One of the aims of modern laboratory studies is searching for dynamic parameters in a small strain of a soil. The reason behind this trend is the higher level of density of urban areas. City traffic, including trams, cars and subway trains, as well as certain types of construction works, can generate seismic waves, which cause a small strain between the soil and the structure to occur. This may lead to an exceeded limit serviceability state. The three main dynamic parameters of the soil are: Young modulus, shear modulus and damping ratio. Estimation of the cohesive soils damping ratio by two methods, namely free-vibration decay and half-power bandwidth, is the main goal of this article. The author performed his own research on two silty clay specimens in a resonant column. The conducted research show that the damping ratio obtained by using the half-power bandwidth method has significant error margin, which is equal approximately 50–60%, yet the curve shapes are very similar to the ones obtained by using the second method discussed in this paper. In author’s opinion, more research on cohesive soils has to be performed in order to introduce necessary corrections to half-power bandwidth equations.

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