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| Titel |
Stress field sensitivity analysis within Mesozoic successions in the Swiss Alpine foreland using 3-D-geomechanical-numerical models |
| VerfasserIn |
Karsten Reiter, Tobias Hergert, Oliver Heidbach |
| 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 |
250127801
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| Publikation (Nr.) |
 EGU/EGU2016-7713.pdf |
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| Zusammenfassung |
| The in situ stress conditions are of key importance for the evaluation of radioactive waste
repositories. In stage two of the Swiss site selection program, the three siting areas of
high-level radioactive waste are located in the Alpine foreland in northern Switzerland. The
sedimentary succession overlays the basement, consisting of variscan crystalline rocks as
well as partly preserved Permo-Carboniferous deposits in graben structures. The Mesozoic
sequence represents nearly the complete era and is covered by Cenozoic Molasse
deposits as well as Quaternary sediments, mainly in the valleys. The target horizon
(designated host rock) is an >100 m thick argillaceous Jurassic deposit (Opalinus
Clay).
To enlighten the impact of site-specific features on the state of stress within the sedimentary
succession, 3-D-geomechanical-numerical models with elasto-plastic rock properties are set
up for three potential siting areas. The lateral extent of the models ranges between 12 and
20 km, the vertical extent is up to a depth of 2.5 or 5 km below sea level. The sedimentary
sequence plus the basement are separated into 10 to 14 rock mechanical units. The Mesozoic
succession is intersected by regional fault zones; two or three of them are present in each
model. The numerical problem is solved with the finite element method with a
resolution of 100–150 m laterally and 10–30 m vertically. An initial stress state is
established for all models taking into account the depth-dependent overconsolidation
ratio in Opalinus Clay in northern Switzerland. The influence of topography, rock
properties, friction on the faults as well as the impact of tectonic shortening on
the state of stress is investigated. The tectonic stress is implemented with lateral
displacement boundary conditions, calibrated on stress data that are compiled in Northern
Switzerland.
The model results indicate that the stress perturbation by the topography is significant to
depths greater than the relief contrast. The impact of fault geometry and frictional properties
is observed within a distance of <1 km. The major impact on the stress state is caused by the
variability of the geomechanical stratigraphy. The stress anisotropy increases when tectonic
shortening is applied to the models. Stress magnitudes and anisotropy are largest within the
stiff formations such as limestone. These stiff formations carry the load due to far field
tectonic forces, whereas weak formations, like the argillaceous target horizon for the waste
disposal, exhibits smaller stress magnitudes. Using the fracture potential as a more
unambiguous indicator, the stiff overburden rocks are closer to failure than the target horizon
for the repository, whereas stiff formations below the target rocks are far from failure. |
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