Architecture and management of a geological repository - Andra

7 – The shafts and the drifts7.7.2.4 Description and creation technique for support backfillLike the drift liner, the retaining plugs may ultimately be altered chemically. As indicated above, thesupport backfill will take over the role of mechanical containment of the swelling core.The principle adopted is to restrict the seal from moving by backfill friction on the drift wall. Thesupport backfill must therefore be given rigidity and friction. A 20 MPa modulus of deformation istherefore considered together with an angle of friction (internal or on concrete) of 40°. Theseproperties can be obtained with a mix, in near proportions, of excavated argillite and sand, with theexcavated argillite being ground and screened to 20 mm as for the standard backfill.The length of the support backfill allows the friction between the backfill and the drift wall to balancethe swelling pressure of the core. This length is about four times the excavated diameter 129 .The same basic techniques are used to install this support backfill as for the standard backfill. Thegrouting (filling) of the drift crown can be improved by clay powder injections.7.7.2.5 Treatment of the argillite zone damaged by the drift excavationThe argillite damaged zone around the seal engineered structure may limit its overall performance, ifthis zone shows inferior hydraulic properties to the rock or the swelling clay core. Andra hasconsidered two design options, based on the presence or otherwise of a more permeable damaged zone(particularly a fractured zone).• Design options consideredThe first option involves removing the liner the entire length of the argillite core and ensuring directcontact between the swelling clay in the seal and the argillite. This solution is appropriate if thedamaged zone is of limited extent and its evolution results in it recovering low permeability. Thepressure applied by the swelling of the core, the creep or relaxation of the argillite may restoreproperties close to the undamaged rock, by reclosing the fissuring. This type of process has beenobserved in the clay in the Mont Terri underground laboratory in Switzerland. This can also be causedby the fissures being healed or clogged chemically.The second option can deal with the appearance of a far more extended fractured zone, for whichuncertainties exist on its ability to recover a permeability close to that of the undamaged rock. Itinvolves interrupting the fractured zone by thin grooves, filled subsequently with swelling clay. Thesehydraulic cut-offs are effective when the potentially fractured argillite is replaced by a less permeablematerial. The technique envisaged to create these grooves limits the appearance of new damagedzones. Nevertheless, should a damaged zone be created, particularly at their extremity, checks havebeen made that the hydraulic cut-offs retain their effectiveness, as these new damaged zones do notconnect with each other or with those already existing. The drift liner could also be maintained eitherside of the grooves.On the site studied, the appearance of a fractured zone is considered possible around the drifts. As aprecaution at the current state of knowledge, Andra is taking the second option as the reference fordrift seals: hydraulic cut-offs, with localised liner removal.• DescriptionThe grooves in question are around 30 cm thick and 1.5 to 3.0 metres deep. This is deeper than theliner thickness and any argillite fractured zone. The groove can therefore anchor itself in an argillitezone that has not been microfissured during creation of the drift.It is planned to space the grooves around 8 m apart; this could be modified in the light of the extent ofthe induced damaged zones, to optimise the hydraulic effectiveness of the cut-offs.129 The excavated diameter, not the useful diameter, is considered here: when the backfill is applied as support, the surrounding liner canalso become degraded; it can then be considered an integral part of the support backfill and resist by its friction on the argillite on theexcavation wall.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM314/495