Effect of soil plasticity on damping ratio at small cyclic strains

In soil dynamics, the most common parameter for measuring soil damping is the equivalent viscous damping ratio, which is derived from the theory of a single-degree-of-freedom system with viscous dashpot. The results of a comprehensive literature review on the equivalent viscous damping ratio at small cyclic shear strain amplitudes, .GAMMA.c, between 0.0001% and 0.01% are presented and discussed. On the basis of the published data from different types of laboratory tests two important trends are established. The first is a significant increase of the scatter of the data points as the plasticity index of the soil, PI, decreases. This indicates that damping in low-plasticity clays and sands is more sensitive to factors such as confining stress, overconsolidation ratio and cyclic loading waveform than the damping in high-plasticity clays. The second trend is an increase of the damping ratio with PI. This trend with PI is opposite from the trend at .GAMMA.c>0.01% established earlier, where damping ratio generally decreases with PI. An explanation is provided that the phenomena responsible for the increase of the damping ratio with PI at small .GAMMA.c are viscous creep and relaxation of the soil. (author abst.)