Effect of prolonged drying at high temperature on the water retention capacity of bentonite (FEBEX-DP samples)

The FEBEX ( Full-scale Engineered Barrier Experiment) in situ test simulated the engineered barrier of a nuclear waste repository and was in operation for 18 years under natural conditions, with a heater simulating the waste container. The water retention curves of bentonite samples retrieved during dismantling of the in situ test (FEBEX-DP samples) were determined in the laboratory with the vapour transfer technique, following wetting paths under isochoric conditions. The water retention curve relates suction (or relative humidity) to bentonite water content. The aim of the tests was to check the effect of prolonged and intense drying on the water retention capacity of the bentonite. To accomplish this, samples from the drier blocks in the barrier –those closest to the heater, whose surface temperature was 100 °C– and from the core of the barrier in cool areas were analysed. For suctions above 10 MPa, the water retention curves determined in the laboratory were affected by the water content of the samples retrieved: samples with higher initial water content tended to reach higher water contents for a given suction. However, the samples that had been taken from locations closest to the heater, which had the lowest initial water contents, reached higher water contents for the lowest suctions (<10 MPa) than the rest of the samples, which attest that the water adsorption capacity was not lost as a result of prolonged drying. The comparison of the water retention curves obtained for the retrieved samples with those for the FEBEX reference bentonite, compacted at similar densities, shows that there were no changes in the water retention capacity as a result of operation, and that the water adsorption capacity of the bentonite under constant volume conditions is mostly conditioned by dry density, irrespective of the previous hydraulic history.