## 3 Results and Discussion  
### Measurement Properties and Experimental Results  
To obtain the first results, measurements with different soil combinations and hydraulic gradients were performed. These measurements are presented in Table 2. The impedance spectra over time are shown in Figure 10. For these tests, only the grain size of the gravel was modified. The sand has a constant grain distribution with a grain size in the range of 0.063-0.25 mm. The impedance spectrum from 40 kHz to 12 MHz was recorded with one sample per second, such that one measurement sequence took 10 min. \begin{table}
\begin{tabular}{l l} \hline
**Frequency Range** & 40 kHz to 20 MHz \\
**Frequency Resolution** & 35 m\(Hz\) \\
**Impedance Range** & 1 \(\Omega\) to 1000 \(\Omega\), best sensitivity at 50 \(\Omega\) \\
**Impedance Resolution** & 20 m\(\Omega\) \\  & 100 Hz @ 10 Points \\
**Sample Rate** & 10 Hz @ 100 Points per frequency sweep \\  & 1 Hz @ 1000 Points \\ \hline \end{tabular}
\end{table} 
Table 1: Specifications and features of the measurement hardware. Figure 9: Wheatstone bridge circuit for complex impedance measurement. After the measurement, the cell was opened and the overall volume change of the material in the proctor mould was determined. In the first test with a grain size of 2-4 mm, material transport should not occur due to the geometry and the low hydraulic gradient. However, suffosion processes cannot be excluded in total. This assumption was supported by the small change in impedance of only 3.65% during the test (Figure 10a). After opening the cell, measuring the volume change and examining the material in the transition area, this assumption was confirmed. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Measurement**} & **Gravel Grain** & **Hydraulic** & **Volume Change** & **Impedance** \\  & **Size [mm]** & **Gradient** & **[\%]** & **Change [\%]** \\ \hline
1 (a) & 2–4 & 7 & 0.26 & 3.65 \\
2 (b) & 4–8 & 7 & 0.37 & 4.33 \\
3 (c) & 4–8 & 9 & 1.56 & 17.21 \\
4 (d) & 4–8 & 11 & 1.68 & 18.17 \\
5 (e) & 4–8 & 13 & 3.11 & 17.93 \\
6 (f) & 4–8 & 13 & 5.34 & 29.83 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Input parameters (gravel grain size, hydraulic gradient) and results (volume change, impedance change). Figure 10: Impedance spectrum over time during down-top water flow corresponding to the measurements 1–6 (**a–f**). In the following, the grain size was changed to 4-8 mm, and the hydraulic gradient was increased with each measurement. In the third measurement, the impedance changed by 17.21% (Figure 10c) during the test. This indicates material transport, which was confirmed by the change in volume and the subsequent examination of the material in the transition area. In the following measurements, this effect intensified due to the increasing hydraulic gradient. The results show that we were able to detect material transport and that the impedance change correlates with the volume change. Especially in Figure 10f, there is also a shift in the resonance frequency. These results validate the previously performed simulations. Figure 11a illustrates the course of the maximum value of the impedance over time at the resonance point. First, an increase of the maximum impedance can be observed, but after about 200 s, a constant value is reached and there is almost no more change. This can be explained by the onset of colmation. This means that the spaces between the coarser material in the transition area have been filled with finer material and therefore that no further material transport is possible. In Figure 11b, the impedance change is plotted over the volume change for 39 individual measurements. The fitted linear graph shows that the relationship could be proportional. However, in order to verify this and to be able to make a statistical statement, further measurements have to be made. Examining the contact zone after the test, it was observed that significant material transport only occurred above a volume change of 1%.