STRAIN-RATE EFFECTS ON SHEAR MODULUS AND DAMPING OF NORMALLY CONSOLIDATED CLAY

A laboratory investigation into the effects of shear strain rate on sh ear modulus and hysteretic damping of normally consolidated clays was carried out. The effects of shear strain rate were examined at cyclic shear strain amplitudes between 10(-6) and 10(-3) in undrained cyclic torsion shear tests. When the frequency of loading was changed between 0.005 and 0.1 Hz, the equivalent shear modulus was insensitive to the rate of shear straining. On the other hand, the hysteretic damping in creased according to the decrease in the shear strain rate. Furthermor e, for shear strains less than about 2 X 10(-5), the maximum stiffness was hardly influenced by the shear strain rate, type of loading, numb er of cycles, and the cyclic prestraining; it can therefore be charact erized as pseudoelastic shear modulus. On the basis of the test result s, it is concluded that when applying the results of laboratory cyclic loading tests to the analysis of in situ cyclic loading problems, the effects of shear strain rate on hysteretic damping should be properly evaluated to match the frequency of loading expected in the field.