Architecture and management of a geological repository - Andra

2 – General Description• Positioning of the repository in the middle of the geological formationThe underground installations are embedded in the geological formation in such a way as to harness itsfavorable properties to form a barrier to water circulation and radionuclide migration. In order tomaximise the thicknesses of argillite located above and below the repository, undergroundinstallations with a low vertical extent, organised on a single level located in the middle of theCallovo-Oxfordian layer are envisioned. The layout of these underground installations may beglobally adapted to the dip in the layer, up to 1.5° 16 ( [6] - Volume 1).Figure 2.4.2Positioning of the repository in the layer. For illustration purposes, the depthand thickness of the clay formation correspond to the location of theunderground research laboratory.• Structures limiting the mechanical disturbance of the geological environmentThe underground structures are designed in such a way as to limit geomechanical disturbance not onlyduring construction and operation, but also following the closure of the repository (cf. § 2.2.1.4).The common profile of the various underground structures (disposal cells and access drifts) isgenerally circular or pseudo-circular 17 . Their excavated diameter is limited in accordance with ourknowledge of argillite and the experience of comparable underground works. A liner is designed toprovide lasting mechanical support. Furthermore, in accordance with frequent practice in undergroundworks, a distance between two adjacent structures of at least five times their diameter has beenadopted to boost their mechanical stability.Backfilling the access drifts at the end of the operating phase helps limit the residual empty space.Such a measure avoids the creation of possible mechanical damage in the argillites in the very longterm, following the loss of liner integrity [38].1617Corresponding to a slope of approximately 3%.In an isotropic stress field, a circular section represents the most stable, least disturbing configuration for an underground structure.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM55/495