Characterization of the failure in Callovo-Oxfordian argillite under plane strain compression using full field measurements

Document available in extended abstract form only. Failure by strain localization is commonly observed in geo-materials. Cracks are detected in Underground Research Laboratories (URL) on the walls of galleries (e.g., Andra in Meuse/Haute Marne laboratory, SCK-CEN in Mol), they have been induced by the stress relaxation that results from the excavation. The cracks represent material discontinuities which could impact the flow properties of the rock mass in some circumstances. Generally, experimental characterization of the localization in rocks is performed with classical axisymmetric triaxial compression tests. The effect of the confining pressure can be observed on several aspects: onset of localization, pattern of localization, porosity evolution inside bands or cracks aperture, grain scale mechanisms of deformation (e.g., [1]). However the history (time evolution) of the localization is not accessible because the observations are post-mortem. Strain field measurement and evolution in time of the strain field are particularly useful to study the strain localization (initiation of the deformation bands) and the post-localization regime. Such tools have been developed for soils (e.g., sand specimens in plane strain condition [2] or in triaxial conditions using X-ray tomography [3]). Similar developments for rocks are still difficult, especially because the pertinent confining pressure to reproduce in situ stresses are higher than for soils. We present here first results obtained in a new true triaxial apparatus that allows observation of the rock specimen under loading and especially the complex development of deformation bands and faults. As for [4] and [5], the three principal stresses are different. However, for selected tests discussed here, the intermediate stress is controlled in order to impose a plane strain condition (zero strain in this direction). Observation of a specimen under load is possible as one surface of the prismatic specimen, which is orthogonal to the plane strain direction, is in contact with a hard transparent window. The deformation of this surface is representative of the deformation in the whole specimen, up to and beyond strain localization. Therefore the evolution of the strain field in the sample can be measured by digital image correlation (DIC) of photographs taken of this surface. It is noted that the cell can sustain a confining pressure up to 100 MPa and so the regime of deformation in soft rocks from brittle through to ductile can be studied. Moreover, the stress path that is imposed to the specimen is not an axisymmetric state and is more realistic with respect to the paths followed in the field. In the selected results, we analysed the process of failure in the Callovo-Oxfordian argillite. First, strain localization under the form of shear bands propagate through the specimen, and then cracks born and propagate in the middle of the band. Complex patterns of cracks, with sets of conjugated cracks are observed in some case. The effect of the mean stress applied during tests on the failure is also observed. The kinetic of crack propagation is studied quantitatively, using the displacement field measured by DIC. The displacement gap on cracks is decomposed in a sliding component and a opening. The mechanical response of argillite specimens is also presented, with the link between post peak response (stress softening) and localization. (authors)