## 6 Conclusions  
In this article, the damping phenomenon of normally and lightly overconsolidated Quaternary cohesive soil from Warsaw was examined using the resonant column device. Different types of clayey Quaternary soils from three various test sites were collected in an undisturbed state. The damping tests in the resonant column apparatus by free vibration method were performed on each specimen. The authors created an empirical model describing the soil damping characteristic in the wide shear strain range. The main conclusions achieved in this research are summarized below, as follows:  
1. As expected, the shear strain had an evident influence on the tested soil damping. The damping ratio was constant and had minimum value in the elastic range of shear strain. Above the nonlinearity threshold strain, the damping ratio started to increase, at first slowly, and after exceeding the plastic strain, it became more noticeable.  
2. The effective stress and the plasticity index also have a significant influence on the damping properties of studied soil. As both parameters increased, the linear damping ratio range (\(D_{MIN}\)) increased. The minimal damping ratio decreased linearly with increasing effective stress. The study of the impact of \(PI\) on \(D_{MIN}\) allowed for the setting of a plasticity threshold on 20%, which indicates a change in the \(D_{MIN}\) trend from a decrease to an increase.  
3. The authors reported the influence of the void ratio on the minimal damping ratio, which is best described by the power function.  
4. The laboratory technique with repeated damping testing significantly improved the reliability of the obtained results. Moreover, thanks to that method, measurement errors are at a low level.  
5. Based on the laboratory tests, empirical Equation (5), describing damping properties, was created. Thanks to the connections between the quadratic function of \(G/G_{MAX}\), the linear function of \(PI\), and the power function of \(p^{\prime}\), the resulting empirical model precisely describes the damping ratio in a wide range of shear strain, with the determination coefficient (\(R^{2}\)) at 0.96 and average relative error at 9.6%.  
6. Using commonly known empirical equations, it is not possible to reliably calculate the damping ratio for the studied cohesive soil. A comparison of the measured and calculated value by the equations from the literature resulted in the very large spread of damping ratio, as well as high values of errors. Therefore, it can be concluded that an empirical model should be applied to the soil, with a similar genesis and mineral composition as the soil for which they were created.  
The proposed formula is still preliminary and needs to be upgraded and confirmed by being applied at different clayey sites.  
Figure 13: Box–plot chart with the average value of damping ratio for the entire result set (the smallest square), average value \(\pm\) standard deviation (box), and minimum and maximum value of damping ratio (whiskers) for measured and calculated damping ratio.  
**Author Contributions:** A.S. and W.S. conceived of and designed the experiments; E.S. and A.G. performed the experiments; E.S. and A.G. analyzed the data and wrote the paper; W.S. and A.S. edited and audited the content.  
**Funding:** This research received no external funding.  
**Conflicts of Interest:** The authors declare no conflicts of interest.