Architecture and management of a geological repository - Andra

5 – Repository ModulesLoading of the lining as a function of time6,05,0Loading pressure (MPa)4,03,02,01,00,00 50 10 15 20 25 30 35t (years)Figure 5.1.19Change in the loading of the lining as a function of timeSensitivity of dimensioning to hypothesesAs previously stated, the desaturation in the cell walls tends to reduce the speed of deferreddeformations. To estimate the sensitivity of dimensioning to this phenomenon, the effect of areduction of 25% 74 in deferred deformation speed was analysed. This analysis shows that the lining asit was dimensioned would be less loaded by 0.5 MPa over the period extending from 100 to 300 yearsafter construction. This load reduction on the lining at a century could allow a reduction in liningthickness of some ten centimetres to be considered.For cells containing weakly exothermic waste, temperature plays a part in the dimensioning, both dueto the effect of thermal expansion of the concrete and the rock, and due to the acceleration of deferreddeformations of the argillites. According to the thermal power of the packages and the duration of theventilation phase of the cell before its closure, it could be necessary to increase the lining’s thicknessfrom 20 to 40%.5.1.3.6 Materials making up the cell and the access drift, plus impact on argilite andpackagesThe material constituting most of the cell and the access drift is concrete. It was chosen for itsdurability and for the favourable physical-chemical environment it offers to the disposal packages.Other materials are required but in smaller quantities, such as steel, or alternatively fibreglass andresin composites.• ConcreteThe concrete envisaged is HPC (high-performance concrete), a type that is now commonly used incivil engineering infrastructures. The mechanical performance of this type of concrete stems from thetype of cement used and the high cement content (on average 400 kg/m 3 ). An admixture (limestonefiller) is introduced into the concrete composition in order to reduce the relatively high release of heatfrom this type of cement during setting, and thus reduce cracking of thermal origin.It should be recalled that the concrete is not designed to act as a hydraulic barrier within the cell. HPChas two major advantages for type B waste cells: thanks to its high mechanical properties, it enablesthe thickness to be reduced, thereby minimising the volume excavated per given useful volume, and itslow porosity ensures long-term strength by minimising interaction with the water from the rock. Theconstruction joints and possible cracking of the concrete will not compromise the durability andstrength of the facility, which is subject solely to compressive stresses.74Which means dividing the Arrhenius coefficient by 3 in Lemaître’s lawDOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM180/495