Architecture and management of a geological repository - Andra

5 – Repository ModulesThe evolution of the swelling pressure with time may be summarised as follows (Figure 5.2.25): Theclay (or mixture) is installed with its retaining plug. The resaturation of the clay plug by the interstitialwaters of the surrounding argillite induces a swelling pressure attaining a first maximum level.Eventually, a degradation of the support materials may induce a certain relaxation. The mostpenalising scenario has been considered in this respect: a piston-like displacement of the retaining plugon the liner intrados, which occurs because the force of the swelling clay exceeds the reaction of thebackfill. The swelling pressure then decreases due to expansion and the reaction of the backfillincreases until a state of equilibrium corresponding to a minimum pressure is reached. Finally, theargillite creep sets the entire system at a second maximum level equal to the effective geostatic stress(7 MPa).Swellingpressure2nd maximumPiston-likedisplacementphase1st maximum1 MPatimeFigure 5.2.25Evolution of the swelling pressure in the swelling clay plug• Choice of clay type and mixture compositionThe deformability of the swelling clays available is high from an industrial point of view and remainshigh for clay and sand mixtures as long as the sand does not form a continuous skeleton (which limitsits proportion to approximately 50%).The swelling pressure of a swelling clay-based material depends on the type of clay chosen, thecomposition of the mixture (proportion of non-swelling granular material), the dry density of the clayand its water content.The swelling clay currently selected is of MX80 type or equivalent (low hydraulic conductivity at lowdry density).The relationship between dry density, water content and swelling pressure constitutes the design basisfor the swelling pressure of the clay plug described below.• Mechanical dimensioning of clay plugA length of approximately 3 m for the clay plug ensures the protection of the disposal cell with respectto alkaline perturbation (Ph). The dimensioning calculations described below confirm that this lengthis sufficient with respect to the possible relaxation of the swelling pressure and the possible long-termeffects of alkaline perturbation.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM225/495