Experimental Study on the Deformation Behaviour of Sandy Opalinus Clay

Abstract:

Clay-rich rocks play an important role in several geoengineering applications, representing suitable cap rocks for the geological storage of carbon dioxide and potential host rocks for the storage of nuclear waste. Due to their diagenetic history, physical and mechanical properties of clay-rich rocks are influenced by a distinct anisotropy, complex mineral composition and rock fabric. Studying the mechanical properties of argillaceous rocks is therefore of great geoscientific interest in order to extend our knowledge of the relevant processes leading to damage and failure across relevant scales and boundary conditions. We investigated the mechanical behaviour of anisotropic Opalinus Clay (OPA), the envisaged host rock formation for nuclear waste in Switzerland. Our study focused on the compositionally heterogeneous sandy facies, characterized by an irregular lamination of quartz-rich lenses with clay-rich interlayers. Using a Paterson-type deformation apparatus, samples prepared at 0°, 45° and 90° to bedding orientation were deformed throughout undrained constant strain rate experiments. Triaxial tests were performed at dry conditions at varying confining pressures (p_c = 50 - 100 MPa), temperatures (T = 25 - 200 °C) and strain rates (ε ̇ = 1*10^-3 - 5*10^-6 s^-1). The influence of water content was investigated at fixed conditions of p_c = 50 MPa, T = 100 °C and ε ̇ = 5*10-4 s^-1, using re-saturated samples. Deformed samples exhibit semibrittle deformation behaviour with strain localization in sub-millimetre wide shear zones. Electron microscopy reveals dominantly cataclastic mechanisms with minor crystal plasticity at the tested conditions. Strength and elastic properties display a weak anisotropy. Samples deformed 45° and 90° to bedding are more compliant and less rigid compared to samples deformed 0° to bedding. Confining pressure and water content are the predominant factors influencing the mechanical behaviour, whereas variations in temperature and strain rate showed minor influence. Here, we found that the compositional and fabric related heterogeneity is more pronounced than the influence of experimental conditions. For example, strain localization and formation of shear zones is mainly influenced by the amount and extend of clay-rich layers, rather than by the thermodynamic boundary conditions.