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

The Opalinus Clay (OPA) is an argillaceous formation that is being considered as potential host rocks for repositories of radioactive waste in Switzerland. If on the one hand its low permeability makes it an excellent sealing horizon, on the other hand the presence of faults inside the formation, and the potential for fluid-induced slip, can compromise its long-term hydrological isolation. We have performed an experimental investigation aimed at exploring the frictional properties of OPA under a variety of boundary conditions such as humidity, grain size and normal stress (5 to 70MPa), in double-direct shear configuration. We observe that first order variation in friction coefficient both for peak friction peak and steady-state friction ss are controlled by both the presence of water and the applied stress. Experiments performed at 100% humidity exhibit lower ss (~0.35) compared to the room humidity ones (~0.41). The increase in normal stress promotes a strong decrease for peak friction and less for ss friction. Samples with a lower initial grain size show higher friction values when compared to higher initial grain size. The analysis of velocity-steps in the light of Rate- and State-friction framework shows that the stability parameter a-b is always positive (velocity-strengthening) and it increases with increasing sliding velocity. At the same time, the critical slip distance Dc decreases with increasing sliding velocity. Both a-b and Dc are systematically higher for experiments conducted at 100% humidity and decrease with normal stress. We also observe a velocity-enhanced dilation for all the experimental conditions, especially strong at low normal stresses. Taken together these results suggest that: 1) humidity decrease steady state friction ss and enhance fault stability. 2) Grain size causes small second order variation of friction, which does not change the general behavior of the frictional parameters. 3) Rate of change of friction with velocity is of the same sign of velocity-enhanced dilation, indeed we observe a linear relationship between a-b and dilation. In light of microstructural observation we build a micromechanical model to relate the deformation mechanisms and the work of dilation to predict the observed evolution of frictional properties.