## 3 Results and Discussion
### Assessment of Saturation Degree and Porosity--In Situ Investigations  
The first example concerns the evaluation of porosity of Pliocene clays and sandy soils based on the resistivity cone penetration test (RCPT). Field studies require a slightly different approach than laboratory investigations; therefore, in order to set the porosity values as close as possible to real ones, the groundwater samples were taken from several depths and analyzed for variation of electrical conductivity. Values of electrical conductivity of groundwater taken using the BAT groundwater monitoring system [47] are presented in Table 3. Assuming that the saturation degree of clays is close to 1, the relationship between porosity and formation factor, F for clays in the tested site is as follows. \[\mathrm{n}=\left(\frac{1}{\mathrm{F}}\right)^{\frac{1}{328}}=\left(\frac{ \mathrm{p_{f}}}{\mathrm{p_{bSAT}}}\right)^{\frac{1}{328}}(\mathrm{R}^{2}=0.79). \tag{7}\]  
Assessment of porosity in cohesive soils is complicated and requires knowledge of the electrical resistivity of groundwater, which can vary with depth [30]. Electrical resistivity measurements assisted to geotechnical exploration in this case, and this simple relationship seems to be sufficient assuming that CEC and B+ are not included. Figure 10 shows the RCPT test results: changes in cone resistance q\({}_{c}\), friction ratio R\({}_{t}\), electrical resistivity profile and porosity calculated according to Equation (7). In addition, the figure presents the porosity values obtained from laboratory tests on undisturbed soil samples. \begin{table}
\begin{tabular}{c c c} \hline
**Depth [m]** & **Soil** & **Groundwater Conductivity [mS/m]** \\ \hline
3.5 & MSa & 161.3 \\
4.0 & MSa & 147.1 \\
4.8 & Cl & 261.8 \\
6.2 & Cl & 274.0 \\
7.2 & Cl & 270.3 \\
7.6 & Cl & 290.7 \\
9.4 & Cl & 308.6 \\
11.6 & siCl & 224.1 \\ \hline \end{tabular}
\end{table} 
Table 3: Results of groundwater electrical conductivity measurements. Figure 9: Analytical relation between electrical resistivity and porosity of sandy clay (sisaCl) based on oedemeter tests. Figure 11 shows the distribution of the saturation degree of sands in the profile. The saturation degree values were calculated based on the Formula (4) and supplemented with the results of laboratory tests. The fully saturated zone starts from the depth of about 3.4 m below the surface; for this zone the porosity of sands was estimated according to Formula (5). Empirical constants appropriate for the given type of soil and proposed in Tables 1 and 2 were adopted in the calculations. Figure 11: Results of RCPT tests; electrical resistivity of sands with calculated/measured saturation degree, formation factor and porosity of sands. Figure 10: Results of RCPT tests; q—cone resistance, R\({}_{\text{f}}\)—friction ratio, electrical conductivity and porosity of clays. The next example presents the RCPT test with a calculated porosity of unsaturated sandy clay (sisaCI) at the SGGW Campus. An undisturbed soil sample was taken from the depth of about 5.5 m and compressed in a modified oedometer. This study allowed to establish a simple relationship between electrical resistivity and porosity presented in Figures 8 and 9. Figure 12 presents the RCPT test results with calculated porosity according to Equation (6). The porosity values in the top and bottom of the layer result from water penetration, which gives higher electrical conductivity in these zones and affects the obtained lower porosity values. Studying the relationship between porosity, saturation degree and electrical resistivity is interesting in both geophysics and geotechnics, e.g., [33; 37]. The presented relationships showed a positive relation in these studies. However, electrical resistivity measurements still remain empirical and site restricted. The application of the above relationships in practice, and especially those used for cohesive soils, is only possible for a specific locality and soil type.