Large-scale failure prediction of clay rock from small-scale damage mechanisms of the rock medium using multiscale modelling

Clay rocks are multiphase porous media whose complex structure is characterised by heterogeneity and possible anisotropy on a wide range of scales. The mesoscopic scale plays a particular role in deformation mechanisms under mechanical loading by cracking. The behaviour of rocks at mesoscale is characterised by the material and morphological (shape and size) properties of its components and their interactions. The accurate reproduction and influence of these mesoscale characteristics on the material behaviour and damage at large scale remain a complex issue. This question becomes crucial when investigating the underground stability during excavation works such as tunnels. In this numerical multi-scale study, the mesostructure characteristics are embedded in a Representative Elementary Area (REA) in a 2D configuration. A double-scale numerical framework, with finite element resolution at both scales (FE²) and computational homogenisation, is considered. The influence of the mesostructural characteristics of a heterogeneous rock and the effect of different inter-granular properties on their macroscopic behaviour, are examined. Additionally, a predictive strategy which is based on the connection between the failure modes of the REA and the failure mechanisms of the macroscale structure is also presented. This study investigates the effect of the mesocracking on the shear banding in a rock specimen during laboratory biaxial shear test and the development of the Excavation Damaged Zone (EDZ) around tunnels. The objective of this work is to explain the failure mechanisms observed up to the engineering scale of underground structures through the morphological and material small-scale characteristics of the REA.