Influence of Dry Density and Salt Content on Hydraulic Conductivity and Drying Shrinkage

Soil is prone to structural degradation under water infiltration, and the combined effects of dry density and salinity further complicate its hydraulic conductivity and drying shrinkage behavior. However, previous studies have primarily focused on single factors, and the interactive mechanisms between compaction state and salinity remain poorly understood. To investigate the hydraulic conductivity and drying shrinkage behavior of soil under different dry densities and salinity levels, this study examined three dry densities (1.30, 1.35, 1.45 g/cm3) and four NaCl contents (0, 0.5%, 2%, 6%). Saturated hydraulic conductivity (ks) and drying shrinkage were systematically measured. The results indicate that dry density is the primary factor controlling pore structure evolution, ks and drying shrinkage behavior. Increasing dry density markedly reduced porosity (up to 15.95%), ks (by 57.14–92.91%), and drying shrinkage. In contrast, salinity exhibited non-monotonic, density-dependent effects. Salts increased porosity through electrochemical interactions and crystallization-induced pore support, but their effects on ks and drying shrinkage displayed threshold and reversal behavior. These coupled effects demonstrate strong nonlinearity and density dependence, providing a mechanistic basis for compaction optimization and the stability assessment of soil under saline conditions.