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Contributions de l’endommagement radoucissant à la zone de rupture autour des tunnels profonds dans les roches quasi-fragiles

Abstract : The French National Radioactive Waste Management Agency (Andra) manages an Underground Research Laboratory (URL) at the Meuse / Haute-Marne Center to study the feasibility of a deep geological repository in the Callovo-Oxfordian claystone (COx). The galleries follow the major and minor principal stress directions. Some galleries show a quasi-isotropic stress state in their cross-section, while others show a greater anisotropy.These works study the short-term failure and fracturing phenomena induced by the underground structures’ excavation. Brittle softening damage is considered as a fundamental failure mechanism. The major part of scientific literature reports an estimation of this area from an elastic post-treatment or based on elastic-plastic analyses. If the first method does not consider the stresses’ redistribution due to dissipative phenomena, elastic-plastic approaches sometimes seem insufficient to explain the geometry of these zones in some cases of deep excavations in quasi-brittle rocks (Pouya et al 2016). Two phenomena are studied, through numerical simulations by 2d Finite Elements: the development of a diffused failures, around the galleries’ cross-section, and the fractures occurrence at their perimeter, along the excavation. While a plane strain analysis is suitable for the first problem, the second one is studied in axial symmetry.For the first part of the research, two isotropic unloading processes, in softening elastic-plasticity or elastic-damage, reproduce a global instability related to the first modelling, whereas localized failure occurs with the second one. The latter model, upgraded to consider intrinsic anisotropies in terms of elasticity, resistance and damage dissipative law, allows a consistent estimation of the short-term failure zone, with values of convergences in accordance to the data, for a gallery considered as a case study. Then, the transition of material’s failure from a brittle to ductile behaviour, with the confinement, is also simulated, according to different boundary conditions of a second gallery. The short-term damaged zones well reproduce the shape and extension of failure systems and the values of the instantaneous convergences are comparable to in-situ measurements (Trivellato et al 2018).The second research axis concerns the elongation and mutual spacing of fractures, simplified as a system of finite and parallel discontinuities. Numerically, they are assimilated to joint elements (Goodman 1976). The cohesive fracture model, based on damage, (Pouya and Bemani 2015) was chosen for these elements. In presence of a single potential fracture, an unstable initiation is observed, possibly followed by a stable evolution, of reduced length. Then, models with a sequences of several joints were used to analyse the fracture periodicity. This work also allowed the study of different numerical techniques simulating the advancement of an excavation front. According to a precise choice of parameters, fractures’ lengths are comparable to the geological and geophysical surveys. Similarly, the activation of one potential fracture among different discontinuities was calculated, showing a periodic occurrence (Trivellato et al 2018).This dissertation constitutes a new approach to reproduce the immediate effects of deep excavations in the Callovo-Oxfordian claystone. They study a short-term failure due to the material’s brittleness, under low confinement. Damage is adopted as the only dissipation mechanism and is integrated by the effects of material’s intrinsic anisotropy as well as its brittle-ductile post-peak transition. Results obtained by every research axis appear favourable to employ these models as complements to excavation studies. In perspective, the integration of plasticity / creep effects, as well as a poro-elastic framework accounting for hydraulic effects, may be considered
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Edoardo Trivellato. Contributions de l’endommagement radoucissant à la zone de rupture autour des tunnels profonds dans les roches quasi-fragiles. Mécanique des matériaux [physics.class-ph]. Université Paris-Est, 2018. Français. ⟨NNT : 2018PESC1170⟩. ⟨tel-02914682⟩

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