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

Abstract

Summary The 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 m2) 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 long-term 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 (< 63 μm and 63 < g.s. < 125 μm), at room (∼25 per cent) and 100 per cent humidity, were slid in double-direct shear configuration, under different normal stresses (5 to 70 MPa). We observe that peak friction, μpeak, and steady-state friction, μss, depend on water vapor content and applied normal stress. Increasing relative humidity from ∼25 per cent RH (room humidity) to 100 per cent RH causes a decrease of frictional coefficient from 0.41 to 0.35. 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 and humidity. The dependence of (a-b) on slip rate is lost as normal stress increases, for each humidity condition. By monitoring the variations of the layer thickness during the velocity steps, we observe that dilation (Δh) is directly proportional to the sliding velocity, decreases with normal stress and is unaffected by humidity. Microstructural analysis shows that most of the deformation is accommodated within B-shear zones, and the increase of normal stress (σn) promotes the transition from strain localization and grain size reduction to distributed deformation on a well-developed phyllosilicate network. These results suggest that: (1) the progressive loss of velocity dependence of frictional stability parameter (a-b) at σn > 35 MPa is dictated by a transition from localized to distributed deformation; (2) water vapor content does not affect the deformation mechanisms and dilation, whereas it decreases steady-state friction (μss), and enhances fault stability.

Hematite Frictional Behavior and He Loss From Comminution During Deformation Experiments at Slow Slip Rates

“… (a) Calculated values of a‐b as a function of μ for hematite from velocity step experiments and other mineral assemblages (1—SAFOD (gouge, wet), Carpenter et al., 2012; 2a—Zuccale Cal‐trem‐clay (foliated rock, room humidity), 2b—Zuccale Cal‐trem‐clay, (gouge, room humidity), 2c—Zuccale Hbl‐bt (gouge, room humidity), Collettini et al., 2011; 3a—Smectite (gouge, room humidity), 3b—Illite shale (gouge, room humidity), Saffer & Marone, 2003; 4a ‐ Chlorite schist (gouge), wet, 4b—Amphibolite (gouge, wet), Fagereng & Ikari, 2020; 5—Opalinus clay (gouge, dry to room humidity), Bigaroni et al., 2023; 6—Calcite/Marble (gouge, wet), Carpenter et al., 2016; 7—Westerley granite (rock, room humidity), Kilgore et al., 1993). Red box is hematite avg ± std.…”

“…In contrast, most of the comparative experiments were conducted on unconsolidated gouge (except Kilgore et al., 1993, and some experiments from Colletini et al., 2011), and many were deformed at water‐saturated conditions (e.g., one experiment from Bigaroni et al. (2023), and experiments from Carpenter et al. (2012, 2016) and Fagereng and Ikari (2020)).…”

“…We note that our experiments were conducted at room humidity on rock surfaces that progressively developed gouge. In contrast, most of the comparative experiments were conducted on unconsolidated gouge (except Kilgore et al, 1993, andsome experiments from Colletini et al, 2011), and many were deformed at water-saturated conditions (e.g., one experiment from Bigaroni et al (2023), and experiments from Carpenter et al (2012Carpenter et al ( , 2016 and Fagereng and Ikari (2020)). For a given material, wet and dry bare surfaces, and wet and dry gouge yield μ values that can differ by 0.4 over a range of normal stresses (e.g., Colletini et al, 2011;Collettini et al, 2009;Mitchell et al, 2016).…”

Importance of Water‐Clay Interactions for Fault Slip in Clay‐Rich Rocks

“…3.1 and 3.2, the stress state at initial H 2 O contact was below the wet frictional slip envelope, suggesting that the alteration of the frictional strength cannot be the only explanation for initial slip and that the effective stress at the saw‐cut interface cannot be fully described by the measured mechanical data (Figures 10b and 10f). Possibly, the build‐up of swelling stress has to be considered in order to represent the stress conditions at the OPA‐BER interface (Bigaroni et al., 2023; Paterson & Wong, 2005; C. L. Zhang, 2017, 2018).…”

“…Hence, the hydro‐mechanical properties of the OPA have been widely investigated. This includes, among others, studies concerning mechanical creep (Naumann et al., 2007; Schulze, 2011), compressive strength (Minardi et al., 2021; Schuster et al., 2021; Winhausen et al., 2022), dry and wet frictional properties (Bigaroni et al., 2023; Orellana et al., 2018b, 2019; Schuster et al., 2023), swelling strain, stress, and its anisotropy (Bossart & Thury, 2008; Minardi et al., 2016; Thury & Bossart, 1999; C. L. Zhang et al., 2010), and the closure of cracks by self‐sealing processes (Bock et al., 2010; Fang et al., 2017; Voltolini & Ajo‐Franklin, 2020; Wenning, Madonna, Kurotori, et al., 2021; C. L. Zhang, 2011). A stress concept for claystones such as OPA has been established, showing that an externally applied load is carried by the bound water of clay particles, that is, by the immobile water of the electrical double layers, and that the OPA may be considered as a colloidal system (Horseman et al., 1996; Rodwell et al., 1999; C. L. Zhang, 2017, 2018; C. L. Zhang et al., 2010).…”