Effect of technological macro voids on the performance of compacted bentonite/sand seals for deep geological repositories

• Three large scale sealing experiments have been installed in the Tournemire URL.
• The analysis focuses on the impact of macro voids on the performance of seals.
• Impact of macro voids, on saturation and swelling pressure kinetics is demonstrated.
• Heterogeneous distribution of bentonite dry density is expected at long term.

Three in-situ performance tests have been installed in the Tournemire Underground Research Laboratory to investigate the impact of technological voids on the long-term performance of the bentonite-based seals. These tests are still in progress, but the collected set of data provides already valuable information on the hydro-mechanical behaviour of the tested seals. The swelling cores consisted of pre-compacted blocks of a natural sodic Wyoming bentonite (MX80 type) mixed with quartz sand in a ratio of 70/30 (in dry mass) with different geometries (monolithic disks or four jointed disks). Two types of technological macro voids existed within the experiments: annular voids between the bentonite-based blocks and the rock and voids between the blocks. Artificially injected water volume, relative humidity, and swelling pressure in both radial and axial directions were monitored. Comparison of the results showed that the presence of technological voids constituted new hydration sources (annular voids) and flow paths (voids between the blocks) that changed the saturation kinetics. The effect of technological voids was also evidenced when comparing the swelling pressure kinetics. The existence of an additional void for one test between the packer and the host-rock greatly influenced the axial swelling pressure evolution, since it constituted a free volume into which the bentonite could extrude. Compared to monolithic disks, the evolution rates of radial swelling pressure were significantly lower in case of four jointed disks. For each test, different evolution of radial swelling pressure was observed, suggesting a heterogeneous structural distribution of bentonite/sand mixture within the blocks.