Evolution of the clay micro-structure in compression and shearing loading paths

The work presents a research approach in which the investigation of the macro-behaviour through element testing is systematically combined with the observation of processes and changes taking place at the micro-scale. The approach is aimed at identifying the main internal features which control, at the micro-scale, the material response, causing given behavioural facets, with the final purpose of assessing the influence of the different aspects of behaviour on model parameter values, hence supporting constitutive modelling and finding a relation between classes of behaviour and corresponding models and classes of clays. The tested materials are two illitic stiff clays of marine origin, Pappadai clay and Lucera clay, that are investigated both when natural and when reconstituted in the laboratory. The microstructural analyses are carried out by means of different techniques: scanning electron microscopy is used to provide a qualitative investigation of the clay fabric and is combined with image processing, which allows to assess quantitatively the fabric orientation by means of a statistical analysis of the orientation of particles; the pore size distribution is analysed by means of mercury intrusion porosimetry, which provides information about the distribution of pores. The microstructure is first analysed for both Pappadai and Lucera clay at initial state, i.e., in the undisturbed state for the natural clay and after consolidation in consolidometer for the reconstituted clay. Then, the evolution of microstructure is explored after 1D and isotropic compression to states pre- and post- gross-yield, up to large pressures. A comparison is made between microstructures of specimens 1D and isotropically compressed to the same void ratio. The evolution of both the natural and the reconstituted clay microstructure is also investigated under shearing loading paths, in both drained and undrained conditions. The microstructural features underlying the macro-response exhibited by the clay are also compared with those recognized in the literature for other natural and reconstituted clays. The constitutive laws adopted by some advanced constitutive models to simulate the effects of structure are evaluated in the light of the observation and interpretation of the features and processes acting at the micro-scale and of their evolution under the different loading paths.