Consideration of anisotropies related to bedding planes in the modelling of thick-walled hollow cylinder tests on Boom Clay

Tunnel excavation in Boom Clay formation, Belgium, highlighted an anisotropic behaviour related to its bedding planes. Hollow cylinder tests mimicking tunnel excavation under laboratory conditions confirmed the isotropic convergence of samples cored perpendicular to the bedding and the anisotropic behaviour of samples cored parallel to the bedding. This article investigates the possibility to reproduce these observations by numerical modelling considering anisotropies related to weakness planes. The behaviour of samples cored perpendicular to the bedding is simulated using an isotropic elastoplastic model. Despites of its simplicity, it shows good agreements with the experimental results. The results highlighted two competing phenomena: the hydro-mechanical coupling occurring during the unloading on the one hand, and the drainage towards the central hole on the other hand. Accordingly, the unloading rate is found to influence the displacement and the plastic zone extent. Strength and hydraulic anisotropies are considered in the simulations of samples cored parallel to the bedding. However, numerical results are opposite to the experimental observations, both in terms of the maximum displacement orientation and the plastic zone extent. Stiffness anisotropy should be considered as it might affect the hydro-mechanical coupling and therefore influence where the plastification begins.