Observation of autogenous sealing of bentonite clay with X-ray computerized tomography

Abstract

Cracking in clay is frequently encountered in geoenvironmental and geotechnical engineering, sometimes causing failures in foundations, landfill liners and road and railway embankments. At the same time, desiccated clay materials are known to spontaneously heal their surface cracks when rehydrated. The autogenous healing capacity of clay has been typically inferred from changes in its permeability and mechanical strength. However, very little direct observation of crack sealing in real time has been reported in the literature. In addition, knowledge of the mechanisms driving self-healing in clay, and the effects of different variables on these mechanisms remain poor. In this paper, micro X-ray computerized tomography (µ-XCT scanning) is used to observe and quantify changes of crack volume in a bentonite clay as a result of swelling driven by hydration. A vertical crack is artificially introduced into an intact cylindrical sample of consolidated bentonite and the crack closure observed. The cracked zone is segmented and quantified by image processing. The effects on the pace and extent of sealing of sealing time, consolidation pressure and boundary constraints, are considered. Results from these experiments, bearing in mind the specific limitations of X-ray imaging, can contribute to a possible coupled chemo-hydro-mechanical interpretation of autogenous sealing of clay.