Experiments related to EDZ evolution at Mont Terri

The excavated damaged zone (EDZ) was identified as an important feature for a repository in clay stone and was investigated from the beginning of the Mont Terri Project. During the last 10 years it was possible to understand the underlying deformation and failure processes and to delineate the main features of the EDZ development during the construction phase of the tunnels. The influence of the bedding planes as given planes of weakness and the importance of natural fractures is shown. lnterestingly, the EDZ development is observed in a very similar way on different scales. The general EDZ geometry seems to be independent from the size of the underground opening - a borehole EDZ of a borehole with a diameter of a few centimetres looks identical to an EDZ of tunnels with a diameter of 5 meters. The rheology of the Opalinus Clay is investigated and a significant time-dependent deformation is observed resulting from changes in mean stress as well as from changes in deviatoric stress. Such processes will lead to on-going convergence during the operational period of the tunnels until they are backfilled. After backfilling, different processes like swelling, disintegration and mechanical loading from buffer swelling and rock creep will lead to significant self-sealing of the initial EDZ causing a reduction of axial flow and of advective transport of radionuclides along tunnels