On Cohesive Soil Damping Estimation by Free Vibration Method in Resonant Column Test

Measurement of soil properties under cyclic and dynamic loading conditions is a critical task in the solution of most geotechnical earthquake engineering problems. The main dynamic properties of soils are usually expressed in terms of shear modulus and damping ratio that are generally obtained from laboratory tests. To date, the resonant column test is considered one of the most accurate ways to determine dynamic soil properties at low to medium-strain levels. Because it allows for investigation of a wide range of strain amplitudes, it is a valuable tool for characterizing the shear soil behavior for a large number of practical geotechnical problems. One of the approaches followed in estimating the damping ratio from the resonant column test is the free vibration method (also called logarithmic decrement or amplitude decay 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 measuring the logarithmic ratio of the amplitude of any two successive peaks of the system damped motion (logarithmic decrement). In this article, the influence on the damping ratio of the shear strain level, the plasticity index, the confining pressure, and the time interval between the peaks selected in calculating the logarithmic decrement is evaluated from the results of resonant column tests performed on 51 undisturbed isotropically consolidated fine-grained soil specimens.