Failure mode transition in Opalinus Clay: a hydro-mechanical and microstructural perspective

Abstract

Abstract. The way rocks deform under changing stress conditions can be described by different deformation modes, which is fundamental for understanding their rheology. For Opalinus Clay, which is regarded as a potential host rock for nuclear waste, we investigate the failure mode as a function of applied effective stress in laboratory experiments. Therefore, we performed consolidated undrained triaxial tests at different effective consolidation stresses from 2.5 to 16 MPa, in which samples were loaded parallel to bedding, and analysed the deformation structures using ion-beam polishing and electron microscopy. With increasing effective confining stress, the results show a transition from brittle-dominated to more ductile-dominated deformations, localising in distinct shear bands. Both effective stress paths and microstructural analysis indicate a tendency towards less dilation in the shear zones for higher effective stresses. Triaxial test results suggest a non-linear failure envelope. The non-linearity of the failure envelope is associated with decreasing dilation with increasing effective stress accompanied by changes in microstructural deformation processes, which explain the decreasing friction angle. For the first time, we can verify that the observed non-linear failure envelope is due to the gradual transition from brittle- to more ductile-dominated deformation on the microscale controlling the bulk hydro-mechanical behaviour of Opalinus Clay.