Frictional Properties of Opalinus Clay: Implications for Nuclear Waste Storage

Abstract

The kaolinite‐bearing Opalinus Clay (OPA) is the host rock proposed in Switzerland for disposal of radioactive waste. However, the presence of tectonic faults intersecting the OPA formation put the long‐term safety performance of the underground repository into question due to the possibility of earthquakes triggered by fault instability. In this paper, we study the frictional properties of the OPA shale. To do that, we have carried out biaxial direct shear experiments under conditions typical of nuclear waste storage. We have performed velocity steps (1–300 μm/s) and slide‐hold‐slide tests (1–3,000 s) on simulated fault gouge at different normal stresses (4–30 MPa). To establish the deformation mechanisms, we have analyzed the microstructures of the sheared samples through scanning electron microscopy. Our results show that peak (μpeak) and steady state friction (μss) range from 0.21 to 0.52 and 0.14 to 0.39, respectively, thus suggesting that OPA fault gouges are weak. The velocity dependence of friction indicates a velocity strengthening regime, with the friction rate parameter (a − b) that decreases with normal stress. Finally, the zero healing values imply a lack of restrengthening during interseismic periods. Taken together, if OPA fault reactivates, our experimental evidence favors an aseismic slip behavior, making the nucleation of earthquakes difficult, and long‐term weakness, resulting in stable fault creeping over geological times. Based on the results, our study confirms the seismic safety of the OPA formation for a nuclear waste repository.

Contrasting Mechanical and Hydraulic Properties of Wet and Dry Fault Zones in a Proposed Shale‐Hosted Nuclear Waste Repository

“…We infer that these stick slips would be expected due to the potential aging of contacts, that is, fault restrengthening, associated with frictional healing processes with slip, such as ploughing, shear-enhanced indentation, shear-enhanced compaction, or a combination of the above (Marone & Saffer, 2015). This observation is also supported by frictional healing rates (β) previously measured in the same OPA gouge samples during slide-hold-slide tests (Orellana et al, 2018b). This observation is also supported by frictional healing rates (β) previously measured in the same OPA gouge samples during slide-hold-slide tests (Orellana et al, 2018b).…”

“…Previous frictional experiments performed on OPA gouge at 100% relative humidity, that is, partially saturated condition (Orellana et al, 2018b), exhibited μ f values between the results of this study (Figure 3b), which were higher than our values of wet friction, that is, μ f, wet < μ f, RH100% < μ f, dry . Taken together, our results highlight the strong effect of water on the frictional weakness of faults hosting OPA, suggesting that small pore fluid or stress field perturbations could result in their activation.…”

“…For comparison, we have included the Byerlee's law and previous results of frictional experiments carried out at 100% room humidity condition(Orellana et al, 2018b). (a) Friction at 2-mm axial displacement and (b) shear stress versus effective normal stress for wet and dry samples.…”

Fault Stability Perturbation by Thermal Pressurization and Stress Transfer Around a Deep Geological Repository in a Clay Formation

“…From the excavation and the experiments performed at the Mont Terri underground research laboratory, it is known that an old, quiet, tectonic fault is present in the Opalinus Clay formation. The fault existence is in apparent contrast with the Opalinus Clay formation properties that should present velocity‐strengthening behavior (Orellana et al, ) and thus impeding the nucleation of a seismic rupture and the formation of such a structure. The aseismic behavior of a fault cutting through a clay formation has seen additional confirmation by the fluid injection experiment performed directly in the Opalinus Clay fault zone at the Mont Terri underground laboratory (Guglielmi et al, ).…”

“…Fault material properties are derived from the recent laboratory investigations performed by Orellana et al (): Their results show that a Mohr‐Coulomb criterion can be fitted to their observations, assuming a friction angle of about 20° and a cohesion value of 0.35 MPa for material under a compressive normal stress of up to 4 MPa. In our model the friction angle is assumed to be strain neutral, while the cohesion drops to 0 as soon as the failure conditions are reached.…”