Long-term mechanical behavior of claystone

For decades, claystone is investigated as a likely host rock material for the deep geological disposal of highly radioactive waste. To ensure safe operation, knowledge about the long-term (thermo-hydro-) mechanical behavior of clay rock is of significant relevance: How will the excavation induced differential stresses gradually dissipate while causing convergence? Laboratory triaxial tests on the mechanical behavior allow for the controlling of major and minor principal stress (differential stress), pore pressure (effective stress), saturation, temperature and loading history. Long-term deformation tests running at low differential stresses reveal gradually declining deformation rates (consolidation + primary creep). With time, the deformation process asymptotically approaches an apparently constant deformation rate at constant volume (secondary creep). At higher differential stresses, accelerated tertiary creep occurs, associated with dilatant deformation and forming of new microcracks, leading to final failure of the specimen. In claystones the primary consolidation process is slow and comparatively well understood: due to low hydraulic conductivity pore fluid is slowly squeezed out, at the same time rearrangement of clay particles and micro grains takes place leading to changes in the microstructure. Moreover, rearrangements in the grain skeleton with a variable deformation rate can occur at constant effective pressure. This creep process is hard to determine and still poorly understood. Only very long creep tests give the possibility to detect secondary creep and to distinguish it from primary consolidation processes. Creep tests run over several months/years on clays from Mont Terri give examples of the settlement mentioned and different types of creep.