Modeling the viscoplastic and damage behavior in deep argillaceous rocks

In order to demonstrate the feasibility of a radioactive waste repository in the Callovo-Oxfordian claystone formation, the French national radioactive waste management agency (ANDRA) started in 2000 to build an underground research laboratory at Bure (East of France). One of the key issues is to understand long term behavior of the drifts. More than 400 m horizontal galleries at the main level of -490 m have been instrumented since April 2005. The continuous measurements of convergence of the galleries are available, allowing a better understanding of the time-dependent response of the claystone at natural scale. Results indicate that the viscoplastic strain rates observed in the undamaged area far from the gallery walls are of the same order of magnitude as those obtained on rock samples, whereas those recorded in the damaged or fractured zone near the gallery walls are one to two orders of magnitude higher, indicating the significant influence of damage or/and macro-fractures on the viscoplastic strains. Based on these observations, a macroscopic viscoplastic model which aims to improve the viscoplastic strain prediction in the EDZ is proposed and implemented in FLAC3D. Both the instantaneous and the time-dependent behavior are considered in the model. The short term response is assumed to be elactoplastic with strain hardening/softening whereas the time-dependent behavior is based on the concepts of viscoplasticity (Lemaitre's model). Finally, the damage-induced viscoplastic strains changes is examined through the plastic deformation (assumed to approach the damage rate). In order to verify both constitutive equations and their implementations, several simulations are performed: (a) triaxial tests at different confining pressures; (b) single- and multi-stage creep tests; (c) relaxation tests with different total axial strain levels, etc. Finally, an example of a blind prediction of the excavation of a drift parallel to the horizontal minor stress, sh (gallery GED) is presented and compared to in situ measurements. The results show the influence of damage on viscoplastic strain rates and the advantages and disadvantages of this new approach are discussed.