Fabric and clay activity in soil water retention behaviour

Modelling the water retention behaviour requires proper understanding of all the processes which affect the amount of water stored in the pore network, depending on the soil state and the soil history. Traditionally, in many applications a single water content – suction curve is used. This approach limits the applicability of the retention data to practical cases, especially when fine grain soils are dealt with, when the deformability and activity of the clay fraction significantly affect the interaction with water. On the other side, water retention is being recognised more and more as a fundamental information in the description of the mechanical response of the soil, as it provides the key connection to the partial volumetric strains in a deformation process. With reference to the work performed at the Politecnico di Milano in the last years, a contribution on the understanding and modelling the coupled water retention- mechanical response in deformable soils is presented. The contribution aims to: (i) summarise the mechanisms which contribute to water retention; (ii) point out the role played by an evolving fabric and the fluid properties on water retention; and (iii) provide an overview on some of the consequences of evolving water retention properties on the mechanical behaviour

Some remarks on single- and double-porosity modeling of coupled chemo-hydro-mechanical processes in clays

Active clays are known to possess an aggregated structure, which justifies the use of double-porosity models to reproduce their behavior. Simulation of chemo-mechanical processes requires instead the introduction of a relevant number of coupled mechanical and transport laws. It follows that double porosity models for coupled chemo-hydro-mechanical require a relevant number of parameters, which are twice those needed by single porosity models. The aim of this work is to evaluate the consequences of using single- and double-porosity frameworks to simulate the transient chemo-mechanical processes in active clays, showing how models based on simple microstructural considerations can help in performing simulations which are a reasonable trade-off between simplicity and accuracy. In particular with single porosity models, it might be necessary introducing parameters having a doubtful meaning to describe adsorption-desorption processes. This type of assumption is not required by double porosity models. While for compacted clays these conclusions are corroborated with microstructural observations, the same hold also when reproducing the behavior of an active clay at a remolded condition. In this latter case the delay of swelling with respect to desalinization, typical of remolded conditions, was satisfactorily reproduced only with double porosity models

A water retention model for compacted bentonites

peer reviewe

A constitutive framework for the chemo-mechanical behaviour of unsaturated non-expansive clays

Both osmotic and matric suction changes have a significant influence on the mechanical behaviour of clays. Despite the different types of interactions at the microstructural level, both suctions havea relevant effect on the fabric of non-expansive clays. Starting from experimental observations at the laboratory scale, it is possible to identify some common features characterizing the mechanical response of non-expansive clays to salinity and degree of saturation changes. This paper presents an elastoplasticframework to reproduce the behaviour of unsaturated clayey soils upon changes in the salt concentration of the pore fluid. In particular, it presents a strategy to include osmotic suction induced by pore fluid salinity in BBM-like models [1]. The model was implemented in the Thebes code and it was calibrated on experimental data performed on Boom clay [2] and remoulded loess [3]

Modelling desiccation cracking in a homogenous soil clay layer: comparison between different hypotheses on constitutive behaviour

Desiccation cracks are usually thought to start from the surface of an evaporating soil layer, and the available simplified models for crack initiation and propagation are based on this hypothesis. On the contrary, experimental results on a Dutch river clay showed that cracks in an evaporating soil layer may start and propagate below the surface, confirming earlier findings by other researchers. A simple one-dimensional model was set up to analyse the consequences of different hypotheses about the material behaviour on the crack onset in a homogenous soil layer undergoing surface drying. The results of the model show that dependence of the material behaviour on the rate of water content change is a necessary requirement for cracks to initiate below the surface. The conclusion suggests that, to properly understand cracking in an evaporating soil layer, an intrinsic time scale for the mechanical response must be accounted for, among all the other factors which were previously highlighted by other researchers. The key factor to predict crack onset below the surface is the dependence of the drying branch of the water retention curve of the compressible soil on the rate of drying, which would be justified by a rate dependent fabric evolution

Un modello di danno fragile per mezzi porosi: esempi di applicazione

Si presenta un approccio accoppiato per modellare il danneggiamento indotto da sollecitazioni idrauliche e meccaniche in ammassi rocciosi. Il danneggiamento del materiale è legato alla formazione a scala microstrutturale di diverse famiglie di fratture parallele, annidate una nell’altra, ciascuna caratterizzata da una propria orientazione e spaziatura. La semplicità della geometria delle fratture permette di esprimere analiticamente la variazione di porosità e di permeabilità causate dal progressivo danneggiamento del materiale. Si illustrano alcune simulazioni, sia a livello di punto di volume sia come problema al contorno, per mettere in evidenza i potenziali campi di applicazione del modello, tra i quali si individuano la stabilità di perforazioni in roccia e l’ottimizzazione di processi di fratturazione idraulica

3D-electrical resistivity tomography monitoring of salt transport in homogeneous and layered soil samples

Monitoring transport of dissolved substances in soil deposits is particularly relevant where safety is concerned, as in the case of geo-environmental barriers. Geophysical methods are very appealing, since they cover a wide domain, localising possible preferential flow paths and providing reliable links between geophysical quantities and hydrological variables. This paper describes a 3D laboratory application of Electrical Resistivity Tomography (ERT) used to monitor solute transport processes. Dissolution and transport tests on both homogeneous and heterogeneous samples were conducted in an instrumented oedometer cell. ERT was used to create maps of electrical conductivity of the monitored domain at different time intervals and to estimate concentration variations within the interstitial fluid. Comparisons with finite element simulations of the transport processes were performed to check the consistency of the results. Tests confirmed that the technique can monitor salt transport, infer the hydro-chemical behaviour of heterogeneous geomaterials and evaluate the performances of clay barrier