Soil Damping Ratio Estimation by Logarithmic Decrement Method in Resonant Column Tests

Measurement of soil dynamic properties is a critical task in the solution of most geotechnical earthquake engineering problems. Dynamic properties of soils are commonly expressed in terms of shear modulus, G, and damping ratio, D, that are generally obtained from laboratory tests. Among them, the resonant column test is the most commonly used, especially at low- and medium-strain levels. One of the approaches followed in estimating damping ratio from the resonant column test is the ‘free vibration’ method (also called ‘logarithmic decrement’ method). It consists of analyzing the time history of the damped free vibrations of the specimen once harmonic loading is cut off after reaching the resonant conditions. The soil damping ratio is determined from the logarithmic ratio of the amplitude of any two successive peaks of the specimen damped motion (logarithmic decrement). In the present paper, the influence on the damping ratio of the shear strain level, the plasticity index, the confining pressure and the time distance between the peaks selected in calculating the logarithmic decrement is evaluated from the resonant column tests performed on 51 undisturbed isotropically consolidated fine-grained soil specimens.