The mechanical behaviour of weak mudstone (Opalinus Clay) at low stresses

Over the last 10 years argillaceous rocks (mudrocks), such as the Jurassic claystone Opalinus Clay, have gained acceptance as a potential host formation for geological disposal of nuclear waste. Observations from tunnels in Opalinus Clay at the Mont Terri rock laboratory, Switzerland, suggest that the excavation-induced response is not linear-elastic. The micro-structure of the Opalinus Clay has been identified as a source for this response. It has been demonstrated that diagenetic processes over the last 180 Ma have been responsible for the formation of strong diagenetic bonds locking latent strain energy into the micro-structure of Opalinus Clay. A conceptual model of the micro-structure, including the effects of diagenetic processes, has been developed as a framework to interpret the mechanical behaviour of the Opalinus Clay. Samples recovered at Mont Terri rock laboratory have been subjected to processes that have resulted in bond breakage and the release of locked-in latent strain energy. This tends to reduce the strength and deformation properties of samples and induces low stiffness non-linearity of the stress–strain curve at low stress levels. Numerical modelling of the sampling stress-path at Mont Terri rock laboratory shows that unloading was the dominant factor resulting in this response. As a result, samples can shed light on the unloading response of the rockmass to excavation. This non-linear behaviour has been quantified by means of routine laboratory compressive tests.