The Paper presents a constitutive model for describing the stress-strain behaviour of partially saturated soils. The model is formulated within the framework of hardening plasticity using two iodependent sets of stress variables: the excess of total stress over air pressure and the suction. The mode1 is able to represent, in a consistent and unified manner, many of the fundamental features of the behaviour of partially saturated soils which had been treated separately by previously proposed models. On reaching saturation, the mode1 becomes a conventional critical state model. Because experimental evidence is still limited, the model has been kept as simple as possible in order to provide a basic framework from which extensions are possible. Tbe mode1 is intended for partially saturated soils which are slightly or moderately expansive. After formulating the model for isotropic and biaxial stress states, typical predictions are described and compared, in a qualitative way, with characteristic trends of the behaviour of partially saturated soils. Afterwards, the results of a number of suction-controlled laboratory tests on compacted kaolin and a sandy clay are used to evaluate the ability of the model to reproduce, quantitatively, observed behaviour. The agreement between observed and computed results is considered satisfactory and confirms the possibilities of reproducing the most important features of partially saturated soil behaviour using a simple general framework.

A constitutive model for partially saturated soils

The Paper presents a constitutive model for describing the stress-strain behaviour of partially saturated soils. The model is formulated within the framework of hardening plasticity using two independent sets of stress variables: the excess of total stress over air pressure and the suction. The model is able to represent, in a consistent and unified manner, many of the fundamental features of the behaviour of partially saturated soils which had been treated separately by previously proposed models. On reaching saturation, the model becomes a conventional critical state model. Because experimental evidence is still limited, the model has been kept as simple as possible in order to provide a basic framework from which extensions are possible. The model is intended for partially saturated soils which are slightly or moderately expansive. After formulating the model for isotropic and triaxial stress states, typical predictions are described and compared, in a qualitative way, with characteristic trends of the...

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A Constitutive Model for Partially Saturated Soils

Soils can rarely be described as ideally elastic or perfectly plastic and yet simple elastic and plastic models form the basis for the most traditional geotechnical engineering calculations. With the advent of cheap powerful computers the possibility of performing analyses based on more realistic models has become widely available. One of the aims of this book is to describe the basic ingredients of a family of simple elastic-plastic models of soil behaviour and to demonstrate how such models can be used in numerical analyses. Such numerical analyses are often regarded as mysterious black boxes but a proper appreciation of their worth requires an understanding of the numerical models on which they are based. Though the models on which this book concentrates are simple, understanding of these will indicate the ways in which more sophisticated models will perform.

Soil Behaviour and Critical State Soil Mechanics

The compressibility and strength characteristics of reconstituted clays are used as a basic frame of reference for interpreting the corresponding characteristics of natural sedimentary clays. The properties of reconstituted clays are termed ‘intrinsic’ properties since they are inherent to the soil and independent of the natural state. The properties of a natural clay differ from its intrinsic properties due to the influence of soil structure (fabric and bonding). Thus the intrinsic properties provide a frame of reference for assessing the in situ state of a natural clay and the influence of structure on its in situ properties. A new normalizing parameter called the void index is introduced to aid in correlating the compression characteristics of various clays. The sedimentation compression curves for most, but not all, natural clays lie well above the corresponding intrinsic compression curves. A consequence of this is that such clays are more sensitive and brittle than the reconstituted material and the...

On the compressibility and shear strength of natural clays

This paper presents a polynomial dimensional decomposition (PDD) method for global sensitivity analysis of stochastic systems subject to independent random input following arbitrary probability distributions. The method involves Fourier-polynomial expansions of lower-variate component functions of a stochastic response by measure-consistent orthonormal polynomial bases, analytical formulae for calculating the global sensitivity indices in terms of the expansion coefficients, and dimension-reduction integration for estimating the expansion coefficients. Due to identical dimensional structures of PDD and analysis-of-variance decomposition, the proposed method facilitates simple and direct calculation of the global sensitivity indices. Numerical results of the global sensitivity indices computed for smooth systems reveal significantly higher convergence rates of the PDD approximation than those from existing methods, including polynomial chaos expansion, random balance design, state-dependent parameter, improved Sobol’s method, and sampling-based methods. However, for non-smooth functions, the convergence properties of the PDD solution deteriorate to a great extent, warranting further improvements. The computational complexity of the PDD method is polynomial, as opposed to exponential, thereby alleviating the curse of dimensionality to some extent. Mathematical modeling of complex systems often requires sensitivity analysis to determine how an output variable of interest is influenced by individual or subsets of input variables. A traditional local sensitivity analysis entails gradients or derivatives, often invoked in design optimization, describing changes in the model response due to the local variation of input. Depending on the model output, obtaining gradients or derivatives, if they exist, can be simple or difficult. In contrast, a global sensitivity analysis (GSA), increasingly becoming mainstream, characterizes how the global variation of input, due to its uncertainty, impacts the overall uncertain behavior of the model. In other words, GSA constitutes the study of how the output uncertainty from a mathematical model is divvied up, qualitatively or quantitatively, to distinct sources of input variation in the model [1].

Reliability Engineering and System Safety

The paper presents a framework for describing the mechanical behaviour of unsaturated expansive clays. It is an extension of an existing formulation developed for unsaturated soils of low activity....

A framework for the behaviour of unsaturated expansive clays

The paper presents an elasto-plastic model for unsaturated soils that takes explicitly into account the mechanisms with which suction affects mechanical behaviour as well as their dependence on degree of saturation. The proposed model is formulated in terms of two constitutive variables directly related to these suction mechanisms: the average skeleton stress, which includes the average fluid pressure acting on the soil pores, and an additional scalar constitutive variable, ξ, related to the magnitude of the bonding effect exerted by meniscus water at the inter-particle contacts. The formulation of the model in terms of variables closely related to specific behaviour mechanisms leads to a remarkable unification of experimental results of tests carried out with different suctions. The analysis of experimental isotropic compression data strongly suggests that the quotient between the void ratio, e, of an unsaturated soil and the void ratio es, corresponding to the saturated state at the same average soil sk...

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An elasto-plastic model for unsaturated soil incorporating the effects of suction and degree of saturation on mechanical behaviour

Presents numerical aspects of the program CODE_BRIGHT, which is a simulator for COupled DEformation, BRIne, Gas and Heat transport problems. It solves the equations of mass and energy balance and stress equilibrium and, originally, it was developed for saline media. The governing equations also include a set of constitutive laws and equilibrium conditions. The main peculiarities of saline media are in the dissolution/precipitation phenomena, presence of brine inclusions in the solid salt and creep deformation of the solid matrix.

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Antonio Gens

Papers

A constitutive model for partially saturated soils

A Constitutive Model for Partially Saturated Soils

A framework for the behaviour of unsaturated expansive clays

An elasto-plastic model for unsaturated soil incorporating the effects of suction and degree of saturation on mechanical behaviour

Numerical formulation for a simulator (CODE_BRIGHT) for the coupled analysis of saline media