Unsaturated Soils

This general report summarises the contributions on unsaturated soil mechanics submitted to the Discussion Session of TC106 – Unsaturated soils –at the 18th International Conference on Soil Mechanics and Geotechnical Engineering held in Paris in September 2013. The thirty-five papers collected under the framework of unsaturated soil mechanics cover a broad spectrum of problems and procedures at varying scales. Much attention is devoted to issues related to experimental techniques and procedures for hydro-mechanical characterisation of unsaturated soils, with special attention to retention behaviour. Swelling, shrinkage and eventually cracking are the processes which seem to capture most of the attention in view of the performance of engineering systems. A few contribution deal with constitutive and numerical approaches, while only a couple of papers introduce unsaturated soil mechanics into engineering practice. While innovative efforts are mainly addressed to experimental techniques in the laboratory, the most challenging issues in future perspective appear to be related to the assessment of unsaturated geotechnical systems in the field, including contaminated soils and mine tailings, besides to more traditional applications dealing with compacted soil structures and soil-atmosphere interaction

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

A laboratory investigation on an undisturbed silty sand from a slope prone to landsliding

A laboratory investigation is presented for undisturbed samples of a silty sand under saturated conditions. The soil was sampled from test pits south of Rüdlingen in North-East Switzerland, where a landslide triggering experiment was carried out on a steep forest slope. The aim of the work was to characterise the behaviour of the soil in triaxial tests, in the light of the possible failure mechanisms of the slope. Conventional drained and undrained triaxial tests were conducted to detect critical state conditions as well as peak shear strength as a function of confining pressure. Soil specimens were also exposed to stress paths simulating in situ water pressure increase to study the stress-strain response and to enhance the ability to predict failure conditions more accurately in the future. Possible unstable response along the stress paths analysed was investigated by means of second order work and strain acceleration. The results show that temporary unstable conditions may be encountered for this soil at stress ratios below ultimate failure and even below critical state line, depending on void ratio, drainage conditions and time dependent compressibility. A modified state parameter is explored as a potentially useful tool to discriminate conditions leading to eventual collaps

Effects of repeated hydraulic loads on microstructure and hydraulic behaviour of a compacted clayey silt

Soils used in earth constructions are mostly unsaturated, and they undergo frequent drying-wetting cycles (repeated hydraulic loads) due to changes in climatic conditions or variations of the ground water level, particularly at shallow depths. After compaction, changes in water content can significantly influence the hydromechanical response of the construction material, which therefore has to be assessed for repeated hydraulic loads. This research investigates the effect of such loads on the microstructure and hydraulic behaviour of a silty soil, typically used in the construction of embankments and dykes, with the aim of providing a better understanding of the consequences of drying-wetting cycles on the response of the material over time. Experimental tests were performed to study the impact of drying-wetting cycles on the water retention, hydraulic conductivity and fabric of compacted specimens. Fabric changes are documented to take place even without significant volumetric strains, promoting an irreversible increase in the hydraulic conductivity and a reduction in the capacity to retain water compared to the as-compacted soil. The fabric changes are interpreted and quantified by means of a hydromechanical model, which accounts for the evolving pore size distribution at different structural levels. The proposed model reproduces quite well the microstructural observations, together with the evolution of the water retention behaviour and of the hydraulic conductivit

A model for coupled electro-hydro-mechanical processes in fine grained soils accounting for gas generation and transport

A theoretical and numerical model is developed for the quantitative analysis of coupled processes taking place in active waste containment systems, such as electrokinetic barriers or fences, in which alow intensity DC current is circulated across the clay barrier to move polar and non-polar contaminants. A novel feature of the proposed approach is the allowance for the presence of air in the pore space. Under unsaturated conditions, all transport coefficients involved in the electrokinetic process are strongly dependent on the degree of saturation of pore liquid. In order to assess the predictive capability of the proposed theory and to appreciate the impact of gas production at the electrodes, a series of numerical simulations of simple onedimensional electrokinetic tests have been performed. The results of the simulations compare reasonably well with data obtained from laboratory experiments performed on an illitic clayey silt. The numerical results indicate that the impact of gas production at the electrodes can be significant, even in low-intensity and short-duration treatments

MODELLAZIONE FISICA DEL FENOMENO DI EROSIONE INTERNA

Fra le cause di collasso di pendii e rilevati in terra soggetti a filtrazione vi è l’erosione interna di particelle, fenomeno evolutivo che può manifestarsi inizialmente in modo diffuso, per poi localizzarsi in canali di flusso, oppure innescarsi lungo discontinuità preesistenti. Questo contributo presenta lo sviluppo di un’attrezzatura di laboratorio per l’osservazione e la misura del fenomeno di erosione interna e formazione di canali di flusso alla piccola e media scala. Consente di analizzare l’influenza del percorso di flusso e di una pressione di confinamento, di condurre prove in regime di filtrazione confinata o non confinata, e di ricostruire modelli fisici in scala ridotta di pendii e rilevati, per strati di qualunque inclinazione

Experimental techniques for multi-scale description of soil fabric and its dual pore network

Fabric affects many aspects of soil mechanical behaviour. When transport processes are of concern, its dual, the pore network, is the key aspect ruling the soil properties of interest. In this chapter, a review of a few widely used techniques, currently adopted to analyse the pore network at increasing scale, is presented, namely mercury intrusion porosimetry, environmental scanning electron microscopy and electrical resistivity tomography. Details on the techniques, their advantages and limitations, are first covered, followed by the presentation of selected test results. The results highlight how these techniques provide an insight into the pore network, and how they can be usefully exploited in the understanding of different hydro-electromechanical processes ordinarily observed at the phenomenological scale. Attention is focused on unsaturated soils with reference to water retention properties, micro / macrostructure interaction, and role of sample heterogeneity.Postprint (published version

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

Gas migration in a Cenozoic clay: experimental results and numerical modelling

Gas migration through a potential host clay formation for the geological disposal of radioactive waste in Belgium is experimentally investigated in the laboratory, and numerical modelling is performed to help in the interpretation of the results. Selected air injection tests under oedometer conditions on initially saturated Boom Clay samples with oriented bedding planes are presented in the paper. Priority in the experimental programme was given to the study of the deformation response along the injection and dissipation stages, as well as to the analysis of the pore network changes, which detect the opening of fissures that can act as preferential air pathways. The experimental results were simulated using a fully coupled hydro-mechanical finite element code, which incorporates an embedded fracture permeability model to account for the simulation of the gas flow along preferential pathways. Clay intrinsic permeability and its retention curve were assumed to be dependent on strains through fracture aperture changes. The numerical results could reproduce upstream/downstream pressures, outflow volume and soil volume change accurately. The experimental results, combined with the numerical simulation, provide good insight into the role of the volumetric response and of the bedding planes on the air transport properties of Boom Clay samples, confirming that fracture aperture occurs during gas injection, which eventually dominates further injection and pressure release stages.Peer ReviewedPostprint (author's final draft

Ageing effects on the small-strain stiffness of a bio-treated compacted soil

The effect of ageing on the small-strain shear stiffness of compacted silty-clayey sand following a ‘soft’ biological treatment is discussed. Samples were prepared by static compaction, adding urea-degrading bacteria to the compaction water. No nutrients were artificially added, relying on the natural availability of urea and calcium ions (Ca2 +) in the humic soil and in the compaction water for the bacteria to precipitate calcium carbonate. After compaction, to replicate different environmental boundary conditions, some of the samples were cured in a system open to vapour transfer and some were cured in a closed system. The small-strain shear stiffness was periodically tracked with bender elements during the ageing period. Tests were run in parallel to investigate the unconfined compression strength, the water retention properties and soil pore size distribution changes during ageing. The test results revealed a small but consistent increase in the small-strain shear stiffness during the ageing period due to the microbiological treatment, in both the closed and open systems. In the latter case, the contribution of the microbiological treatment to the increase in stiffness could be estimated after correcting the data for the suction increase due to evaporation.Peer ReviewedPostprint (author's final draft