The paper proposes a constitutive model for structured soils (MSS) which describes the engineering effects of structure development and degradation, such as: high intact stiffness and strength, appreciable reduction of stiffness and strength due to de-structuring, and evolution of stress- and structure-induced anisotropy. A key feature of the model is the treatment of pre-consolidation as a structure-inducing process and the unified description of all such processes via a ‘bond strength envelope’ associated with the onset of appreciable de-structuring and distinguished from the onset of plastic yielding. Other features include: a damage-type mechanism to model volumetric and deviatoric structure degradation, the evolution of stress- and bond-induced anisotropy using a fading memory scheme, adaptable predictive capabilities depending on the sophistication of the available test data, modularity to extend its applicability in several soil types, and mathematical formulation in a general tensorial space to facilitate its incorporation in finite element codes. The predictive capabilities of the model are evaluated against the results of laboratory tests on the stiff overconsolidated Vallericca clay: (a) isotropic and anisotropic consolidation tests up to very high pressures; and (b) anisotropically consolidated triaxial shearing at both low pressures (structured material response) and high pressures (de-structured material response).

The elastic behaviour of granular materials is non-linear, in that the small-strain tangent stiffness depends on the stress level. The elastic moduli typically vary as power functions of the mean stress. Simple models of this nonlinearity can result in behaviour that violates the laws of thermodynamics. To guarantee that an elasticity model is thermodynamically acceptable it must be possible to derive the elastic behaviour from a free energy potential (or alternatively from a complementary energy potential). In this paper elasticity models are derived that allow for variation of elastic moduli as power functions of mean stress, while guaranteeing thermodynamic acceptability. The important issue of the dependence of secant stiffness on strain amplitude (a phenomenon related to dissipation processes in the soil) is acknowledged but not addressed here.

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The mechanical behaviour of geomaterials is complex and, as a consequence, material models form an important part of any numerical analysis in geotechnical engineering. There are so many constitutive models already available that an external observer might well question whether further constitutive models should be developed or, rather, existing models should somehow be compared and evaluated. There is no consensus within the geotechnical engineering community in addressing this question. Practising engineers are at the mercy of the model developers as they try to discover which model might be suitable for which purpose. The developers themselves are rarely impartial in their evaluation: they will typically extol the virtues of their own modelling framework while at the same time recommending further enhancement.However, there is, in our opinion, a logical way to respond to the question. The evaluation of constitutive models should be in the hands of researchers and practitioners who wish to make use of the models for solving practical problems; leaving the developers to respond to their objective conclusions and use them for further improvement of the models. Unfortunately, the current state of constitutive modelling does not permit this line of thinking to be followed. Users of constitutive models generally have neither the time nor the expertise to implement the models into finite element (FE) codes by themselves and therefore their choice of models remains confined to the few (often primitive) models that happen to be already available in commercial FE codes or, perhaps, they may have access only to particular models that are being developed at their own research institutions. *

Small-strain stiffness of reconstituted clay compressed along constant triaxial effective stress ratio paths INTRODUCTION Any calculation of ground movements around an engineering structure requires a knowledge of soil stiffness. Even at relatively small strains, the stress±strain behaviour of soils is highly non-linear and the stiffness can vary signi®cantly over the range of strains of interest in civil engineering. The assessment of reliable stress±strain relationships for soils, for both static and dynamic deformation problems, requires a correct evaluation of the shear stiffness at small strains, G 0 , and of the shape of the stiffness degradation curve (e.g. see Burghignoli et al., 1991). By assuming that the mechanical behaviour of the soil is linear elastic inside a relatively small