2019 Volume 7 Issue 2 Pages 615-626
In the last years, X-ray computed tomography (CT) has been found to be a valuable tool to visualize and analyze structures in granular media from a microscopic point of view. With a sufficient resolution (voxel size) and a sequence of images over time, CT-images also allow to visualize processes in unsaturated soils, where microscopic features, such as the phase distribution (water-air-solid), the distribution of pores or air clusters as well as the interfaces between the phases, e. g. the interfacial area between the non-wetting and the wetting phase, can be studied. In the experimental program presented here, CT-imaging is applied to obtain 3D-data sets of two different soils in the unsaturated state. The hydro-mechanical behavior of free-standing unsaturated soil columns, only kept together by capillary cohesion, is investigated in two test series. In the first test series, soil columns, made of a coarse to medium coarse sand and of a packing of glass beads, are submitted to evaporation, while their macroscopic degree of saturation is monitored by weighing over time. The evaporation process is virtually stopped at selected time points by sealing the specimen from the outer atmosphere. Then, the specimens are placed in a micro CT-scanner to investigate the unsaturated state changed by evaporation from a microscopic point of view. The distribution of pore water, the change in interfacial area as well as the shape of capillary bridges can be analyzed with focus on the loss of capillary cohesion within the specimen on its way to collapse due to drying. In the second test series, a recently developed miniature compression device is used to perform uniaxial compression tests on free-standing cylindrical unsaturated sand and glass bead specimens. The compression device is also placed in a CT-scanner to obtain 3D-images for different compression stages, i. e. for different axial strain. The measured CT-data of the evaporation and compression tests are finally analyzed with the focus on differences between the hydraulic and mechanical behavior of irregular shaped sand packings and ideal glass bead packings. Furthermore, the microscopic and macroscopic behavior during a change in degree of saturation by evaporation and during mechanical compression can be compared.
