The proportion of external water intake in the water vapor migration experiments with the different fines contents is greater for liquid and vapor coupling migration. Moreover, water vapor migration in coarse-grained saline soil is mainly influenced by the fines content. However, in the liquid and vapor coupling migration experiments with the different initial moisture contents, the external water intake and fluorescence migration height are greater than those in the experiments with the different fines contents. Notably, liquid water migration is mainly influenced by the initial moisture content.
(3)
A 6% initial moisture content imposes a greater inhibitory effect on water vapor migration and liquid and vapor coupling migration than a 15% fines content, indicating that higher initial moisture content plays a more significant role in inhibiting soil water migration.
(4)
The initial moisture content inhibits the migration of both water vapor and liquid water, while the fines content, although also inhibiting water vapor migration, is conducive to liquid water migration. Notably, liquid and vapor coupling migration indicates varying degrees of moisture and salt migration and accumulation. With increasing initial moisture content, the accumulation height gradually decreases. In contrast, with increasing fines content, the accumulation height gradually increases.

Author Contributions

Conceptualization, H.Y. and J.L.; methodology, H.Y.; investigation, J.L.; resources, J.L.; data curation, J.M.; writing—original draft preparation, H.Y.; writing—review and editing, Y.W.; supervision, J.Z.; project administration, B.W.; funding acquisition, J.Z., Y.W., and B.W. 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

Authors Yang, H.; Liu, J.; Ma, J.; Zhang, J. were employed by the Xinjiang Transportation Planning Survey and Design Institute Co., Ltd. Authors Wang, Y.; Wang, B. were employed by the 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. Fluorescein solution evaporation test: (a) soil sample containing a fluorescein solution; (b) heating; and (c) glass cover plate with color-less water vapor.
Figure 1. Fluorescein solution evaporation test: (a) soil sample containing a fluorescein solution; (b) heating; and (c) glass cover plate with color-less water vapor.
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Figure 2. Diagram of the experimental liquid and vapor coupling migration device.
Figure 2. Diagram of the experimental liquid and vapor coupling migration device.
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Figure 3. External water intake system setup: (a) Mariotte bottle diagram and (b) porous plate diagram.
Figure 3. External water intake system setup: (a) Mariotte bottle diagram and (b) porous plate diagram.
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Figure 4. Distribution of the final water and salt content in the soil samples with the different initial water contents: (a) final moisture content and (b) final salt content.
Figure 4. Distribution of the final water and salt content in the soil samples with the different initial water contents: (a) final moisture content and (b) final salt content.
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Figure 5. Actual results at the end of the water vapor migration test under test condition 1.
Figure 5. Actual results at the end of the water vapor migration test under test condition 1.
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Figure 6. Distribution of the final water and salt content in the soil samples with different fines contents: (a) final moisture content and (b) final salt content.
Figure 6. Distribution of the final water and salt content in the soil samples with different fines contents: (a) final moisture content and (b) final salt content.
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Figure 7. Curves of the external water intake schedule for the liquid and vapor coupling migration tests with the different initial moisture contents.
Figure 7. Curves of the external water intake schedule for the liquid and vapor coupling migration tests with the different initial moisture contents.
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Figure 8. Distribution of the final moisture content and the salt content in the liquid and vapor coupling migration tests with the different initial moisture contents: (a) final moisture content and (b) final salt content.
Figure 8. Distribution of the final moisture content and the salt content in the liquid and vapor coupling migration tests with the different initial moisture contents: (a) final moisture content and (b) final salt content.
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Figure 9. Liquid and vapor coupling migration fluorescein tracer results for the different initial water contents: (a) test condition 1; (b) test condition 2; (c) test condition 3; (d) test condition 4; and (e) test condition 5.
Figure 9. Liquid and vapor coupling migration fluorescein tracer results for the different initial water contents: (a) test condition 1; (b) test condition 2; (c) test condition 3; (d) test condition 4; and (e) test condition 5.
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Figure 10. Curves of the external water intake schedule for the liquid and vapor migration tests with different fines contents.
Figure 10. Curves of the external water intake schedule for the liquid and vapor migration tests with different fines contents.
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Figure 11. Distributions of the final moisture and salt contents in the liquid and vapor coupling migration tests with different fines contents: (a) final moisture content and (b) final salt content.
Figure 11. Distributions of the final moisture and salt contents in the liquid and vapor coupling migration tests with different fines contents: (a) final moisture content and (b) final salt content.
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Figure 12. Liquid and vapor coupling migration fluorescein tracer results for the different fines contents: (a) test condition 6; (b) test condition 7; (c) test condition 8; (d) test condition 9; and (e) test condition 10.
Figure 12. Liquid and vapor coupling migration fluorescein tracer results for the different fines contents: (a) test condition 6; (b) test condition 7; (c) test condition 8; (d) test condition 9; and (e) test condition 10.
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Figure 13. Schematic diagram of road structure layer.
Figure 13. Schematic diagram of road structure layer.
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Table 1. Experimentally determined coarse grain size distributions of the saline soil samples.
Table 1. Experimentally determined coarse grain size distributions of the saline soil samples.
Sieve pore diameter (mm)2~51~20.5~10.25~0.50.075~0.25<0.075
Passing rate/%5.433.455.2516.4755.3514.05
Table 2. Ion content of the saline soil samples.
Table 2. Ion content 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 saline sulfate soil
20.5630.0790.0170.0160.0270.2130.3450.818
Table 3. Experimental conditions for water–salt migration in the coarse-grained saline soil samples.
Table 3. Experimental conditions for water–salt migration in the coarse-grained saline soil samples.
Water Migration ModeExperimental ConditionsInitial Moisture Content/%Fines Content/%
Water vapor migration/liquid and vapor coupling migration1015
21.5
33
44.5
56
665
77.5
810
912.5
1015
Table 4. External recharge in the water vapor migration tests with the different initial moisture contents.
Table 4. External recharge in the water vapor migration tests with the different initial moisture contents.
Test ConditionExternal Water Intake/mL
C1 (initial moisture content of 0%)1564
C2 (initial moisture content of 1.5%)1134
C3 (initial moisture content of 3%)630
C4 (initial moisture content of 4.5%)504
C5 (initial moisture content of 6%)378
Table 5. External recharge in the water vapor migration tests with different fines contents.
Table 5. External recharge in the water vapor migration tests with different fines contents.
Test ConditionExternal Water Intake/mL
C6 (fines content of 5%)848
C7 (fines content of 7.5%)722
C8 (fines content of 10%)602
C9 (fines content of 12.5%)471
C10 (fines content of 15%)378
Table 6. Total external water intake in the water vapor migration and liquid and vapor coupling migration tests under different initial moisture content conditions.
Table 6. Total external water intake in the water vapor migration and liquid and vapor coupling migration tests under different initial moisture content conditions.
Test ConditionExternal Water Intake of Water Vapor Migration/LExternal Water Intake of Liquid and Vapor Coupling Migration/LPercentage of Water Vapor Migration in Liquid and Vapor Coupling Migration/%
C1 (initial moisture content of 0%)1.56423.456.67
C2 (initial moisture content of 1.5%)1.13421.205.35
C3 (initial moisture content of 3%)0.63018.953.32
C4 (initial moisture content of 4.5%)0.50417.302.91
C5 (initial moisture content of 6%)0.37815.202.49
Table 7. Total external water intake in the water vapor migration and liquid and vapor coupling migration tests under different fines content conditions.
Table 7. Total external water intake in the water vapor migration and liquid and vapor coupling migration tests under different fines content conditions.
Test ConditionExternal Water Intake of Water Vapor Migration/LExternal Water Intake of Liquid and Vapor Coupling Migration/LPercentage of Water Vapor Migration in Liquid and Vapor Coupling Migration/%
C6 (fines content of 5%)0.8485.7514.75
C7 (fines content of 7.5%)0.7227.709.38
C8 (fines content of 10%)0.60210.805.57
C9 (fines content of 12.5%)0.47113.303.54
C10 (fines content of 15%)0.37815.202.49
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Yang, H.; Liu, J.; Ma, J.; Wang, Y.; Wang, B.; Zhang, J. Experimental Study on the Influences of the Fines Contents and Initial Moisture on the Water and Salt Migration of Coarse-Grained Saline Soil Subgrades. Sustainability 2024, 16, 11280. https://doi.org/10.3390/su162411280

AMA Style

Yang H, Liu J, Ma J, Wang Y, Wang B, Zhang J. Experimental Study on the Influences of the Fines Contents and Initial Moisture on the Water and Salt Migration of Coarse-Grained Saline Soil Subgrades. Sustainability. 2024; 16(24):11280. https://doi.org/10.3390/su162411280

Chicago/Turabian Style

Yang, Haoyuan, Jie Liu, Jianyong Ma, Yong Wang, Bo Wang, and Jiangpeng Zhang. 2024. "Experimental Study on the Influences of the Fines Contents and Initial Moisture on the Water and Salt Migration of Coarse-Grained Saline Soil Subgrades" Sustainability 16, no. 24: 11280. https://doi.org/10.3390/su162411280

APA Style

Yang, H., Liu, J., Ma, J., Wang, Y., Wang, B., & Zhang, J. (2024). Experimental Study on the Influences of the Fines Contents and Initial Moisture on the Water and Salt Migration of Coarse-Grained Saline Soil Subgrades. Sustainability, 16(24), 11280. https://doi.org/10.3390/su162411280

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