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A relationship between elastic anisotropy, as typically observed in clayey soils subjected to shear wave propagation tests, and plastic anisotropy, detected at yielding and leading to rotated yield loci, is proposed. Such a relationship is expected because both elastic and plastic anisotropies can be ascribed to the same directional ingredients that characterise the fabric of the soil at the microscale. The relationship takes the form of an analytical relation between an elastic and a plastic fabric tensor, the former entering a hyperelasticity theory while the latter is the rotational hardening variable of a clay plasticity theory. The elastic anisotropy can be measured experimentally by wave propagation along orthogonal planes, identifying the ratio of the corresponding elastic shear moduli while a sample is compressed at fixed stress ratio, and paired with plastic anisotropy obtained by the integration of its plastic fabric tensor evolution equation during the foregoing compression. Such experiments were available and used to calibrate and validate the proposed elastic–plastic anisotropy relationship. The findings have a two-way beneficial effect for the solution of a geotechnical boundary value problem, where one can easily measure initial elastic clay anisotropy in the field, which can be used to initialise the plastic anisotropy for the subsequent analysis of the problem, while the evolving plastic anisotropy can be used to update the elastic fabric tensor during deformation. [ABSTRACT FROM AUTHOR]

Expansive clayey soils, which are prevalent across the surface sediments of the earth, are considered problematic for the structures and infrastructure founded in them. These soils are also challenging for environmental engineers involved in the design and construction of municipal and nuclear waste liners, petroleum engineers dealing with shale oil and gas, and agricultural scientists. Because their overall behaviour is controlled by molecular-level processes and interactions, a behaviour model encompassing the contributions from these interactions is required to predict the behaviour of these soils. This interaction becomes increasingly complex for natural expansive clays where a complex composition of clay and non-clay minerals exists. Unfortunately, none of the existing constitutive models pertaining to expansive clays comprehensively incorporates all the factors contributing to volume change behaviour of naturally occurring expansive soils. This study led to the development of a nano-level constitutive model for natural expansive clays through a three-phase study consisting of macro-level swell potential testing on laboratory control and natural field samples, micro-investigation of the pre- and post-swell specimens, and molecular level modelling through molecular mechanics, molecular dynamics and Monte Carlo techniques. This swell potential behaviour model incorporates all the major contributing factors such as moisture content, density, cation exchange capacity, exchangeable cations and interaction of clay minerals with non-clay minerals. The developed models have been successfully validated through close prediction of the swell potential behaviour of the laboratory control samples and natural field samples. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The paper presents a chemo-mechanical model for expansive clays that takes into account the effects of cation content and cation exchange. These factors play a key role in the mechanical behaviour of very active clays, particularly with regard to volumetric behaviour. The model is based on an existing double-structure formulation that distinguishes specifically between microstructure and macrostructure. Chemical effects are defined at the microstructural level, the seat of the basic physico-chemical phenomena affecting highly swelling clays. The microstructural model accounts for changes in both osmotic suction and in cation content. Microstructural strains are considered to be reversible; material irreversibility arises from the interaction between the two structural levels. The formulation is developed for general unsaturated conditions; saturation is considered as a limiting case. The model is successfully applied to the reproduction of experimental behaviour observed in oedometer tests on saturated bentonite subjected to chemo-mechanical loadings, and in hydration tests of unsaturated bentonite performed using different solute concentrations. [ABSTRACT FROM AUTHOR]

Bentonites are clay soils characterised by a high specific surface and a permanent negative electric charge on their solid skeleton. Their common use as hydraulic and contaminant barriers for landfill and soil remediation applications, including the final disposal of nuclear waste, needs to be supported by adequate theoretical modelling of their mechanical behaviour and transport properties, in order to assess the expected performance in the long term. To this end, a theoretical approach has been proposed in order to derive constitutive equations for their coupled chemical-hydraulic-mechanical behaviour. The phenomenological parameters that govern the transport of electrolyte solutions through bentonites - that is, the hydraulic conductivity, the reflection coefficient (which is also called the chemico-osmotic efficiency coefficient), and the osmotic effective diffusion coefficient - have been measured through laboratory tests on a bentonite with porosity of 0·81, over a range of sodium chloride concentrations in the pore solution that varied from 5 mM to 100 mM. The relevance of the osmotic phenomena has been shown to decrease when the salt concentration increases. The obtained results have been interpreted by assuming that the microscopic deviations of the pore solution state variables from their average values are negligible. In this way, it is possible to interpret the macroscopic behaviour on the basis of the physical and chemical properties of the bentonite mineralogical components. [ABSTRACT FROM AUTHOR]

Engineering issues for which the understanding of the chemo-mechanical behaviour of soils is relevant include wellbore stability problems, the salinification of groundwater, and nuclear waste storage. However, despite the vast number of situations in which couplings between chemistry and mechanics occur, the available constitutive models rely on limited experimental evidence. This paper presents the results of an experimental programme on the chemo-mechanical behaviour of a non-swelling illite. The osmotic suction is controlled through the ion concentration of sodium chloride in the pore water. Stress paths include mechanical loading at a constant osmotic suction, and an increasing osmotic suction at a constant mechanical stress. The experimental results point out a correlation between the osmotic suction and initial oedometric modulus, as well as between the osmotic suction and yield stress. A constitutive framework for soils is extended to take the observed chemo-mechanical couplings into account. The numerical model has been calibrated for the illite using the parameters obtained through the tests under mechanical loading at a constant concentration, and validated using more elaborate stress paths. The presented experimental and constitutive investigation builds a basis for the assessment of engineering issues in which pore liquid chemistry plays a major role. [ABSTRACT FROM AUTHOR]

In this paper a new anisotropic model for time-dependent behaviour of soft soils is presented. The formulation is based on a previously developed isotropic creep model, assuming rotated Modified Cam Clay ellipses as` contours of volumetric creep strain rates. A rotational hardening law is adopted to account for changes in anisotropy due to viscous strains. Although this will introduce some new soil parameters, they do not need calibration as they can be expressed as functions of basic soil parameters through simple analytical expressions. To start with, the one-dimensional response of the model is discussed, making it possible to explore how the model is capable of capturing key features of viscous soft soil behaviour. Subsequently, the three-dimensional generalisation of the model is presented, followed by comparison with experimental data, showing good agreement in both triaxial undrained compression and extension. In the authors' opinion, the simple formulation of the model makes it attractive for use in engineering practice. [ABSTRACT FROM AUTHOR]

Microstructural and mineralogical observations of clay-stones reveal the presence of clay aggregates bonded by a skeleton of inert minerals such as calcium carbonate. The behaviour of these natural materials evolves from a rock-like behaviour, when undisturbed, to clay-like soil when weathered or when subjected to straining. A model has been developed to simulate the constitutive behaviour of these transitional materials. Following the proposal made by Vaunat & Gens, the material is conceived as a composite medium made of a clay matrix and a quasibrittle bonding microstructure. The basic model has been modified and extended to reproduce the expansive behaviour of the clay matrix, a fundamental aspect to simulate weathering effects, induced largely by drying-wetting cycles. The clay matrix reacts to stress and suction changes, whereas the bond component is not affected by suction changes. An elasto-plastic double-structure model for expansive clay soils describes the clay matrix. The bonding structure follows a damage model. The interaction between the two constitutive models derives from strain compatibility conditions and energy considerations. The model performance is first illustrated by means of a sensitive analysis that explores the effect of initial bond strength, bond damage rate, bonding concentration, and wetting-drying cycles. Simple stress paths (uniaxial deformation; triaxial compression) are used to highlight the features of the formulation and the role of significant parameters. Some published tests are also reproduced with the model. They have been selected to show some relevant features of evolving soft clay rocks: stiffness and strength degradation during loading and enhanced rebound during unloading, and the effect of drying-wetting cycles on subsequent stiffness and strength degradation. [ABSTRACT FROM AUTHOR]