## 5 Test Results and Discussion
### G/G\({}_{0}\) Curves  
In practice, \(G_{0}\) can be measured in situ, while laboratory measurements are used to determine the \(G-\gamma\) and \(G/G_{0}\) curves. In this study, the \(G/G_{0}\) curves were determined using best-fit parameters according to the modified Hardin-Drnevich model. The results of the model fitting are the parameters \(G_{0}\) and \(\gamma_{0.7}\). Figure 11 shows the \(G/G_{0}\) curves for all 20 samples (Table 2) tested under different stress conditions. The modified Hardin-Drnevich model was originally developed and tested primarily for sandy soils [62]. Figure 12 shows that soil with a \(PI=15\%\) matches the model almost perfectly, while soil with a \(PI=33\%\) shows a more pronounced plateau in the range of \(\gamma\) smaller than \(5\times 10^{-5}\). Better agreement can be achieved if the power factor coefficient is included (Equation (3)), as recommended by [PERSON] [27]. However, such a formulation of the hyperbolic model is not included in the HSS model. Figure 11: \(G/G_{0}\) for all soil samples under different stress conditions. The circles outlined in black are test results performed under anisotropic stress conditions. Figure 10: Comparison between \(G_{0}\) (model) and \(G_{0}\) (BE), where the solid black lines are \(\pm 10\%\) away from the identity line. Two outliers that were excluded from regression analysis are marked with X. The comparison between the established empirical models (Table 1) and measurements (Figure 13) is presented in this study in the form of \(\gamma_{0.7}\). The empirical \(G/G_{0}\) curves for different \(PI\) were also fitted according to the modified Hardin-Drnevich model to determine \(\gamma_{0.7}\), and then exponential trend lines were generated for each model (Figure 13). Both the empirical \(G/G_{0}\) curves and the measurements show that \(\gamma_{0.7}\) increases with \(PI\). However, discrepancies in the \(\gamma_{0.7}\) exist not only between the empirical \(G/G_{0}\) curves and the measurements, but also between the empirical \(G/G_{0}\) curves of different authors. When analyzing the results in Figure 13, the empirical models of [PERSON] (2016), [PERSON] et al. (2018) and [PERSON] et al. (2018), which also take \(p^{\prime}\) into account, show that the influence of \(PI\) is less pronounced compared to the other models (the slope of the line is less steep). It can be seen from the measurement results that the results for the local soils can deviate from the general equations in the literature. This study also confirms that \(p^{\prime}\) should not be completely neglected in relation to \(\gamma_{0.7}\). In the cases where only four consolidation stages were measured, there was no significant increase in \(\gamma_{0.7}\) with \(p^{\prime}\). This could be due to random measurement errors or other influences on \(\gamma_{0.7}\). To further investigate the effect of stress state on stiffness at small strains, some specimens were subjected to additional isotropic consolidation stages with stresses 2 or 4 times higher than those of the fourth consolidation stage. The result was a significant increase in \(\gamma_{0.7}\) (Figure 14). It was also found that anisotropic stress conditions  
Figure 12: Example of fitting modified Hardin–Drnevich model: \(G/G_{0}\) curves for a specimen (**a**) with low and (**b**) with high \(IP\). Figure 13: The comparison between the empirical models and the measurements represented by \(\gamma_{0.7}\), with the black lines deviating \(\pm 25\%\) from the trend (17,22,29, 2018; 2018). did not significantly change \(\gamma_{0.7}\). Isotropic and anisotropic tests at the same \(p^{\prime}\) (second lowest \(p^{\prime}\)) are so close to each other in Figure 14 that they can hardly be distinguished. Comparing measurements with the empirical model of [PERSON] (2018) and [PERSON] et al. (2019), a comparable increase in \(\gamma_{0.7}\) can be observed. A comparison shows that the empirical model proposed by [PERSON] et al.