Effect of Salt Solution on Water-Retention and Microstructure Characteristics of Ningming Expansive Soil

The pore solution in expansive soil contains numerous chemical components that can significantly affect the soil’s water-retention properties, strength, and deformation. This study focuses on Ningming expansive soil and investigates the effects of varying concentrations of NaCl solutions on its water-retention characteristics. The soil–water characteristic curve of expansive soil over the full suction range was obtained using the pressure plate method. The microstructure of expansive soil was analyzed using mercury intrusion porosimetry and scanning electron microscopy. The results indicate that the water-retention capacity of expansive soil increases with higher concentrations of NaCl at equivalent suction levels. MIP tests demonstrated that, regardless of changes in pore solution concentration, the interparticle pores in pre-consolidated samples consistently dominate the pore structure, while agglomeration pores play a secondary role. Importantly, variations in the pore solution concentration primarily alter the characteristics of interparticle pores without significantly affecting their overall structure. SEM analysis revealed that the microstructure of pre-consolidated samples exposed to different pore solution concentrations exhibited less development than that of compactable samples, with a notable reduction in macropores. Furthermore, the arrangement of soil particles became increasingly uniform, and the stratification within the soil matrix was more pronounced. In addition, the Brooks–Corey (BC) model and van Genuchten (VG) model were employed to fit the measured data. It was found that the air entry values predicted by the two models were closely aligned with the measured data; therefore, it is recommended to utilize the average value as the air entry value corresponding to the changes in pore solution concentration of pre-consolidated Ningming expansive soil.