Architecture and management of a geological repository - Andra

7 – The shafts and the drifts7.7 Closure of underground facilitiesClosure operations may be carried out over a number of stages that take into account the rationale ofreversibility and in particular, the possibility of going backwards. See chapter 10.For the purpose of repository closure operations, structures are designed so as to minimise long-termmechanical argillite deformation, limit water flow and compartmentalize the repository (seesection 7.1) [38].These functions are assigned to various types of structure. Minimising mechanical argillitedeformations essentially relies on backfilling of all drifts. Limiting water flow relies on localisedimpermeable structures known as seals. Various seals are installed at points that enhance theireffectiveness and redundancy, thereby compartmentalize the repository.The building of these structures forms part of a stage-based approach to disposal process management,with each type of structure marking a stage of the process.Indeed, it is planned for cell sealing operations to be followed by backfilling of the drifts that servethem: connecting drifts inside a B-waste disposal area, access drifts to C-waste cells within eachmodule.The decision could then be taken to close a disposal area. This would require the sealing of connectingdrifts providing access to this area: seals at the entrance to a B-waste disposal area, separation seals fora C-waste disposal area.The last stage would entail backfilling of connecting drifts linking disposal areas to shafts, as well asthe sealing of shafts. Seals built inside drifts located close to the shafts would reinforce seals builtinside the shafts.Mining experience shows that backfilled (or sealed) drifts can be reopened as long as precautions aretaken to restore support and lining integrity, as the drift is gradually cleared out.7.7.1 Typical drift backfill7.7.1.1 Design principlesTypical backfills must be able to support argillite pressure, considering the possible rupture of driftlining long after closure. This entails minimising long-term rock deformation in order to prevent orcurtail spreading of the damaged area around the structures.Lining rupture results in argillite convergence. After vacuums left by backfill operations are filled in,the backfill subsides and offers increasing resistance (its deformation module increases together withstress) until equilibrium is once again reached between the argillite and the backfill.Potential argillite damage during this process depends on two factors: the magnitude of totaldeformation and argillite loading rate. In this case, the latter is the decisive factor: instantaneousrelaxation could result in significant damage in the case of minor deformations; very slow changesmight not damage the rock, even in the case of major deformations.Two criteria are therefore necessary for backfill operations: (i) vacuums must be kept to a minimum;(ii) Backfill material must have sufficient load-bearing capacity from the time it is put in. Its hardnessthen increases as it becomes more tightly packed through the effects of the argillite. Research showsthat an initial deformation module of 10 MPa is sufficient in this respect.The amount of backfill is considerable (several million cubic meters). Research was thereforeconducted into possible reuse of excavated argillite as a basic backfill material. This option would beeconomically beneficial, while also reducing the need for filler materials.In close proximity to the seals, the backfill also fulfils a mechanical containment function. This aspectis dealt with in section 7.7.2 on drift seals.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM308/495