From Particle Gradation to Soil Water Retention and Suction Stress Characteristic Curves of Sands Using the Unsaturated Discrete Spheres Method

The soil-water retention curve (SWRC) and suction stress characteristic curve (SSCC) are fundamental to understanding the hydromechanical behavior of unsaturated soils. However, direct laboratory measurement of these curves is often challenging due to cost, time, expertise, and equipment requirements. Therefore, previous studies have attempted to estimate the SWRC or SSCC through numerical or analytical methods, but these methods have limitations that restrict their accuracy and applicability, particularly regarding particle size variations. This study introduces a new method called the unsaturated discrete spheres method (UDSM) for determining the SWRC and SSCC of unsaturated sands based on particle gradation. Analytical equations are developed to calculate matric suction, volumetric water content, and suction stress for a two-particle system using soil particle gradation characteristics. These expressions are then extended to a representative particle assembly using the discrete element method (DEM). The proposed method’s accuracy is validated against experimental SWRC and SSCC data from 11 different sandy soils. Results show strong agreement between predicted and measured values, demonstrating that the UDSM can reliably capture the hydromechanical response of unsaturated sands. The UDSM offers a practical, physics-based, and scalable solution for estimating SWRC and SSCC when laboratory testing is not feasible, providing a valuable tool for geotechnical analysis and design involving unsaturated soils.