Architecture and management of a geological repository - Andra

7 – The shafts and the driftsFigure 7.7.6Diagram of a B waste disposal cell sealAt the front end of the seal, on the cell side, the geometry of the retaining plug is adapted to that of thecell head (see Section 5.1). In addition, the dimensions of the drift giving access to the cell allows theseal core to be installed away from the influence of the damaged zone in the connecting drift.7.7.3 Shaft seals7.7.3.1 Design principlesA shaft is closed with two seals and backfill (Figure 7.7.7).A first seal in the upper part of the argillite formation being studied, isolates the repository from theoverlying geological formations. The aim is for its permeability to be as low as possible. Its core isbased on swelling clay, like the drifts. This core lies on a concrete retaining plug set at the bottom ofthe shaft, the station and extending several tens of metres along the drifts that are linked to it. Themechanical robustness of this plug ensures the stability of all the closing engineered structuressuperposed in the shaft. The weight of the overlying materials may ultimately contribute to thecontainment of the swelling clay; a retaining plug is nevertheless built above the core to play amechanical role during the resaturation phase.The first seal is covered by backfill up to the top of the Oxfordian limestone, using identical materialas for the drift backfill.A second seal is built above the backfill separating the most permeable layers of the overlying strataformations. These levels consist of three hydraulically independent assemblies: the Oxfordianlimestone, the Kimmeridgian (two limestone beds) and the Tithonian. They are not really aquifers onthe site in question; the hydraulic conditions have to be restored nevertheless close to this initial state.The separation seal could be located in the lower Kimmeridgian marls, 10 to 15 metres high. Its makeup is similar to the repository isolation seal, detailed later.Backfilling continues in the upper part of the shaft right up to the surface.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM316/495