Sandra L. Houston - US grants

The objective of this work is: (1) to identify the most suitable method of sampling and of sample preparation for collapsible soils found throughout the Southwest U.S.; and (2) to develop and test a relatively simple procedure (modified Jennings & Knight odometer test) for determining the extent of wetting of soils in the field.

The purpose of the project is to develop a down-hole in-situ loading plate test under various wet conditions, which will provide data that can be used to anticipate settlement damage to structures resting on collapsible soils; and the development of a simplified analysis technique which can be used efficiently by practicing engineers. The workplan includes: 1) development of hardware; 2) development of testing procedures; 3) development of procedures for extrapolating the test data to the prototype; and 4) checking the system by applying it to a medium-scale field test.

Higher, thicker landfills with steeper side slopes raise legitimate concerns about slope stability and liner integrity, particularly during earthquakes. The most important landfill properties for seismic safety are moduli and shear strength. The objective of this research is: a. to investigate techniques for characterization of landfills and other highly nonhomogeneous materials containing large aggregate; b. develop correlations between seismic velocity measurements and landfill shear strength measurements, and use these correlations to develop three- dimensional profile of shear strength variation within a landfill section; c. apply the methodology to one existing landfill.

9307787 Houston When collapsible soils area wetted they fail and produce differential settlement which may damage structures. The purpose of the project is to: further refine a down-hole in-situ test developed in a previous project, so it can be used inside the hollow stem auger; develop techniques for predicting the extent and intensity of wetting due to accidental wetting; enhance the understanding of the flow mechanisms when water infiltrates an initially dry soil; and develop techniques for combined wetting and mud jacking for the mitigation of collapsing soils under existing structures. The results of this research will improve the basic understanding of the behavior of collapsible soils under certain conditions and this new knowledge will directly benefit the geotechnical engineering practice. ***

It is recognized that our infrastructure to a very large extent is founded on unsaturated soils. In fact, construction in unsaturated soils is typically preferred when practical, due to reduced costs and effort. However, the geotechnical research community has not yet solved all of the issues of implementation of unsaturated soil theory. One of the primary difficulties in bringing the relatively new fundamental theories of unsaturated soil mechanics into engineering design, analysis, and construction is the difficulty, time, and cost associated with unsaturated soil characterization. Two major elements are needed to bring unsaturated soil mechanics and fundamental analyses into practice: (1) estimates or measurements of soil suction, and (2) moduli or coefficients relating soil suction to performance parameters such as soil shear strength and compressibility. In this research, a range in sophistication of approach will be employed for unsaturated soil characterization. Hierarchical Level 1 will be developed for extremely high-level, large budget projects for which significant expenditures are likely. Hierarchical Level 4 will be developed for lower budget, routine projects for which very limited testing and analysis are common. This research is an important step in bringing unsaturated soil mechanics into present day geotechnical engineering and construction practice. The implementation of unsaturated soils theory would result in more fundamentally sound analyses and design and would lead to substantial savings in construction costs for many applications.

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 (ASU?s advanced unsaturated soils testing equipment and SDSU?s 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.

Current engineering practice for determining the volume change behavior of unsaturated expansive soils are mostly based on simplified tests, and correlations with index properties. Such practices can lead to uneconomical and distress prone foundation designs. Hence, there is a fundamental research need to review the current characterization practices in expansive soils and to revise them to reflect the current state of knowledge of unsaturated soil mechanics. The research team from both UT Arlington and Arizona State University are working towards accomplishing this research need by performing various experimental and modeling tasks. Experimental tasks include investigations on eight types of expansive subsoils to determine Soil Water Characteristic Curves (SWCCs) over a wide range of soil suction, measure volumetric swell strain and swell pressure with consideration of clay mineralogy, moisture content, dry unit weight, and stress history. Modeling tasks include development of analytical formulations to predict swell characteristics using SWCC attributes, clay mineralogy, pore void data, and study flow and deformation based finite element modeling to simulate volume change behavior. The final outcome of this research is the development of methods for better predictions of expansive soil properties using unsaturated soil mechanics principles.
The broader impacts of the research will focus on training graduate and undergraduate students in unsaturated soil mechanics and expansive soil characterization; incorporating research findings into undergraduate and graduate courses; strengthening understanding of volume change behavior of expansive soils and providing better characterization tools for design of infrastructure foundations on expansive soils; involving women and minority students in research tasks; and teaching simple experimental modules in summer classes to K-12 students to enhance their interests in civil engineering.

Most of our physical infrastructure is built on unsaturated soils, yet for both technical and historical reasons, educators continue to reserve the teaching of unsaturated soil mechanics to the graduate level curriculum. Billions of dollars are at risk annually from moisture-sensitive soil damage to infrastructure. At the other extreme, billions of dollars are wasted annually due to overly conservative structures that are designed based on an assumption of saturated soil properties. Improving student understanding of unsaturated soil behavior could result in large cost savings in both the public and private sectors. In addition, a recent survey indicates that a majority of geotechnical faculty think that unsaturated soils mechanics should be introduced at the undergraduate level. Students' depth of understanding of soils as an engineering material would be greatly enhanced via introduction of geotechnical principles for unsaturated conditions, with the saturated soil case being presented as a subset of the broader theory. This project involves developing student-centered lecture and laboratory modules on the basic principles of unsaturated soils theory and the application of these principles to problems of movement of structural foundation systems. The modules emphasize solving geo-hazard problems of collapsible and expansive soils. Emerging geotechnical challenges in the energy sector are used as additional motivational examples. These modules are being pilot-tested on students at seven universities, with a large and diverse set of students to ensure that the modules are not biased toward a particular subset of students.

Approximately 320 million hectares of the world's land is covered with expansive clays that are subject to shrinkage and swelling in response to changes in soil moisture conditions. Damage to U.S. public infrastructure from expansive clays is estimated to be $15 B/yr. For every billion dollars in damage from unconservative design, perhaps $10B is lost from overly conservative design. Expansive clays are unsaturated soils in that the void space between clay particles is filled partly with air and partly with water. However, common approaches to dealing with expansive soils do not fully consider the unsaturated soil aspects that drive infrastructure damage. This award supports the development of an innovative approach to bringing unsaturated soil mechanics theory into wide use in dealing with infrastructure development on expansive clay profiles. Due to the magnitude of expansive soils problems, and the involvement of 10s of thousands of practitioners addressing these problems, this study will provide societal benefit through saving billions of dollars annually on U.S. infrastructure. The project has numerous outreach components, including cross-training between researchers and practicing engineers that will improve engineering education. Participation of underrepresented students and returning-veteran students will broaden the impact of this study through development of more diverse and well-trained work force.

A dual approach will be taken for estimation of expansive soil swell-shrinkage movements, wherein a fundamentals-based soil suction and net normal stress approach will be linked to a surrogate-based approach. The method will embrace principles of unsaturated soils theory, yet expand applicability through well-founded yet commonly-measured soil suction surrogate parameters. Climatic effects (including site-specific 'microclimate') will be incorporated into surrogate parameter determination, consistent with recent research on soil surface-atmosphere interactions. This research fills a gap between theory and the vast amount of available field data. A fundamentally-based soil suction surrogate will facilitate use of the huge database of soil parameters available from past expansive soil investigations. Simple functions of water content and index properties will be used - in lieu of soil suction - to evaluate expansive soil movements for partial wetting or drying under field net normal stress conditions. The primary goal is to develop a soil suction surrogate approach that yields exactly the same, or essentially the same, expansive soil movement results as the complete-stress-state based suction approach, and which allows geotechnical engineers to take advantage of their vast experience from a more fundamental perspective.