Tome Architecture and management of a geological repository - Andra

4 - General architecture of the repository in a granite mediumThis is because it enables sections of different shapes to be constructed and lends itself well to thegradual adaptation of the detailed repository architecture in response to the exploratory findings asconstruction progresses. However and whenever the connecting drift lengths are long (severalkilometres), using tunnel borers is considered as an alternative, regardless of the architecture.Mechanical excavation using a tunnel borer presents the advantage over the drilling-blasting methodas it only generates slight damage to the excavation walls. Furthermore, in certain conditions (longstretches to be excavated in a straight line, homogenous geology, etc.) excavation productivity may bebetter than that achieved with drilling-blasting.Given the dimensions of the main, secondary drifts and modules, solid seam blasting may be carriedout.The excavation sequencing is identical to that presented for ramp construction, namely in four stages:- developing the blasting pattern, drilling the round and charging the mines;- blasting the round and extracting the blasting gases by ventilation;- removing the broken muck (mucking) and removing the unstable blocks in the roof (purging);- making the walls safe (occasional or routine ground support).As in ramp construction, pressure-grouting is used where water-conducting faults are crossed.4.2.2.6 Drift seals and backfillingIt may be decided to close parts of the repository and lastly the access openings as part of the stagedmanagement of the disposal process. This essentially entails emplacing backfill and seals in the driftsto protect the repository modules from water circulations inside them (cf. chapter 4.2.1: designprinciples).• Seals in the repository driftsVery low permeability seals are systematically planned in the connecting drifts between the waterconductingfaults and the repository modules installed in low permeability granite "blocks". They aremade up of a swelling clay core mechanically confined by two concrete abutments anchored to the rock.The seals are installed in portions of the drifts where the rock is not or slightly fissured and whereexcavating has been performed with methods limiting wall damage.At the seal level, the damaged zone is removed by cutting an over-excavation using ornamental masonrytechniques (diamond-tipped cable or disk sawing) [15]. Cutting is continued in the granite beyond thedamaged zone through a thickness of about 50 cm.The coupling between the seal core and the "sound" granite develops as clay swells. This coupling isfavoured by the presence of retaining abutments which confine the clay into the drift axis.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY71/228