Physico-mechanical properties of halite and of gypsum-mudstone crushed rock backfills of the Mercia Mudstone Group

Abstract: Permanent disposal of radioactive waste in deep geological repositories (DGRs) relies on engineered barriers, such as crushed rock backfills, to ensure wastes are isolated and contained. The performance of crushed rock backfill, during construction, operation and closure, relies on optimizing its mechanical properties. This paper explores how density and composition influence the strength of (i) crushed halite and (ii) crushed gypsum-mudstone mixtures of the Triassic Mercia Mudstone Group which is a potential DGR host rock in the UK. Unconfined compressive strength (UCS) testing of compacted halite demonstrated that strength correlated with compaction density, with UCS increasing from 220 kPa at a density of 1.72 g cm⁻³ up to 6600 kPa at the maximum achieved density of 1.83 g cm⁻³. Direct shear testing revealed an angle of internal friction for crushed halite samples of 39–40° at a density of 1.5 g ⁻³. Direct shear testing of gypsum-mudstone composites from 100% gypsum to 100% mudstone revealed that higher gypsum percentages provide greater frictional shearing resistance, whilst higher mudstone percentages promote more cohesive strength. Data are also presented on bulk materials characterization by XRF, SEM-EDS and optical microscopy. This research provides insights into how the strength properties of compacted crushed halite and gypsiferous mudstone backfills can be engineered through control of density and composition.