Frictional properties of Opalinus Clay: influence of humidity, normal stress and grain size on frictional stability

International audienceThe Opalinus Clay (OPA) is a clay-rich formation considered as a potential host rock for radioactive waste repositories and as a caprock for carbon storage in Switzerland. Its very low permeability (10 −19 to 10 −21 m 2) makes it a potential sealing horizon, however the presence of faults that may be activated during the lifetime of a repository project can compromise the longterm hydrological confinement, and lead to mechanical instability. Here, we have performed laboratory experiments to test the effect of relative humidity (RH), grain size (g.s.) and normal stress on rate-and-state frictional properties and stability of fault laboratory analogues corresponding to powders of OPA shaly facies. The sifted host rock powders at different grain size fractions ( 35 MPa is dictated by a transition from localized to distributed deformation and (2) water vapour content does not affect the deformation mechanisms and dilation, whereas it decreases steady-state friction (μ ss), and enhances fault stability