Design and experimental characterization of potential processed clay rock-based backfill and sealing materials

The German generic concept envisages disposing of high-level nuclear waste (HLW) in pre-Tertiary clay rock formations located about 600 m below the ground (BGE 2022), and is based on the safety and safety assessment methodology presented in Jobmann et al. (2017). The methodology presents generic designs of backfill and sealing elements, which must fulfill safety functions, such as stabilizing the surrounding rock formation, limiting the propagation of and/or compressing the excavation damaged zone (EDZ), limiting the advective fluid flow between the biosphere and the emplacement units, and preventing gas overpressure. The safety functions require materials that maintain stiffness, develop swelling capacity, reduce hydraulic conductivity with respect to water, and preserve hydraulic conductivity with respect to gas. Volume-change and fluid-transfer properties must remain stable irrespective of stress state, saturation state, temperature, pore water chemistry, or time. The generic concept proposes crushing and sieving clay rock chips produced during drift and shaft excavation and reusing the processed material either pure or mixed with bentonite or sand as backfill and sealing materials. Because processed clay rock typically contains little smectite, mixing is applied to enhance volume-change and fluid-transport properties. The (partial) reuse is advantageous due to its low costs, high availability and mineralogical compatibility with the host rock formation. Previous studies (e.g., Zhang et al. 2023) have shown that the properties of processed clay rock-based materials can meet the hydraulic and mechanical requirements for backfill and sealing elements. Backfill and sealing elements are installed by either compacting granular material on site using conventional methods (e.g., vibration, rolling) or by placing pellets or blocks being pre-compacted off site. The methods differ in the compaction energy that they can apply. In comparison to conventional methods, the pre-compaction using a hydraulic press enables much higher initial dry densities (?d). Because the installation method determines the initial dry density and thus governs the volume-change and fluid-transport properties, its appropriate selection is essential in order to fulfill the hydraulic and mechanical safety functions. The study experimentally investigates the compaction, volume-change and fluid-transport properties of processed clay rock and its mixtures with bentonite, focusing on the effects of bentonite content and compaction state. The results can then be used to identify potential processed clay rock-based backfill and sealing materials that will be subject to further in-depth analyses.