Using Local Second Gradient Model and Shear Strain Localisation to Model the Excavation Damaged Zone in Unsaturated Claystone

Abstract

The drilling of galleries induces damage propagation in the surrounding medium and creates, around them, the Excavation Damaged Zone, EDZ. The prediction of the extension and fracture structure of this zone remains a major issue especially in the context of underground nuclear waste storage. Experimental studies on geomaterials indicate that localised deformation in shear band mode usually appears prior to fractures. Thus, the excavation damaged zone can be modelled by considering the development of shear strain localisation bands. In the classical finite element framework, strain localisation suffers of a mesh-dependency problem. Therefore, an enhanced model with a regularisation method is required to correctly model the strain localisation behaviour. Among the existing methods, we choose the coupled local second gradient model. We extend it to unsaturated conditions and we include the solid grain compressibility. Furthermore, air ventilation inside underground galleries engenders a rock-atmosphere interaction that could influence the damaged zone. This interaction has to be investigated in order to predict the damaged zone behaviour. Finally, a hydro-mechanical modelling of a gallery excavation in claystone is presented and leads to a fairly good representation of the EDZ. The main objectives of this study are to model the fractures by considering shear strain localisation bands and to investigate if an isotropic model accurately reproduces the in situ measurements. The numerical results provide information about the damaged zone extension, structure and behaviour that are in very good agreement with in situ measurements and observations. For instance, the strain localisation bands develop in chevron pat-B. Pardoen * · S. Levasseur · F. Collin Université de Liège, Département ArGEnCo, Chemin des Chevreuils 1, 4000 Liège, Belgium Tel.: +32-4-3662033 E-mail: b.pardoen@ulg.ac.be * FRIA, F.R.S.-FNRS scholarship holder tern during the excavation and rock desaturation is observed close to the gallery due to air ventilation.

Shear banding modelling in cross-anisotropic rocks

“…This indicates that the block at gallery 895 wall located in the horizontal direction continues to move significantly towards the gallery center after the end of excavation, which is due to the progressive drainage of the rock around the gallery and to hydro-mechanical coupling. Such increase of convergence after the end of excavation was also observed by Pardoen et al (2015a).…”

“…Previous numerical studies indicate that for an isotropic triggering of strain localisation and to the creation of the excavation fractured zone during the drilling Pardoen et al, 2015a;Salehnia et al, 2015). Moreover, this fractured zone has an elliptical extension with a significantly larger extent in one direction, depending on the stress anisotropy.…”

Permeability evolution and water transfer in the excavation damaged zone of a ventilated gallery

“…Theses modelling often include separately hydro-mechanical coupling [6,7], permeability variation [8], flow transfers [9], or strain localisation [10]. For 40 strain localisation approaches, there is a lack of numerical modelling which takes into account these different aspects simultaneously, at large scale, and which reproduce in situ experimental measurements.…”

“…Nowadays, it is commonly assumed that underground drilling process engenders cracks and eventually fractures [1] that deteriorates the hydro-mechanical properties of the surrounding host material. These modifications take place in a zone called Excavation Damaged Zone (EDZ), located around the galleries, which is affected by important modifications of the mate- 10 rial flow characteristics such as permeability increase [2]. Since a low hydraulic conductivity is required to ensure a safe long-term disposal, the EDZ behaviour is a major issue because it may constitute a preferential flow path for radionuclide migration.…”

Inputs from in situ experiments to the understanding of the unsaturated behaviour of Callovo-Oxfordian claystone

“…Pardoen et al [26] shows that the fractures induced by excavation process in the gallery vicinity could be reproduced by shear banding, with strain localisation properly reproduced by means of a regularisation method. Taking into account intrinsic permeability evolution in the shear band conduct with localisation methods seems to be promising to better reproduced the behavior of GED gallery, with its anisotropy, its long-term evolution and the influence of desaturation.…”