Development of a viscoplastic constitutive model for deep rocks: concepts And application to the MHM argillite

Document available in extended abstract form only. With its long experience in applied numerical modeling, EDF-CIH initiated, ten years ago, the development of a specific constitutive model to reproduce the rock's mechanical behavior during underground cavity excavation. However, this model is not only dedicated to underground openings behavior simulation but it is also established on classical concepts admitted in soils and rocks mechanic. The need to develop this constitutive model, called L and K, first appeared in finding that modeling usually applied by engineers and research units to design underground openings were not be able to find physical behavior and phenomena observed on openings and sites, such as: - The state of stress influence on the development of particular ultimate mechanisms (spalling or squeezing), - The difficulty to identify interpretation criteria for considering, with a pragmatic point of view, the opening security level, according to a stability criterion, - The difficulty to understand and to find delayed degradation and rupture, - The difficulty to simulate the behavior of supports, like grouted bolting, for which the behavior is highly linked to the massif's one. This model was the subject of three thesis: two consecutive ones to establish the concepts and the mathematical formulation, and a third one destined to apply the model in and Hydro-Mechanical configuration. From a conceptual point of view, the constitutive model has to reproduce 'reference' behaviour, characteristic of rock mass, observed on laboratory tests or on feedback from openings. These types of behaviour are mainly: - A non-linear maximum strength threshold, with a friction and dilation angle depending on the state of stress, - The specific evolution of the dilation angle, with a maximal value corresponding to a stress threshold physically linked to the initiation of a bifurcation mode, - The total cancellation of the cohesion for high strains, - The total cancellation of volumetric strains rate, in particular of dilation for high shear strains, - An ultimate friction angle related to the critical state angle, - Volumetric strains contractant or dilatant, depending on the state of stress. The delayed behaviour is implemented by the definition of a long term strength threshold, and allows taking into account: - The effect of the excavation average rate, - The effect of a stopping in excavation works, - The long-term behavior of an unsupported face heading. This advanced constitutive model is described in the paper from a conceptual point of view. The aim is not to define theoretical equations but only to expose the main physical concepts implemented. To illustrate the relevance of the model, an application to the Meuse/Haute-Marne Argillite is given from a simple example. (authors)