## 3 Experimental Program
### Methodology  
Bender elements have been widely utilized to measure \(V_{S}\) at low strain levels, namely \(\gamma<0.001\) percentage [45]. The simple operation of these piezoceramic transducers, together with their small size, has stimulated their use in a big variety of geotechnical equipment since their first appearance [46]. BEs have been incorporated into a wide range of laboratory device (i.e., triaxial [47], true triaxial, hollow cylinder, simple shear, shear box, oedometer, resonant column), as well as into boundaries of laboratory model test apparatuses. Originally, BEs were applied to determine \(G_{\text{max}}\), but with the passage of time and their greater usage, they have been employed to measure P-wave velocity (\(V_{P}\)), to investigate fabric and structure, additionally in particular cementation and its degradation, degree of saturation or anisotropy, to assess sample's quality and control multi-stage testing [48]. However, BE testing is not yet a standard, due to the variability of its results in comparison with standard resonant column testing [45]. The reliability of BE technique is influenced by different factors, e.g., near-field effects [49], directivity [48], travel distance [50], boundary effects [51], sample geometry and size [52], cross-talking [53]. The difficulty lies as well in accurately evaluating the travel time (\(tt\)) of shear wave from the source BE to the receiver BE, whereas a distance between elements (\(L_{TT}\)), accepted as the tip-to-tip distance, can be precisely measured. \(V_{S}\) is then calculated in accordance with the formula:  
\[V_{S}=\frac{L_{TT}}{tt}. \tag{2}\]  
The operating principles of a BEs system have been described in many previous studies of the authors, e.g., [54]. The time domain and frequency domain analysis were applied in this work to estimate the travel time. The time domain methods are the mostly widely used. These techniques are referred to an \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & \multicolumn{4}{c}{**Pure RCA**} & \multicolumn{4}{c}{**RCA with Rubber Chips**} \\ \cline{3-8}
**Description** & **Unit** & **No. 1** & **No. 2** & **No. 3** & **No. 4** & **No. 5** & **No. 6** \\ \cline{3-8}  & & \multicolumn{4}{c}{**Value**} \\ \hline Initial water content & \% & 6.93 & 14.91 & 6.67 & 9.59 & 8.54 & 9.64 \\ Bulk density of soil mass & g/cm\({}^{3}\) & 1.75 & 1.88 & 1.79 & 1.32 & 1.36 & 1.32 \\ Soil particle density & g/cm\({}^{3}\) & & & & 2.60 & & \\ Bulk density of soil & g/cm\({}^{3}\) & 1.64 & 1.63 & 1.68 & 1.20 & 1.25 & 1.20 \\ skeleton & - & 0.59 & 0.59 & 0.55 & 1.16 & 1.08 & 1.17 \\ Saturation ratio & \% & 0.31 & 0.66 & 0.32 & 0.21 & 0.21 & 0.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Initial properties of pure RCA specimens and mixtures of RCA with rubber chips. analysis made from signals represented along the axis of time in which the input and/or output waveforms are plotted. Two points of these graphs can be selected following a given criterion, where their time difference is defined as the travel time between the transmitter and receiver of the analyzed wave [55]. A typical output signal gathered from a step input signal is presented in Figure 5. Estimation of \(tt\) by the identification of the first direct arrival in the output signal was the initial assumption in this method. This assumption is an intuitive interpretation following the method used on in situ geophysical measurements and was the interpretation method initially adopted [46]. The method remained unchanged for nearly 10 years until a travel-time determination based on the first inversion of the received signal was proposed [56]. What is, however, the appropriate first arrival point: A, B, C or D? Suggested criteria and recommendations are various and differ depending on installation, application, and input signal [53].