Swelling Behavior of Bentonite-Based Backfilling Materials in Nuclear Waste Repository Conditions

Abstract

During the postconstruction phase, the stress equilibrium conditions within the proposed underground nuclear waste repositories may alter because of various factors, such as far-field mining activity, fluctuation of the groundwater table, and seepage erosion. A reduction in the transmitted stress from the host rock may destabilize the isochoric condition, leading to swelling displacement of backfilling and buffer materials. In the current study, the swelling behavior of bentonite-based backfilling materials was studied. Laboratory tests were carried out on compacted bentonite (dry density=1.26 Mg/m3) and bentonite-sand (70% bentonite, 30% sand, dry density=1.50 Mg/m3) specimens in a strain-controlled oedometer. First, compacted specimens were hydrated with distilled water under constant volume condition. Next, predetermined displacements were introduced in a step-wise manner that caused expansion of the specimens. The swelling pressures of the specimens were measured under a constant-volume condition at each induced strain. The test results showed that induced strains caused a decrease in the swelling pressures of the saturated backfilling materials. A decrease in the swelling pressure for a maximum induced strain of approximately 1.6% was found to be approximately 19% for the bentonite specimen and approximately 30% for the bentonite-sand specimen. It was shown how the swelling index of backfilling materials can be used to assess the recoverable swelling pressure following any induced strain.