Zhang Jianxun, Mao Xuesong
Highway Academy, Chang'an University, Xian, 7100000, People's Republic of China.
Sci Rep. 2023 Nov 23;13(1):20542. doi: 10.1038/s41598-023-47985-x.
Moisture migration in unsaturated soils is a result of the interaction between temperature and soil microstructure. In order to reveal the mechanism of moisture increase of subgrade soils under diurnal cycle conditions, a series of macro and microscopic tests were carried out on the unsaturated silty clay and sand soil, including liquid-vapor mixed migration tests simulating a one-dimensional subgrade, environmental scanning electron microscope (ESEM), and matrix suction test. Then, the soil microstructure in microscopic images was investigated using the particle (pores) and cracks analysis system (PACS). Next, the relationship between the thermal effects of the soil-water characteristic curve (SWCC) and changes in soil microstructure was analyzed. Finally, the change mechanism of liquid-vapor mixed migration based on the change in soil microstructural under thermal effects was analyzed. The results showed under the diurnal cycle, both the silty clay and sand soil columns appeared in the phenomenon of a "diurnal cycle of water vapor migration", which led to moisture accumulation at the top of the soil layer. In silty clay soil column, moisture was primarily driven by water vapor pressure and migrated upwards. Additionally, moisture redistribution led to changes in soil microstructure, which in turn influenced the process of moisture migration. The moisture content in the upper soil layer increased making both inter-aggregate and intra-aggregate pores decrease. The moisture content in the lower soil later decreased, leading to the water-holding capacity of the lower soil layer to increase. So, the moisture migration gradually decreased at night. In the sand soil column, moisture migration was mainly driven by gravity potential and migrated downwards. Moisture redistribution made inter-aggregate pore and matrix suction of the upper soil layer increase, leading to an increase in moisture migration at night.
非饱和土中的水分迁移是温度与土壤微观结构相互作用的结果。为揭示昼夜循环条件下路基土水分增加的机理，对非饱和粉质黏土和砂土进行了一系列宏观和微观试验，包括模拟一维路基的液-气混合迁移试验、环境扫描电子显微镜（ESEM）试验和基质吸力试验。然后，利用颗粒（孔隙）和裂缝分析系统（PACS）对微观图像中的土壤微观结构进行研究。接着，分析了土水特征曲线（SWCC）的热效应与土壤微观结构变化之间的关系。最后，基于热效应下土壤微观结构的变化分析了液-气混合迁移的变化机理。结果表明，在昼夜循环作用下，粉质黏土柱和砂柱均出现了“水汽迁移昼夜循环”现象，导致土层顶部水分积聚。在粉质黏土柱中，水分主要受水汽压力驱动向上迁移。此外，水分再分布导致土壤微观结构发生变化，进而影响水分迁移过程。上层土壤层中的含水量增加，使得团聚体间孔隙和团聚体内孔隙均减小。下层土壤层中的含水量降低，导致下层土壤层的持水能力增加。因此，夜间水分迁移逐渐减少。在砂柱中，水分迁移主要受重力势驱动向下迁移。水分再分布使上层土壤层的团聚体间孔隙和基质吸力增加，导致夜间水分迁移增加。
