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

Abstract

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.

Full text

Geophys. J. Int. (2023) 233, 211–228 https://doi.org/10.1093/gji/ggac457
Advance Access publication 2022 November 22
GJI Rock and Mineral Physics, Rheology
Frictional properties of Opalinus Clay: influence of humidity, normal
stress and grain size on frictional stability
Nico Bigaroni ,1Marco Maria Scuderi,1Fr´
ed´
eric Cappa,2Yves Guglielmi,3
Christophe Nussbaum,4Luca Aldega ,1Giacomo Pozzi5and Cristiano Collettini1,5
1Dipartimento di Scienze della Terra, Sapienza Universit`
a di Roma, 00185 Rome RM, Italy. E-mail: nico.bigaroni@uniroma1.it
2Universit´
eC
ˆ
ote d’Azur, CNRS, Observatoire de la Cˆ
ote d’Azur, IRD, G´
eoazur, 06560 Valbonne, France
3Hydrogeology Department, Lawrence Berkeley National Laboratory, Energy Geosciences Division, Berkeley, 94720 CA, USA
4Federal Office of Topography, Swissotopo, Mont Terri Consortium, St-Ursanne, Switzerland
5Istituto Nazionale di Geofisica e Vulcanologia, INGV, Roma, Italy
Accepted 2022 November 18. Received 2022 November 17; in original form 2022 July 28
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–
70 MPa). We observe that peak friction, μpeak and steady-state friction, μss, depend on water
vapour 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 (h) 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)atσn>35 MPa is dictated by a transition from localized to distributed deformation and
(2) water vapour content does not affect the deformation mechanisms and dilation, whereas it
decreases steady-state friction (μss), and enhances fault stability.
Key words: Fault zone rheology; Friction; Dynamics and mechanics of faulting; Microstruc-
tures; Rheology and friction of fault zones; Opalinus Clay; Geological repositories.
1 INTRODUCTION
The Opalinus Clay (OPA) is a clay-rich formation that is being
considered as potential host rock for radioactive waste repositories
(Bossart et al. 2018) and as a caprock for carbon storage (Zappone
et al. 2021). To avoid contamination of air and underground water,
the goal of a geological repository is to guarantee a permanent
confinement and isolation for high-level nuclear waste, as well as
for the CO2injected at depth (Tsang et al. 2015). OPA can act as a
fluid barrier because of very low permeability (10−19 to 10−21 m2;
Orellana et al. 2019), adsorption properties (clay minerals are well
suited to retain radionuclides, Bossart et al. 2018), and self-sealing
behaviour by swelling capacity of certain clay minerals (Bock et al.
2010; Voltolini & Ajo-Franklin 2020).
C
The Author(s) 2022. Published by Oxford University Press on behalf of The Royal Astronomical Society. This is an Open Access
article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which
permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. 211
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212 N. Bigaroni et al.
The hydrogeological, geochemical and geotechnical characteri-
zation of OPA is conducted at the Mont Terri Laboratory (MTL),
an underground research facility located at about 340 m depth in
the northern part of Switzerland (Thury & Bossart 1999;Nussbaum
et al. 2011). The laboratory is situated within the Opalinus Clay, in
the southern limb of the Mont Terri anticline and its tunnels cross-
cut a 1.5–3 m thick non-active fault zone namely the ‘Main Fault’
(Nussbaum et al. 2018; Guglielmi et al. 2020; Fig. 1a). The fault
zone contains a variety of structures such as fractures, calcite veins,
undeformed rock, bedding planes, scaly clay and OPA Clay-fault
gouge (Nussbaum et al. 2011;Bossartet al. 2018; Laurich et al.
2018; Fig. 1c).
In the context of geological repositories, the presence of faults
represents a critical element that has to be taken into account dur-
ing the site characterization process of a storage formation (Tsang
et al. 2015). Faults can act as a possible pathway for fluids, favour-
ing the premature loss of CO2or radioactive material dissolved in
the water in the form of radionuclides, by migrating in the sur-
rounding aquifer, and thus polluting the environment (Wang et al.
2001;Tsanget al. 2012,2015). The risk of any damage to the
Opalinus Clay formation caused by the mechanical re-activation of
faults must be evaluated during the diverse life stages of the geolog-
ical repository. During the repository construction phases, recent
studies highlight how new mining activities can cause mechanical
instability on nearby fault zones (Hopp et al. 2022). Similar ex-
cavation damages can be expected all through the operational and
closing phases of underground galleries (Nussbaum et al. 2011;
Popp et al. 2008). In the context of carbon capture and storage
(CCS), a potential risk factor is associated with the reactivation of
ancient tectonic fault systems during injection operations of large
volumes of CO2(Rutqvist et al. 2016). Therefore, even of mod-
est magnitude, damage due to induced seismicity can compromise
the long-term confinement and containment capabilities of the OPA
formation (Bossart et al. 2018). Thus, understanding the frictional
and hydromechanical properties of faults within OPA Clay it is of
relevance for geological carbon storage and nuclear waste disposal.
Previous laboratory studies have focused on exploring how scaly
clays and the clay-fault gouge control the mechanical behaviour of
the faults within the OPA formation and therefore also of the ‘Main
Fault’ (Fang et al. 2017,2018; Orellana et al. 2018a,2018b,2019).
Laboratory friction experiments at subsurface conditions and sub-
seismic slip velocities showed that scaly clays have either velocity
strengthening or velocity weakening behaviour that might have the
potential to control and nucleate seismicity within the OPA forma-
tion (Orellana et al. 2018a). The OPA Clay-fault gouge, however,
shows a velocity strengthening behaviour that would favour aseis-
mic slip, resulting in stable fault creeping over geological timescales
(Fang et al. 2017,2018; Orellana et al. 2018b). Recently, Orellana
et al. (2019) observed that, under laboratory conditions of controlled
water saturation (dry and wet), low effective normal stresses and
subseismic velocities, OPA Clay-fault gouge upon velocity down-
step has the tendency of shear-enhanced compaction coupled with
a decrease in permeability (similar observations in other materials
by Zhang & Cox 2000;Segallet al. 2010; Scuderi et al. 2017a;Im
et al. 2018).
Decametric scale in-situ fluid injection experiments performed
directly within the ‘Main Fault’ structure were able to reactivate
the ‘Main Fault’ with small displacement (i.e. from tens to about
a hundred of microns) and associated seismic events (Guglielmi
et al. 2020; Jeanne et al. 2018; Cappa et al. 2022). During these
in situ experiments, by measuring the pressure of the fluids and
displacement signals directly from the fault, it was possible for
the Authors to estimate the evolution of fault-parallel displacement
(i.e. slip) and fault perpendicular displacement (i.e. dilation) and to
measure how the fault permeability evolved as a function of time
and pressure increase (Guglielmi et al. 2020). Similar results have
been observed from laboratory experiments aimed at replicating
fault reactivation by fluid injection at similar boundary conditions
of the MTL, in shale-bearing (Scuderi & Collettini 2018) and lime-
stone fault gouges (Cappa et al. 2019). Taken together these recent
experiments (both in situ and in the laboratory) show that, after the
reactivation by fluid injection of a critically stressed fault (sensu
Walsh & Zoback 2016), the slip starts as an accelerating aseismic
creep coupled with fault dilation. As the slip and fluid injection
continue further, the frictional sliding is mainly controlled by a rate
strengthening behaviour that promotes stable aseismic slip.
In this paper, we present a comprehensive study of the rate-and-
state frictional properties of OPA Clay-fault powder considered a
gouge analogue, over a wide range of normal stress (5–70 MPa),
grain size (<63 μm and 63 <g.s. <125 μm) and relative hu-
midity conditions (∼25 per cent). Our main goal is to improve our
understanding on how these variables influence fault strength and
frictional stability upon reactivation of OPA Clay. The influence of
grain size and applied normal stress above 30 MPa on the OPA Clay
friction are experimental conditions that have not been tested pre-
viously, as well as the quantification of dilation following velocity
up-steps. We expand the boundary conditions of normal stresses
above those expected to occur at typical depths of geologic reposi-
tories, to obtain a more comprehensive view of OPA Clay frictional
properties. Thus, our results could also generate a benchmark for
studies applied to CO2sequestration in deep reservoirs, caprocks
integrity and seismic behaviour of decollement zones. To obtain the
OPA Clay friction properties, we performed velocity step sequences
and slide-hold-slide (SHS) tests in a double direct shear configu-
ration, monitoring the evolution of layer thickness as a proxy for
the volumetric deformation of the gouge layers. Finally, we discuss
our results under the light of microstructural observations (i.e. SEM
analysis), to develop a conceptual model that couples the deforma-
tion mechanisms and the mechanical data.
2 MATERIALS AND METHODS
2.1 Experimental samples
To study the frictional properties of the Opalinus Clay gouge, we
sheared pulverized samples of undeformed OPA Clay retrieved from
the monitoring drill core BFS-1 (0.14 m in diameter; Guglielmi
et al. 2020) 37.40 m below the MTL galleries, just outside the
‘Main Fault’ zone, in the host rock (Fig. 1).
We prepared the gouge samples by crushing the undeformed OPA
sample (Fig. 1c) with a ball mill for less than two seconds to avoid
any thermal damage to the mineral grains due to the heat generated
by friction during milling spin. To understand the influence of grain
size (g.s.), the powdered sample was subdivided into two batches at
different grain sizes, a coarser one sieved between 63 and 125 μm
and the finer one having g.s. <63 μm.
The mineralogical assemblage was determined by XRD analysis
on the same powders used for the friction experiments (g.s. <63 μm
and 63 <g.s. <125 μm). The analysis was conducted using a
Bruker D8 Advance X-ray system equipped with Lynxeye XE-T
silicon-strip detector at the Department of Earth Sciences, Sapienza
University of Rome (Italy). The instrument was operated at 40 kV
and 30 mA using CuKαradiation (λ=1.5406 ˚
A). Samples were run
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Frictional properties of Opalinus Clay 213
Figure 1. (a) Vertical cross-section showing the ‘Main Fault’ (bounded in red) at Mont Terri Laboratory (MTL). The grey patches are the scaly clay fabric
within the ‘Main Fault. The blue rectangle shows the location of the optical log of BFS-1 (modified from Guglielmi et al. 2020). The black bar on the core log
shows the exact location where the core sample of intact host rock of OPA was retrieved. (b) Detail of the core rock sample used during lab experiments. (c)
Outcrop picture of the shear zone at the top ‘Main Fault’ surface in tunnel 98, where the gouge appears as thin discontinuous lenses between scaly clay and
undeformed OPA (modified from Laurich et al. 2018).
between 2◦and 70◦2θwith step sizes of 0.02◦2θwhile spinning
the sample. Data were collected with variable slit mode to keep
the irradiated area on the sample surface constant and converted
to fixed slit mode for semiquantitative analysis. Semiquantitative
estimation of mineral phases was performed by calculating peak
areas and using mineral intensity factors as calibration constants
(Moore & Reynolds 1997).
X-ray semiquantitative analysis (Table 1) shows that the OPA
Clay gouges are mainly made up of sheet silicates (55–58 per cent)
including kaolinite, chlorite, illite/muscovite and mixed layers illite-
smectite, calcite (21–22 per cent), quartz (14–15 per cent) and
pyrite (4 per cent) without substantial difference in mineral content
between the two grain size fractions tested. K-feldspar, albite and
ankerite do not exceed 2 per cent. Such mineralogical composition
and weight per cent of mineral phases is consistent with the results
shown by Klinkenberg et al. (2009) and Fang et al. (2017).
2.2 Experimental procedure
We performed experiments using a biaxial, versatile, rock defor-
mation apparatus (BRAVA, Collettini et al. 2014; Fig. 2a). The
apparatus was used in the double- direct shear configuration which
consists of two layers of powdered gouge samples sandwiched in
a three-steel block assembly (Fig. 2b). This configuration is char-
acterized by two stationary side blocks and a sliding central block
with a nominal frictional contact area of 50 mm ×50 mm. During
the assembly preparation, for all the experiments, each layer has
been constructed maintaining a constant initial thickness of 5 mm.
To explore the influence of relative humidity (RH) conditions on
the frictional behaviour of the OPA Clay we prepared samples at
100 per cent RH, and at room humidity. We monitored the water
vapour content with a hygrometer during each experimental phase.
For the experiments at room humidity, we measured values between
∼20 and ∼30 per cent RH so, from here on, we refer to ∼25 per
cent RH as an intermediate value for room humidity experiments.
For each of the 100 per cent relative humidity experiments, we
prepared the sample assembly (i.e. forcing blocks plus gouge) the
day before and left overnight in a controlled humid environment
for 24 hr (humidifier cell). The humidity of the environment was
logged using a hygrometer and ensured that was maintained at 100
per cent. To evaluate the amount of humidity prior to the experiment
we measured a ∼15 per cent weight increase during the overnight
humidification procedure, that is a reasonable value for adsorbed
water under a 100 per cent humid environment (e.g. Morrow et al.
2017). To avoid dehydration of swelling minerals, samples were
constructed fast (it usually takes about 5 min for this configuration)
and immediately placed in the apparatus vessel. We conducted the
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214 N. Bigaroni et al.
Tab le 1. XRD data for the simulated gouges derived from core samples of monitoring hole (BFS-1) at 37.40 m of depth from
the Mont Terri Laboratory (MTL) in Switzerland.
Grain-size (μm) Bulk mineral composition (wt. per cent)
Ph Qz Cal Py Ab Kfs Ank
g.s. <63 58 per cent 14 per cent 21 per cent 4 per cent 1 per cent 1 per cent 1 per cent
63 <g.s. <125 55 per cent 15 per cent 22 per cent 4 per cent 2 per cent 1 per cent 1 per cent
Note: Ph-phyllosilicate minerals (kaolinite, chlorite, illite/muscovite, mixed layers illite-smectite), Qz-quartz, Cal-calcite, Py-
pyrite, Ab-albite, Kfs- k-feldspar, Ank-Ankerite.
Figure 2. (a) Sketch of the Rock Deformation Apparatus BRAVA at INGV of Rome (not in scale). (b) Schematics of the assembly. Normal stress and Shear
stress are applied by two different hydraulic rams. The dilation (h) and compaction are measured using an on board LVDT.
whole experiment with the humidifier inside the vessel, closed on
both sides by plexiglass panels to ensure 100 per cent humidity. All
experiments were run at room temperature (∼25 ◦C).
The experimental dataset was designed also to study the influence
of the grain size on frictional properties. For this reason, for each
experimental condition (of normal stress and relative humidity) we
carried out two experiments, one with an initial grain size <63 μm
and the other with grain size between 63 and 125 μm (for more
details see Table S1).
The deformation apparatus BRAVA uses two servo-controlled
hydraulic rams to apply the horizontal and vertical loads (Collettini
et al. 2014). The applied load (both horizontal and vertical) was
measured via strain gauge load cells (accuracy ±0.03 kN) posi-
tioned at the extremity of the hydraulic rams in contact with the sam-
ple assembly. To measure the horizontal and vertical displacements
we used two linear variable displacement transformers (LVDT), one
for each ram, with an accuracy of ±0.1 μm. We corrected for the
stretch of the vertical frame with a value of 0.116801 MPa μm–1
when the shear stress is <10 MPa, and with 0.30146 MPa μm–1
when it is >10 MPa. To account for the stretch of the horizontal
frame we corrected with a value of 0.12536 MPa μm–1 when the
normal stress is <20 MPa, and with 0.41656 MPa μm–1 when it is
>20 MPa. At any time of the experiment, as needed, the two pistons
can be controlled either through a load feedback control mode, to
maintain a constant load, or a displacement feedback control mode,
to apply a constant displacement rate.
At the beginning of each experiment, we advanced the horizon-
tal piston in displacement control at velocity of 1 μms
–1,upto
1 kN of horizontal load. We then switched the control feedback
in load control mode, increasing the horizontal load stepwise by
1 kN every minute. Once we reached the target value, we waited
30 min to allow the two gouge layers to compact to a steady-state,
constantly monitoring the total thickness using the horizontal LVDT
(Fig. 2b). We start shearing the sample by advancing the vertical
ram in displacement control at a constant velocity of 10 μms
–1 until
steady-state sliding. Initially, the sample deformed quasi-elastically
up to a peak friction value (blue square in Fig. 3)followedbya
frictional evolution to a steady-state sliding frictional strength (blue
circle in Fig. 3). To study the velocity dependence of friction and in-
fer frictional stability with accumulated shear strain, we performed
two sets of velocity step tests where we changed the shear velocity
quasi-instantaneously stepwise from 1 to 300 μms
–1 with a three-
fold increase. The velocity step series were separated by a series of
slide-hold-slides (SHS), with hold time ranging from 1 to 3000 s
that we use to calculate frictional healing. This range of tested val-
ues is motivated by previous studies that showed this experimental
protocol is well adapted for the estimation of frictional properties
of gouge materials (e.g. Marone 1998).
To estimate the frictional properties and fault slip stability, the
rate-and-state (R&S) friction framework was used (Dieterich 1979;
Marone 1998; Scholz 1998). These constitutive law describes the
evolution of the coefficient of friction (μ) in terms of a constant ini-
tial friction (μ0) suitable for steady-state slip at a reference velocity
(V0), a term that describes the direct effect aand a term to express
the evolution effect b, over a critical distance Dcand an evolving
state parameter θ, that can be interpreted as the average life-time of
contacts (Dieterich 1979; Ruina 1983; Marone 1998):
μ=μ0+aln V
V0+bln V0θ
Dc.(1)
The evolution law for the state variable θis given by:
dθ
dt=−
V0
Dc
ln Vθ
Dc.(2)
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Frictional properties of Opalinus Clay 215
Figure 3. Representative frictional experiments sheared at high (70 MPa) and low (10 MPa) normal stress, under different relative humidity (RH) conditions.
The blue square inside the ‘run-in’ phase (0–4 mm) and the blue circle, are the points where in all the experiments were taken respectively peak and steady-state
friction. (a) Example of slide-hold-slide test in time. (b) Example of one velocity step where the best fit of the inversion model (in red) over the experimental
data (in black) is plotted with fault dilation (in blue). Inversion results with standard deviations are reported in the box.
Eq. (1) coupled with eq. (2) is usually referred to as the Ruina or
slip law. The choice of this evolution law is motivated by recent stud-
ies showing a good fit to experimental frictional data (Bhattacharya
et al. 2017; Scuderi & Collettini 2018). The steady-state velocity
dependence of friction is described by the constitutive parameter
(a–b)(eq.3):
a−b=μss
ln V
V0,(3)
where μss is the change in the steady-state friction upon an im-
mediate change in sliding velocity from V0to V(Marone 1998). If
friction increases with increasing velocity, (a–b)>0, the material
is velocity strengthening and slip is inherently stable, leading to
aseismic fault creep (Fig. 3b). However, if the material is velocity
weakening, (a–b)<0, frictional strength decreases with slip ve-
locity and slip may be unstable, and any perturbation on the fault
can potentially promote slip acceleration. We modelled our labora-
tory frictional data using an iterative least square loop based on a
fifth order Runge–Kutta method (Reinen & Weeks 1993;Saffer&
Marone 2003). The results of the iterations for each velocity step
allow us to determine the best fit parameters of the velocity step
test, that is the critical slip distance Dc, the parameters aand b,and
their respective variances.
The amount of frictional healing, μ, was measured as the dif-
ference between the peak friction measured upon reshear after each
hold and the post-hold steady-state friction (Marone 1998; Fig. 3a).
Frictional healing rates βwere calculated as:
β=μ
(th).(4)
Volumetric changes in the simulated fault gouge during deforma-
tion are evaluated by analysing the evolution of the layer thickness of
the fault gouge deforming at constant frictional contact area. During
the experiments gouge layers undergo continuous bulk compaction
due to material loss related to the geometry of the DDS configu-
ration (e.g. Scott et al. 1994; for more details see the evolution of
the layer thickness during shear sliding, Fig. S2). We calculate the
amount of dilation (h) during velocity steps (Fig. 3b) by removing
the bulk compaction trend in layer thickness by using a least square
fit to the data in an interval following the transient phase of velocity
steps, that is after Dc(for more details see Fig. S3), as outlined
by Samuelson et al. (2009) (see also Marone et al. 1990;Mair&
Marone 1999; Scuderi et al. 2017b; Giacomel et al. 2021).
2.3 Microstructural analysis
To obtain information on the deformation mechanisms that mod-
ulate slip behaviour of OPA, we performed microstructural anal-
yses using a scanning electron microscope (SEM). After selected
experiments we collected the gouge material of the post-mortem
samples, carefully detaching it from the steel forcing blocks us-
ing a copper foil (0.8 mm thick), and then impregnated with two
components epoxy resin. Once the resin had hardened, the samples
were smoothed in dry conditions with sandpaper up to 5000 grit
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216 N. Bigaroni et al.
Figure 4. (a) Evolution of friction (μ) as a function of normal stress. Squares and circles represent, respectively, the μpeak and the μss. Brown is for room
humidity (RH) experiments; blue is for the 100 per cent RH ones (light for μpeak and dark for μss). Empty symbols are experiments at higher initial grain size
(63 <g.s. <125 μm); full symbols are g.s. <63 μm. (b) Shear strength in a Mohr-Coulomb space. Frictional coefficient of OPA Clay is the slope of the linear
best fit for τss values (brown for room humid; blue for 100 per cent RH), with the shaded area that corresponds to ±1σ. For comparison, we have included
Byerlee’ s law and previous results of frictional experiments carried out under dry and 100 per cent humidity conditions on OPA Clay gouges by Orellana et al.
(2018b,2019).
and coated with graphite. The samples thus prepared, were then
analysed with a scanning electron microscopy (SEM), model FEI
Quanta 400, using backscattered diffraction (BSD) images.
3 RESULTS
3.1 Frictional strength
The typical experimental curves that we observe in response to
the application of shear deformation usually show a peak in shear
strength (μpeak) that is followed by a strain weakening stage until
the achievement of a steady-state shear strength (μss; Fig. 3; e.g.
Samuelson & Spiers 2012; Den Hartog & Spiers 2013; Haines et al.
2013; Kohli & Zoback 2013). The value of steady-state friction (blue
dot in Fig. 3) was taken just before the first hold of the slide-hold-
slide (SHS) test. This is because, after the first cycle of velocity-steps
(at ∼9 mm of displacement and velocity of 10 μms
–1), almost all
experiments show a steady-stable sliding, which does not always
occur after the peak friction and subsequent strain weakening (at
the end of the ‘run-in’ phase). In Fig. 4we show the evolution of
peak (μpeak) and steady-state (μss ) friction as a function of normal
stress (σn). We observe that μpeak is normal stress dependent and
decreases as the normal stress (σn) increases for all the boundary
conditions of room humidity and grain size (Fig. 4a). Differently,
μss is weakly normal stress dependent and is constant for all the
boundary conditions.
The major variation in frictional strength is related to the presence
of water (i.e. relative humidity) in our sample, causing a system-
atic reduction in friction. Room humidity experiments show values
between ∼0.38 and ∼0.55 for μpeak and ∼0.42 to ∼0.45 for μss.
However, in experiments conducted at 100 per cent RH, μpeak varies
between ∼0.30 and ∼0.46, whereas μss is almost constant at ∼0.35.
The fault gouge with g.s. <63 μm(Fig.4a), shows an average re-
duction of ∼19 per cent in μpeak and of ∼22 per cent in μss, with
the increase in relative humidity. The experiments with gouge grain
size 63 <g.s. <125 μm, displayed an average reduction due to
RH increase of 17 per cent for μpeak and 18 per cent, for μss.The
friction coefficient calculated in a Mohr–Coulomb space (Fig. 4b)
using a linear fit of the shear stress values at steady-state (τss), is
0.41 ±0.01 for room humidity experiments and 0.35 ±0.01 for
100 per cent humidity.
3.2 Velocity dependence of friction
The results obtained from the modelling of velocity step experi-
ments are summarized in Fig. 5. This figure shows the evolution of
the stability parameter (a–b) as a function of the sliding velocity
and applied normal stress for room humidity (Fig. 5a) and for 100
per cent RH (Fig. 5b) experiments (see also Table S1 and Fig. S5).
For all the experiments and investigated boundary conditions, the
stability parameter (a–b) shows a velocity strengthening behaviour
(a–b >0) suggesting stable aseismic creep (Fig. 5). In general, our
results show that the increase in RH and normal stress has a ma-
jor influence on controlling fault stability, while the starting grain
size has little to no influence. Under room humidity conditions
and low normal stress, the evolution of (a–b) is strongly controlled
by the shear velocity, starting from near velocity neutral values
at low velocity (<10 μms
–1) and increasing to marked velocity
strengthening at the highest velocities (>30 μms
–1; Fig. 5a). As
the normal stress increases the dependence of (a–b) on the sliding
velocity vanishes with values that stabilize around 0.002. Adding
water causes an overall increase of the (a–b) values toward the
velocity strengthening field when compared to the room humid-
ity experiments (Fig. 5b). As it was observed under room humid
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Frictional properties of Opalinus Clay 217
Figure 5. Evolution of RSF constitutive parameters (a–b) with normal stress and velocity (see the colourmap), under (a) room humidity and (b) 100 per cent
RH conditions. Full and empty symbols show respectively data from initial grain size <63 μm, and 63 <g.s. <125 μm. (c) Detail of the frictional response
at low normal stress (10 MPa) between room humid (grey) and 100 per cent RH (blue) for velocity up-steps of 1–3 μms
–1 and (d) 100–300 μms
–1. (e) Detail
of the frictional response at high normal stress (70 MPa) between room humidity (grey) and 100 per cent RH (blue) for velocity up-steps of 1–3 μms
–1 and
(f) 100–300 μms
–1.
conditions (a–b) shows a strong dependence on shear velocity
(μms
–1) at normal stress <35 MPa, evolving towards velocity in-
dependent constant values of ∼0.0035 at normal stress >35 MPa.
The experimental curves show that, at low normal stress
(10 MPa), the frictional response following a velocity up-step, of
1–3 μms
–1 (Fig. 5c) and 100–300 μms
–1 (Fig. 5d), differs signif-
icantly with increasing RH. We observe an increase in the direct
effect (i.e. the parameter a in RSF laws), especially at low slip
velocities (Fig. 5c). This behaviour is at the origin of the high val-
ues of (a–b) that we report at low normal stresses. In contrast, for
high normal stress (70 MPa) the friction curves do not significantly
differ between a velocity up-step of 1–3 μms
–1 (Fig. 5e) and 100–
300 μms
–1 (Fig. 5f), both for room humidity and 100 per cent RH
conditions.
For each velocity step we have also calculated fault dilation (h)
as a proxy of volumetric strain changes (e.g. Marone et al. 1990;
Marone & Kilgore 1993; Samuelson et al. 2009). Like the evolu-
tion of the stability parameter (a–b), the amount of dilation increases
with sliding velocity at low values of normal stress, under room hu-
midity and 100 per cent RH conditions (Figs 6a and b, respectively).
As the normal stress increases, the absolute values of dilation de-
crease and become less dependent on shear velocity. The dilation
data for the experiments conducted with an initial grain size of
63 <g.s. <125 μm (empty symbols in Fig. 6), show values similar
to those observed for the g.s. <63 μm, following the same inverse
relationship with normal stress.
3.3 Frictional healing
To evaluate the potential for fault restrengthening during quasi-
stationary contact we quantify the amount of frictional healing (μ)
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218 N. Bigaroni et al.
Figure 6. Evolution of dilation (h) as a function of normal stress and up-step velocity, both for the (a) room and (b) 100 per cent RH experiments. Full and
empty symbols show respectively data from initial grain size <63 μm, and 63 <g.s. <125 μm.
as a function of hold time (th; Fig. 7) to calculate the frictional heal-
ing rate (Fig. 8). Our data show that frictional healing is mainly
controlled by the hold time and applied normal stress, and subordi-
nately by the presence of water and grain size (Fig. 7). For all the
boundary conditions, μ increases linearly with the logarithm of
hold time and normal stress (see the colour bar in Fig. 7). The pres-
ence of water affects the increase in μ with normal stress, at any
hold time. Experiments conducted at room humidity (Figs 7aand
d) show that at low normal stress, the frictional healing has a weak
dependence on hold time. As the normal stress is increased above
20 MPa fault strengthening increases with the hold time showing a
positive relation with the applied normal stress. Under 100 per cent
RH, frictional strengthening always increases as a function of hold
time at all normal stresses. This implies that the major differences
due to the increase in water vapour content are observed at high
hold times. At g.s. <63 μm and hold time of 3000 s, an increase
in normal stress from 10 to 70 MPa, results in an increase of μ
from ∼0.0007 to ∼0.0098 at room humidity (Fig. 7a), and from
∼0.0027 to ∼0.0056 at 100 per cent RH (Fig. 7b). For experiments
at 63 <g.s. <125 μm(secondrowinFig.7), the evolution of μ
with normal stress and hold time is similar to that observed for the
smaller grain size. With the bigger grain size, for room humidity
experiments (Fig. 7c), as the normal stress increases above 20 MPa,
the amount of μ is lower than the values measured at lower grain
size. Instead, for the experiments at 100 per cent RH we measured
similar μ values for both the grain sizes.
We performed a best log-linear fit of the frictional healing val-
ues to retrieve the frictional healing rate (β). Under any humidity
conditions and grain size, we observe a systematic increase of heal-
ing rate with normal stress (Fig. 8). For the experiments at g.s. <
63 μm, the addition of water causes the healing rate to decrease
at any normal stress values, with the only outlier at 10 MPa. Un-
der these conditions, the largest differences in the frictional healing
rate are observed at the highest normal stress (70 MPa), where (β)
drops from 0.0025 at room humidity, to 0.0014 at 100 per cent RH
conditions. The influence of grain size differs depending on the hu-
midity condition. At 100 per cent RH conditions we observe similar
(β) values at any grain size tested (blue curves in Fig. 8). At room
humidity conditions, as the normal stress increases above 20 MPa,
we observe lower (β)valuesat63<g.s. <125 μm, compared with
experiments at smaller grain size (brown curves in Fig. 8).
3.4 Microstructural analysis
To couple the frictional behaviour with the details of strain accom-
modation as a function of normal stress, humidity, and grain size, we
analyze SEM images of postmortem samples. The analysis of the
strain accommodation mechanisms (i.e. localized or distributed) is
based on the geometric relationship described by Logan et al. (1992)
for granular fault gouges (see also: Tchalenko 1970; Marone et al.
1992). In detail, we use the term B-shear to describe a continuous
shear band, parallel to the slip direction, located at the contact with
the indentation of the lateral forcing steel blocks, often found in both
sides of the gouge layer. With R-shears we identify shear bands that
fully or partially cross, at a low angle, the bulk gouge volume. Both
B- and R-shears are characterized with variable degree of grain-size
reduction and localization. Within these shear zones we recognize
a S-C type fabric, characterized by S-shaped foliation tilted toward
the shear direction, and bounded by a continuous alignment of platy
minerals, parallel to slip direction (C-planes). With P-like foliation
we identify alignments of anisotropic grain, starting from the B-
shears, that penetrate the bulk volume of the gouge with opposite
inclination to the R-shears (Tchalenko 1970; Passchier & Trouw
2005).
At low-normal stress of 10 MPa (Fig. 9), most of the defor-
mation is accommodated within tens of microns thick B-shears,
either under room humidity (Fig. 9e) or 100 per cent RH conditions
(Fig. 9f). These B-shears are parallel to the slip direction and are
characterized by grain size reduction and misaligned sheet silicates
that surround bigger clasts (Figs 9e and f). In the bulk volume of
the gouge, we report the presence of OPA Clay aggregates (i.e. mi-
croliths, Laurich et al. 2017) distributed in a matrix composed of
sheet silicates, quartz, and calcite grains (Figs 9b and d). The OPA
Clay aggregates have heterogeneous grain size, typically between
20 and 60 μm, and are constituted by undeformed vestigial OPA
minerals assemblage, that indicate little deformation in the bulk (see
also Fig. S4 in Supporting Information). Increasing humidity has
little influence on how deformation is accommodated, being char-
acterized by localized cataclasis and grain size reduction within the
B-shears (Figs 9c and d). Compared to the samples at room humid-
ity we observe two B-shears (Fig. 9c), characterized by grain size
reduction combined with development of incipient S-C type fabric
(Figs 9d and f). Starting from the edges of the sample structure,
we report the presence of incipient R-shears, which start from the
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Frictional properties of Opalinus Clay 219
Figure 7. Evolution of the frictional healing μ as a function of hold time (th) and normal stress (see the colormap). In the first row we plot the evolution of
μ with thfor the experiments at g.s <63 μm, under (a) room humidity and (b) 100 per cent RH conditions. In the second row we plot the corresponding μ
values for the experiments at 63 <g.s <125 μm, under (c) room humidity and (d) 100 per cent RH conditions.
B-shears at low angle (∼25◦) and fade out within the bulk losing
their continuity (Fig. 9c).
At high normal stress (70 MPa, Fig. 10) the deformation is dis-
tributed throughout the bulk volume via incipient P-like foliation
and low angle R-shears (Figs 10a and b), as also supported by
the total absence of OPA Clay aggregates. The P-like foliation and
R-shears are high shear-strain zones, characterized by grain size re-
duction combined with the development of S-C fabric (Figs 10band
d). The P-like foliation forms an angle of approximately 45◦with
the B-shears and is marked by the alignment of pyrite framboids
(Fig. 10e) or elongated calcite grains (Fig. 10f). In experiments
conducted at low normal stress, the increase of water vapor content
does not change the overall texture of the microstructures (Figs 10c
and d). Looking closely, we can only see small variations in fabric,
with C-planes of the S-C type fabric, better organized and continu-
ous in 100 per cent RH conditions (Fig. 10f) than in room humidity
conditions (Fig. 10d).
4 DISCUSSION
4.1 The origin of frictional strength and water induced
weakening
The frictional strength of OPA measured in our laboratory experi-
ments is generally low, ranging between 0.35 <μ
ss <0.45, when
compared with the classic view proposed by Byerlee’s results (i.e.
0.6 <μ
ss <0.8; Byerlee 1978) and is consistent with previous re-
sults on the same material (Orellana et al. 2018b,2019) and mixtures
of clay and granular materials (e.g. Bos et al. 2000;Ikariet al. 2007,
2009; Tembe et al. 2010; Moore & Lockner 2011; Kohli & Zoback
2013). The systematic evolution of the peak and steady-state friction
can provide important information on the micromechanics related
to strain accommodation that controls the frictional strength of the
fault gouge (e.g. Saffer & Marone 2003; Behnsen & Faulkner 2012;
Haines et al. 2013). From our experiments emerges that the applied
normal stress influences the mode of strain localization while the
increase of water vapour content causes a marked decrease in the
absolute value of frictional strength (Fig. 4). At low normal stress
(<35 MPa) friction evolves to a marked peak during the early stages
of deformation, followed by a gradual strain weakening to steady-
state values (Fig. 3). As the normal stress exceeds 35 MPa, sample
yielding is followed by a short strain hardening stage that leads to
steady-state friction (Fig. 3). This behaviour is common under dif-
ferent humidity conditions and initial grain size (for more details
see Fig. S1). Coupling our microstructural observations with the
mechanical data, we can infer the micro-mechanism governing the
initial stages of fault deformation. At low normal stress, during the
loading stage, the peak friction is the result of the interaction be-
tween misaligned phyllosilicate minerals with the angular-granular
(quartz and calcite) grains, resulting in a high peak strength (e.g.
Saffer & Marone 2003; Haines et al. 2009,2013). As deformation
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220 N. Bigaroni et al.
Figure 8. Evolution of Healing Rate (β) with normal stress. The data points from room humidity experiments are shown in brownish colors, whereas the 100
per cent RH ones are in blueish colors. Empty symbols (and lighter colors) are experiments at higher initial grain size (63 <g.s. <125 μm); full symbols (and
darker colors) correspond to the experiments at g.s. <63 μm. The error bars on each symbol refer to the standard deviation of the semi-log-linear fit obtained
from the plots in Fig. 7.
continues after the peak friction, shear localization occurs along
incipient B-shear zones (Fig. 9) causing strain weakening. This is at
the origin of lower steady-state frictional strength favoured by slid-
ing along an incipient network of phyllosilicates (e.g. Tchalenko
1970;Ikariet al. 2009;Morrowet al. 2017; Collettini et al. 2019).
At high normal stress (>35 MPa), the absence of a peak in fric-
tion and the low frictional strength are coupled with the fabric of
the experimental fault zone characterized by deformation being ac-
commodated by R-shear (Figs 10a and c) and P-foliation (Figs 10b
and d). Here, a well-developed network of phyllosilicates with S-C
type fabric is in geometrical continuity with the B-shears (Figs 10e
and f). This type of texture has been widely observed for a range
of phyllosilicate rich fault rocks (e.g. Bos et al. 2000; Tembe et al.
2010;Haineset al. 2013;Morrowet al. 2017).
The effect of water vapour content in lowering the frictional
strength of phyllosilicate-rich gouge powders has already been ex-
tensively studied (e.g. Saffer & Marone 2003;Crawfordet al. 2008;
Tembe et al. 2010; Moore & Lockner 2011; Behnsen & Faulkner
2012; Ruggieri et al. 2021). Our data clearly show that under 100 per
cent RH conditions the friction coefficient is systematically lower
than that from room humidity experiments (∼25 per cent RH), as
much as 0.1 (Fig. 4). This water-assisted weakening mechanism
is typically observed in clay-rich fault gouges (e.g. Israelachvili
et al. 1988;Safferet al. 2001;Ikariet al. 2007;Morrowet al.
2017; Orellana et al. 2019) whereas, for pure quartz powders, the
results by Frye & Marone (2002) show that increasing humidity
has no effect on the steady-state coefficient of friction at a given
normal stress, even if this is not always observed (e.g. Dieterich
&Conrad 1984). Several processes have been invoked to explain
this weakening in clay-rich fault gouges, likely related to the nature
of the weak chemical bonds that hold together the platy foliae of
phyllosilicates (e.g. Bird 1984;Morrowet al. 2000). The increase
of water vapour content can lubricate the mineral surface, weaken-
ing the chemical bonds between phyllosilicate foliae and, in turns
decreasing the shear strength (Israelachvili et al. 1988;Morrow
et al. 2017). We further note that the chemistry of the water can
also play a crucial role in controlling frictional strength. In fact,
water enriched with different solutes (calcium, salt and silica) can
act as a catalyst for chemical reactions between minerals control-
ling the bulk mechanical response. In the context of nuclear waste
disposal and carbon dioxide sequestration, the water in the repos-
itory is salty and highly charged with ions, potentially adding a
further control on the frictional response of the OPA Clay. Alter-
natively, due to the anastomosed structure along the shear planes
the water can be trapped, resulting in undetected interstitial high
pore pressures that may lower the effective normal stress locally
(i.e. σn—Pf; e.g. Saffer & Marone 2003; Faulkner et al. 2018;Orel-
lana et al. 2018b). This phenomenon can also be related to the
separation of clay particles as a swelling effect of OPA Clay (Pa-
terson & Wong 2005). It has been widely documented the ability
of OPA Clay to swell in the presence of water (e.g. Klinkenberg
et al. 2009;Bocket al. 2010;Tsanget al. 2015;Fanget al. 2017;
Voltolini & Ajo-Franklin 2020), giving rise to substantial increase
of pore pressure, in the order of several MPa (Zhang et al. 2010).
Taken together, the presence of water can cause changes in the
mechanical behaviour of fault gouge, thus being a controlling fac-
tor for decreasing fault strength (Fang et al. 2017; Orellana et al.
2018b). From our experiments emerge that, under the same stress
boundary conditions, water exerts a systematic weakening from the
early shear stages (i.e. peak friction) to the steady-state of frictional
sliding. Comparing the microstructures resulting from experiments
under ∼25 per cent RH conditions with those performed under 100
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Frictional properties of Opalinus Clay 221
Figure 9. SEM images showing the microstructures developed in post-mortem samples, after being deformed at 10 MPa of normal stress. (a) and (c) are
panoramic views of the experiments run under room humidity (∼25 per cent RH) and 100 per cent RH conditions, respectively. Here, the B-shears (shaded in
yellow) shows different degree of shear localization, compared to the bulk deformation. In (b) and (e) are shown close-ups of the microstructures developed
under room humidity conditions. (d) and (f) are the close-ups related to 100 per cent RH.
per cent RH conditions, no significant differences in strain accom-
modation mechanisms are observed at either low (Fig. 9) or high
normal stresses (Fig. 10). The coupling of these observations high-
lights that frictional sliding along phyllosilicate networks is an effi-
cient mechanism for accommodating deformation and the increase
of water vapor content decreases friction without changing the
style of deformation (i.e. localized or distributed) of the OPA Clay
fault.
It is interesting to note that the grain size has little effect on the
absolute values of peak and steady-state friction (Fig. 4,seealso
Fig. S1). When considering purely granular material, it has been
shown that the grain size has a major effect in controlling mechanical
parameters and fault stability (e.g. Marone & Kilgore 1993; Bedford
& Faulkner 2021). Comparing the X-ray semiquantitative analysis
of the experimental fault gouges at different grain sizes it emerges
that the weight percentage of sheet silicates is similar for both
grain sizes (55–58 per cent). Based on this data and microstructural
observations we posit that, if the percentage of phyllosilicates is
high, shear localization and frictional sliding are favoured along
phyllosilicate foliae forming a continuous network (e.g. Moore &
Lockner 2011), independently of initial grain size, that is at the
origin of similar values of μpeak and μss.
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222 N. Bigaroni et al.
Figure 10. SEM micrograph showing the microstructures developed in post-mortem samples, after being deformed at 70 MPa of normal stress. (a) and (c)
are panoramic views of the experiments run under 25 per cent RH and 100 per cent RH conditions, respectively. Here, the B-shears (shaded in yellow) shows
different degree of shear localization, compared to the bulk deformation. In (b) and (e) close-ups of the microstructures developed under room humidity
conditions are shown. (d) and (f) are the close-ups related to 100 per cent RH.
4.2 The micromechanics of fault stability
The frictional stability parameter (a–b) measured for the OPA Clay
always shows positive values for every initial boundary condition
(i.e. humidity and grain size) and for the whole range of applied
normal stress (Fig. 5, see also Fig. S5). These data therefore de-
note a typical velocity-strengthening behaviour that is commonly
observed in mixtures of granular minerals and phyllosilicates over a
critical percentage of 20–30 per cent (e.g. Niemeijer & Spiers 2006;
Crawford et al. 2008; Tembe et al. 2010;Ikariet al. 2011; Moore
& Lockner 2011; Carpenter et al. 2015; Giorgetti et al. 2015; Rug-
gieri et al. 2021) as well as in previous experiments conducted on
OPA Clay (Fang et al. 2017; Orellana et al. 2018b,2019). From our
data emerges that the parameter (a–b) mainly depends on sliding
velocity and applied normal stress and subordinately on humidity
conditions (Fig. 5).
At normal stress lower than 35 MPa, (a–b) strongly depends
on slip velocity, which causes an increase from values close to a
velocity neutral behaviour to a strong velocity strengthening as the
shear velocity increases (Figs 5a and b, see also Fig. S5). This
evolution of (a–b) parameter is accompanied by a positive rate
dependence of fault dilation with sliding velocity (Fig. 6). At higher
normal stress (>35 MPa) we observe a severe reduction of the
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Frictional properties of Opalinus Clay 223
velocity dependence of the (a–b) parameter (Fig. 5), dilation values
(Fig. 6) and critical slip distance Dc(see Fig. S6). The influence of
the normal stress on the rate-dependence of (a–b) can result from the
transition from localized (Fig. 9) to distributed deformation (Fig. 10)
with increasing normal stress, as illuminated by microstructural
analyses.
These observations have main implications for the coupling of
microphysical mechanism(s) of shear accommodation with the ori-
gin of fault frictional sliding stability. The velocity dependence of
the rate parameter (a–b) is commonly associated with the evolution
from an asperity population, at V0, to a new population at V1, that is
formed over a critical slip distance Dc(Rice & Ruina 1983;Segall
& Rice 1995; Marone 1998). Our mechanical data coupled with
microstructural observations suggest that above a critical normal
stress of 35 MPa, there is a switch in how deformation is accommo-
dated. We posit that when σn<35 MPa, the strain is accommodated
in a localized but heterogenous boundary shear zone, where larger
microliths prevent the formation of a continuous foliation plane
(Figs 9e and f). The preservation of some microliths testifies the
lower efficiency of cataclasis due to the low normal stress, which
produces the marked μpeak in the initial stages (Fig. 4a). Under
these conditions, upon velocity up-step, the competition between
granular deformation and sliding along the phyllosilicate foliae in
the heterogeneous B-shears causes the fault to dilate, promoting
stabilization by the increase of (a–b) (e.g. Marone et al. 1990;Chen
&Spiers2016;Ikariet al. 2016; Chen & Niemeijer 2017). When
σn>35 MPa, fault gouge exhibits distributed deformation that
is accommodated by wider B-shears and within the bulk volume
by P-like foliation (Fig. 10). B-shear zones are also finer in grain
size and display S-C fabric (Figs 10e and f). Under these boundary
conditions, upon velocity perturbation, sliding is favoured along
the continuous C-planes, thus decreasing the dilation of the gouge
(e.g. Niemeijer & Spiers 2006; Den Hartog & Spiers 2014), which
possibly explains the insensitivity of (a–b) to velocity (e.g. Saffer
et al. 2001;Ikariet al. 2007).
The increase of water vapour content enhances fault stability by
increasing the (a–b) parameter, both at low (Figs 5c and d) and high
normal stress conditions (Figs 5e and f). By observing the dilation
values (h) upon velocity up-steps (Fig. 6) and the microstruc-
tural texture of post-mortem samples, no significant differences
between room and 100 per cent humidity conditions occur. This
evidence suggests lubrication of phyllosilicate foliae can promote
an increase of (a–b). Alternatively, in Section 4.2. we explored the
idea of local increase of pore pressure in a well-developed phyl-
losilicate network, as a possible cause for lowering μss values. After
a sudden dilation upon velocity up-step the local pore pressure can
be released, increasing the effective normal stress and thus, pro-
moting frictional stability by dilatancy hardening (Segall & Rice
1995; Brantut 2020). In general, the increase of water vapour con-
tent produces the frictional strength reduction (Fig. 4) and enhances
the velocity strengthening behaviour possibly inhibiting frictional
instabilities (Fig. 5b; see also Ikari et al. 2009; Tembe et al. 2010;
Morrow et al. 2017; Orellana et al. 2019).
As reported for the absolute values of frictional strength, the grain
size does not affect the friction velocity dependence of the OPA Clay
gouge, contrary to what it has been shown by previous works on
granular material (e.g. Marone & Scholz 1989; Scuderi et al. 2017b;
Bedford &Faulkner 2021). This evidence further supports the idea
that, once a well-developed phyllosilicate network is formed, the
frictional properties are controlled by the continuity of the fab-
ric within the principal slip zones, regardless of the initial grain
size.
From the slide-hold-slide tests we observe low values of fault
healing (0 <μ<0.01, in Fig. 7) and healing rate (0.0001 <β
<0.0025, in Fig. 8) that are consistent with previous studies con-
ducted on the OPA Clay fault gouge (Orellana et al. 2018b)and
on phyllosilicate-rich gouges (e.g. Niemeijer & Spiers 2006; Beeler
2007; Carpenter et al. 2011; Tesei et al. 2013;Chenet al. 2015;
Giorgetti et al. 2015; Okamoto et al. 2020). The increase of normal
stress causes a progressive increase of healing and healing rate at
any experimental condition. This suggests that much of the contri-
bution to frictional healing depends on time-dependent mechanical
compaction (Scholz 2019), such as interlocking of clay foliae with
granular grains (Den Hartog & Spiers 2014; Orellana et al. 2018b).
4.3 Implication for geological repositories
In the context of geological repositories, the choice of the geolog-
ical formation designated for the storage of radioactive waste, or
high volumes of injected CO2must consider not only static physical
and chemical properties of the host rock but also the mechanical
behaviour of the faults occurring within the formation. Our results
on the frictional strength show that a simulated fault gouge in the
Opalinus clay is mechanically weak, with low friction coefficients
(∼0.35 to ∼0.41), for a wide range of normal stresses (10–70 MPa).
The increase of water vapour content reduces the frictional strength
under any boundary stress condition (see blue curves in Figs 11a
and b). We observed that the friction curves at low normal stress are
characterized by a marked μpeak (Fig. 11a) that disappears at high
normal stress (Fig. 11b). In the light of microstructural analysis,
we observed that this disappearance corresponds to the transition
from strain localization to a more distributed deformation at σn>
35 MPa (compare Figs 11e and f). Considering a stress gradient
between 20 and 24 MPa km–1 at the MTL (e.g. Thury & Bossart
1999; Corkum & Martin 2007), this transition should take place
at depth >1.5 km, or deeper than the operational depth of a geo-
logical repository (between 300 and 1000 m, IAEA 2012). These
results could also be of importance to better constrain the even-
tual instability processes at the wall of galleries in the MTL. Here,
during the operational and closing phases of underground galleries,
the mining activities develops a so-called excavation damage zone
(EDZ), where it has been long observed processes of desaturation
and resaturation (Popp et al. 2008) which affects the efficiency
of the geological barriers (Tsang et al. 2015). The EDZ fracture
network is characterized by Mode I and Mode II fractures in the
inner shell that is typically air-filled, and an outer shell which is
partially water-saturated with mainly Mode I extensional fractures
(Nussbaum et al. 2011). Under the light of our results, high relative
humidity (RH) inside the underground galleries could favour the
development of excavation-induced shear fractures (Mode II) by
decreasing the frictional strength of OPA Clay (Fig. 11a). In addi-
tion to the effects on the near field (i.e. EDZ), a recent study by Hopp
et al. (2022) reported small displacements (∼200 μm) at slow rates
(up to 1.5 nm s–1 ), tens of meters away from the excavation site of
new tunnels. The Authors located these mechanical instabilities at
the boundary of the fault zone, where frictionally weak clay flakes
are abundant. These findings are consistent with the localized de-
formation and presence of OPA Clay aggregates that we observe at
normal stress <10 MPa (Fig. 11e), coupled with values of (a–b)ap-
proaching neutral at slow sliding velocities (<3μms
–1) and under
∼25 per cent RH conditions (Fig. 11c). Of particular importance is
the mechanical behaviour upon fault reactivation (tested via veloc-
ity up-steps and SHS). Our results are in line with previous studies
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224 N. Bigaroni et al.
Figure 11. Conceptual model for the OPA Clay fault gouge, that summarizes the evolution of mechanical and microstructural observations as a function of
normal stress. Evolution of friction with progressive axial displacement, at (a) low and (b) high normal stress. Evolution of fault stability parameter (a–b)at(c)
low and (d) high normal stress. The increase of water vapour content decreases the friction (blue curves), increases the fault stability parameter (a–b) and does
not affect the microstructural deformation features of the OPA Clay fault gouge. (e) Microstructural model of localized deformation under low normal stress
in the B-shears, with microliths in the bulk volume. (f) Microstructural model of distributed deformation in the fault gouge under high normal stress, with a
well-developed S-C type fabric and incipient P-like foliation in the B-shears. The two insets are close-ups on the micromechanical behaviour in the B-shear
(dotted line) upon up-step velocity condition. At low normal stress (e), the stability is dominated by competition between granular deformation and sliding
along the phyllosilicate foliae in the heterogeneous and ondulated B-shears, which promote dilation (h). At high normal stress (f), the frictional stability is
controlled by frictional sliding on well oriented phyllosilicate lamellae and dilation is reduced (see Section 4.2, for more details).
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Frictional properties of Opalinus Clay 225
(Fang et al. 2017; Orellana et al. 2018b,2019) that show positive
values of (a–b) and low healing rate. Therefore, the experimental
fault gouge in the OPA Clay is frictionally stable with few re-
strengthening capabilities upon re-shear. Overall, our experimental
results support the hypothesis where shear strain is accommodated
by aseismic slip that may not radiate seismic energy (Orellana et al.
2018b). This behaviour can be compared to OPA Clay fault gouge
with similar mineralogical composition and clay fabric as our sam-
ples. Further, the gouge is present as patches only in the top of the
‘Main Fault’, therefore the induced seismicity observed at the MTL
(Guglielmi et al. 2020) can be attributed to structural and miner-
alogical heterogeneity of the fault hosted in OPA Clay. However,
few key considerations should be made by considering the effect
of shear velocity and normal stress. At low σn(<35 MPa) the sta-
bility parameter (a–b) is dependent on sliding velocity, with values
that (under room humidity conditions) approach velocity neutral
behaviour at 1 μms
–1 evolving to a strong velocity strengthening at
300 μms
–1 (Fig. 11c). At high σn(>35 MPa), while maintaining a
velocity strengthening behaviour, the velocity dependence of (a–b)
is severely decreased (Fig. 11d). This different evolution of (a–b)
with normal stress can be important for earthquakes triggered by
nearby unstable fault patches (Perfettini & Avouac 2004;Imet al.
2020). In the OPA Clay formation, patches of scaly clay fabric
could be potentially unstable (i.e. velocity-weakening behaviour),
as reported by Orellana et al. (2018a and 2019). Our result suggests
that, at σn<35 MPa, the higher rate-strengthening behaviour at
higher slip velocity (Fig. 11c) may be efficient in preventing nu-
cleation of earthquakes, quenching possible acceleration with an
increase of (a–b) (Cappa et al. 2019). As the normal stress in-
creasesupto70MPa,(a–b) becomes independent of slip velocity
(Fig. 11d), easing the propagation of seismic slip in weak and veloc-
ity strengthening phyllosilicate fault patches in decollement zones
(e.g. Wibberley & Shimamoto 2003; Faulkner et al. 2011;Rowe
et al. 2011; Bullock et al. 2015; Tarling et al. 2018). Further studies
need to be done in clay-rich fault gouges to physically constrain
the evolution of frictional stability parameter (a–b) with sliding ve-
locity under a wide range of applied normal stress and pore-fluid
pressures. Indeed, as observed in a recent experimental study by
Jia et al. (2021), the increase in fluid pressure in shale samples can
change the frictional stability, with potential transition from velocity
strengthening to velocity weakening.
5 CONCLUSIONS
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
from1to300μms
–1 and slide-hold-slide from 1 to 3000 s.
Our experiments confirm that the OPA Clay is weak, with friction
coefficients at steady-state of ∼0.35 and ∼0.41, for 100 per cent RH
and ∼25 per cent 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; at higher normal stresses the (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 increasing normal stress, from strain
localization and grain size reduction to distributed deformation on
well-developed and pervasive phyllosilicate networks. The amount
of relative humidity does not affect deformation mechanisms (i.e.
localized or distributed), whereas decreases fault strength and in-
creases fault stability. We hypothesize that this is due to a possible
interplay of OPA Clay swelling and lubrication, caused by the weak-
ening of chemical bonds between phyllosilicate foliae. Notably, the
initial grain size (<63 μmor63<g.s. <125 μm) does not af-
fect 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 con-
text of geological repositories, 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 storage in the deep
subsurface. Indeed, the OPA Clay shows the characteristics of a
low-permeability caprock, but mechanical instabilities on the pre-
existing faults can potentially generate undesired seismicity and
create new hydraulic pathways that limit the sealing capacity.
ACKNOWLEDGMENTS
We thank C. Marone for useful discussion about this manuscript, D.
Mannetta (Sapienza thin section laboratory) for the thin sections of
the samples and G. Volpe for assistance during experimental proce-
dures. FC, YG and CN provided the samples for the experiments.
NB and GP conducted the experiments. NB and MMS analysed the
results and wrote the main manuscript. LA performed the miner-
alogical analysis. All authors contributed to the writing and revision
of the manuscript. This research was founded by four partners of
the Mont Terri Project that contributed to the FS experiment: the
Swiss Federal Office of Topography (Swisstopo), the Swiss Fed-
eral Nuclear Safety Inspectorate (ENSI), the Japan Atomic Energy
Agency (JAEA) and the U.S. Department of Energy. The Mont Terri
Project is an international research project for the hydrogeological,
geochemical and geotechnical characterizations of a clay forma-
tion (Opalinus Clay). We wish to thank Brian Kilgore and Antonio
Pio Rinaldi for their constructive and thoughtful comments on the
manuscript.
DATA AVAILABILITY
The data underlying this paper are available in Zenodo, at https:
//doi.org/10.5281/zenodo.6685286 under the open access creative
commons attribution 4.0 international license. The authors declare
that they have no conflict of interest. For any other request, please
contact the corresponding author at nico.bigaroni@uniroma1.it.
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SUPPORTING INFORMATION
Supplementary data are available at GJI online.
Tab l e S1. Experimental boundary conditions, frictional strength
and parameters.
Figure S1. Frictional evolution curves for all the experiments.
Figure S2. Evolution of the layer thickness during shear sliding.
Figure S3. Calculation of the dilatancy coefficient.
Figure S4. Microstructural analysis of 5 post-mortem specimens.
Figure S5. Details of velocity dependence of RSF parameters.
Figure S6. Evolution of critical slip distance (Dc) with normal
stress and humidity.
Please note: Oxford University Press is not responsible for the con-
tent or functionality of any supporting materials supplied by the
authors. Any queries (other than missing material) should be di-
rected to the corresponding author for the paper.
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