Experimental study on influences of different loading frequencies on dynamic modulus and damping ratio

There is not a consistent conclusion on the influences of loading frequency on dynamic modulus and damping ratio, and the quantitative results are few. Through a series of tests using the new cyclic triaxial (CT) divice with high precision, the main objective of which is to evaluate the influences of excitation frequency on the shear modulus and damping ratio of typical sand and clay samples are studied. The necessity of considering the frequency-dependent modulus and damping ratio is discussed from the perspective of ground motion on the basis of the tests. The results show that: (1) For the sand, the shear modulus is almost independent of the loading frequency. The damping ratio slightly decreases due to the increase of vibration frequency, but the deviations can be ignored. (2) For the clay soil, the loading frequency has significant influences on its shear modulus and damping ratio. The shear modulus increases, but the damping ratio decreases with the increase of the loading frequency. (3) The CT tests indicate measurable growth in the reference shear strain γ_r with the loading frequency in the form of an increasing exponential function. The dynamic properties are very sensitive to the excitation frequency when f ≤ 1 Hz. In the range of 1 Hz <f ≤ 3 Hz, the influences are clear. When the frequency exceeds the upper band of 3 Hz, the shape is nearly on the same level. (4) The maximum damping ratio D_max decreases with the vibration frequency in a form of exponential function. When the frequency is lower than 10 Hz, the influences are obvious. If f>10 Hz, the influences gradually vanish. (5) The ground motion of a simplified ideal clay site is calculated using two sets of shear modulus and damping ratio curves under two different test loading frequencies, 0.1 Hz and 3 Hz. The peak ground accelerations and the response spectra show serious dissimilarities, which are increasingly severe as the intensity of ground shaking is enhanced. This study therefore demonstrates that considering the frequency correlation of shear modulus and damping ratio is of extreme necessity.