Architecture and management of a geological repository - Andra

7 – The shafts and the drifts7.7.2.2 Design and production technique for a swelling clay core• Design and justificationThe length of about 40 metres adopted for the swelling clay core guarantees performance with littlehydraulic transmissivity; several hydraulic cut-offs can be created in the aureole of the argillitedamaged by the excavation. The motivations for these cut-offs and their definition are discussedfurther on, in § 7.7.2.5.To ensure sufficient, long-term swelling pressure under all circumstances, the clay is given an initialswelling pressure of 3 MPa.The swelling clay being considered is MS80 127 or equivalent. This industrially-used clay has aparticularly low permeability of less than 10 -13 m/s at the scale of the material. In addition, thispermeability seems less dependent on the dry density than most other industrial clays (it remains loweven at low densities). This is a clear advantage in this situation, where restricted swelling pressure isimportant.To avoid fracturing the argillite on the excavation wall, the core swelling pressure must not exceed13 MPa, as for the C waste cell plugs (see Chapter 5). In addition, the support engineered structures(retaining plug and bakfill) will be less under stress the lower the swelling pressure. Nevertheless,there must be sufficient pressure to ensure leaktightness in contact with the drift wall: to have aswelling reserve, the intention is that the core swelling pressure is always higher than or equal to1MPa during the evolution of the engineered structure.The principle of this evolution is similar to the C waste cell plugs (Chapter 5). The core swellingpressure starts with a first maximum after resaturation; this initial swelling pressure depends entirelyon the formulation and installation conditions of the clay (dry density, water content, clearances leftfor installation). A relief period may follow, reducing the swelling pressure, if an alteration in theretaining plugs causes the core to expand laterally: in this pessimistic situation, it is important tomaintain the swelling pressure above 1MPa. Ultimately, the pressure will increase to a state ofequilibrium with the rock, corresponding to the effective geostatic stress (7 MPa at the depth of theunderground research laboratory).For the pessimistic situation adopted for the dimensioning, the following figure illustrates (i) theprogressive reduction in the core swelling pressure, with its volume expansion, and (ii) the increase inthe mechanical containment capability of the support backfill as it is compacted by the coredeformation. It shows that based on an initial swelling pressure of around 3 MPA, the backfill canblock the core deformation whilst it shows a residual swelling pressure of 1MPa.127 It is a natural, sodium smectite mined in Wyoming (United States)DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM311/495