Microstructural Changes Underlying the Macro-response of a Stiff Clay

The paper presents a research approach in which the investigation of the macro-behaviour of a natural stiff clay through element testing is systematically combined with the analysis of the microstructural features of the clay and of the changes taking place at the micro-scale. The objective is that of recognizing internal features and processes causing specific behavioural facets and assess their influence on algorithms and parameter values adopted by models, with the final purpose of connecting classes of behaviour and corresponding models to classes of clays. The microstructural features of the undisturbed natural Pappadai clay are analysed first and then, by comparison, the microstructure evolution is checked under different loading paths. In the present paper, the microstructural assessment of the natural clay after one-dimensional compression to medium and large pressures is discussed. The clay fabric is qualitatively investigated by means of scanning electron microscopy (SEM); a statistical analysis of the orientation of particles is carried out by means of image processing, allowing to quantify the fabric orientation. The bonding strength is assessed by means of chemical micro-probing in the SEM and indirectly through the effects of on purpose strain paths affecting it. The pore size distribution of the clay is investigated by means of mercury intrusion porosimetry (MIP).