Laboratory soil-column investigation of thermo-hydro-saline migration in saline soil with sand replacement layers under evaporation
GAO Junli1,2, LI Zihao1, LIU Feiyu1,*(), DAI Zili1, GAO Jinbo1, ZHANG Zhen2
1School of Mechanics and Engineering Science, Shanghai University, Shanghai 200072, China 2Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education, Tongji University, Shanghai 200092, China
In arid regions, saline subgrades are highly susceptible to deterioration caused by evaporation-driven water-salt migration, which can induce salt accumulation, cracking, and long-term loss of stability. To investigate the role of sand replacement layers in regulating thermo-hydro-saline (THS) migration under evaporation, we conducted indoor soil-column experiments in combination with microstructural observations. The effects of sand type (coarse, medium, and fine sand) and replacement ratio (0.00%, 20.00%, 35.00%, and 50.00%) were systematically examined under simulated high-temperature and strong-evaporation conditions typical of northwestern China, with continuous monitoring of temperature, relative humidity (RH), and electrical conductivity (EC). The results show that sand replacement effectively inhibited capillary rise, reduced surface salt accumulation, and alleviated shrinkage cracking. Among the tested sand types, coarse sand exhibited the strongest inhibitory effect on upward water-salt migration, whereas fine sand showed the weakest effect because its smaller pores and stronger capillary continuity facilitated upward water-salt migration. Under the medium sand condition, increasing the replacement ratio was associated with stronger suppression of surface salt accumulation, with the 50.00% replacement ratio showing the strongest effect. However, the influence of replacement ratio was not monotonic across all response indicators. A replacement ratio of approximately 35.00% maintained relatively continuous pathways for heat and moisture transfer, whereas higher replacement ratios produced a looser soil skeleton and weaker capillary continuity. Microstructural observations further revealed that salt crystals mainly accumulated near the evaporation front and the lower replenishment zone, while coarse sand tended to form larger pores and reduce matric suction, thereby disrupting upward migration pathways. These findings provide a theoretical basis and technical support for optimizing saline subgrade design and mitigating salt-related damage in arid regions.
Received: 31 December 2025
Published: 30 June 2026
Conceptualization: LI Zihao; Methodology: GAO Junli, LI Zihao; Formal analysis: DAI Zili; Writing - original draft preparation: LI Zihao; Writing - review and editing: LIU Feiyu; Funding acquisition: GAO Junli, LIU Feiyu, ZHANG Zhen; Resources: DAI Zili; Supervision: GAO Junli; Project administration: GAO Junli, LIU Feiyu; Visualization: LI Zihao, ZHANG Zhen; Validation: DAI Zili; Investigation: GAO Jinbo; Data curation: GAO Jinbo. All authors approved the manuscript.
GAO Junli, LI Zihao, LIU Feiyu, DAI Zili, GAO Jinbo, ZHANG Zhen. Laboratory soil-column investigation of thermo-hydro-saline migration in saline soil with sand replacement layers under evaporation. Journal of Arid Land, 2026, 18(6): 1031-1058.
Fig. 2Particle size distribution of the saline soil. d10, d30, and d60 are the particle diameters corresponding to 10.00%, 30.00%, and 60.00% cumulative volume fractions, respectively; Cu, coefficient of uniformity; Cc, coefficient of curvature.
Sample type
Optimum moisture content (%)
Maximum dry density (g/cm3)
Minimum dry density (g/cm3)
Liquid limit (%)
Plastic limit (%)
Internal friction angle (°)
Saline soil
16.20
1.54
1.27
24.10
13.90
26.13
Table 1 Index properties of the saline soil
Sand sample
Particle size range (mm)
Coefficient of curvature (Cc)
Coefficient of uniformity (Cu)
Maximum dry density (g/cm3)
Minimum dry density (g/cm3)
Coarse sand
0.50000-1.00000
2.23
4.48
1.92
1.50
Medium sand
0.25000-0.50000
1.62
3.06
1.85
1.44
Fine sand
0.10000-0.25000
1.35
2.54
1.76
1.32
Table 2 Basic physical properties of river sand
Fig. 3Schematic diagram (a) and photograph (b) of the water-salt migration test
Fig. 4Relationship between electrical conductivity (EC) and salt content. In the fitted equation, x denotes the NaCl solution concentration.
Fig. 5Process flow diagram of the water-salt migration test
Test ID
T1
T2
T3
R1
R2
R3
C
Sand type
Coarse sand
Medium sand
Fine sand
Medium sand
Medium sand
Medium sand
None
Sand replacement ratio (%)
35.00
35.00
35.00
20.00
35.00
50.00
0.00
Table 3 Experimental design
Fig. 6Sampling locations for scanning electron microscopy (SEM) observation within the soil column
Fig. 7Evolution of macroscopic surface crack patterns on the circular soil-column surface under different conditions during evaporation after 1 d (a, d, g, j, m, and p), 3 d (b, e, h, k, n, and q), and 5 d (c, f, i, l, o, and r) heating. The small circular marks visible in some photographs are contact marks caused by microscope positioning and do not affect the identification of macroscopic crack patterns.
Fig. 8Evolution of surface salt crystallization on local areas of soil columns under different sand replacement conditions after 1 d (a, d, g, j, m, and p), 3 d (b, e, h, k, n, and q), and 5 d (c, f, i, l, o, and r) heating. The white particles or patches shown in the images are surface sodium chloride (NaCl) crystals.
Fig. 9Temperature profiles along the soil-column height from the initial state through the first 5 h of heating on Day 1 under different medium sand replacement ratios. (a), initial; (b), 1 h; (c), 2 h; (d), 3 h; (e), 4 h; (f), 5 h.
Fig. 10Variation in soil temperature at a height of 29.0 cm during the first 5 h heating period on Day 1 under different medium sand replacement ratios
Fig. 11Temperature responses at heights of 10.0 (a), 15.0 (b), 20.0 (c), and 26.0 cm (d) measured at the end of each daily 5 h heating cycle over the 5 d test period under different medium sand replacement ratios. In this figure, 0 denotes the initial state before heating, whereas 1-5 denote the measurements recorded at the end of each daily 5 h heating cycle on Days 1-5, respectively.
Fig. 12Temperature profiles along the soil-column height from the initial state through the first 5 h of heating on Day 1 for different sand types. (a), initial; (b), 1 h; (c), 2 h; (d), 3 h; (e), 4 h; (f), 5 h.
Fig. 13Variation in soil temperature at a height of 29.0 cm during the first 5 h of heating on Day 1 for different sand types
Fig. 14Temperature responses at heights of 10.0 (a), 15.0 (b), 20.0 (c), and 26.0 cm (d) measured at the end of each daily 5 h heating cycle over the 5 d test period under 35.00% coarse, medium, and fine sand replacement. In this figure, 0 denotes the initial state before heating, whereas 1-5 denote the measurements recorded at the end of each daily 5 h heating cycle on Days 1-5, respectively.
Fig. 15Variation in relative humidity (RH) along the soil-column height from the initial state to the end of each daily 5 h heating cycle over the 5 d test period under different medium sand replacement ratios. (a), initial; (b), 1 d; (c), 2 d; (d), 3 d; (e), 4 d; (f), 5 d.
Fig. 16Variation in RH at a height of 29.0 cm over Days 1-5 under different medium sand replacement ratios. At the beginning of the test, the RH profile remained close to saturation, with the initial RH being approximately 100.00% throughout the column.
Fig. 17Increase in RH at a height of 29.0 cm during the four non-heating periods under different medium sand replacement ratios
Fig. 18Variation in RH along the soil-column height from the initial state to the end of each daily 5 h heating cycle over the 5-day test period for columns with 35.00% coarse, medium, and fine sand replacement. (a), initial; (b), 1 d; (c), 2 d; (d), 3 d; (e), 4 d; (f), 5 d.
Fig. 19Variation in RH at a height of 29.0 cm measured at the end of each daily 5 h heating cycle over Days 1-5 for columns with 35.00% coarse, medium, and fine sand replacement
Fig. 20Increase in RH at a height of 29.0 cm during the four non-heating periods for columns with 35.00% coarse, medium, and fine sand replacement
Fig. 21Variation in soil EC along the soil-column height from the initial state to the end of each daily 5 h heating cycle over the 5 d test period under different medium sand replacement ratios. (a), initial; (b), 1 d; (c), 2 d; (d), 3 d; (e), 4 d; (f), 5 d.
Fig. 22Variation in EC at a height of 29.0 cm measured at the end of each daily 5 h heating cycle over Days 1-5 under different medium-sand replacement ratios
Fig. 23Variation in soil EC along the soil-column height from the initial state to the end of each daily 5 h heating cycle over the 5 d test period for columns with 35.00% coarse, medium, and fine sand replacement. (a), initial; (b), 1 d; (c), 2 d; (d), 3 d; (e), 4 d; (f), 5 d.
Fig. 24Variation in EC at a height of 29.0 cm measured at the end of each daily 5 h heating cycle over Days 1-5 for columns with 35.00% coarse, medium, and fine sand replacement
Fig. 25SEM images of the soil column with 35.00% medium sand replacement at 10.0 cm (a), 15.0 cm (b), 20.0 cm (c), and 26.0 cm (d). The left and right columns correspond to 500× and 2000× magnifications, respectively.
Fig. 26SEM images at a height of 20.0 cm for soil columns under different replacement conditions. (a), without replacement; (b), 35.00% coarse sand; (c), 35.00% medium sand; (d), 35.00% fine sand; (e), 20.00% medium sand; (f), 50.00% medium sand. The left and right columns correspond to 500× and 2000× magnifications, respectively.
Fig. 27Mechanism of crack formation and salt crystallization in saline soil under evaporation, and typical cracking and salinization distress observed in subgrades of saline soil regions. (a), ground cracking; (b), continuous evaporation in saline soil; (c), soil salinization.
Fig. 28Evaporation front downward shift under continuous evaporation in saline soil subgrade
Fig. 29Schematic diagram of the "liquid bridge" phenomenon. The thin arrows indicate local heat transfer between soil particles and the surrounding water films, while the thick arrow represents the enhanced heat-transfer pathway formed by the liquid bridge.
Fig. 30Schematic diagram of electrical double layer compression
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