Engineered barrier of bentonite pellets and compacted blocks: State after reaching saturation

• Large-scale in situ testing of a barrier of bentonite blocks and pellets.
• Bentonite barrier artificially hydrated with Pearson water at isothermal conditions
• After 10.5 years of operation the bentonite was saturated and the test was dismantled.
• The pellets looked homogeneous, every void and gap was sealed.
• Dry density and water contents of blocks and pellets were similar.

The EB experiment was a large-scale test performed in the Underground Research Laboratory of Mont Terri (Switzerland) for the demonstration of an engineered barrier concept for nuclear waste disposal consisting of the simultaneous use of high-density bentonite blocks and a lower-density bentonite pellets mixture (the granular buffer material, GBM). For that purpose, a gallery was excavated in the Opalinus clay and a dummy waste canister was placed on a bed made of bentonite blocks and surrounded by the GBM material. The bentonite barrier was artificially hydrated with Pearson water and after 10.5 years of operation at isothermal conditions it was considered that the bentonite was completely saturated and the dismantling of the barrier was undertaken. A sampling campaign was done to assess the final state of the bentonite barrier with regards to dry density and water content.Upon dismantling, the GBM looked perfectly homogeneous, with every void and gap between the different elements (blocks/GBM, GBM/host rock, GBM/canister, etc.) having been sealed. Full saturation had been reached all through the barrier. Moreover, the dry density of the blocks had decreased to values similar to those of the GBM, and the average water contents for both kinds of materials were similar. Nevertheless, the initial conditions of the system did have a certain impact on the final distribution of dry density and water content: the bottom of the barrier had a chance to quicker and higher water uptake (due to the heterogeneities in the initial porosity and characteristics of the artificial hydration system), which gave place to immediate swelling that resulted irreversible, with permanent higher water contents and lower dry densities towards the floor and back of the gallery, particularly in the GBM. Despite these heterogeneities, the water contents and dry densities of the whole barrier (GBM and blocks) were much more homogeneous than at the beginning of the test and remained within a relatively narrow range. The bentonite degree of saturation was homogeneous and very close to 100% all through the barrier.The feasibility and performance of this kind of initially heterogeneous barrier was proved in that it had an optimal sealing capacity and developed acceptable swelling pressures between 1.3 and 2.2 MPa.