ABSTRACT Digital Image Correlation techniques are applied to sequences of images of argillite rocks samples at various saturation states at both the global scale of the sample and the local scale of the microstructure. Not only it is confirmed that the clay matrix deforms much more than the other mineral inclusions, but it also appears that the deformation is very inhomogeneous in the matrix, with some areas almost not deformed, while others exhibit deformation twice the average overall strain, depending on the local distribution of the inclusions. Strain heterogeneities are also detected at the scale of the sample of saturated rocks and can be correlated with the presence of a network of cracks. On such samples, DIC analysis shows that the overall strain results both from the bulk deformation of the sound rock and the closing or opening of these cracks, which is confirmed by the analysis of acoustic emissions.
1 INTRODUCTION One of the major objectives of the hydro-mechanical behaviour studies of nuclear-waste storage in deep geological formations is to assess their stability and performance under variable hygrometry atmospheres. These perturbations may induce damage (opening of existing or new cracks) in the drift wall, and change the confinement properties of the rock. In France, an underground research laboratory is built in the argillite layers of Callovo-Oxfordian age in the Eastern Paris Basin (Meuse/Haute-Marne site, France). Argillite rock is sensitive to the presence ofwater in terms of mechanical response due to the pore state and the interaction of water and the clay phase. The moisture transfer between atmosphere and rock induces deformation (swelling and shrinkage) at the interface. To understand the multi-scales-effects of the moisture conditions on argillite rocks, an experimental program was conducted to determine the effects of water saturation on the global physical and mechanical properties. Micromechanical investigations aim at detecting the actual physical deformation and damage mechanism active at a microscopic scale, the complex averaged interactions ofwhich determine the macroscopic behaviour of materials. They allow a physically-based extrapolation of experimental data to situations not accessible to experiment, by means of adequate scale transition models. Such an approach might be very useful for the determination of the long term hydro-mechanical behaviour of argillite rocks. Qualitative observations of these micromechanisms by means of mechanical tests inside the chamber of a scanning electron microscope (SEM), as those currently used for the analysis of metals and composites, cannot however be transposed directly to geomaterials, especially because the exposition of the samples to vacuum may noticeably modify the physical properties of the material (microstructure, water content…). To circumvent these difficulties, direct optical observations of samples with controlled degrees of saturation and mechanically loaded on conventional testing machines, have been preferred. By means of specifically designed optical microscopes, appropriate lighting devices and digital image correlation techniques very small evolutions of the microstructures can be detected.