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| Titel |
Fault Frictional Stability in a Nuclear Waste Repository |
| VerfasserIn |
Felipe Orellana, Marie Violay, Marco Scuderi, Cristiano Collettini |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250123196
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| Publikation (Nr.) |
 EGU/EGU2016-2403.pdf |
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| Zusammenfassung |
| Exploitation of underground resources induces hydro-mechanical and chemical perturbations
in the rock mass. In response to such disturbances, seismic events might occur, affecting the
safety of the whole engineering system. The Mont Terri Rock Laboratory is an
underground infrastructure devoted to the study of geological disposal of nuclear waste
in Switzerland. At the site, it is intersected by large fault zones of about 0.8 – 3
m in thickness and the host rock formation is a shale rock named Opalinus Clay
(OPA).
The mineralogy of OPA includes a high content of phyllosilicates (50%), quartz (25%),
calcite (15%), and smaller proportions of siderite and pyrite. OPA is a stiff, low permeable
rock (2×10-18 m2), and its mechanical behaviour is strongly affected by the anisotropy
induced by bedding planes.
The evaluation of fault stability and associated fault slip behaviour (i.e. seismic vs.
aseismic) is a major issue in order to ensure the long-term safety and operation of the
repository. Consequently, experiments devoted to understand the frictional behaviour of
OPA have been performed in the biaxial apparatus “BRAVA”, recently developed at
INGV.
Simulated fault gouge obtained from intact OPA samples, were deformed at different
normal stresses (from 4 to 30 MPa), under dry and fluid-saturated conditions. To estimate the
frictional stability, the velocity-dependence of friction was evaluated during velocity steps
tests (1-300 μm/s). Slide-hold-slide tests were performed (1-3000 s) to measure the amount
of frictional healing. The collected data were subsequently modelled with the Ruina’s
slip dependent formulation of the rate and state friction constitutive equations. To
understand the deformation mechanism, the microstructures of the sheared gouge were
analysed.
At 7 MPa normal stress and under dry conditions, the friction coefficient decreased from a
peak value of μpeak,dry = 0.57 to μss,dry = 0.50. Under fluid-saturated conditions and same
normal stress, the friction coefficient decreased from a peak value of μpeak,sat = 0.45 to
μss,sat = 0.34. Additionally, it has been observed that the weakening distance Dw is
smaller under fluid- saturated conditions (∼4 mm) compared to dry conditions (∼6
mm).
Results showed a linear decrease of both peak friction and steady state friction when
normal stress increases. When fluid- saturation degree of gouges is reduced, gouge samples
underwent a transition from velocity strengthening to velocity weakening behaviour, thus
indicating a potentially unstable frictional behaviour of the fault. Furthermore, under both
saturated and dry conditions, the frictional healing rate showed a low recovery of the friction
coefficient under different holding times.
Our experiments indicate that the frictional behaviour of Opalinus Clay is characterized
by complex processes depending upon normal stress, sliding velocity, and saturation degree
of the samples. This complexity highlights the need for further experiments in order to better
evaluate the seismic risk during long-term nuclear waste disposal within the OPA clay
formation. |
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