International audienceAn experimental study of the swelling anisotropy of the Callovo-Oxfordian argillaceous rock under hydration is presented. The investigation, which combines environmental scanning electron microscopy (ESEM) and digital image correlation techniques, has been carried out at the micrometric scale of the composite microstructure of the rock. Specimens were hydrated in the ESEM over a wide range of relative humidity and observations conducted on two planes: plane 1 parallel to the bedding plane, and plane 2 perpendicular to it. The observations reveal that the local swelling (which can be quantified at a local gauge length of about 5 µm) is strongly anisotropic in both planes. The global swelling, measured over areas of about 500 µm in width, is also clearly anisotropic in plane 2 (with major swelling direction normal to the bedding plane), but not in plane 1. The global isotropy in plane 1 arises from the uniform distribution of the orientation of anisotropic local strains, while the anisotropic swelling in plane 2 is due to a preferred local orientation. © 2013 Mineralogical Society

Abstract. In order to investigate the micromechanical mechanisms of argillaceous rocks under combined mechanical load and varying moisture, and observe very low strain rates of about 10 -10 s -1 , i.e. about 10 -4 per week, a specific optical setup for DigitalImage Correlation (DIC) has been developed at the Solids Mechanics Laboratory, Ecole Polytechnique. This paper mainly presents two procedures to optimize the experimental setup and enhance the full-field strain measurement accuracy. The first one allows us to minimize the systematic DIC errors by choosing an optimal optical setup, including appropriate lens aperture and light source with small wavelength, in combination with a specific grey level interpolation method (bilinear, bicubic, biquintic) in DIC algorithms. The second procedure aims at reducing the errors induced by overall out-of-plane motions of the sample. This procedure makes use of a specifically designed optical microscope, which tracks the few tens of micrometers out-of-plane motions of the sample all along the several months lasting test, to compensate the magnification fluctuations which disturb the macroscopic measurements. The current optimized optical setup gives a better than 10 -5 accuracy at macro scale (3 mm gage length) and 10 -4 accuracy at microscale (about 100 µm gage length).

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