Abstract Research interest in the thermo-mechanical behavior of geomaterials is growing as a result of an increasing number of geomechanical problems involving thermal effects (including reservoir engineering, high-level nuclear waste disposal, heat storage, geothermal structures). A unified thermo-plastic/viscoplastic constitutive model, based on a generalized Hoek-Brown criterion, was formulated to describe the time independent and the time dependent behaviors of geomaterials under temperature effects. This constitutive model includes the evolution of the yield limits (elastoplastic and viscoplastic) and the evolution of the fluidity coefficient with temperature. The elastoplastic part of the model is described by an isotropic strain hardening and by an isotropic strain softening. The viscoplastic part of the model is described by the overstress concept of Perzyna. Even if typical responses to simple test-cases are briefly discussed in the last paragraph, the paper focuses on the concepts and on the mathematical formulation of the model.
1. Introduction The effect of temperature on the behavior of geomaterials is a crucial issue in geotechnical engineering. There are many applications based on the understanding of the thermo-mechanical behavior of rocks and soils, notably for high-level nuclear waste disposal, heat storage, geothermal structures, petroleum drilling, zones around buried high-voltage cables and others related to bituminous materials.
Within the context of the storage of nuclear waste, rocks and soils will be exposed to an elevated temperature during many years and, therefore, will suffer from changes in their mechanical properties. For this reason, particular attention has recently been paid to the thermo-mechanical behavior of Boom Clay (Sultan et al. 2002, Delage et al. 2010, Baldi et al. 1988),Tournemire argillite (Masri 2010) and Callovo- Oxfordian argillite (Gasc-Barbier et al. 2004, Zhang 2007, Delage 2013) among others.
Since Prager's first works (Prager 1958) about nonisothermal plasticity, many constitutive models have been developed to describe the brittle-plastic behavior of geomaterials at high temperatures (Hueckel et al. 1994, Modaressi & Laloui 1997, Laloui & Cekerevak 2008, Zhou et al. 2011, Dizier 2011). Most of these models are based on cap models extended to non-isothermal conditions. They are mainly used to describe the short term behavior of soft and indurated geomaterials under thermo-mechanical loads. Thus, except the models developed by Modaressi & Laloui (1997) and Zhou et al. (2011), delayed thermomechanical effects are often not duly coupled to rateindependent plasticity even if the long term behavior is the key to ensure the safety and the stability during design and construction analysis.