## 4 Results
### Deformability Characteristics  
The deformability of the samples can be quantified by analysis of the strain development in comparison to the deformation in the first cycle. In other words, we represent the deformability as a plastic strain normalization parameter \(P_{x}\) (14). The concept of this quotient was previously developed by [PERSON] et al. [48] as a concept of the degradation index \(\delta\) which is represented as Equation (19):  
\[\delta=\frac{G_{SN}}{G_{S1}}[-], \tag{19}\]  
where \(G_{S1}\) and \(G_{SN}\) are secant moduli at cycles 1 and \(N\), respectively. This relation was later extended by [PERSON] and [PERSON] [49] to the cyclic stress-controlled tests and redefined as follows in Equation (20):  
\[\delta=\frac{G_{SN}}{G_{S1}}=\frac{\frac{\delta_{u}}{\epsilon_{a}^{pN}}}{ \frac{\delta_{u}}{\epsilon_{a}^{pN}}}=\frac{\epsilon_{a}^{p,1}}{\epsilon_{a} ^{p,1}}[-], \tag{20}\]  
This expression is the exact definition of plastic strain normalization parameter \(P_{x}\) which in such test conditions is also a cyclic stiffness degradation characteristic. In Figure 8, the plastic strain normalization parameter \(P_{x}\) versus the number of cycles is presented. The \(P_{x}\) value changes as the cyclic loading follows. We represent the \(P_{x}(N)\) characteristics by a well-known power function proposed by [PERSON] in Equation (21):  
\[P_{x}=a_{pX}N^{p_{pX}}[-], \tag{21}\]  
Figure 8: Plastic strain normalization parameter in confining pressure equal to 18, 90 and 135 kPa (**a**), and for the samples consolidated with confining pressure around to 45 kPa (**b**). where the \(a_{PX}\) and \(b_{PX}\) parameters can be modeled by taking into account the stress conditions and physical parameters, with the use of regression analysis, we found the \(a_{PX}\) and \(b_{PX}\) parameter values for the presentation of plastic strain characteristics. The least-square method as a quality of the regression of fit is employed. The results of nonlinear regression analysis are given in Table 15 in Appendix A. During the procedure of P\({}_{x}\) characteristic fitting, we found out that the parameters \(a_{PX}\) and \(b_{PX}\) have a weak correlation with the soil properties (Table 15 in Appendix A). We can draw the conclusion that the equations mentioned in the literature can model the plastic strain development accurately but the parameters in these models have to be derived experimentally. What is more, for the correlation analysis, the increase in plastic strain in the first cycle (\(\Delta\varepsilon_{1}\)) and \(10^{\text{th}}\) cycle (\(\Delta\varepsilon_{10}\)) was also examined, and there is no correlation between these parameters and the \(b_{PX}\) value. In Figure 9, the \(T_{x}\) parameter is presented versus the number of cycles. The total strain normalization parameter \(T_{x}\) change with the number of cycles has similar characteristics. The samples consolidated with \(\sigma^{\prime}3\) equal to around 45 kPa. The \(T_{x}\) parameters in tests 1.1 and 1.8 have significantly different characteristics when Figures 8 and 9 are compared. Both cases do not change in the same way. The 1.1 sample seems to accumulate plastic strains in the first few cycles of loading, and after that, we observe a stiff response. The reason for that might be that these are not full saturation conditions as some air in pores was left after the consolidation phase. If we assume that the soil in full saturation conditions would respond to cyclic loading as the rest of the soil specimens do, the \(P_{x}\) and \(T_{x}\) parameter change would be similar to the rest of the samples. The same situation occurs in the case of sample 1.1, but this time, the amount of cycles is lower (around 2-3 cycles to achieve undrained conditions). The normalization parameters \(P_{x}\) and \(T_{x}\) are highly dependent on the strain in the first cycle. Therefore, the rule regarding which normalization parameters are related to the first strain value might be misleading.