Séverine Levasseur

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

AbstractThe 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 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 that develop in chevron pattern during the excavation and rock desaturation, due to air ventilation, are observed close to the gallery.

Numerical post failure methods in multiphysical problems

La rupture dans les geomateriaux est souvent precedee par la formation de fines bandes de localisation des deformations. La formation de ces bandes de localisation est un processus non negligeable, etudie a la fois sur le plan experimental et sur le plan theorique. Cet article resume les principaux phenomenes observes sur les processus localises et propose quelques outils theoriques et numeriques necessaires a la caracterisation de ces processus de localisation. Afin de tenir compte des interactions entre les differentes phases des milieux poreux, une technique de regularisation basee sur des modeles de type second gradient est etendue aux couplages mutiphysiques.

On micromechanical damage modeling in geomechanics: Influence of numerical integration scheme

Tunnel excavations in deep rocks provide stress perturbations which initiate diffuse and/or localized damage propagation in the material. This damage phenomenon can lead to significant irreversible deformations and changes in rock properties. In this paper, we propose to model such behavior by considering a micromechanically-based damage approach. The resulting micromechanical model, which also accounts for initial stress, is described and assessed through the numerical analysis of a synthetic tunnel drilling in Opalinus Clay. A particular emphasis is put on the numerical integration of the model. In particular, an appropriate choice of the latter is required to ensure the numerical stability and a confident prediction of excavation damaged zone around tunnels.

On Water Transfer and Hydraulic Connection Layer During the Convective Drying of Rigid Porous Material

The convective drying of a natural porous material, limestone, is investigated in this study, with both experimental and numerical approaches. The first experimental campaign, which focuses on the influence of samples’ slenderness, suggests the presence of a hydraulic connection layer between the porous water and the external environment, in spite of the very fine pore structure of the material. This hydraulic transfer enables the fast water evaporation at the beginning of the drying test, when external conditions drive the kinetics. Furthermore, the results show that this layer does not exceed 30 mm deep from the external surface, given the drying conditions of the test. A second experimental campaign aims to analyse, by mean of an X-ray tomography tool, the internal water content during the drying. It confirms that water transfer takes place within the limestone in two distinct stages. The first stage being faster than the second one with a homogeneous desaturation along the sample, it is consistent with the hypothesis of the hydraulic connection layer. Finally, the finite element modelling makes possible to identify the main mechanisms of water transfer, namely liquid convection and vapour diffusion.

Using Shear Strain Localisation to Model the Fracturing Around Gallery in Unsaturated Callovo-Oxfordian Claystone

Galleries drilling leads to damage propagation, fracturing and properties modifications in the surrounding medium. The prediction of the damaged zone behaviour is an important matter and needs to be properly assessed. To do so the fractures can be modelled using shear strain localisation. The coupled local second gradient model is used under unsaturated conditions to correctly model the strain localisation behaviour. The permeability evolution and the rock desaturation due to air ventilation in galleries are considered. Finally, a hydro-mechanical modelling of a gallery excavation in Callovo-Oxfordian claystone is performed leading to a fairly good representation of the damaged zone.

Numerical modelling of the excavated damaged zone in Boom Clay

There is an international consensus on the advisability of storing high-level nuclear waste in the deep geological repository with low permeability. Boom Clay which is characterized by low hydraulic conductivity and the self-sealing capacity is known as one of the potential host rock for this geological disposal process. The excavation process in deep geological host rocks is expected to create a perturbed zone around the underground structures in rock masses in which the significant irreversible deformations and significant properties changes can be occurred. Modelling of the fracturing system and extension of the so-called Excavation Damaged Zone EDZ as an essential issue is focused in this study using the strain localization approach as a classical mode of failure of geo-materials. Numerical modelling of strain localization needs a specific approach to overcome the practical problem of mesh size dependency within the framework of classical finite elements. The second gradient method is used in this study in order to overcome this problem. Coupled numerical modelling of a gallery excavation in in Boom Clay host rock is performed. The simulation provides information about the strain localization in shear bands mode accomplished by the evolution of EDZ during the gallery excavation.

Hydro-Mechanical Modelling of the Boom Clay Excavation, Convergence and Contact with Concrete Lining

The Boom Clay is considered as one of the potential host rock formation in Belgium for radioactive waste repository in deep geological layers. Gallery excavations will induce large hydro-mechanical disturbances around disposal system that need to be well understood and characterised. This study discusses particularly the role of interactions between the lining of the galleries and the host formation in the numerical characterisation of excavations in Boom Clay. The excavation and the convergence of the connecting gallery of the HADES underground research facility in Mol is modelled in a hydro-mechanical framework. Zero-thickness interface elements are used to manage numerically the contact between the host rock and the lining. Numerical predictions are compared with strains measurements recorded within the concrete segments of the lining in the underground research laboratory in Mol. The study highlights the impact of the anisotropic behavior of the host rock on the response of the model.

Experimental Investigation of Hydromechanical Coupling During Clay Drying

Boom clay is studied as a potential host rock for underground nuclear waste storage. This experimental work analyzes its response under the convective drying conditions that may be applied in case of contact with the ambient atmosphere (during disposal drilling, for instance) or with gallery ventilation (because of damage in gallery lining). The drying kinetics is first established. In a second phase, the cracking and shrinkage onset and development are focused on, thanks to an X-ray microtomography device. The results show a phase of ideal shrinkage and a phase of shrinkage with desaturation, which are closely related to the kinetics. Cracking and shrinkage progressively develop from the drying surface, up to values representing about 3 and 12% of the cross section area, respectively.

Applications to Geotechnical Problems of the Micromechanical Modeling of Damage

The mechanical behaviors of geomaterials are significantly affected by the presence of voids or crack-like defects. The modeling of such behavior is classically performed by considering purely macroscopic or micromechanically-based damage models. In the perspective of applications in civil engineering or in geomechanics, we propose in this paper to evaluate a homogenization approach, based on Mori-Tanaka scheme, applied to micro-cracked materials. In order to provide an appropriate interpretation of the nonlinear behavior at macro-scale, the crack-induced damage is coupled to friction phenomena on closed cracks lips. The predictions of the coupled model are first analyzed on laboratory tests performed on Callovo-Oxfordian Clay. Then, they are extended to a numerical analysis of excavation damaged zones around tunnels.

Tunnel excavation modeling with micromechanical approaches

A zone with significant irreversible deformations and significant changes in flow and transport properties is expected to be formed around underground excavation in the deep geological layers considered for the high level radioactive waste disposal. The present study concerns the modeling of this phenomena by a micromechanical damage model, based on a Mori-Tanaka homogenization on a cracked media. This anisotropic model is derived from Eshelby homogenized scheme, on which a coupling between damage and friction is taking into account on cracks. Compared to elastoplastic model on tunnel drilling modeling, micromechanical modeling seems very promising: both approaches provide similar EDZ sizes and shapes even if they do not have the same effects on perturbed mechanical behavior; micromechanical model also overcomes the elastoplastic one by a realistic characterization of crack processes.