Architecture and management of a geological repository - Andra

7 – The shafts and the driftspressure (MPa)3,02,52,01,51,00,50,0Swelling clay pressureBackfill reaction0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0lengthening of clay core (m)Figure 7.7.3Equilibruim between the pushing of the core swelling and the reaction of thebackfill (initial move of 1 m before placing the backfill under stress and backfillstiffness of 1 m/MPa)Note that for the C waste cell plugs, a greater initial swelling pressure is sought (see Chapter 4), giventhe restricted length of these plugs.• Constructing, shaping and installing the core materialTo achieve an initial swelling pressure of 3 MPA, the knowledge acquired on swelling clay MX80[47] shows that the dry clay density should be 1.4 to 1.5 approximately (in a mix, this is the density ofclay in the mix 128 ). This is an average value on the engineered structure given the voids at installation[38].Two implementation techniques are possible to achieve this. The first involves the factory prefabricationof pressed, compacted blocks, then their dry assembly in the drift being sealed. To adjustthe swelling pressure, the blocks consist of a mix of clay and sand (with 20 to 30% sand). The secondtechnique is to spray high-density aggregate (or pellets), possibly mixed with pulverulent clay. Forexample, the average dry density sought can be achieved with compacted clay pellets with a density of1.8 and a void coefficient between pellets of 20%, representative of traditional spraying techniques.The use of pre-compacted blocks has been full-scale tested in the Tunnel Sealing Experiment (TSX) inthe Canadian Underground Research Laboratory (Figure 7.7.4). To improve filling of the drift andobtain a more homogenous core, the peripheral voids, particularly in the cell, were filled with claypowder (or a mix of pellets and powder).128 Clay mass/water mass occupying the same volume as the mixDOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM312/495