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

Abstract : An 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.