## 4 Results
### Direct Shearing Tests
Although clay shear strength increased with each percentage of PET added, the most significant variation was at an addition of 2% PET. For the Cl2 PET admixture, the shear strength showed an increase of 30.51% for \(\sigma\) = 100 kPa, 22.54% for \(\sigma\) = 200 kPa and 19.40% for \(\sigma\) = 300 kPa, compared to the shear strength of the unimproved clay (Cl). When the amount of PET in the sample was doubled (Cl4 PET), the shear strength increased compared to the Cl2 PET admixture, but the increase was not as significant as in the Cl2 PET admixture. For the Cl4 PET admixture, the shear strength showed an increase of 15.05% for \(\sigma\) = 100 kPa, 7.57% for \(\sigma\) = 200 kPa and 4.34% for \(\sigma\) = 300 kPa, compared to the shear strength of the Cl2 PET mixture. For the Cl6 PET mixture, the shear strength showed a slight decrease of 4.30% for \(\sigma\) = 100 kPa and an increase of 5.49% for \(\sigma\) = 200 kPa and 10.15% for \(\sigma\) = 300 kPa, compared to the shear strength of the Cl4 PET admixture. The values of internal friction angle and cohesion increased with the addition of PET to the studied samples. Analysing the results in Table 5 and the graphs in Figures 6 and 7, it can be observed that the peak shearing parameters of the clay increased as shredded PET waste was added. From an initial value of the internal friction angle of 18.83\({}^{\circ}\) for unimproved clay, it reached a value of 20.93\({}^{\circ}\) for Cl2 PET, 20.20\({}^{\circ}\) for Cl4 PET and 24.40\({}^{\circ}\) for Cl6 PET (Table 5). The best value of the internal friction angle was for the Cl6 PET admixture, an increase of 5.57\({}^{\circ}\) compared to the value of the unimproved clay (Figure 6). In the case of cohesion, the value increased for all Cl + PET admixtures. The highest result was recorded for the Cl4 PET admixture, with a cohesion value of 29.96 kPa, an increase of 19.59 kPa compared to the value of unimproved clay (Table 5). For the Cl2 PET admixture, the cohesion value was 19.79 kPa and for Cl6 PET it was 18.53 kPa, both values being higher than the value of the simple, unimproved clay (Figure 7). When it comes to Cl6 PET, the cohesion value decreased from a value of 29.96 kPa for Cl4 PET to a value of 18.53 kPa (Table 5). This decrease may be caused by the high percentage of PET present in the sample, which could have caused a possible sliding of clay particles on the shredded PET. This issue can be investigated in further research through a microscopic analysis of the contacts between clay particles and PET. Analysing the residual shear parameters, the best result was registered for the cohesion value of the Cl4 PET admixture, which was 15.49 kPa. The cohesion value also increased for the Cl2 PET and Cl6 PET admixtures, but it was lower than the value for the Cl4 PET admixture. For the internal friction angle, the values for the three Cl + PET admixtures were slightly lower than the value for the unimproved clay. \begin{table}
\begin{tabular}{c c c c c c} \hline  & & & & & \\ \hline  & & **Cl** & **Cl2 PET** & **Cl4 PET** & **Cl6 PET** \\ \hline  & & & & & \\ \hline \(\Phi^{\prime}\) & (\({}^{\circ}\)) & 18.83 & 20.93 & 20.20 & 24.40 \\ \(\mathrm{c}^{\prime}\) & (kPa) & 10.37 & 19.79 & 29.96 & 18.53 \\ \hline  & & & & & \\ \hline \(\Phi_{\mathrm{res}}\) & (\({}^{\circ}\)) & 12.37 & 12.21 & 11.33 & 11.92 \\ \(\mathrm{c}_{\mathrm{res}}\) & (kPa) & 0.50 & 9.70 & 15.49 & 13.15 \\ \hline \end{tabular}
\end{table} 
Table 5: Shear strength parameters of unimproved and improved clay. The shear strength parameters \(\phi\) and c should not be considered individually but in conjunction with each other, as they together describe the shear strength of the soil. For example, in the case of the Cl6 PET admixture, even if the cohesion value decreased compared to Cl4 PET admixture, the shear strength increased for the range of vertical stresses applied (\(\sigma\) = 100-200 kPa). In 2017 [PERSON] et al.