Microstructure-Based Effective Stress Formulation for Unsaturated Granular Soils

Abstract

The principle of effective stress states that the strength and volume change behaviors of soil are governed by intergranular forces expressed in terms of a continuum quantity called effective stress. Although the principle of effective stress is regarded as one of the most fundamental concepts in soil mechanics, its applicability to unsaturated soil has been questioned. The central issue is whether a measure can be developed for three-phase soils that plays an equivalent role as the effective stress does for two-phase soils. Combining the techniques of microstructural analysis and image processing, this study formulated the effective stress for unsaturated granular soils. A novel suction-controlled experimental setup was integrated with an X-ray computed tomography (CT) scanning system and used to image and model microstructural features. A tensorial quantity, called the fabric tensor of the liquid phase, that characterized the complex fabric resulting from saturated pockets and networks of liquid bridges was identified and introduced in the proposed formulation. The trend in the variation of the fabric tensor of the liquid phase as a function of suction (or saturation) was captured for both the wetting and drying paths of the partial saturation. It was observed that the fabric tensor of the liquid phase had an intrinsic association with the evolution of the effective stress tensor. It is concluded that, for unsaturated granular soils, consideration of the fabric tensor of the liquid phase is imperative in effective stress formulations.