Correlation between shape parameters of particle size distribution and soil water retention curves

Abstract The Soil Water Retention Curve (SWRC) is a critical property for understanding water movement in unsaturated soils, influencing hydraulic conductivity and shear strength. While empirical models like those of Brooks and Corey and van Genuchten are widely used, they often lack a direct connection to soil microstructure. This study introduces shape parameters $$\nu$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>ν</mml:mi> </mml:math> (for particle size distribution, PSD) and $$\mu$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> (for SWRC) to establish a direct correlation between soil grading and water retention. Using the UNSODA database, we analyzed 301 soil samples across various textures and void ratios. Results show a strong correlation between $$\nu$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>ν</mml:mi> </mml:math> and $$\mu$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> , particularly in soils with lower void ratios, where denser packing enhances the influence of PSD on water retention. Coarser soils exhibited stronger correlations, while finer soils showed more complex behavior due to additional hydraulic mechanisms. The proposed empirical formula provides a practical tool for predicting SWRC based on PSD, offering a balance between simplicity and physical relevance. Validation using independent datasets confirmed the model’s robustness for granular soils but highlighted limitations in fine-grained soils, where clay content introduces divergence. This study advances the understanding of soil–water interactions and provides a valuable framework for geotechnical and hydrological applications.