| dc.description.abstract | The volumetric deformations associated with expansive soils pose a risk to infrastructure, costing billions in mitigation annually. Many engineering designs assume the soil is in either the dry state or fully saturated. However, clays used as barrier systems and foundations are in the unsaturated state for the majority of the design life. Therefore, it is necessary to understand the mechanism underlying expansive soil behavior, particularly in response to changes in moisture along cycles of wetting and drying. There is substantial evidence demonstrating that the engineering behavior of these clays is controlled by microscopic, physicochemical forces, i.e., the van der Waals attractive force, double layer force, interlayer force, and mechanical contact forces. The physicochemical forces at the microscale that ultimately govern macroscale behavior, however, are not well understood and there is a need for a conceptual framework to quantify changes in fabric as the soil structure interacts with pore fluid. In order to provide better insight into the evolution of soil fabric under pore fluid changes and the controlling microscale mechanisms that cause swelling, two innovative approaches were utilized with the goal of providing a conceptual framework for quantifying microstructural forces and parameters present that affect macroscale behavior: (1) Discrete Element Modeling (DEM) and (2) Fractal Theory.
Discrete element modeling was employed to provide a method to better predict expansive soil swell pressures. The DEMClay program was adapted to consider physicochemical forces including the van der Waals attractive force, double layer repulsive, and mechanical contact forces in addition to microscale parameters (cation exchange capacity, specific surface area, Hamaker constant) to model microscale forces of expansive clays. The model was calibrated with a single mineral Na-montmorillonite and validated using natural specimens of montmorillonite-bearing soils with widely variable cation exchange capacity (CEC) and specific surface area (SA) parameters. DEMClay provided accurate predictions of swell pressures and can be employed by practitioners with knowledge of a few key soil parameters.
In addition to implementing a valid model to predict swell pressures of single-mineral and natural soils using physicochemical parameters, concepts from fractal geometry were used to provide the missing quantitative element describing surface features and structure of clay particles. Fractal geometry offers a numerical understanding of the patterns of fabric, structure, and pore networks found on high resolution environmental scanning electron microscopy (ESEM) images of expansive single-mineral clays and natural soils at specific densities and suction. Quantitative assessments of microstructural changes were made using fractal geometry parameters, including fractal dimension and lacunarity, where the fractal dimension describes the complexity of the soil surface and lacunarity describes the texture, or pattern of voids of the fabric. For example, fractal dimension was lower for single-mineral clays than natural clays but lacunarity was higher for the artificial clays than the natural clays. These parameters allow engineers to better understand the evolution of surface complexity of expansive clays with varying mineralogy and physicochemical properties under cycles of hydrologic hysteresis and can be employed as parameters in models focused on predicting expansive soil behavior.
This research advances the understanding of unsaturated expansive soils through analyzing microscale mechanisms including surface complexity, pore fluid hysteresis and physicochemical forces. The outcome of this study provides engineers with a model that can accurately predict macroscale behavior of expansive soils, e.g., swell pressures, based on void ratio, intrinsic physicochemical parameters like SA and CEC, mineralogical parameters, and suction state and discusses methods in which this model can be further improved. It also details a novel way in which to consider surface complexity of clay minerals through fractal geometry that provides quantitative relationships with many important soil parameters and helps explain the evolution of behavior through suction states. | en_US |
