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Because of their mineralogy, active clays are very sensitive to changes in the type and concentration of species dissolved in the pore water. At a molecular level, this sensitivity can be explained by variations in the short range interaction forces acting between clay platelets immersed in the pore water. While models based on equilibrium of short range interaction forces can be used to predict the volumetric behavior of dilute clay suspensions, different approaches must be invoked for real soils. Natural and compacted soils react differently from dilute suspensions to chemical changes because of their fabric. It is, therefore, not possible to derive reliable relationships (at least from the quantitative point of view) describing the chemo-mechanical behavior of active soils solely on the basis of mineralogical and chemical properties. Both microscopic experimental evidences and modeling approaches are used here to analyze the effects imparted by fabric on the coupled hydro-chemo-mechanical behavior of an active clay soil.

The electrical conductivity of a soil can be related to the electrical conductivity of its solid and fluid constituents by means of theoretical and empirical relationships, taking into account the effect of porosity, saturation degree, fabric and clay content. Hence monitoring the evolution of the electrical conductivity inside a soil sample can provide useful information concerning the progress of hydro-chemomechanical processes and the subsequent effects on both soil skeleton and pore water. With this aim a laboratory apparatus for 3D electrical resistivity tomography has been recently developed. Some applications of this apparatus to the monitoring of different geotechnical processes in which the knowledge of the water content and of ionic concentration is particularly relevant are presented.

This work presents an insight into double-structure effects on the coupled chemo-hydro-mechanical behaviour of a compacted active clay. In the first part, selected pore size distribution curves are introduced, to highlight the influence of solute concentration on the evolution of the microstructure of compacted samples. An aggregated structure with dual-pore network is induced by compaction even at relatively high water contents. This structural arrangement is enhanced by salinisation, and has a notable influence on transient volume change behaviour – that is, the occurrence of different stages of swelling upon pore water dilution and higher volume change rates upon salinisation. A coupled chemo-hydro-mechanical model, taking into consideration double-structural features from a chemo-mechanical viewpoint, is described and then used to interpret these behavioural responses and present complementary information on local transient processes. The model is designed to identify an intra-aggregate and an inter-aggregate domain, and assigns different values of hydraulic pressure and osmotic suction to each domain. Distinct constitutive laws for both domains are formulated, and the flow of salt and water between the two domains is accounted for by a physically based mass exchange term. The model is used to simulate salt diffusion tests run in an oedometer at constant vertical stress. Parameters used in the formulation are calibrated based on separate experimental evidence, both through direct test results and through back-analyses of laboratory experiments.