Physicochemical Controls on Initiation and Evolution of Desiccation Cracks In Sand-Bentonite Mixtures: X-Ray Imaging and Stochastic Modeling.

The shrink-swell behavior of active clays in response to changes in physicochemical conditions creates challenges for management of agricultural and natural resources as well as for construction of geotechnical barriers for hazardous waste isolation. Initiation and evolution of desiccation cracks in active clays are strongly dependent on the physicochemical factors and boundary conditions. To investigate effects of bentonite content (20, 40, 60%), pore water chemistry (0.05 and 0.5 M NaCl) and drying rates (40 and 60^oC) on cracking behavior, well-controlled dehydration experiments were conducted and X-ray Computed Tomography (CT) was applied to visualize and quantify topological and geometrical features of evolving crack networks, including crack porosity, specific surface area, and crack aperture distribution (CAD). A stochastic model based on the Fokker-Plank equation was adopted to describe the CAD and assess the impact of the physicochemical factors on cracking behavior. The analysis of crack porosity and crack specific surface area showed that both clay content and temperature had larger impact on cracking than pore water concentration. More cracks formed at high bentonite contents (40 and 60%) and at high temperature (60^oC). The drift, diffusion and source terms derived from stochastic analysis indicated that only the temperature effect was clearly recognizable in samples exposed to 60^oC, which led to a faster stabilization of the CAD and a wider range of crack sizes at the end of the experiment.