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.

Themed Lecture

A Unified Two Independent Stress Variable Approach to Shrink-Swell and Collapse

From discovery in the early to mid- 1900’s, expansion (or shrink-swell) and collapse responses have been viewed as uniquely separate geohazard events.   Not only are these two volume change responses viewed separately for naturally occurring soil deposits, but the late 1980’s saw geohazards terms of hydrocompression and hydroexpansion introduced for compacted soils.  Although descriptive of actual volume change processes, such added-terms present a picture of a phenomena that is somewhat unusual or unique – perhaps even requiring separate methods of analyses.  However, depending on density, any fine-grained soil with clay content can expand or collapse, or exhibit essentially no volume change, depending on the net total stress and soil suction state and change of state (Justo et al., 1984). Although there are circumstances wherein a deposit can be viewed as strictly expansive or strictly collapsible for engineering purposes, such heavy emphasis on separation of these two volume change responses to wetting is somewhat of a slippery slope that can and has resulted in failures or poor infrastructure performance.
In 1968, Matyas and Radhakrishna introduced the concept of a void ratio state surface, clearly demonstrating that unsaturated soil volume change is dependent on the two independent stress state variables of net normal stress and suction.   For decades now the basic theory of unsaturated soil mechanics, established in terms of two independent stress variables has been known (Fredlund and Morgenstern, 1977; Alonso, et al. 1990; Gens and Alonso, 1992).  A holistic view of the elastoplastic response of unsaturated soils makes it clear that a stress-path based, two independent stress state variable method is required to quantify volume change of unsaturated soils (Houston and Zhang, 2021). Still, reliance on pre-unsaturated soil mechanics era methods persists, particularly within the geotechnical practice community and especially for volume change.
Unsaturated soil theory forms the basis for compelling arguments for discarding long-held efforts to pigeonhole soils into one classification or another (collapsible versus expansive). The goal of this presentation is to encourage a bigger picture, more fundamental and unified approach to thinking about volume change of unsaturated soils.  Implications of the geotechnical engineers’ continued reliance on expansive and collapsible soil classifications, typically based on index-based correlations and non-stress-path appropriate laboratory testing, are explored.