Fault creep behavior and the frictional response of the Opalinus Clay formation

Abstract:

High-level nuclear waste disposal in deep geological formations is critical for several countries. In Switzerland, the Opalinus Clay (OPA) formation has been studied as an effective natural barrier by the Mont Terri Rock Laboratory, an underground research facility used for research purposes only. The OPA formation is a shale that has favorable hydro-chemo-mechanical barrier properties for the retention of radionuclides. However, faults intersecting the OPA formation have the potential to facilitate the migration of radionuclides through the formation via fault reactivation and induced earthquakes.

To better understand fault reactivation in the OPA formation, we study the creep and frictional response of wet samples of its three subunits: 1) the clay-rich shaly (~50% clays, ~25% quartz, and ~15% calcite), 2) the quartz-rich sandy (~25% clay, ~50% quartz, and ~10% calcite), and 3) the carbonate-rich (~20% clays, ~35% quartz, and ~35% calcite) facies.

In this work, creep experiments were carried out using HighSTEPS, a new state-of-the-art biaxial apparatus used for the study of earthquake mechanics. We performed shear experiments on synthetic fault gouge samples (<125 m) of the three units. The samples were first saturated for 1 hour at 0.5 MPa normal stress and then loaded to a normal stress of 8.5 MPa. At this point, we waited 1 hour for sample consolidation. After, we sheared the samples at 1 m/s for 5 mm of displacement. At this point friction (µ) values were obtained. Shearing was then stopped for 30 min to achieve the relaxation stress. After this initial step, a differential shear stress was stepped by a small increment of 0.3 MPa for 30 min long, thus the sample can deform again until a new, higher creep strain rate. We repeated the cycle until fault acceleration is observed.

Our results show different values of friction for the three units: µ_shaly=0.24 µ_sandy=0.43 and µ_carb=0.47. Creep strain rates evolve with different stress levels and are variable for one sample to another highlighting the effect of clay content on the creep response of the different subunits of the OPA formation. Further, for nuclear waste storage, fault creep can be of importance in association with the static stress changes that can occur during tunnel construction and/or long-term thermal-induced stress changes adjacent to faults.