Laboratory X-ray tomography: A valuable experimental tool for revealing processes in soils

The use of high-resolution tomographic techniques has allowed for unprecedented observations and a renewed understanding of geomaterials and processes. A laboratory x-ray scanner is used to explore the potential of the technology in the context of complex geotechnical systems. Tests benefit from the fast and non-destructive nature of x-ray measurements and the micrometer-scale resolution that is attainable. Several first-time observations are reported here. In this paper we demonstrate the following: subsurface volume loss in sandy soils can cause the formation of sharply defined low-density pipes; cryogenic suction consolidates sediments next to ice lenses during ground freezing; root growth involves transverse expansion, and the stress relaxation at the tip facilitates further longitudinal invasion; blade insertion causes successive shear localizations; and the incipient formation of desiccation cracks is not necessarily along a planar front - in fact, the fracture plane may split as it encounters heterogeneities at the tip. Finally, it is shown that x-rays can be used to monitor chemical processes that cause coupled mechanical effects, such as osmotic consolidation induced by ionic diffusion and mineral dissolution. Although brief events may not be tomographically imaged, single x-ray radiographs can be analyzed and compared to gain extensive process information.