Frictional Properties of Opalinus Clay: Influence of Humidity, Normal Stress and Grain-size on Frictional Stability

We designed frictional experiments to characterize the effect exerted by humidity, grain size and normal stress on frictional behaviour of the Opalinus clay fault gouge. We explored a wide range of normal stresses, ranging from 5 to 70 MPa performing velocity up-steps from 1 to 300 μm/s and slide-hold-slide from 1 to 3000s. Our experiments confirms that the OPA clay is weak, with friction coefficients at steady-state of ~0.35 and ~0.41, for 100% RH and 25% RH experiments, respectively. The OPA clay is velocity strengthening over the entire range of applied normal stress. We observe a direct relationship between frictional parameter (a-b) and slip velocity up to 35 MPa where, from there on, (a-b) parameter seems to be velocity independent. As evidenced by the microstructural analysis, we suggest that this behaviour is due to the progressive transition with normal stress, from strain localization and grain size reduction to distributed deformation on well-developed phyllosilicate networks. The amount of relative humidity does not affect deformation mechanisms (i.e. localized or distributed), whereas decreases fault strength and increases fault stability. We hypothesize that this is due to a possible interplay of OPA clay swelling and lubrication, caused by the weakening of chemical bonds between phyllosilicate foliae.  Notably, the initial grain size (< 63 µm or 63 < g.s. < 125 µm) does not affect either the frictional strength or stability, with similar values of dilation upon velocity up-step. Collectively, our mechanical and microstructural observations have allowed us to build a conceptual model that summarizes the main mechanical features of the OPA clay fault gouge. In the context of deep geological repositories (DGR), our results confirm that slow aseismic slip is the most likely slip behaviour for a fault gouge hosted in the OPA clay, with similar mineralogical composition and clay fabric as our samples. Beyond the context of deep geological repositories, this study has also implications for carbon capture and geological storage in the deep subsurface. Indeed, OPA has the characteristics of a low permeability caprock, but faulted, and the integrity of a sealing caprock overlying a storage reservoir can evolve after fault reactivation, potentially generating undesired seismicity and new hydraulic pathways.

The data are uploaded are structured as follow:

1) A .txt file of the datafile that is recorded from the machine (raw data)

2) A file in .txt format containing the elaborated data (data_rp)  

The data are analyzed using rawPy that can be found at https://github.com/marcoscuderi/rawPy

For any additional information please do not hesitate to contact the corresponding author Nico Bigaroni at [e-mail]