Architecture and management of a geological repository - Andra

5 – Repository ModulesAs indicated earlier, the swelling pressure sought lies between a minimum value of 1 MPa and amaximum (initial) value of 7 MPa. For the permeability of the clay plug (or core), the range of valuessought is in the order of 10 -12 to 10 -11 m/s.The calculation method is illustrated based on the example of MX80 swelling clay, without taking intoaccount the effect of the sand (simplified approach).The maximum swelling pressure is determined in Figure5.2.26. One of the curves shows the evolutionof the clay swelling pressure during degradation of the retaining plug (as the clay core swells, itincreases its length by pushing the degraded concrete and backfill constituting the support). The othercurve shows the reaction of the backfill, which increases as the backfill settles. Mechanicalequilibrium is attained where the two curves intersect. The curve representing the swelling pressure isadjusted based on the initial dry density so as to obtain an equilibrium point at 1 MPa. Pessimistichypotheses have been taken into account regarding the deformability of the degraded concrete and theaccess drift backfill. Equilibrium with the backfill is obtained for an initial swelling pressure (prior toconcrete degradation) in the order of 4.5 MPa (4.1 MPa in the figure), corresponding to an initial drydensity of approximately 1.5.54Initial swelling pressure beforeincrease of the length of the clayplug ; dry density 1.5Pressure (Mpa)Pression (MPa)32Change in swelling pressure when thelength of the clay plug increases ; drydensity decreasesEquilibrium1Reaction of the backfillas a function ofdisplacement of the clay/ backfill interface00.0 0.2 0.4 0.6 0.8 1.0Allongement Increase in length noyau of clay d'argile plug (m)Equilibrium between swelling pressure of the clay plug and reaction of backfill (degradedconcrete), assuming a 0.1 m displacement before reaction of backfill and a stiffness of0.5 m/Mpa (MX80 type clay, sand not taken into account)Figure5.2.26Equilibrium between the swelling force of the clay plug and the reaction of thebackfillA similar calculation for a mixture with a sand content of 30% yields an initial dry density 90 ofapproximately 1.7 [65].90Initial dry density target values for swelling clay compacted in situ (according the hypothesis currently considered for plug construction).If prefabricated swelling clay components are used, the initial dry density will need to be higher so as to compensate the presence of thecavities installed.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM226/495