The disposal of highly radioactive spent nuclear fuel in deep geological media will require excavating a large number of galleries in low-permeable rocks, altering initial rock integrity at the repository site. The FE-tunnel excavated in Opalinus Clay at the Mont Terri Underground Research Laboratory (Switzerland) is a unique full scale experiment to study this process. We conducted a numerical study of the excavation of the FE-tunnel in a coupled hydro-mechanical Finite Element framework and employing an anisotropic plasticity coupled with damage constitutive model. A second gradient of dilatancy formulation is employed to avoid spurious mesh dependent behavior originating from the softening of the coupled damage-plasticity model. The approach is validated by comparing numerical predictions and in-situ observations during and after tunnel excavation in terms of displacements, pore water pressure evolution and degradation of elasticity. Mechanical parameters are calibrated using laboratory experiments and values available in the literature. The model well reproduces the coupled hydro-mechanical processes induced by excavation, giving a good agreement between numerical predictions and experimental in-situ monitored data. Furthermore, the evolution of the Excavation Damaged Zone is correctly predicted. Thus, this modeling approach is suitable for the purpose of simulating tunnel excavation in lowpermeable anisotropic quasi-brittle shales.