Hydro-mechanical behavior of claystone-based backfill materials under geo-environmental conditions

In the context of the Cigeo-project, the French agency in charge of radioactive waste management (Andra) studies claystone-based materials as to whether they can be potentially used to backfill shafts and drifts of the future repository for intermediate - and high-level radioactive waste located in the clay-rich Callovo-Oxfordian (COX) sedimentary rock formation at a depth of around 500 m. The installation of backfill in shafts and drifts generally aims to ensure the integrity of the repository system upon its closure. In particular, it is installed to stabilize the geological formation and to limit the propagation of the damaged/ disturbed zone evolving around the excavations. Potential backfill materials must thus sustain the overburden pressure and exhibit swelling pressure when they saturate under constant-volume conditions. In addition, the closure of hydraulic conductive voids ensues through the swelling under constant-volume conditions. Crushed and sieved COX-claystone spoil and its mixtures with MX80-bentonite are potential backfill materials as they contain smectite, which typically exhibits such a hydromechanical behavior. On account of the fraction of smectite, potential backfill materials and their hydro-mechanical behavior are affected by different geo-environmental conditions. In this laboratory experimental study, it is of general interest to analyze how variations in geoenvironmental conditions affect the performance of potential claystone-based backfill materials, in particular their volume change and hydraulic conductivity behavior. Relevant geo-environmental conditions are the fraction of smectite in the materials, the maximum grain diameter of the bentonite fraction in the mixture, the as-compacted/ initial dry density, the degree of saturation, the saturating solution chemistry, in particular its pH, and their combinations. Variations in the as-compacted/ initial dry density are expected to be of greatest relevance as conventional compaction techniques might be employed to compact the backfill material in-situ. The hydro-mechanical behavior of processed COX-claystone spoil and its mixture with MX80- bentonite are analyzed by means of constant-volume swelling pressure, free-swell potential, one-dimensional compression/ oedometer and constant-head hydraulic conductivity experiments. Initially, it is evaluated how the fraction of smectite in the materials, the maximum grain diameter of the bentonite fraction, the initial dry density and the saturating solution chemistry affect individually and combined the evolution of the swelling pressure of claystone-based backfill materials. The individual and combined impact of the fraction of smectite in the materials as well as of the saturating solution chemistry on the evolution of the hydraulic conductivity are subsequently assessed. In order to analyze the impact of the pH of solutions, the hydraulic conductivity experiments are complemented by microstructural and textural analysis. The volume change behavior of the processed COX-claystone/ MX80-bentonite-mixture are evaluated not only in the saturated but also in the unsaturated state allowing to identify possible hydro-mechanical path dependencies. By considering samples compacted to different initial dry densities, it is assessed whether the initial dry density affects possible dependencies of the volume- change behavior on the hydro-mechanical path. Based on the performed investigations, conclusions regarding the hydro-mechanical behavior of claystone-based backfill materials are drawn, and suggestions for future studies are made. (author)