Effective Stress in Unsaturated Soil: Accounting for Surface Tension and Interfacial Area

A pore‐scale model for water retention and the specific air–water interfacial area is developed to quantitatively assess the contribution of interfacial forces to effective stress in coarse‐grained unsaturated soils. The simulated air–water interfacial area for several sand‐sized materials shows non‐monotonic behavior as a function of saturation, with the peak interface area between 10 and 25% saturation. Comparisons are made between suction stress characteristic curves calculated with and without accounting for interfacial forces calculated using surface tension and the simulated interfacial area. The contribution of interfacial forces to effective stress is most significant at low saturation ( S < ~0.2) and for wetting processes because the air–water interface area is largest. The percent difference between suction stress for drying processes calculated with and without accounting for interfacial forces is zero at 100% saturation, negligible for saturation greater than ~70%, increases to ~10% at 50% saturation, and reaches a maximum of ~50% at 10% saturation. During wetting, the difference increases to as much as 140% at 10% saturation. While these differences can be appreciable for some geotechnical problems, the practical implications of neglecting interfacial forces in effective stress formulations for the mechanical behavior of unsaturated soil depend on the range of saturation, suction stress, and normal stress encountered in typical field environments.