Evaluation of damage-induced permeability using a three-dimensional Adaptive Continuum/Discontinuum Code (AC/DC)

Digging a shaft or drift inside a rock mass is a common practice in civil engineering when a transportation way, such as a motorway, railway tunnel or storage shaft is to be built. In most cases, the consequences of the disturbance on the medium must be known in order to estimate the behaviour of the disturbed rock mass. Indeed, excavating part of the rock causes a new distribution of the stress field around the excavation that can lead to micro-cracking and even to the failure of some rock volume in the vicinity of the shaft. Consequently, the formed micro-cracks modify the mechanical and hydraulic properties of the rock. In this paper, we present an original method for the evaluation of damage-induced permeability.ITASCA has developed and used discontinuum models to study rock damage by building particle assemblies and checking the breakage of bonds under stress. However, such models are limited in size by the very large number of particles needed to model even a comparatively small volume of rock. In fact, a large part of most models never experiences large strains and does not require the accurate description of large-strain/damage/post-peak behaviour afforded by a discontinuum model. Thus, a large model frequently can be separated into a strongly strained “core” area to be represented by a Discontinuum and a peripheral area for which continuum zones would be adequate.Based on this observation, Itasca has developed a coupled, three-dimensional, continuum/discontinuum modelling approach. The new approach, termed Adaptive Continuum/Discontinuum Code (AC/DC), is based on the use of a periodic discontinuum “base brick” for which more or less simplified continuum equivalents are derived. Depending on the level of deformation in each part of the model, the AC/DC code can dynamically select the appropriate brick type to be used.In this paper, we apply the new approach to an excavation performed in the Bure site, at which the French nuclear waste agency, ANDRA, is building an underground experimental laboratory. The modelling aims at predicting the damage-induced variations of permeability at the periphery of a shaft or drift during the first two years of its opening. In the first part of this paper, we present the concepts that have been developed to build a numerically efficient mixed model while minimizing the effect of continuum/discontinuum boundaries. We also describe the particle model developed to reproduce the short- and long-term behaviours of Callovo-Oxfordian argillite. In the second part of the paper, we present the numerical model built to simulate shaft excavation. We summarize the calibration process used to define the short- and long-term parameters, as well as the assumptions considered. Finally, we present an interpretation in terms of crack-induced permeability at the periphery of the shaft. First, a (crack transmissivity vs. opening) relation is calibrated using laboratory results and then applied to the crack network generated during the mechanical simulation. As the underground laboratory currently is being excavated, we will be able to compare our results with experience in the future. The predictions are plausible and show expected orders of magnitude.