Unsaturated soils play an essential role in a variety of natural earth processes and engineered earthen systems. The pore water in an unsaturated soil system forms a complex fabric consisting of saturated pockets of water under negative pressure and a network of liquid bridges formed near the particle contact points. Water influences bulk soil behavior by modifying intergranular stress through negative pressure in the saturated pores, and by providing an intergranular bonding force through the liquid bridges. The magnitude and relevance of each mechanism, however, is highly dependent on the pore water fabric, which is readily altered with changes in suction, saturation, wetting direction, external stress, and global or localized deformation. It is evident that changes to unsaturated soil microstructure under mechanical or hydraulic loading will influence macroscopic soil behavior, but the difficulties associated with its characterization have limited development of microstructure-based frameworks for predicting macro soil response. This collaborative project seeks to observe and quantify the multiphase fabric of unsaturated soils by making use of recent advances in non-destructive imaging techniques. Microfocus X-ray computed tomography will be integrated with a series of special loading stages designed to image the microstructure of unsaturated sand specimens under controlled suction and stress conditions and over a wide range of saturation and strain. Images will be analyzed to characterize salient features of the multiphase fabric, including 3D grain orientation, particle contact normals, liquid bridge configurations, and the distribution of liquid- and gas-saturated voids. Tensors describing these features will be quantified and their evolution tracked as specimens are subject to controlled changes in suction, wetting direction, compression, and shear. Grain size, density, anisotropy, suction, confining stress, and strain rate will be treated as experimental variables. Microstructural observations will be integrated into a new constitutive framework for unsaturated soil behavior that explicitly accounts for elements of the solid, liquid, and gas fabric. The research will work to resolve the links between unsaturated soil microstructure and macroscale response, and will implement them through a new constitutive platform for predicting engineering behavior. Observations of fabric evolution with hydraulic and mechanical loading will provide direct evidence to address the bottleneck issues that currently limit our predictive understanding of unsaturated soil behavior, including wetting-drying hysteresis, coupling between suction, saturation and deformation, liquid bridge rupture, dilation, and rate effects. Understanding multiphase interactions in packed particles with wetting fluids is also critical to other scientific fields that deal with physical phenomena such as filtration, drying, pharmaceutical and ceramic agglomeration, and oil recovery. Teaching and diversity will be enhanced through graduate and undergraduate student involvement and educational module development, including activities targeted specifically for women and minorities at the University of Missouri and Washington State University.
