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@PHDTHESIS{Oelker:771048,
author = {Oelker, Anne},
othercontributors = {Urai, Janos and Viggiani, Gioacchino},
title = {{D}eformation properties of {B}oom {C}lay :
{I}mplementation of a multi-scale concept},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
reportid = {RWTH-2019-09913},
pages = {1 Online-Ressource (181, LXXL Seiten) : Illustrationen,
Diagramme, Karten},
year = {2019},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2020; Dissertation, Rheinisch-Westfälische
Technische Hochschule Aachen, 2019},
abstract = {The quantitative understanding of grain-scale deformation
mechanisms in clay is important in hydrocarbon and water
exploration as well as in the evaluation of clay formations
as potential host rock for long-term underground
repositories for high-level radioactive waste. Analyses on
the nano-scale allow understanding the underlying
microphysical processes and therefore form the basis for
microphysics based constitutive laws for transport and
deformation, which can be confidently extrapolated to
conditions outside those in experiments. A microphysical
understanding of the transport and deformation properties is
important, because the properties of Boom Clay are known to
be complex: deformation is anisotropic, after a certain
amount of strain deformation tends to localize, and the
transport properties are also expected to be dependent on
deformation. In addition, one of the favourable properties
of Boom Clay is that fractures have the tendency to
self-seal, but a microphysical understanding of this process
is so far not available. This study examines the development
of microstructure during triaxial tests on Boom Clay freshly
collected at HADES level with σ1 applied parallel and
perpendicular to the bedding, to various total axial
strains. We used a range of methods integrated in a
multi-scale analysis. First, the samples were saturated and
deformed in consolidated-undrained (CU) triaxial tests
starting at 2.2 MPa effective stress, combined with in-situ
micro computed tomography (μ-CT). The μ-CT data with a
resolution of 13.5 μm/pixel were analysed by 3D digital
image correlation (DIC) to compute the incremental
displacement fields, and the evolution of the strain field
in the sample. Deformed samples were slowly dried,
sectioned, and the microstructure studied by optical and
scanning electron microscopic (SEM) imaging with resolutions
down to a few nanometres. The stress-strain curves our
experiments are in good agreement with previous studies by
Coll (2005); Sultan et al. (2010); Deng et al. (2011b);
Bésuelle et al. (2014). The orientations of the shear zones
(SZ) with respect to the shortening direction are 40 to
45°, in reasonable agreement with published values of
friction angles. The behaviour is slightly anisotropic. The
initial pore water pressure increase Δu of samples
shortened perpendicular to the bedding (S⊥B) is higher
than the values measured in samples deformed with stress
parallel to the bedding (S ‖ B), which is in agreement
with what is expected from microstructure. DIC analysis
shows that the evolution of strain is also different in
S⊥B and S ‖ B samples, although most samples localize
the strain at about $2\%$ axial shortening. From this point
on, more and more of the axial shortening is taken up by
movements along the SZ and the distributed strain in the
sample decreases. Non-localized strain in samples S⊥B is
highest (up to 3 $\%)$ in cone shaped zones close to the top
and bottom of the samples. In samples S ‖ B, strain is
more homogeneously distributed prior to strain localization,
which occurs at slightly higher axial strain than in S⊥B.
Because of the prominence of the evolving SZ, the shear
strength at high shear strains of S⊥B and S ‖ B is
similar. Based on the DIC maps, we selected representative
regions with different styles of deformation for imaging at
high resolution to understand the evolution of
microstructure and porosity. We defined four different
structural domains:1. OSZ: shear strain < 3 $\%,$
non-localized deformation and a microstructure comparable to
undeformed samples;2. OSZ-HS: higher shear strain than in
OSZ (≥ 3 $\%),$ but still non-localized deformation, in
S⊥B: Microstructure comparable to undeformed samples, in S
‖ B: Microstructure characterized by numerous micro-kinks
and -folds; in both, S⊥B and S k B, OSZ-HS are present in
significant parts of the samples more so in S ‖
B;3.OSZ-TZ: S k B at both boundaries of the SZ, shear strain
comparable to OSZ-HS (≥ 3 $\%),$ non-localized deformation
with a strongly altered microstructure characterized by
micro-kinks;4.ISZ: SZ with a shear strain between 5 and 50,
strongly localized deformation with a shape preferred
orientation (SPO) of elongated grains, reduction of porosity
and pores parallel to the shearing direction. Evolved SZ in
S ‖ B always have a kink-zone surrounding them. The
thickness of SZ varies between 20 and 200 μm, increasing
with increasing shear strain. The internal structure of ISZ
is similar in S ‖ B and S⊥B. Microstructures show
evidence for frictional/granular deformation mechanisms
(grain rotation, grain sliding, pore collapse and
reorientation, mica grain bending) and no evidences for
cataclastic processes. This is in agreement with
microstructure which contains about 20 $vol.\%$ silt-size
hard grains (calcite, feldspar and quartz) embedded in a
highly porous clay matrix. It is also generally observed
that quartz and feldspar grains in SZ are much smaller than
in the bulk of Boom Clay. This is not because they were
fragmented to smaller pieces, but because the SZ develop in
regions where are no large grains. This study provides a
microstructural basis for the construction of a
microphysics-based model for the deformation of Boom Clay,
which forms the basis for a microphysics based constitutive
law, which can be extrapolated to conditions outside those
used in our experiments.},
cin = {531220 / 530000},
ddc = {550},
cid = {$I:(DE-82)531220_20140620$ / $I:(DE-82)530000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2019-09913},
url = {https://publications.rwth-aachen.de/record/771048},
}
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