Rate and Creep Effect on the Stiffness of Soils

The shear modulus of two undisturbed Italian clays was measured in the laboratory by means of a Resonant Column/Torsional shear apparatus for a strain range of 0.0001 % up to 1 %. Three different kinds of tests were performed on hollow cylindrical specimens reconsolidated to the in situ geostatic stress: a) static monotonic loading tests at constant stress rate, b) cyclic loading tests at constant strain rate, c) Resonant Column tests. Moreover the small strain shear modulus G., determined at strain level less than 0.001 % was measured during the drained creep following reconsolidation. The increase of Go with time during the drained creep was therefore assessed and compared with available correlations from literature. The data obtained from the above mentioned research activity enabled one to examine the influence of the strain rate, loading conditions (cyclic vs. monotonic) and strain level on the deformation moduli of the tested clays. It was possible to assess that the normalised curves (G/Go vs. g) are dependent on loading conditions and strain rate.