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Argillaceous rocks are candidate host and (or) cap formations for the geological disposal of nuclear wastes in many countries, including Canada, France, and Switzerland. The understanding of the long-term mechanical behaviour of such rocks is an essential requirement for the assessment of their performance as a barrier against radionuclide migration. The French Institute for Radiological Protection and Nuclear Safety (IRSN) operates an Underground Research Laboratory (URL) in Tournemire, France, in a rock formation known as the Tournemire shale. Many types of experiments are conducted at the Tournemire URL to better understand the physical and chemical behaviour of this shale and its interaction with seal materials intended to be used in the geological disposal of radioactive wastes. The Canadian Nuclear Safety Commission (CNSC) collaborates with the IRSN and CanmetMINING laboratories to perform experimental and theoretical research on the mechanical behaviour of the Tournemire shale. Using data from creep tests, and monotonic and cyclic triaxial tests performed at CanmetMINING Laboratories, we developed constitutive relationships for the mechanical behaviour of the Tournemire shale. The model is based on the theory of plasticity, and takes into consideration the inherent anisotropy due to the existence of bedding planes, hardening behaviour before the peak strength, and viscosity.Key words: shale, viscoelastoplasticity, anisotropy, triaxial test, geological disposal.Résumé : Dans de nombreux pays comme le Canada, la France ou la Suisse, les roches argileuses sont des formations candidates pour le stockage géologique des déchets nucléaires. La compréhension du comportement mécanique à long terme de ces roches est une condition essentielle pour l'évaluation de leur performance comme une barrière contre la migration des radionucléides. Mots-clés : schiste, viscoelastoplasticité, anisotropie, essai triaxial, stockage géologique.