Studying the mechanical behaviour of an anisotropic clay rock around gallery intersections and the effect of the support stiffness

Gallery intersections are frequently encountered in underground construction of deep geological repositories for nuclear waste. Their three-dimensional nature, combined with the excavation sequence and primary support installation, makes them a complex challenge to address. The redistribution of stress and strain that occurs, plays a key role in the response of the surrounding rock. Furthermore, a high in-situ stress state and an anisotropic nature of the material could have a substantial influence on the response of the rock. In this context, the present study investigates the behaviour of perpendicularly intersecting supported galleries excavated at great depth in an anisotropic clay rock through 3D finite element analyses. A conventional step-by-step progressive excavation is modelled as well as the phasing of the intersection excavation. The surrounding rock is a sedimentary indurated claystone, the Callovo-Oxfordian (COx) clay rock, which is assumed to follow an anisotropic Drucker-Prager elastoplastic constitutive law with shear strength hardening. Different stiffnesses for the elastic support are used to account for a flexible, an intermediate, and a more rigid support. The obtained results focus on the impact of each support stiffness on the stress distribution, plastic strain, and the generated plastic zone in the surrounding rock mass.