Application of x-ray computed tomography to study initiation and evolution of surface cracks in sand-bentonite mixtures

Sodium saturated bentonite is an expandable montmorillonitic clay that is commonly used as major constituent of earthen liners at hazardous and municipal waste disposal facilities. However, questions remain as to the long-term durability of clay-based barriers because of interactions between clay and waste leachate and exposure of clay to life-cycle stresses such as desiccation and freeze-thaw actions.;Several field and experimental studies have been conducted to investigate formation of desiccation cracks in expansive soils. However, many of these studies, were destructive in nature, limited to two-dimensions, and/or used boundary conditions that were not clearly defined. Furthermore, none of the projects considered the dynamic evolution of desiccation cracks with time.;In this dissertation research project well-controlled dehydration experiments with clearly defined boundary conditions were conducted to investigate the effects of bentonite content (20, 40, 60%), pore water chemistry (0.05 and 0.5 M NaCl) and drying rates (40 and 60°C) on initiation and evolution of desiccation cracks. In addition, potential impacts of polypropylene fiber additives on crack formation in sand-bentonite mixtures were investigated. Swelling and permeability tests were conducted by means of volume-change measurements and fully-automated flexible wall permeametry. X-ray computed tomography (CT) in conjunction with a specifically developed MATLAB code were employed for automated image processing and to visualize and quantify the topological and geometrical crack features (crack-porosity, -specific surface area, and -size distribution). Due to the crucial importance of the segmentation step prior to image analysis, which was widely ignored in previous studies, several global and locally adaptive segmentation techniques were evaluated to find the optimum method. A stochastic model based on the Fokker-Planck Equation (FPE) was adopted to characterize the crack aperture distribution (CAD).;The analysis of the crack porosity and specific surface area showed that all examined physicochemical factors significantly influenced the formation of desiccation cracks. In particular, bentonite content and temperature had the greatest impact with the largest cracks being observed in samples with higher clay contents (40 and 60%) and exposed to 60°C temperature. The terms extracted from the FPE indicated that temperature had a significant impact on cracking in that samples exposed to 60°C showed a faster stabilization of CAD and, by the end of the experiment, a wider range of apertures. Results for fiber-reinforced samples clearly indicated a dramatic reduction of desiccation cracks for all observed sand-bentonite mixtures. The results of the swelling and permeability tests indicated that the physicochemical factors had a major impact on the hydraulic properties of all observed mixtures.