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Clayey soils exhibit a very pronounced time-dependent behaviour owing to their viscous properties. To analyse and to quantify these properties, two rounds of tests have been performed on a compacted clay. The first round, consisting of constant-strain-rate compression, stress relaxation and creep, shows that viscous properties are not influenced by different levels of stress and strain. Moreover, the influence of the loading frequency on the viscoelastic modulus is evidenced by the cyclic tests performed at very small strain amplitudes. The results of the second round, consisting of creep and stress-relaxation tests performed at different levels of stress and strain in triaxial loading-unloading-reloading phases and analysed by way of the viscoplastic mechanism terms from Perzyna's over-stress theory, show the existence of a primary yield mechanism activated at the maximum loading point. A kinematic mechanism becomes activated during the unloading and reloading phases, controlling from this moment on the viscoplastic behaviour within the primary yield locus. [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]