Architecture and management of a geological repository - Andra

5 – Repository ModulesIt must be noted that this drift lining is thicker than that of the B waste disposal cells, despite theirhigher effective cross-section. The difference is mainly due to the thermal effect. The higher thicknessof the C-module access drift lining results from the integration of creep acceleration due to thermalrelease from C waste.5.2.3.5 Assessment of impact on argillite in the immediate environment of the disposal cellThe need to minimise damage to the geological formation in the immediate environment of thedisposal cells leads to verifying the low impact of their construction on the argillite.• Short-term impactThe design is intended to minimise short-term impact, on the one hand by orienting the disposal cellsparallel to the major horizontal stress (to reduce the extent of the fractured zone around the cells) and,on the other hand, by sufficiently spacing the cells (to reduce mechanical interactions among them).For this purpose, a minimum centre-to-centre distance corresponding to 5 cell diameters has beenadopted.The extent of the fractured zones 85 and microfissured zones 86 surrounding the structures when openedis first evaluated assuming that the rock is not supported, and ignoring the effects of the neighbouringaccess drift. The results of the purely mechanical calculations [66] are shown in Table 5.2.1 below.They indicate that the construction of C waste disposal cells at a depth of 500 m does not induce afractured zone.Table 5.2.1 Extent of fractured and microfissured zones in the short term (depth: 500 m)Extent measured from structure wall (ratioGeological horizon Cwith respect to radius R)(circular structures assumed isolated)Fractured zone Microfissured zoneDisposal cell (oriented parallel to major stress) None 0.5 RAccess drift (oriented perpendicular to majorstress)0 to 0.1 R(vertical aspect ratioellipsoid)0.3 to 0.7 R(vertical aspect ratioellipsoid)In order to prevent the interaction with the access drift from increasing the damage facing the usefulpart of the disposal cell and its sealing plug, the latter are located outside the fractured zone of thedrift. Due to their orientation perpendicular to the major horizontal stress, the access drifts aresurrounded by an ellipsoidal microfissured zone whose main axis is vertical and whose smaller axis(horizontal) amounts to 0.3 times the drift radius R [66]. Considerations regarding the operation of theconcrete plug have led to prescribing a length of 4 m for the latter. The swelling clay plug is thereforelocated 4 m from the access drift, in a zone unaffected by the damage induced by the latter.• Long-term impactIn the longer term, the argillite creeps freely until reaching the disposal cell sleeve, and then thepackages when the sleeve ruptures (after corrosion). The various annular cavities must therefore beminimised so as to limit argillite deformation. Given the various mechanisms involved and the smalldiameter of the C waste disposal cell, the appearance of a fractured zone of significant thickness isconsidered highly unlikely.8586Fractured zone: Geomechanical zone above the works, where the rupture threshold may be exceeded. It is characterised by theappearance of fractures possibly connected, leading to a significant increase in rock permeability (5.10 -9 m/s, as opposed to 5.10 -14 -5.10 -13 m/s for unaltered rock).Microfissured zone: Zone appearing either near the works (if no fractured zone has been produced) or behind the fractured zone (ifpresent). It is characterised by diffuse, scarcely connected microfracturing leading to only a limited increase in permeability (5.10 -11m/s).DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM213/495