Rock behaviour conditions the stability of deep underground exploitations. The excavations generate deformations, damage, and fractures in the surrounding rock. At engineering scale, phenomenological constitutive models are often considered; however, macroscale behaviour takes its origin from small-scale properties. Their influence on material deformations and damage across scales remains complex. Therefore, the behaviour of a clay rock is modelled at two scales. The excavation and damaged zones around galleries are reproduced at large scale. The excavation-induced fractures are modelled with shear bands. The approach is enriched with microstructural characteristics of mineral inclusions and clay matrix. The material mesostructure and behaviour are embedded in a representative elementary volume. A double-scale numerical framework (FEM×FEM) with computational homogenisation relates small- and large-scale behaviours, as deformations and failures. The damage and cracking developments at micro/meso scales allow to predict macroscale shear banding. The results highlight the possibilities of double-scale computing to predict underground structure behaviour.
The long-term management of high-level nuclear wastes is envisaged by deep geological repository. Low-permeability clay rocks are considered as favourable host media. Due to their safety function, the behaviour of the Excavation Damaged Zone (EDZ) that develops around galleries during their drilling can affect the safety of repository facilities (Armand et al. 2014). In fact, the underground excavation process induces rock damage and the development of an EDZ dominated by fracturing process. Understanding and predicting the latter are of paramount importance. Among the different envisaged media, the Callovo-Oxfordian (COx) claystone is studied in France (Andra 2005). It is an indurated sedimentary clay rock which has a complex microstructure exhibiting multi-scale heterogeneities. At meso-scale (i.e. mineral inclusion scale), the COx claystone is composed of several types of minerals with scattered mineralogical characteristics. It is well known that large-scale phenomena (deformation, failure, etc.) take their origin from small-scale processes, as at mineral inclusions and clay matrix scales for clay rocks. Questions have risen on how micro- and meso-structural characteristics of heterogeneous rocks can enrich the macroscale constitutive behaviour to predict deformation and failure processes. Consequently, the numerical modelling of the mechanical behaviour of the clay rock is realised by considering its micro- and meso-structural characteristics. The behaviour of the COx claystone and its damage under mechanical solicitations are modelled at several scales. Firstly, an insight of micro-damage and meso-cracking is obtained by small-scale modelling. Then, macroscale shear failure is reproduced at the scale of laboratory samples. Finally, the macroscale behaviour of the EDZ is studied with the development of shear bands in relation to the failure types at mesoscale and to the damage at microscale.
