# Dynamic Characterization of Cohesive Material Based on Wave Velocity Measurements
## 4 Conclusions  
In this study, the characterization of the dynamic properties of cohesive material, namely silty clays, was inspected. The conducted experiments included the dynamic tests in the GDS Resonant Column Apparatus equipped with bender elements. The authors aimed to verify the dynamic behavior of clayey soil in connection with wave propagation produced by piezoelectric transducers. Shear wave velocity is related to the shear modulus of soil. Consequently, the measurements of the S-wave velocity provided a convenient method for determining soil stiffness. Additionally, the calculation of the P-wave velocity and, subsequently, the dynamic Poisson's ratio allowed for the examination of [PERSON]'s modulus with regard to the tested cohesive material. Figure 19: Small strain shear modulus _versus_ mean effective stress at the loading and unloading stage. Figure 18: Shear wave velocity _versus_ period at the loading and unloading stage. The findings of this research, in line with the literature in relation to the methods of interpretation of bender element test results, indicate that, for tests with piezoelectric transducers installed in the resonant column apparatus on silty clays, both analyzed techniques used as a part of the time domain methods, _i.e._, peak-to-peak and start-to-start, yield similar results. They are also considered as the most comprehensive and appropriate methods to identify the travel time of the shear waves propagating through the sample. The authors, however, did not inspect any other methods, like the cross-correlation or \(\pi\)-point phase, but the velocities established by those methods are comparable to the findings from the resonant column measurements [25]. The dynamic parameters obtained by the start-to-start method were always slightly larger (up to about 20%) than those obtained using the peak-to-peak method. According to the authors, this is a consequence of the broadening of the excitation pulses after they are propagated through the sample. The lowest wavelength in the presented experiments was about 1.0 mm, which was at least 10-times bigger than the largest grain size of soil sample (that was less than or equal to 100 \(\upmu\)m). In addition, no inhomogeneous pockets were found in the sample constitution after a destructive analysis of the material. In these conditions the macroscopic sample was expected to behave as a conducting media, and no enhanced scattering was expected at the grain boundaries. Since the studied sample was homogeneous, in terms of the wavelength, the difference between the results from the two discussed techniques can be explained by taking into account different wave velocities and different attenuations of the frequency components that constitute the excitation peaks. Although each excitation pulse used in the experiments was considered to have a well-defined frequency, in reality, the pulses are composed of a continuity of different frequencies with different amplitudes. This happens due to the finite size of the excitation pulse in the time domain, which adds additional frequencies to it. In the start-to-start method, the results were strongly influenced by the fastest travelling wave, while in the peak-to-peak method, the frequency component with the highest amplitude was the one considered. From the Figures 9 and 10 it can be clearly noticed that the wave velocity from the S-S method had an almost flat dependency as a function of frequency; however, in the P-P method, it has a change of about 20% in the frequency range experimented. This suggests that, in the S-S method, the frequency component responsible for the excitation of the detector is independent of the frequency chosen for the excitation pulse (due to the unintentional frequencies present in the excitation pulse). In certain experimental circumstances, such as during the measurements of \(G\), the peak-to-peak method led to more scattered results, which were probably connected to the interpretation of the findings made by the authors.