RHEOLOGICAL CHARACTERIZATION OF A CLAY FORMATION FROM DRIFTS EXCAVATION : ELASTIC AND ELASTOPLASTIC APPROACH

International audienceAn extensive scientific programme has been carried out by Andra (French Agency in charge of radioactive waste management) for investigating feasibility of High Level Activity Waste disposal in deep geological formation. An Underground Research Laboratory (URL) is currently being constructed in North-eastern France to assess the adequacy of a hard-clay argillite layer (Callovo-Oxfordian formation) situated between 420 m and 550 m of depth. Geotechnical measurements have been carried out during the shafts and drifts excavation and particularly upon the main level of the laboratory (-490 m). The drifts are “horseshoe section” type with about 17 m² in area mainly supported by metallic ribs and rock bolts. The digging has been performed with classical pneumatic hammer. Measurement sections have been instrumented very close to the front face using convergencemeters and radial extensometers. This paper presents a comparison between in situ measurements and numerical modelling. Elastic calculations are not in agreement with the measured deformations. An elastoplastic constitutive model considering damage and using Hoek & Brown criteria has been developed and implemented in the FLAC3D numerical code. Mechanical parameters came from lab tests performed on core samples. For the first meters, model provides consistent displacements. Beyond 4 meters, a time dependent convergence takes place and has to be integrated in the model to take into account creep and/or hydromechanical behaviour

Hydromechanical response to a mine by test experiment in a deep claystone

International audienceIn order to demonstrate the feasibility of radioactive waste repository in deep geological formation, an underground research laboratory is being constructed by Andra (French national radioactive waste management agency) in eastern France, in a Callovo-Oxfordian claystone. 15 boreholes were drilled from a drift at -447 m to install sensors around the shaft (6 m diameter) at depth -460 m to -474 m in order to record the hydromechanical behaviour of the claystone during a shaft sinking (drill and blast method). This paper is devoted to the analysis of the mechanical and hydromechanical behaviour observed during the shaft sinking. Analytical approach used herein allows to realistically evaluate the undrained response of the shaft neighboring with agreement with the in situ measurements. For the transient phase, prediction is qualitatively in accordance with measurement. In addition, deformation and displacement measurements are successfully compared to a simple 3D elastic calculation performed with the real face advancing. This emphasizes the quality of the data set which would be compared in the Modex-Rep European project with complex numerical modelling (poro-elasto-plasic-damage models, creep behaviour,...).En France, l'Andra (Agence Nationale de gestion des déchets radioactifs) est en charge des études pour la faisabilité d'un stockage de déchet radioactif haute activité à vie longue, dans une formation géologique profonde comme des argiles. Pour cela, l'Andra a construit le laboratoire de recherche souterrain de Meuse Haute Marne dans le Nord-Est de la France (à 300 km environs de Paris) dans une formation d'argilite du Callovo-Oxfordien qui se trouve entre 420 m et 550 m au niveau du puits principal. La première expérimentation géomécanique réalisée dans le laboratoire est un " mine by test " autour du puits principal (diamètre 6 m). A partir d'une galerie se trouvant à -445 m, 15 forages ont été réalisés pour installer des capteurs et mesurer le comportement hydromécanique de l'argilite entre -460 et -470 m lors du creusement du puits principal. Les évolutions des mesures in situ sont présentées et comparées avec la solution analytique poro-élastique du creusement d'un puits infini. Les mesures de déplacement sont comparées avec les résultats d'un calcul élastique en 3D. Ces analyses simples montrent la cohérence et la qualité des différentes mesures in situ qui serviront de données de référence dans le projet Européen ModexRep, où le creusement du puits est simulé avec des modèles 3D complexes (poro-elasto-endo-plasticité, fluage,...)

Modelling of the hydromechanical response of a shaft sinking in a deep claystone

International audienceIn the framework of the feasibility study of a radioactive-waste repository in a geological formation, Andra (French radioactive waste management agency) has been built an underground research laboratory within a Callovo-Oxfordian argillite formation located in Eastern France. During the sinking of the laboratory's access shaft, the hydromechanical behaviour of the argillites was monitored through an in situ experimental program called REP. The experimental zone is located between 460 and 476m depth. From a drift located at -445 m, 15 instrumentated boreholes were drilled downwards and equipped with 120 mechanical and hydraulic sensors. A predictive modelling of the shaft sinking has been performed in the framework of the European Modex-Rep project, using a poro-elastoplastic model based on a generalized Hoek and Brown criterion. Results of blind prediction emphasize that the model reproduces the in situ phenomena, but is not able to reproduce the amplitude of the drop in pore pressure during the shaft sinking. This article presents the analysis of this discrepancy and discusses new approaches aimed at improving the previous model. Finally, permeability changes around the shaft based on the in situ measurements and/or damage-induced permeability changes implemented in the model allow to better reproduce in situ data

Determination of the thermal conductivity of opalinus clay via simulations of experiments performed at the Mont Terri underground laboratory

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