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.

Early methods for unsaturated soils often employing saturated soil effective stress, but with suction assumed to be zero.  Alternatively, total stress analyses, coupled with tests following field stress paths, were used. Particularly with publication of Soil Mechanics for Unsaturated Soils (Fredlund and Rahardjo, 1983), which embraced works of Matyas and Radhakrishan (1968) and Fredlund and Mogenstern (1976), unsaturated soil mechanics began to root. Subsequently, elastoplastic modeling emerged with the Barcelona Basic Model (BBM) (Alonso,et al., 1990).  Zhang and Lytton (2009 to 2012) contributed an elastoplastic framework, the Modified State Surface Approach (MSSA), which established a link between the State Surface Approach of Fredlund and Rahardjo and the BBM. Unsaturated soil models have evolved, but net stress and soil suction have consistently been shown to control shear strength and volume change response and use of these two separate stress state variables in modeling remains unquestioned (Gallipoli, et al., 2018). The net stress and suction-based MSSA, providing a big-picture view from unsaturated to saturated and expansive to collapsible, is used to compare and contrast various models and as an aid to judge the suitability and limitations of early modeling approaches, as well as simplified methods commonly used in routine practice today.