Architecture and management of a geological repository - Andra

7 – The shafts and the driftsThe ground pressure on the structures progressively increases over time with the creeping andrelaxation of the argillite. In parallel, a decrease in the mechanical resistance of the liners withchemical alterations must be taken into account. These two phenomena are such that they lead to abreakage of the liners a long time after the closure. The result is that deformations resume in theargillite, which may cause or reactivate damaged zones (fractured and microfissured) around thestructures.In order to limit these deformations, Andra has decided to carry out a systematic backfilling of thedrifts during the closure.With respect to water circulation limitations, chapter 6 shows how they are based, first of all, on theargillite’s characteristics. Installing seals in the shafts and drifts allows reducing even more over thelong term the flow of water percolating in the repository and the flow rate of this water.The drift seals are also aimed at compartimentalizing the repository. The principle ofcompartimentalization consists of limiting an altered situation to a fraction of the repository. Thecompartimentalization must limit as much as possible the effects of such a situation on thesurrounding modules of the directly implicated module.7.2 Retained design principlesThe disposal cells are accessed by two types of structures :- The connecting structures between the surface installations and the underground installations(shafts, ramps) ;- The connecting and access underground drifts.7.2.1 Shaft or rampIn order to access the underground installations of the repository, two types of structures can beimagined, shafts and ramps :- Shafts are vertical structures. They are normally equipped with a machinery allowing loads to belowered and raised by cages suspended on steel cables ;- Ramps are inclined drifts, normally at a slope less than 15 % ; they can be used by tyredvehicles 116 .These two types of structures are currently in widespread use in the mining industry : shafts have beenused for more than a century and ramps for several decades.The reference solution retained for the underground repository uses only shafts. This solution has theadvantage of simplicity (only one type of structure) ; the shafts can be adapted to large flows to betransported.The possibility of using a mixed shaft – ramp solution will be discussed below in section 7.3.4.Among the existing underground repositories or those in project, the WIPP (United States) and theGorleben project (Germany) use only shafts ; these are sunk to respective depths of 650 m and 900 min sedimentary soils down to the base salt formation. In Sweden and Finland, mixed solutionscombining shaft and ramp are planned to depths of approximately 500 m in granite.116 There are also inclined shafts whose slope is between 20% and the vertical. This type of structure is only used under special conditions(inclined layers, mountainous topography, etc.).DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM291/495