A cross-anisotropic constitutive framework for modelling gallery intersections excavated in the Callovo-Oxfordian claystone

Abstract

This paper presents the development of a constitutive law tailored for cross-anisotropic rocks and its application on the intersection of two perpendicular unsupported galleries excavated in the the Callovo-Oxfordian (COx) claystone, considered as a potential geological formation to host an underground radioactive waste repository in France. An anisotropic Drucker-Prager elastoplastic law with shear strength hardening is developed to describe the rock matrix behaviour, and the fabric tensor method is used to account for the anisotropy of material strength related to its sedimentary layered structure. The effects of material anisotropy and in-situ stress state anisotropy on the short-term mechanical behaviour of the rock surrounding the intersection area are investigated, and the extent and shape of the plastic zone is presented for different gallery orientations and construction stages. In particular, the results indicate that the distribution of orthoradial stresses and plastic strains are mainly influenced by the anisotropic nature of the rock, leading to a preferential horizontal extension of the plastic zone regardless of the orientation of the galleries. Additionally, the zone affected by the excavation of the intersection is noticeable at a distance of maximum two diameters from the walls of the main gallery.