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 considered 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 consolidation stresses 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.
Highlights
Many shales and other clay-rich rocks are considered as natural barriers in geo-engineering applications such as the disposal of nuclear waste (e.g. Sellin and Leupin, 2013)
For Opalinus Clay, which is considered as a potential host rock for nuclear waste, we investigate the failure mode as a function of applied effective stress in laboratory experiments
The non-linearity of the 20 failure envelope is associated with decreasing dilation with increasing effective stress accompanied by changes in microstructural deformation processes, which explain the decreasing friction angle
Summary
Many shales and other clay-rich rocks are considered as natural barriers in geo-engineering applications such as the disposal of nuclear waste (e.g. Sellin and Leupin, 2013). Recent high-resolution microstructural studies on clay-rich rocks deformed under triaxial compression provide insight into the 65 deformation structures and associated mechanisms, which include micro-cracking, grain bending and rotation, as well as particulate flow localised within distinct shear zones (Desbois et al, 2017; Oelker, 2020; Schuck et al, 2020; Winhausen et al, 2021). Winhausen et al (2021) found from microstructural analysis of an Opalinus Clay sample deformed under triaxial compression that brittle and ductile failure mechanisms at grain-scale may coexist, i.e. micro-cracking and bending of phyllosilicates. This coexistence and the effective stress-dependent deformation microstructures found in other shales (Ibanez 70 and Kronenberg, 1993; Petley, 1999) suggest a transitional failure behaviour on the microscale likely due to increasing effective confining stresses. This study attempts to relate the abovementioned processes on the microscale to the hydro-mechanical deformation behaviour of Opalinus Clay under various effective stresses
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