Shot-clay MX-80 bentonite: An assessment of the hydro-mechanical behaviour

• An innovative technique for bentonite emplacement is considered: shot-clay.
• Hydro-mechanical behaviour of shot-clay granular bentonite is analysed.
• The performance of shot-clay bentonite and compacted granular bentonite are compared.
• Dry density at emplacement is the main influence on bentonite performance.

This study presents the results of an experimental study conducted to characterise the hydro-mechanical behaviour of shot-clay MX-80 bentonite. In the shot-clay process, granular bentonite was mixed continuously with water and shot on the walls of a tunnel section at the Grimsel Underground Research Laboratory (URL), in Switzerland. The shot-clay was placed to create a layer of bentonite in direct contact with the host rock to avoid preferential water and/or gas flow along the tunnel wall. Samples for an experimental programme were collected during the shooting. The index properties, microstructural features, swelling potential and water retention properties of the shot-clay were analysed. An experiment was then conducted under controlled total suction to analyse the hydro-mechanical behaviour of the material along a predefined stress path involving suction and confining stress variations. Based on the results of this test, the expected behaviour of the shot-clay bentonite when subjected to the environmental conditions in the repository was determined. The test results were compared with data on the observed behaviour of compacted MX-80 granular bentonite to assess the effects of the shot-clay emplacement technique on the behaviour of the MX-80 bentonite. The results highlight the role of the emplacement dry density on the behaviour of bentonite.