Damping at small strains in cyclic simple shear test

Cyclic tests were conducted to study damping properties of two reconstituted sands and three laboratory-made clays at small cyclic shear strain amplitudes of gamma(c) approximate to 0.001-0.04%, employing a recently developed constant-volume equivalent-undrained direct simple shear device for small-strain testing. The tests were strain-controlled with an approximately sinusoidal shape of cyclic straining. The effects of cyclic strain amplitude (gamma(c)), frequency of cyclic loading (f), plasticity index (PI), silt content, vertical effective consolidation stress (sigma(vc)') and overconsolidation ratio (OCR) on the equivalent viscous damping ratio, lambda, were investigated. The results show that, for a given gamma(c), lambda decreases with sigma(vc)' and OCR, but both of these effects become smaller if PI increases. The effect of f on lambda was not observed for f approximate to 0.01-0.1 Hz. The results also show that below gamma(c) approximate to 0.005%, lambda for clays is larger than lambda for sands, which is exactly the opposite of the trend above gamma(c) approximate to 0.005% established previously. Such a reversal of the trend of lambda with respect to the type of soil is explained by the relative contributions of soil nonlinearity and soil viscosity to the area of the hysteretic cyclic stress-strain loop at small versus large cyclic strains.