Pellet mixtures in isolation barriers

Abstract

Granular mixtures made of high-density pellets of bentonite are being evaluated as an alternative buffer material for waste isolation. Ease of handing is an often-mentioned advantage. The paper describes the experimental program performed to characterize the hydromechanical behaviour of compacted pellet’s mixtures. Grain size distribution was adjusted to a maximum pellet size compatible with the specimen’s dimensions. Dry densities of statically compacted specimens varied in most of the cases in the range: 1.3 to 1.5 Mg/m3. Pellets had a very high dry density, close to 2 Mg/m3. To outstanding characteristics of these mixture is its discontinuous porosity. Pore sizes of the compacted pellets vary around 10 nm. However the inter-pellet size of the pores is four to five orders of magnitude higher. This double porosity and the highly expansive nature of the pellets controlled all the hydraulic and mechanical properties of the mixture. Test performed include infiltration tests using different water injection rates and mechanisms of water transfer (in liquid and vapour phases), suction controlled oedometer test and swelling pressure tests. The interpretation of some of the tests performed required backanalysis procedures using a Hydro-Mechanical(HM)computer code. Material response was studied within the framework of the elastoplastic constitutive model proposed by Alonso et al.(1990) (Barcelona Basic Model, BBM). Parameters for the model were identified and also a set of hydraulic laws necessary to perform coupled HM analysis. A large scale in situ test (the “EB” test in Mont Terri, Switzerland) is described and analyzed. Rock barrier parameters were adjusted on the basis of available test. The test excavation, barrier emplacement and forced hydration was simulated by means of the CODE_BRIGHT program. The comparison between measurement and computed results include data on relative humidity in the rock and the buffer, swelling pressures and displacement.