Structure and mechanical response of sub-Apennine Blue Clays in relation to their geological and recent loading history

The paper discusses the intrinsic properties, geological history, natural structure and mechanical behaviour of stiff Italian clays, part of the sub-Apennine Blue Clay formation, which are located at different sites within the marine basin where the clay formation was deposited in the early Pleistocene. For all the clay deposits being considered, diagenesis, unloading due to erosion, and weathering of the top strata represent the main stages of the geological history. However, the clays have undergone different levels of unloading, and have developed different levels of structure due to their different locations in the basin. These differences are accounted for in the analysis of their current states and behaviour. Based upon a large experimental database, the structure and mechanical behaviour of the clay samples from the different sites are compared and related to the clay history. In particular, the soil structure is characterised based upon SEM analysis results and compression–swelling test results. The comparison of the response to compression–swelling cycles of the natural clay with that of the same clay when reconstituted leads to the evaluation of parameters such as the stress sensitivity S_σ and the swell sensitivity C*_s/C_s, which are used to assess the structural strength of the natural clay with respect to that of the reconstituted clay. The stress–strain behaviour of the different clays along different constant stress-ratio compression paths and shear paths (from low to high pressures) is discussed in detail. The variations in stress–strain response of samples subjected to different pre-shear consolidation paths are outlined. The corresponding trends of the plastic strain increment ratios are investigated. As a result of the analysis, the features of the gross yield curves of the different clay horizons are also assessed. Finally, the study provides elements for the characterisation of the hardening properties of stiff clays, and therefore of the evolution of the material anisotropy during plastic deformation.