Impact of reconstitution on the hydro-mechanical behaviour of Opalinus clay shale

Abstract This study investigates the impact of clay shale reconstitution on its hydro-mechanical behaviour, using grain size distribution as a key parameter to assess the degree of sample preparation. This topic is of practical relevance, as the characterisation of fine-grained soils often relies on their intrinsic properties, typically measured after reconstitution. While fine-grained soils that can be homogenised with minimal mechanical effort follow a fairly uniform preparation process, this is not the case for materials requiring initial pulverisation. To address this, three batches of remoulded Opalinus Clay samples (fine, medium, and coarse) were prepared. Their hydration curves, swelling pressure, and compressibility behaviour were analysed. Additional classification tests, pore size distribution measurements, and X-ray diffraction (XRD) analyses were performed to support physical characterisation. XRD revealed no preferential breakage of specific clay minerals or segregation of grain sizes. Test results indicated that preserved bonding in coarser grains acts as an additional attractive force, limiting hydration potential and reducing water adsorption. In contrast, finer materials showed higher hydration and swelling pressure due to increased surface area. While compressibility behaviour remained similar across samples at high stress, coarse-grained samples exhibited overconsolidated behaviour at low stresses, attributable to the structural integrity of larger grains.