New challenges in experimental unsaturated soil mechanics. Experimental upscaling of an engineered gas-permeable seal

Abstract

An example of upscaling phenomena with experimental techniques is presented and discussed within an engineered seal concept for a future deep geological repository for low and intermediate radioactive waste mainly produced by the energy/medicine/industry sectors. The seal will undergo a long-term saturation stage and subsequent gas transport through such a barrier. Consequently, gas entrapment and generation processes (dominated by the degradation of organic substances and metals) are expected to occur in the emplacement caverns along the pollutant waste storage. The case concerns a compacted 80/20 (dry mass) sand/bentonite S/B mixture with a complex microstructure that significantly evolves on hydration affecting the gas transport properties. This type of seal limits the gas pressure by increasing its gas transport capacity (gas-permeable seal). The large-scale and demonstration experiment GAST (Nagra's GTS, Switzerland) focuses on the S/B response to saturation and the gas transport capacity in the later gas invasion phase. A series of laboratory experiments running parallel to the in situ test and bridging different scales (from bentonite inter-sand filling microstructural tests and point tests to dm-scale mock-ups) are discussed within the saturation, gas breakthrough pressure and gas dissipation process of the gas-permeable seal context.