Combined impact of selected material properties and environmental conditions on the swelling pressure of compacted claystone/bentonite mixtures

Mixtures composed of 70% crushed Callovo-Oxfordian claystone and 30% MX80-bentonite are considered as materials, that could be used for backfilling a future radioactive waste repository in deep sedimentary rock formations. Their characterization is of interest, as the replacement of fractions of crushed claystone by bentonite enhances the chemo-hydro-mechanical performance of backfill. The materials are envisaged to be installed directly in the drifts and shafts by means of conventional compaction techniques. The hydro-mechanical behavior of materials containing expansive mineral phases, and especially their swelling behavior, is known to be significantly affected by the initial material properties and environmental and stress conditions. The present study aimed to assess the combined impact of variations in the material properties and environmental conditions, particularly the grain size distribution, dry density and saturating solution chemistry, on the swelling pressure of the mixtures, by conducting a comprehensive laboratory experimental program. The results revealed that the adjustment of the grain size distribution of employed bentonite enhanced the compaction behavior and, in turn, the swelling behavior of the mixtures. Generally, swelling pressures of mixtures were less affected by the employed saline and alkaline solutions than those of crushed claystone. The measured swelling pressures were exponentially related to the initial dry density of the expansive mineral phase, regardless of the grain size distribution. Based upon the finding that the expansive mineral phase being present in crushed claystone contributed to measured swelling pressures, a new approach was introduced to calculate the dry density of the expansive mineral phase in bentonites and their mixtures with non-expansive or less-expansive materials.