Hydro-mechanic coupled modeling of an experimental gallery with viscoplastic approach and permeability function of rock degradation

Document available in extended abstract form only. The long term behavior of underground cavities is technically and economically a great stake for EDF, especially in the framework of underground storage of nuclear waste. The EDF-CIH has so developed a constitutive model, called L and K, which is able to describe delayed mechanical effects of rocks. Besides, this model offers a coupling between instantaneous and delayed behavior, and an influent variation of dilation which governs the volumetric strain of the material during a solicitation. Furthermore, the specificities of nuclear waste storage imply to take into account water pressure effects, thanks to hydro-mechanical coupled equations. One of the difficulties of such a modeling is the coexistence of two simultaneous time-dependant phenomena: the mechanical creep and the hydraulic flow. Plassart (2011) has highlighted the ability of the L and K model in hydro-mechanical conditions to reproduce in situ measurements, for both displacements and water pressure evolutions. These modeling have now been improved and the new results obtained are the subject of this paper. Computations have been conducted on the geometry of the GMR gallery dug in the LSMHM experimental site for the radioactive waste storage. The gallery has a mean radius of 2,3 m and ends on a bed of approximately 1,6 m. Calculations have been realized with the plane deformations hypothesis which allows the use of the Convergence-Confinement method to simulate the digging taking into account its progress for a given section. The constitutive model used to characterize the behavior of the rock in place is the L and K model and the hydraulic conditions are saturated. A regularization method must be used in modeling in order to limit the impact of the spatial discretization on the results obtained in post-localized computations. The method considered is the second gradient of dilation available on CodeAster software (developed by EDF-R and D) which is well adapted to dilatant geo-materials. The coefficient of regularization is taken equal to the one defined by Plassart (2011) for Callovo-Oxfordian clays and the L and K model. First results have been obtained with a constant permeability for the rock in place showing high negative pore pressures on the wall of the gallery. These suctions are due to the high dilatancy of the rock and its low permeability. Such negative pore pressures are not in agreement with experimental observations. The idea of the work performed in continuity with first results is to try to define permeability as a function of the material damage via the differential of porosity and quantify the impact on pore pressures. This approach brings down the negative pressures in the gallery wall and is in a rather good agreement with in situ data. However, the impact of changes in the permeability remains concentrated on a thin layer near to the wall and differences with constant permeability are not so obvious at the measure point, two meters away from the wall. (authors)