Quantification of Grain, Pore, and Fluid Microstructure of Unsaturated Sand from X-Ray Computed Tomography Images

A comprehensive series of three-dimensional x-ray computed tomography (XCT) imaging experiments was conducted to quantitatively assess the multiphase particle- and pore-scale properties of fine Ottawa (F-75) sand. The specimens were prepared to saturations ranging from approximately 5 % to 80 %. Specimens were doped with 10 % CsCl pore fluid solution and imaged using a monochromatic synchrotron x-ray source at energies below and above the Cs x-ray absorption k-edge to allow for high contrast between the solid, liquid, and air phases. Multi-phase properties quantified from the XCT images included individual particle sizes and areas, as well as grain size distribution, pore shape and size distribution, water menisci distribution, solid, liquid, and gas surface areas, and particle contact coordination number. At low saturations, pore water is distributed primarily in the form of pendular rings and liquid bridges located between individual grains and in the smallest pore throats and bodies. A highly discontinuous water phase is evident as a large number of separately identifiable water units having very small volume. As the water saturation increases, the number of individual water units decreases; as expected, the average volume of these units increases significantly as the pore water coalesces into larger and larger units. Results obtained using SEM imaging and conventional geotechnical testing methods for particle-size distribution and soil water retention were compared with those derived from analysis of the XCT images. Results compare very well in each case, typically within a few %. It is shown that the XCT is a reliable and non-destructive method to quantify pore-scale information vital to advance understanding of the hydrologic and mechanical behavior of unsaturated soils at the macroscale.