Architecture and management of a geological repository - Andra

7 – The shafts and the driftsThen, the head frame, the extraction machines and the electric equipment are installed. Finally, thecables and the shaft cages are installed.7.5.2 Construction of the driftsThe drift construction techniques are similar to those used for the construction of the B wasterepository’s cells and are described in section 5.1. However, since the connecting drift sections aresmaller, the drifts can be sunk in full section and not in a divided section.7.5.2.1 Current driftsThe connecting drifts and the access drifts of the C waste modules (and spent fuel modules) areconstructed in four phases by sections of several hundred metres :- Excavation by a roadheader 124 and ground support by rockbolts and shotcrete of the drift (steelarches and shotcrete in the sealing zones) ;- Pouring of the concrete slab ;- Pouring of the concrete liner ;- Drift equipment (utilities, rails or roadway, ventilation duct, etc.).7.5.2.2 Drift intersectionsDrift intersections are built in a number of stages:- Excavation and support of the larger diameter drift;- Excavation of vertical grooves at the four corners of the future intersection, followed by filling ofthese grooves with reinforced concrete so as to form four pillars with a cross-section ofapproximately 5 sq. meters in order to reinforce intersection roof support;- Excavation and support of the smaller diameter drift;- Pouring of concrete slab for both drifts;- Pouring of concrete lining into both drifts. This lining is integral with the pillars.7.6 Durability of structuresReversible disposal implies that shafts and drifts must last for as long a time as is decided to keepunderground installations open.Industry experience in terms of civil engineering tunnels and mining installations shows that suitablydesigned and built structures can be kept in service for more than a century. Keeping these structuresin service generally implies that they will need to undergo maintenance. Indeed, requirements for theuse of a structure are more stringent than requirements governing its overall stability. In fact, localisedproblems may prevent safe operation of the structure, without however jeopardising its overallstability 125 .The scale of maintenance operations on underground structures depends on support and lining design.At the current stage of study, preference has been given to a civil-engineering type approach:installation of a thick lining in order to be able to operate drifts and shafts with less maintenance for atleast one hundred years. Beyond that point, the stability of these structures could be maintained forseveral hundreds of years through monitoring and increased maintenance if needed.124 For constrcution work taking palce away from nuclear operations and outside sealing areas, drill and bast could be considered.125 It is this overall stability, and not operational safety requirements, that is taken into consideration regarding long-term behaviour ofstructures following storage closure.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM307/495