Suction Stress Characteristic Curve for Unsaturated Soil

TLDR

Suction stress depends on saturation, water content, or matric suction, paralleling the soil–water characteristic curve and hydraulic conductivity function for unsaturated soils. The paper introduces the suction stress characteristic curve (SSCC) for unsaturated soil and proposes that various interparticle forces can be combined into a macroscopic suction stress. The authors use particle‑scale equilibrium analyses to classify interparticle forces—active forces transmitted through soil grains, active forces at or near contacts, and passive counterbalancing forces at or near contacts—and experimentally validate the SSCC by analyzing unsaturated shear‑strength data across various soils, demonstrating its characteristic nature and a method for its determination. Experimental evidence indicates that the SSCC can represent both Mohr–Coulomb and critical state failures and offers a simple, practical way to describe stress state in unsaturated soil.

Abstract

The concept of the suction stress characteristic curve (SSCC) for unsaturated soil is presented. Particle-scale equilibrium analyses are employed to distinguish three types of interparticle forces: (1) active forces transmitted through the soil grains; (2) active forces at or near interparticle contacts; and (3) passive, or counterbalancing, forces at or near interparticle contacts. It is proposed that the second type of force, which includes physicochemical forces, cementation forces, surface tension forces, and the force arising from negative pore-water pressure, may be conceptually combined into a macroscopic stress called suction stress. Suction stress characteristically depends on degree of saturation, water content, or matric suction through the SSCC, thus paralleling well-established concepts of the soil–water characteristic curve and hydraulic conductivity function for unsaturated soils. The existence and behavior of the SSCC are experimentally validated by considering unsaturated shear strength data for a variety of soil types in the literature. Its characteristic nature and a methodology for its determination are demonstrated. The experimental evidence shows that both Mohr–Coulomb failure and critical state failure can be well represented by the SSCC concept. The SSCC provides a potentially simple and practical way to describe the state of stress in unsaturated soil.