Tome Architecture and management of a geological repository - Andra

4 - General architecture of the repository in a granite medium• On-going block characterisationThe first on-going exploratory phase concerns characterising the modules host blocks. Surveyingactivities are included in the general repository construction planning.Hence the first surveys can be carried out during the construction of the last common infrastructures ofthe repository, in the shaft and main connecting drift zone. They concern granite massif zones wherethe first repository modules are to be installed.Subsequent surveying are carried out in other volumes of the massif, while repository moduleconstruction work is going on in the blocks that have already been surveyed and qualified. Thesuccessive worksites can be accessed by the construction and commissioning of the first mainconnecting cluster drifts. These connecting drifts are constructed ahead of the surveying sites. Theirlay-out is defined on the basis of previous surveys that have pinpointed the zones conducive toaccommodating the repository structures in the granite massif.The exploratory drifts are likely to intersect faults or water-conducting fractures (with hydraulic headat initial hydrostatic pressure). Dealing with the most water-conducting fractures by grout injectionahead of the working face or after excavation, and selecting the cementitious grout will be suited to theposition of the permeable zone in relation to the blocks to be surveyed.These adjustments aim at not disturbing any measurements to be performed while still maintainingfavourable conditions for continuing excavation work (wall stability, checking drained water volumes,etc.).These structures are considered temporary with the exception of the exploratory drifts (cf. § 4.2.2.4)whose installation in a block makes them suitable for conversion into connecting drifts in the moduleto be constructed. They are backfilled and sealed at the edge of the block at the end of the surveyingwork. All boreholes drilled in the block are plugged prior to exploratory drift closure operations.• On-going validation of the cell layoutThe second on-going exploratory phase entails identifying the disposal cells location in the module.This work is essentially conducted by boreholes. The distance in between boreholes and their lengthdepend on the geometry of the cells to be constructed (cf. § 5.6 for B waste, and § 6.6 for wastedisposed of in borehole).This exploration work is carried out between the end of connecting drifts construction and disposalcells excavation. Carrying out surveying in part of a module while simultaneously excavating inneighbouring drifts must be assessed in terms of possible perturbations on measurements.As in the case of exploratory drifts, boreholes are considered as temporary. Those drilled within thevolume of a suitable location for disposal cell construction are not plugged (as their bored volume isincluded in the volume of the cell to be excavated).Boreholes that have identified fractures not meeting the selection criteria defined at the end ofprevious exploration phases are plugged (low pH cementitious grout) or sealed in the vicinity of thedrift from where they were drilled (swelling clay plugs and low pH cement plugs).Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY79/228