• Title of Presentation

    Stress State Variable Selection and Terminology: Why it Matters

  • Abstract

    A well-accepted concept in unsaturated soil mechanics is that two stress variables are needed to fully represent soil behavior. For modeling unsaturated soils, a Bishop-type equation can be used in conjunction with a second variable, such as suction or modified suction; alternatively modelling of unsaturated soils may be done using a net stress and suction approach. Taking a macro-level perspective, arguments are presented that the net stress and suction approach is a more fitting selection of stress variables for modelling, even where transition from saturated to unsaturated conditions is considered. It is further argued that the fundamental theory of unsaturated soil mechanics should be presented on the experimentally validated controlling stress state variables of net stress and suction. Included is a review of various unsaturated soil effective stress approaches found in the literature. Terzaghi’s saturated soil effective stress is volume change based, but effective stress equations for unsaturated soils are found to categorize into one of four constructs: (1) volume change, (2) yield, (3) shear strength, and (4) degree of saturation. Each category of unsaturated soil effective stress definition is reviewed within an elastoplastic framework using the Modified State Surface Approach. It is shown that single-valued unsaturated soil effective stress equations lead to uncomfortable results, including that volume change and/or yield occurs along constant effective stress paths, and that required  values are stress path and stress history dependent, differ among that various definition categories, and range from positive to negative. Thus, the term “effective stress” for a Bishop-type equation is misleading because use of such equations results in glaring violations of the concept of Terzaghi’s effective stress which is foundational in the field of geotechnical engineering. Mainstream adoption of unsaturated soil mechanics within the geotechnical community, where a certain expectation of the term “effective stress” is entrenched, requires clear communication that two stress variables are required for modelling of unsaturated soils, and that the behavior-controlling stress variables are net stress and suction.

  • Biography

    Sandra Houston is Professor Emerita in the School of Sustainable Engineering and the Built Environment at Arizona State University. Professor Houston’s contributions to the field of geotechnical engineering focus on unsaturated soils and arid region problem soils, including in particular collapsible and expansive soils and unsaturated flow. Sandra has served in numerous leadership positions in the American Society of Civil Engineers (ASCE), Geo-Institute (GI), and the International Society of Soil Mechanics and Geotechnical Engineering (ISSMGE). She is a recipient of the 2017 ASCE Terzaghi Award, the 2004 William H. Wisely American Society of Civil Engineers Award, the 2018 Distinguished Lecturer for the Pan-American Unsaturated Soils Conference series, and the 9th Pedro de Alba lecturer. Professor Houston has also served as president of the Geo-Institute, and chair of the ASCE Board-level Committee on Diversity and Inclusion. She was the formational Chair of the GI Committee on Unsaturated Soils and served for many years as a USA representative and secretary of the TC106 Committee on Unsaturated Soils. within the International Society for Soil Mechanics and Geotechnical Engineering.

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