Investigation of desaturation in an old tunnel and new galleries at an argillaceous site

A numerical investigation of the desaturation process at the argillaceous Tournemire site has been carried out. This desaturation is initialized by the contact of the saturated rock with the ambient air in excavated openings. The used hydraulical model is based on the Richards’ approximation for unsaturated one phase flow coupled with the deformation of a porous medium with anisotropic linearly elastic behavior. In relation to the extent of the desaturated zone around an excavated opening, the intrinsic permeability and the relative permeability have been identified to be the most important model parameters. The mechanical deformation process itself, the seasonal influences and the tunnel lining are less important for the formation of the desaturated zone. The comparison with measured saturation values bares some difficulties but indicates the principle capacity of the applied finite element codes to simulate the desaturation process. The consideration of seasonal changes in humidity in the ambient air results in a constantly recurring desaturation–resaturation cycle in the near field of the openings. This seasonally influenced zone amount 1–2 m and is nearly independent from time and from a variation of model parameters within a reasonable range. The possibility of material weakening in this zone is of special interest, since claystone is a potential host rock for the disposal of radioactive waste.