Architecture and management of a geological repository - Andra

7 – The shafts and the drifts7.7.3.2 Description and design of the repository isolation seal• Installation and heightThe seal is installed as high as possible in the upper Callovo-Oxfordian. The argillites are strongermechanically here than in the middle of the formation. These stronger levels are at least about thirtymetres thick (38 m perpendicular to the underground laboratory [6] - Volume 1). Acquired knowledgesuggests that the creation of the shaft does not cause a fractured zone to appear [38]. The intention isto remove the liner when the seal is created over the entire core height. The swelling material thuscomes into direct contact with the argillite.• Swelling coreThe core is based on MX80 swelling clay or its equivalent. This choice is dictated by the sameconsiderations as for the drift seals. The swelling pressure targeted at installation (after closure ofinstallation voids) is 7 MPa. This value corresponds to the main minor stress (effective). This highlevel is compatible with mechanical robustness of the containment engineered structures (retainingplug and backfill).The pressure sought may be obtained with a dry material density 130 of 1.8 and 10% installation voids.As for the drift seals, two implementation methods are possible: pellets (or mix of pellets and powder)and pre-compacted blocks. Spraying of pellets and powders leads firstly to densities in the order of 1.4to 1.5; mechanical compacting is then possible to obtain the targeted densities (dry densities of 1.70were obtained by tipping in bulk and compacting into place during full scale tests such as the onecarried out in the shaft of the Salzdetfurth mine in Germany). The targeted densities are obtaineddirectly, however, if pre-compacted blocks are used. Figure 7.7.8 illustrates a possible layout of huge,pre-compacted blocks, each one weighing nearly 4,500 kg.Figure 7.7.8System of swelling clay bricks for a shaft seal130 For a clay-sand mix, this value relates to the density of the clay fraction in the mix; thus, for a mix of 70% type MX80 clay and 30%sand, the corresponding density of the mix will be 1.95.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM318/495