Architecture and management of a geological repository - Andra

7 – The shafts and the drifts7.7.1.2 DescriptionBackfill is composed of excavated argillite which, following storage at the surface, is reconditioned bybeing crushed to a thickness of 20 millimeters. Its water content is close to that of the normal Proctoroptimum (NPO) 126 , i.e. 10 to 15%. The aim is to have a dry backfill density of at least 1.6 at the timeof installation, with a swelling potential of one percent or slightly more.7.7.1.3 Installation proceduresBackfill is initially prepared at the surface before being taken to the backfill site. It can be installed byhorizontal compacting in the lower section of the drift, with inclined sloped layers in the upper part ofthe drift. Horizontal tamping is done using standard roadwork equipment. For oblique layers, it wouldbe possible to use a rammer placed at the end of the articulated arm of an earth mover; obliquecompacting is undergoing a demonstration test.A hydraulic ram was therefore tested on backfill made up of pure argillite or argillite-sand mix in aconcrete structure simulating a drift (see Figure 7.7.1). Dry densities in excess of 1.7 were obtained.Figure 7.7.1Hydraulic ram in action in the backfill demonstration model7.7.2 Drift sealsEach connecting drift seal consists of a core of swelling clay, which gives the engineered structure itsvery low permeability, and the concrete retaining plugs which give the core its mechanicalcontainment (see Figure 7.7.1). The mechanical properties of the drift backfill are reinforced comparedwith the standard backfill over twenty to thirty metres approximately on both sides, to contribute to themechanical containment of the engineered structure.This principle has to be adapted to the special configuration of the B waste disposal cells when sealingthe drifts giving access to these cells.126 The NPO, or normal Proctor optimum, is a standard test used to determine a material’s most favourable water content for compactionpurposes. Backfill mechanical resistance is favoured by slightly higher water content than NPO.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM309/495