Architecture and management of a geological repository - Andra

5 – Repository ModulesConnection between ground support and lining - summary of the problemThe role of the ground support is to ensure the safety of the worksite during the construction phase.The role of the lining is to ensure the stability of the structure throughout the period of operation.The dimensioning of the ground support must enable it to withstand instantaneous deformationsdirectly associated with the progress of excavating the drift, and deferred deformations due to theviscous behaviour of the rock, before the lining is installed. The dimensioning of the lining mustenable it to resist deferred deformations after its installation.Delaying fitting the lining offers the advantage of encouraging the expansion of the terrain, thuslimiting the forces to be subsequently taken up by the lining, but the drawback of prolonging thefunctioning period of the ground support, which must consequently be reinforced. A compromise musttherefore be found.The viscoplastic law adopted means deferring fitting the lining in order to avoid using overly largeamounts of concrete. Fitting the lining 6 months after the excavation is proposed. The ground supportis dimensioned to last for a year, thus allowing a flexibility of several months for fitting the lining.Using a cautious approach, and following normal practice, the contribution of the ground support tothe long-term stability of the structure has not been taken into account in the dimensioning of thelining.Results for the ground supportThe results below apply to the following ground support options: for safety reasons the bolts are fittedjust behind the face, while the shotcrete is cast some ten metres from the face, since, according to thecalculations, the shotcrete is not stable if it is set very close to the face.The 2.4 m long rockbolts are installed close to the front in a 1.5 m x 1.5 metre pattern and arecombined with the welded mesh. They provide a shallow anchorage inside the microfissured argillite.The length of these bolts must be finalized taking into account underground laboratory tests inparticular.The B40 shotcrete is designed with a thickness of 25 cm at the cell for a waiting time of one year. Thisthickness would only be 15 cm for a waiting time of 6 months.Results for the liningThe results given here correspond to a lining fitted at 6 months and dimensioned, like current largecivil engineering structures, for a lifetime of about a hundred years. For a drift with an excavateddiameter of 10.50 m (average value over all the cells), and in the shape of a horseshoe, using B60concrete, we obtain a thickness at the cell of 70 cm. The thickness is 95 cm in the side walls. Thethickness of the concrete reaches 1.40 m at the junction between the sidewalls and the base slab, whichis the most stressed zone.With the chosen shape, the lining is stressed only in compression and therefore does not need anyreinforcing bars.Analysis of the durability performance of the liningFigure 5.1.19 shows the change in the load exerted by the terrain on the lining as a function of time.We see that the pressure exerted by the terrain on the lining is of the order of 3.5 MPa at 100 years.The increase in pressure after this century is less than 0.5 MPa per century 73 . The lining couldtheoretically withstand a pressure of the order of 8 MPa. We find that the margins of safety anddimensioning used allow for the cell to be stable over several centuries, in association with the slowrate of increase in the pressure of the terrain on the lining.73The choice of conservative values to represent the deferred deformations of the argillites means overestimating the pressure undergoneby the lining at 100 years and its increase after that.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM179/495