Strain Rate Effect on Stiffness and Damping Ratio of Clays

This paper describes the results of a laboratory study which was carried out in order to determine the stress-strain behaviour of two Italian clays (Augusta and Pisa clays), with special attention being paid to the variation of stiffness parameters and damping ratio with strain rate. The experimental study was carried out using a Resonant Column/Torsional Shear apparatus and a Triaxial Cell. Monotonic loading torsional shear tests (MLTST), cyclic loading torsional shear tests (CLTST), resonant column tests (RCT) and monotonic compression loading triaxial tests (TX) were performed on specimens of Augusta and Pisa clays. The obtained results showed a little influence of strain rate on stiffness parameters in the small strain range. While the stiffness parameters for medium and large strains are strongly influenced by the strain rate. The measured damping ratio values resulted to be influenced by strain rate much more than stiffness. A comparison of the stress-strain curves obtained from monotonic and cyclic tests allowed one to verify the effectiveness of a modified second Masing rule.