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.

Clayey sand is a typical transitional coastal alluvial soil controlled by both coarse-grain friction and fine-grain cementation. Current studies focus mostly on remolded samples, lacking systematic understanding of water-induced structural degradation and quantitative physico-mechanical correlations for natural undisturbed clayey sand. In this work, 74 intact undisturbed specimens (0.5–23.0 m depth) were tested via basic physical tests, one-dimensional consolidation and consolidated-undrained triaxial shear tests. Pearson correlation analysis was performed to establish prediction relationships between routine physical indices and mechanical parameters. Results show the soil is classified as SC clayey sand with 39.70% fines and an average natural water content of 23.17%. Natural water content dominates soil engineering performance, presenting strong linear correlations with dry density and void ratio (|r| = 0.90). Higher water content and void ratio increase compressibility and reduce shear strength. The compression coefficient and compression modulus exhibited a consistent nonlinear relationship, reflecting the inherent linkage between these two compression parameters. Burial depth has little influence on soil properties, and plasticity index only serves for soil classification. Mechanistically, increasing moisture may thicken adsorbed water films, weaken interparticle contact and matric suction, and the fine particle-filled skeleton may further enhance the water sensitivity of the soil. The established prediction models support fast evaluation of soil mechanical behaviors, offering theoretical and practical support for geotechnical design of similar coastal clayey sand strata.