Modelling the excavation damaged zone in Callovo-Oxfordian
claystone using shear strain localisation
B. Pardoen1,2, S. Levasseur1, F. Collin1, D. Seyedi3
1
ArGEnCo Department, University of Liège, Liège, 4000, Belgium,
FRIA, F.R.S.-FNRS scholarship holder. E-mail: b.pardoen@ulg.ac.be
3
Andra, R&D Division, 92298 Chatenay-Malabery, France.
2
The solution currently studied for long-term management of radioactive waste is the repository in deep
argillaceous geological media with low permeability. The drilling of underground galleries in these
clayey rocks induces damage propagation in the surrounding medium. This excavation process creates
a damaged zones in which the material properties are modified (Blümling et al. 2007). The prediction
of the fracture structure and material properties evolution within this zone is a major issue especially
in the context of underground disposal.
Experimental works have highlighted that the damage around galleries in Callovo-Oxfordian claystone
is localised and develops mainly as extensional and shear fractures (Armand et al. 2014). We propose
to represent these shear fractures with strain localisation in shear band mode. In order to properly
represent the shear strain localisation behaviour, an enhanced model introducing a regularization
method and an internal length scale is needed. Among the different regularization methods, the
coupled second gradient local model is chosen (Collin et al. 2006).
The constitutive mechanical law used for the clayey rock is an elasto-viscoplastic model. Due to their
mode of deposition, argillaceous rocks are anisotropic materials with a stratified structure. These
materials are therefore characterised by a cross-anisotropy, with an isotropic behaviour in the
stratification planes. This cross-anisotropy is taken into account both in the elastic (Graham and
Houlsby 1983) and plastic behaviours of the material. The material plasticity is defined by a nonassociated Van Eeckelen yield surface including friction angle hardening, cohesion softening as well
as cohesion anisotropy. The latter is modelled by introducing a microstructure fabric tensor
(Pietruszczak et al. 2002). The creep deformation of the material is described by a viscoplastic process
that corresponds to a delayed plastic deformation (Zhou et al. 2008). Furthermore, a flow model is
used to reproduce transfers in partially saturated porous media. The different parameters used in the
model are obtained from the literature and from experimental data fitting. The experimental data are
provided by the French national radioactive waste management agency, Andra.
To illustrate the development of the damaged zone, a two-dimensional plane strain state hydromechanical modelling of a gallery excavation is performed (Lagamine code, ULg) for two orientations
of the gallery in the Callovo-Oxfordian claystone. Firstly, a gallery oriented along the minor horizontal
principal stress is considered. In this direction, the anisotropy of the stress state in the plane
perpendicular to the gallery is the predominant factor leading to the elliptical shape of the damaged
zone, with a significantly larger extent in the vertical direction. Secondly, a gallery oriented along the
major horizontal principal stress is modelled. The stress state in the plane perpendicular to the gallery
is quasi-isotropic and does not lead to shear strain localisation unless the material anisotropy is
considered. This material anisotropy seems to be the predominant factor leading to the elliptical shape
of the damaged zone, with a larger extent in the horizontal direction. Moreover, the modelling
provides information about the fracture structure and evolution around the gallery. It exhibits a
chevron fracture pattern corresponding to in situ experimental measurements of shear fractures for
both considered gallery orientations (Armand et al. 2014).
References :
Armand G, Leveau F, Nussbaum C, de La Vaissiere R, Noiret A, Jaeggi D, Landrein P, Righini C
(2014) Geometry and properties of the excavation-induced fractures at the Meuse/Haute-Marne URL
drifts. Rock Mech Rock Eng 47(1):21–41
Blümling P, Bernier F, Lebon P, Martin CD (2007) The excavation damaged zone in clay formations
time-dependent behaviour and influence on performance assessment. Phys Chem Earth 32(8–14):588–
599
Collin F, Chambon R, Charlier R (2006) A finite element method for poro mechanical modelling of
geotechnical problems using local second gradient models. Int J Numer Meth Eng 65(11):1749–1772
Graham J, Houlsby GT (1983) Anisotropic elasticity of a natural clay. Geotechnique 33(2):165–180
Pietruszczak S, Lydzba D, Shao JF (2002) Modelling of inherent anisotropy in sedimentary rocks. Int
J Solids Struct 39:637–648
Zhou H, Jia Y, Shao JF (2008) A unified elastic-plastic and viscoplastic damage model for quasibrittle rocks. Int J Rock Mech Min, 45:1237–1251
Please tag the appropriate:
 oral presentation
 poster presentation
 intended to be proposed as full paper of publication in Proceedings
Topic of scientific programme:
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General strategy for clay based repository concepts
Geology and clay characterisation
Geochemistry
Mass transfer
Alteration processes
Geomechanics
Monitoring