Assumptions of extremes of wetted state of unsaturated soils during infrastructure lifetime (saturated or dry properties) have significant implications for design, construction, functionality, safety, and structural longevity. Assessment of the degree of saturation that occurs in the subsurface requires understanding and quantifying actual surface flux, a complex function of soil surface conditions, including in particular soil suction, degree of cracking, and slope geometry. There are two major related research issues yet to be adequately addressed in geotechnical applications: (1) the effect of surface cracking in volume-change-sensitive clays on unsaturated flow property functions, and (2) the effect of drainage conditions (well-sloped surface versus poorly drained) on surface flux of cracked and intact clays. Seasonal cracking of soil results in poor estimates of runoff and infiltration due to the changing soil storage conditions (Arnold et al, 2005). Prediction of soil suction profiles requires substantial improvements in current capabilities, both from a soil property and numerical modeling perspective. Several conditions present numerical solution challenges: (i) strong nonlinearities in soil properties, (ii) abrupt changes of moisture conditions at the surface boundary and wetting front, and (iii) the presence of surface runoff conditions (Scanlon et. al., 2002). The behavior of unsaturated cracked soil is quite different from that of intact soil, further complicating evaluation of surface flux conditions for clays.
This study addresses key remaining questions rarely, or only superficially, discussed in the geotechnical literature, and is geared toward transformation of surface flux modeling capabilities for cracked and intact clays. The geotechnical research team will work with a co-investigator in Applied Math towards addressing these needs through development of: (1) data and models for unsaturated soil properties of cracked clays including volume change of both the cracks and matrix; (2) data and models for run-off for well-inclined (sloped) and level-grade cracked and intact (3) improved solution methods for surface flux, including run-off, and (4) evaluation of field damage related to drainage and cracking for consistency with modeling and data. A key element of this research is the collaboration and sharing of physical resources among partners (Arizona State University's [ASU] advanced unsaturated soils testing equipment and San Diego State University's [SDSU] unique tilt table).
A wide range of problems arise from unsaturated soils. Damage to infrastructure from expansive clays alone is estimated to be as much as 15 billion/yr (Nuhfer et al., 1993; Wray and Meyer 2004). Krohn and Slossom (1980) estimated that 20% of surface soils of the U.S. are subject to shrink-swell (and cracking). Concerns over movement of contaminants to great depth have heightened interest in understanding unsaturated flow and the complex interactions between climate, human surface activities and unsaturated soil subsurface conditions and processes. Researchers have demonstrated the role of unsaturated soil behavior in rainfall-induced slope failure (Toll, 1999; Yin, 1998). Surface flux and surface runoff are critical to the performance of slopes, and little is known about the influence of cracks on the flux or on slope stability itself. In California in 1982, over 18,000 slides swept down slopes with little warning, damaging homes and killing 14 residents (Ellen and Wieczorek, 1988). U.S. costs for landslide repairs exceed $2 billion/yr and landslides, nearly all rainfall-induced, result in 25-50 deaths/yr (Spike and Gori, 2003). This research will have impact on solutions to all of these problems through enhancing our understanding and modeling of surface flux and unsaturated flow.
Students will be trained and will be engaged in dissemination, including conferences and publications. Findings will be presented in CE classrooms at ASU and SDSU, and integrated into ASU's Math program where students will perform numerical simulations and compare with existing codes and data. On-going ASU recruiting programs focused on teachers and underrepresented students will be used to bring aspects of this study into junior and high school classrooms. A set of lectures will be developed on the research process and presented at a San Diego high school having an 82% female and non-white male student body. Students will also tour labs. Working with pre-engineering teachers, Co-Is will develop a means of attracting students to engineering and research.
National Science Foundation Division of Civil, Mechanical, and Manufacturing Innovation
August 2008 - March 2013