This grant provides funding for fundamental study on the capillary effects in a volume of partially saturated clayey soils. Specifically, the magnitude and molecular sources of soil suction, and curvature and contact angles of air-water capillary meniscus between clay particles are of interest. The studies on these topics will provide answers to outstanding fundamental questions concerning the sources of very high suction in clayey soils (as high as 100 times the atmospheric pressure) and the sources of hysteresis of soil-water characteristic relation. The study will be conducted with the aid of numerical and experimental methods. In regard to the numerical methods, a particle-based modeling technique known as the discrete element method and an atom-based modeling technique known as the molecular dynamics will be used. Concerning the experimental methods, the pressure-plate suction testing method, Fourier Transform Infrared spectroscopic method and Environmental Scanning Electron Microscopic method will be employed. The numerical studies are computationally intensive. The difficulty of obtaining unlimited and readily-available computational power has hindered such computationally intensive studies in the past. The recent developments in the use of Graphic Processing Units (GPUs) for computational purposes make possible having teraflop-computational power (basically a parallel supercomputer) right on an office workstation at an affordable price. The confluence of the above recent developments (the advancements in discrete element modeling, molecular dynamics and GPU-based computing) provides an opportunity for tackling the challenging fundamental research of interest in this study.
If successful, the study will lead to fundamental scientific knowledge and methodologies needed for designing geotechnical structures involving unsaturated fine-grained soils. For example, the study will lead to rational methods for controlling landslides occurring on partially saturated mountainous slopes triggered by torrential rain and/or earthquakes.
A large part of the natural earth contains unsaturated soils – soils whose pores are only partially saturated with a fluid like water. When such a soil becomes saturated (i.e., when the pores are completely filled with a fluid), the behavior changes drastically, leading to, in some cases, catastrophic failures. For example, when an unsaturated hill becomes saturated with water during a torrential rain, a landslide could be triggered. An important part of controlling the behavior of structures involving unsaturated soils is quantitative prediction. There are several advanced numerical models for this purpose; e.g., the finite element method. A key ingredient to such predictive models is mathematical model describing its constitutive behavior called the constitutive models. In the case of unsaturated soils, the key component of the constitutive models is a relationship known as the Soil Water Characteristic Curve (SWCC). Development of this relationship, including its significant features such as the very high suction at low saturations and hysteretic behavior, requires full understanding at the microscopic level (particle and nano-scale levels). In the current study, the modeling techniques including the molecular dynamics (MD) and discrete element method (DEM) were further refined and developed to explore the fundamental behavior of the unsaturated soils. Using the molecular dynamics, the following goals were achieved: (a) understanding of the meniscus between two partially-saturated clay particles, (b) quantitative information on meniscus curvature and contact angle, and (c) forces between two partially-saturated clay particles. The outcomes of these studies were then used in DEM to investigate and develop specimen-level SWCC along with hysteresis. Intellectual Merit: While researchers have forwarded hypotheses on why the measured suction is very large in some cases and why a hysteretic soil-water relation is observed, the hypotheses largely remain unverified to this day. This is due to the fact that, in clayey soils, suction and hysteresis are essentially molecular phenomena and the only sure way of studying them is to use molecular level methods. MD is one such technique and DEM allows bridging the scales. The proposed integrated study has solved some outstanding issues in unsaturated soil mechanics, and brought modern numerical methods to the forefront of unsaturated soil analyses. Broader Impact: One of the key factors governing soil suction is capillary, which is of interest in many fields of science besides soil physics and geotechnical engineering. Using the NSF-REU support, two undergraduates (one male and one female) were engaged in research through this project. Some graduate-level courses were modified to incorporate materials that resulted from the study. In particular, materials on molecular dynamics, discrete element method and the new findings on the mechanisms of swelling were added.
