MICROSTRUCTURAL ANALYSIS OF UNSATURATED GRANULAR SOILS USING X-RAY COMPUTED TOMOGRAPHY
Manahiloh, Kalehiwot Nega
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This study presents microstructural characterization of partially saturated granular soils using X-ray computed tomography (CT). In the funicular and pendular saturation regimes of unsaturated soils, pore-water assumes a complex fabric consisting of saturated pockets of water under negative pressure and a network of liquid bridges. Characterization of this complex fabric is essential to the development of effective stress relationship for partially saturated soils. A novel experimental setup for real time monitoring of microstructure inside an X-ray computed tomography system is designed with modifications to the Tempe type cell and hanging water column method. The experimental apparatuses are integrated with X-ray CT scanner to generate three dimensional images of partially saturated soil microstructure under controlled saturation direction and suction. Analytical approaches are combined with digital image processing techniques to quantify microstructural parameters of interest. Algorithms and macros are developed to quantify the degree of saturation and the fabric tensor of the liquid phase from X-ray CT images. A standalone image processing program is developed as an integral part of the scanning and image pre-processing routines. A new formulation for effective stress in partially saturated soil is developed based on virtual work principles incorporating explicitly a tensor measure that characterizes the complex fabric resulting from saturated pockets and networks of liquid bridges. When the soil becomes fully saturated, the formulation reduces to the Terzaghi's effective stress. The fabric tensor of the liquid phase, however, is observed to vary throughout the saturation and desaturation processes. It is therefore essential that the new formulation, coupled with the experimental methods proposed here, is needed to fully describe the effective stress in partially saturated soils.