Strain rate dependent behaviour of a natural stiff clay.

The experimental results reported in this paper were obtained performing monotonic, cyclic and dynamic torsional shear tests on a stiff Italian clay in the pre-failure stress-strain range. The experimental results clearly demonstrated that the small strain values of shear stiffness and damping ratio are significantly affected by strain rate: while the initial shear modulusG0 continuously increases with strain rate, the equivalent viscous damping ratio D0 initially decreases with strain rate, then it reaches a minimum constant value and thereafter it increases significantly. The experimental data have also demonstrated that, if the strain rate is properly accounted for, a linear behaviour for the soil is no more distinguishable, even at strain levels as low as 10-4%. As a whole, these effects could have meaningful practical implications, thus requiring to be accounted for in modelling soil behaviour in those problems where the rate of loading is significantly varying.