<p>Sedimentary clayey rocks are of growing interest since several European countries considered these rocks as potential host rocks for deep geological repository for high level long life nuclear wastes. In situ observations have highlighted the influence of the inherent anisotropy of these rocks on the features of the excavation damaged zone around the underground drifts. Excavation induced fractures are observed either in tensile or in shear modes. Therefore, this article seeks to numerically investigate the failure mechanism within the Callovo-Oxfordian claystone, which is considered as a host rock in France, as well as the confining pressure dependence of its mechanical behaviour. The latter is a quasi-brittle and transversely isotropic rock. This work aims at developing a three-dimensional model using the 3-dimensional Distinct Element Code (3DEC®) with a polyhedral rock material discretisation to simulate the failure process at the sample scale. The Grain-Based Method allows to reproduce the crack initiation by the decohesion between two discretisation entities. The initiation of cracks to the formation of macroscopic fractures are then naturally captured with this method. The rock anisotropic mechanical behaviour is reproduced, in this study, either by applying morphological criteria to the discretisation pattern, or by employing an anisotropic Mohr–Coulomb failure criterion for the potential cracks. The comparison with results from laboratory compression tests highlights the model ability to reproduce the stress level influence on the rock mechanical behaviour, and the classical anisotropic U-shaped variation of the macroscopic shear strength as a function of the loading direction towards the bedding planes.</p>