In drained compression tests, saturated specimens of claystone, collected by ANDRA (the French agency in charge of the management of radioactive waste disposal) from samples taken at 500 m depth, exhibit a visco-elasto-plastic behaviour and are also susceptible to damage. This viscous behaviour includes the viscosity of both the skeleton and the water. In existing models, the creep phenomena are attributed to the water permeability, to the skeleton visco-plasticity or sometimes to both. Using Biot's theory, the development reported here assumes a damageable visco-elasto-plastic argillite skeleton saturated by water. This model was used to simulate an excavation from ANDRA's underground laboratory (located in Sure, France), where increasing permeability with respect to crack opening was considered using Poiseuille's law. The proposed application explains how both viscous phenomena combine at each step of the calculation. Just after the excavation, water overpressure decreases near the gallery, approaching zero due to the damage, and thus increases permeability. The viscosity is then controlled by the solid skeleton creep rate. Later, the redistribution of hydraulic pressure becomes more important and permeability again plays a major role.