Architecture and management of a geological repository - Andra

6 – Overall underground architectureTotal flow:A few metrescubed per yearin the absenceof sealsFigure 6.3.5Dead end type tree architectureThe flow of water drained into each dead end is limited to the flow that this element can exchangewith the argillite. As the elements of the same tree structure level run parallel to the trajectory of thewater, inflow from other elements is avoided in each case. Thus, the flows circulating in the structuresand, consequently, the rates of advection are low closest to the cells. The flows drained are onlycombined progressively towards the main connecting drifts and the shafts.It should be noted that, this principle allows a certain degree of flexibility in the detailed configurationof the structures compared with the architecture presented.6.3.3.3 Water flows trapped by the shafts in overlying formationsDepending on the direction of the hydraulic head gradient between the Oxfordian and the Dogger,connecting shafts could capture the water flows in geological formations passed through and carrythem into the repository. These flows would then be added to those drained into the argillite. The sealsplanned for these shafts, and present also in the drifts, prevent this process by the hydraulic resistancethat they provide against the circulation of the water.If the role of these seals is not taken into account (failure situation), the grouped arrangement of theshafts and their location away from repository zones would, however, make it possible to limit theflow of water that would be trapped in the overlying formations and which could flow into thedrifts [80].This option can be compared with that of distant shafts as illustrated in the diagram in Figure 6.3.6. Adifference in hydraulic head between the distant shafts could induce a U-shaped flow passing throughthe repository and would bring it into contact with the overlying formations. In this situation, the flowsof water trapped by the shafts would not be limited by the low permeability of the argillite.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM269/495