A calculation model for unsaturated permeability and soil–water characteristic curve of sandy soils considering pore distribution

This study develops a pore-scale model to analyze how particle arrangement and variable cross-sectional pore geometry on the hydraulic behavior of unsaturated soils. The particle-size distribution is partitioned into groups, within which pore systems of three, four, and five-particle arrangements were constructed. Using the pressure-difference method for connected channels and the Young-Laplace equation, a calculation framework for the unsaturated permeability coefficient and soil-water characteristic curve was established, explicitly considering soil micropore distribution. Model performance was verified with 18 soil datasets from the Unsaturated Soil Hydraulic Database and further examined through variable-head permeability tests on extremely fine sands with varying silty fine sand and fine-particle contents. Results show that the model reliably predicts the unsaturated permeability coefficient and Soil Water Characteristic Curve (SWCC) for loamy sand, sand, and sandy loam across both high and low water contents when suitable particle arrangement systems are applied. The five-particle system is more effective for high water content in permeability calculations and for coarse-grained soils in SWCC prediction, whereas the three-particle system better represents low water contents and fine-grained soils. The model requires only particle-size distribution, dry density, saturated permeability coefficient, and saturated water content, without introducing additional empirical parameters, thereby offering a concise and practical approach for characterizing unsaturated soil hydraulic properties.