Tome Architecture and management of a geological repository - Andra

4 - General architecture of the repository in a granite mediumAmong available technical solutions, construction of low permeability seals combined with the use ofslightly "tamped" backfilling materials is currently considered.Seals of a similar design to those used in the repository connecting drifts are installed in all driftsserving an access structure (shaft and/or ramp). The shaft base is cemented to a height of about twentymetres above the upper level of the repository. The volume over this concrete filling is entirelybackfilled up to a level about twenty metres below the leak-tight lining emplaced at the top of the shaftwhere a seal is constructed. The backfilling of the upper part, above the seal, completes the closureoperation.• Seals in the access structuresAt the current stage of the study the seals comprise a swelling clay core mechanically confined by twoconcrete abutments anchored in the formation.The seals are installed in the portions of the shaft or ramp excavated with methods limiting walldamage.The concrete lining and damaged zone at the edge of the drifts and right above each seal are cut out(diamond-tipped cable or disk sawing). Cutting is continued in the granite beyond the damaged zonethrough a thickness of about 1 m.The coupling between the seal core and the "sound" granite develops as the clay swells. The seal core,tens of metres high, also includes a buffer thickness to compensate for any deferred tamping of thebackfill, in the event of concrete abutment failure. Two solutions are envisaged for clay coreemplacement: tailored stacking of pre-compacted bricks or in situ filling with compacted pellets. Inboth cases, provision is made for inserting a device to collect any sweeping water on the shaft wall(gutters, weepholes), upstream of the sealing zone, in order to maintain favourable conditions for theconstruction of the clay core.The core is mechanically limited by two concrete abutments, dimensioned to withstand the swellingpressure of the clay and the effective mass of the backfill materials emplaced higher up inside theshaft. Dimensioning these retaining plugs (shape and height) has yet to be made.The seal for the ramp is similar to those planned for the repository drifts• Backfilling the access structuresOnly mechanical performance is envisaged at this stage for the access structures backfill. The particlesize of the granite aggregate should minimise the voids after in situ compacting.The addition of a fraction of swelling clay to compensate for the deferred tamping efects may beenvisaged. Resort to this option should consider a draining water collection system upstream of thecompacting worksite.4.3 Disposal processThe industrial commissioning of the installations starts with the reception of the first packages on thesite and their emplacement in the first constructed disposal cells. Simultaneously, construction work ofnew structures could gradually start while packages are emplaced in disposal cells. Operation of areversible disposal facility is characterised by the coexistence of construction activities (including ongoinggeological surveying) and package emplacement, and also by the progressive feature of theseactivities.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY77/228