A rheological model to describe the time-dependent behaviour of Vallericca clay

Experimental results obtained on Vallericca clay clearly show that, at small strain, shear stiffness, G0, and damping ratio, D0, are significantly affected by strain rate. In particular, it was observed that G0 continuously increases with frequency, while D0 draw a u-shaped dependence on frequency. Various attempts has been made to design a model able to provide satisfactory predictions of the behaviour observed in terms of both G0 and D0 versus frequency. In this paper, starting from the analysis of two well known rheological models, Maxell and Kelvin-Voigt model, a simple scheme is proposed in which the two above mentioned frames act in serie. The rheological scheme proposed explicitly accounts for creep effects, assumed to be responsible for the damping increase at low frequency and, in the mean while, allows the best fitting of experimental data at high frequency, in terms of both initial stiffness and damping ratio.