A Unified Two Independent Stress Variable Approach to Moisture-Change-Induced Unsaturated Soil Volume Change

Abstract

In 1968, Matyas and Radhakrishna introduced the concept of the state surface, demonstrating that unsaturated soil volume change is dependent on two independent stress state variables, net total stress and suction. For decades the basic theory of unsaturated soils has been known, and a holistic view of the elastoplastic response of unsaturated soils, based on a modified state surface approach (MSSA), makes it clear that a method accounting for independent roles of net total stress and suction is required to quantify volume change of unsaturated soils. Still, reliance on pre-unsaturated-soil-mechanics-era methods persists, particularly within the geotechnical practice community. Unsaturated soil theory forms the basis for compelling arguments for discarding long-held efforts to classify unsaturated soils as exclusively expansive or exclusively collapsible with respect to volume change response. A more fundamental and unified approach to thinking about volume change of unsaturated soils supports the use of a consistent Stress Path Method to practice-based volume change analyses. Implications of geotechnical engineers’ continued reliance on expansive and collapsible soil classifications, often based on index-based correlations and non-stress-path appropriate laboratory testing, are explored. Recommendations for laboratory testing and modeling for moisture-change induced unsaturated soil volume change are made.