Effect of anisotropic creep on the convergence of deep drifts in Callovo-Oxfordian claystone

Abstract

The French National Radioactive Waste Management Agency (ANDRA) is conducting a research program including in-situ experiments at the Meuse/Haute-Marne Underground Research Laboratory (MHM URL), which aims to demonstrate the feasibility of constructing and operating a deep geological radioactive waste disposal in the Callovo-Oxfordian (COx) claystone formation. A drift network of about 2km has been excavated essentially following minor and major horizontal stress directions in the MHM URL since 2000. Continuous monitoring shows that the convergence of the drifts has not stabilized yet, but the convergence rate decreases with time. Moreover, the convergence response strongly depends on the drift orientation. Understanding the time-dependent behavior of the host rock is the key issue for optimizing the design of the disposal. This paper aims to present a constitutive model including threefold anisotropies (elasticity, plasticity and viscosity) to describe the behavior of COx claystone, with a focus on the time-dependent behavior. Transverse isotropic elasticity is considered. Anisotropic Mohr–Coulomb criterion is used to represent the plastic behavior of the host rock. Anisotropic Lemaitre creep law is proposed to model the time-dependent behavior of the COx claystone. Anisotropy is elaborated by introducing an equivalent stress tensor. Model parameters are calibrated based on laboratory tests on sample scale and then adjusted against the field observation in MHM URL. The proposed model is finally applied for modeling the anisotropic convergence observed in the drifts GED and GCS which are respectively excavated in the direction of the minor principal horizontal stress and in the direction of the major principal horizontal stress. The model proved to be able to reproduce the observed convergences for the two directions of the drifts.