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Thermo-hydro-mechanical behavior of the Callovo-Oxfordian claystone : Effects of stress paths and temperature changes

Abstract : Extensive research is carried out by the French National Radioactive Waste Management Agency (Andra), in order to characterize the Callovo-Oxfordian (COx) claystone, a candidate host rock for a deep geological radioactive waste repository in France. The hydromechanical behaviour of the rock due to the excavation of the galleries are studied, as well as the thermo-hydro-mechanical (THM) response arising from heat generated by the exothermic waste packages. A laboratory programme was carried out in this work to characterize the response of COx claystone to different THM loadings within the framework of transversely isotropic thermo-poro-elasticity.Due to the very low permeability of the COx claystone, laboratory experiments have to be adapted for long saturation and drainage durations. Analytical solutions are presented for the time dependent pore pressure field in a specimen submitted to various loading paths and different rates. This provides a simple and efficient tool for the estimation of the conditions that must hold for reliable determination of material parameters. It allows as well an optimization of various test conditions.Based on this approach, a new transient step loading procedure was developed for isotropic tests in drained and undrained conditions, under both thermal and mechanical loading. This protocols render experiments on low permeable rocks more time efficient, giving access to several THM parameters and permeability measurements in a single test.In isotropic compression tests, pore pressure tests, and deviatoric loading tests parallel and perpendicular to the bedding plane, poroelastic properties were investigated on saturated rock specimens. The performed experiments provide compatible material parameters at different stress levels, evidencing a significant transverse isotropy, which had little effects on the back-calculated Biot’s coefficients and more effects on Skempton's coefficients.Thermal loads were exerted on COx specimens along different heating and cooling paths. Drained and undrained thermal expansion coefficients along both the transverse isotropic directions were determined. The measurement of pore pressure changes yielded the thermal pressurization coefficient, with a stress and temperature dependency identified.Thermo-hydro-mechanical loading paths corresponding to the paths expected in situ at the symmetry axis between two microtunnels, in which canisters are placed, were mimicked in the laboratory. Using a specially developed novel triaxial device, samples were heated with no radial strain allowed, until thermally induced pore pressures caused effective tensile stresses, which ultimately fractured the material at axial effective stresses around -3.0 MPa. Fracturing under different lateral total stresses allow to describe the failure with Fairhurst's generalized Griffith criterion. Using the THM properties evaluated earlier in the study, one is able to satisfactorily reproduce the observed deformations under tension.Last of all, a thermo-poroelastic model was implemented in the finite element solver Freefem++. The THM behaviour around parallel microtunnels in periodic layout is simulated in a 2D configuration. This helps to better understand the spatial arrangement of THM processes, heat and fluid transport, and the change of stress states with respect to the presented failure criterion
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Submitted on : Tuesday, February 18, 2020 - 2:19:13 PM
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Philipp Braun. Thermo-hydro-mechanical behavior of the Callovo-Oxfordian claystone : Effects of stress paths and temperature changes. Géotechnique. Université Paris-Est, 2019. English. ⟨NNT : 2019PESC1004⟩. ⟨tel-02483143⟩

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