Architecture and management of a geological repository - Andra

5 – Repository ModulesFigure 5.2.24Metal plug for C waste disposal cell• Swelling clay plugIn order to ensure its hydraulic function, the material must have low permeability and gooddeformability. These properties do not require high swelling pressures. Moderate pressures are sought(a few MPa) to limit the mechanical load applied on the supports.The various design choices made with respect to swelling pressure, swelling clay type, mixturecomposition and associated dimensioning are specified below.• Swelling pressureSwelling pressure refers to the effective pressure exerted by the hydrated clay laminations 89 .The swelling pressure must be comprised between 1 and 13 MPa (or lower, if possible, for the reasonsexplained below).The low value (1 MPa) provides a reserve of swelling capacity ensuring impermeability to contactwith the argillite. It covers the uncertainties regarding the relaxation of the clay (following thedegradation of the retaining plug) and the at least partial transfer of mechanical confinement stressesfrom the clay plug to the access drift backfill.The high value (13 MPa) corresponds to the tensile rupture of the argillite on the excavation wall(cancellation of its effective tangential stress). It is therefore an imperative limit. A value below 7 MPais looked for, as this will be the ultimate value obtained in equilibrium with the site’s radial geostaticstresses at a depth of 500 m (a total stress of 12 MPa due to the weight of the soils, minus 5 MPa ofinterstitial pressure, resulting in an effective stress of 7 MPa); it is therefore pointless to attempt todevelop higher swelling pressures, as they will only produce unnecessary stress on the supports beforereturning to 7 MPa. An even lower pressure is in fact preferable, provided that the minimum pressureof 1 MPa after relaxation is guaranteed.89 The effective pressure is the total pressure less the pressure exerted by free waterDOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM224/495