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In the context of the disposal of high‑level nuclear waste (HLW) in clay rock formations, crushed clay‑rock‑based materials can be used to backfill repository drifts and shafts and to construct sealing elements, as their expansive clay content provides favourable properties, such as the development of swelling pressure and the reduction of hydraulic conductivity. The installation of backfill and sealing elements serves to limit the propagation of the excavation damaged zone (EDZ) by stabilizing the surrounding rock formation and by inhibiting fluid transport between the emplacement units and the accessible biosphere. Once installed, these elements complement the host rock and contribute to ensuring the long‑term integrity of the repository.The behavior of backfill and sealing elements - particularly their volume‑change and fluid transfer behavior - is significantly controlled by the initial (or as‑compacted) dry density. This parameter, in turn, depends on the material’s mineralogy, grain‑size distribution, initial (or as‑compacted) water content, and the applied compaction energy.This study presents a simplified approach using the compressibility index to derive the initial dry density of crushed clay‑rock‑based materials from the applied axial stress, initial water content, and expansive clay content.The approach is validated by static compaction experiments in a drained oedometer setup. Prior to compaction, crushed clay rock is mixed with sodium bentonite at three wet‑weight ratios, and each mixture is prepared to six target water contents. The results show that, for low expansive clay contents, the compressibility index exhibits a quadratic dependence on the initial water content, which transitions to a linear dependence as the expansive‑clay content increases. Overall, the findings underline the relevance of this simplified approach for the installation of backfill and sealing elements, particularly with respect to the selection of appropriate installation techniques.