One-year monitoring of desiccation cracks in Tournemire argillite using digital image correlation

Abstract

Digital image correlation (DIC) was used for the first time in an underground gallery to monitor desiccation cracks during an annual climatic cycle. This experimental in situ investigation was carried out on a study area of 344×275mm², located on the East96 gallery front at the Tournemire experimental station, during which the relative humidity and temperature were continuously measured for more than one year, from March 2011 to March 2012.Our results demonstrate the ability of the non-invasive DIC method to monitor clay–rock strains for at least four months, and to monitor the opening and closing of desiccation cracks for more than one year. Moreover, our study provides the following phenomenological results. As observed in the laboratory, the hydric strains were anisotropic; the strains perpendicular to the desiccation cracks were almost homogeneous and much larger than those parallel to the same cracks. The changes in crack apertures calculated from the displacement fields (at an accuracy of approximately 26.9µm) and the strain fields were clearly correlated and concomitant with changes in relative humidity and temperature (with 25%<RH<99% and 6°C<T<14°C). Contrary to direct measurements acquired at the Mont-Terri site, the crack apertures of the desiccation cracks were reversible after one year of data acquisition. Moreover, although the main desiccation cracks were sub-horizontal and associated with the direction of bedding planes, our work demonstrates the existence of sub-vertical cracks. The qualitative interpretation of our entire dataset has emphasized the need for a multi-scale approach to understand the desiccation cracking mechanisms at the scale of an underground facility.