## 4 Results  
### Direct Shearing Tests  
The shear strength has two characteristic values: peak and residual. Corresponding to these parameters, there will be two distinct soil-characteristic lines, with which the shear parameters \(\phi^{\prime}\) and \(\mathsf{c}^{\prime}\) as the peak values and \(\phi_{\text{res}}\) and \(\mathsf{c}_{\text{res}}\) as the residual values can be determined. Analysing the direct shear tests results (Table 4, Figures 4 and 5) realized with Wykeham Farrance equipment, model 27-WF0215/B, it can be observed that the shear strength increased at an addition of 2% and 4% PET, both for the peak and residual values. In the case of peak parameters, for a vertical stress of 100 kPa, the shear strength increased progressively from an initial value of 44.47 kPa for Cl to 58.03 kPa for CI2 PET, then to 66.76 for CI4 PET, decreasing to a value of 63.89 kPa for the Cl6 PET admixture. The same increasing upward trend can be observed for vertical stresses of 200 kPa and 300 kPa. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & **CI** & **CI2 PET** & **CI4 PET** & **Cl6 PET** \\ \hline  & 100 & 44.47 & 58.03 & 66.76 & 63.89 \\  & 200 & 78.57 & 96.28 & 103.56 & 109.25 \\  & 300 & 112.67 & 134.52 & 140.36 & 154.61 \\ \cline{2-5} \(\sigma\) (kPa) & \multicolumn{4}{c}{\(\tau_{\text{f}}\) (kPa) for Residual Shearing Parameters} \\ \hline  & & & & \\ \cline{2-5}  & 100 & 22.43 & 31.34 & 35.54 & 34.26 \\  & 200 & 44.36 & 52.99 & 55.58 & 55.36 \\  & 300 & 66.29 & 74.63 & 75.63 & 76.47 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The shear strength (\(\tau_{\text{f}}\)) for the four types of admixtures. An improvement in the clay shear strength was observed by adding a small percentage of PET. In the case of the Cl2 PET admixture, the shear strength increased with 13.57 kPa for \(\sigma=100\) kPa, 17.71 kPa for \(\sigma=200\) kPa and 21.86 kPa for \(\sigma=300\) kPa, compared to the shear strength of the unimproved clay (Cl). For the Cl4 PET admixture, the shear strength increased more, with a value of 8.73 kPa for \(\sigma=100\) kPa, 7.29 kPa for \(\sigma=200\) kPa, and 5.84 kPa for \(\sigma=300\) kPa, compared to the shear strength of the Cl2 PET admixture. For the Cl6 PET admixture, the shear strength slightly decreased, compared to the shear strength of the Cl4 PET admixture, with a value of 2.87 kPa for \(\sigma=100\) kPa. But for \(\sigma=200\) kPa, the shear strength increased, with a value of 5.69 kPa, and for \(\sigma=300\) kPa the shear strength increased again, with a value of 14.25 kPa, compared to the shear strength of the Cl4 PET admixture. For the residual parameters, the shear strength followed the same trend as for the peak parameters. It increased as the percentage of PET added to the admixture increased, up to an addition of 6% PET, where the shear strength started to decrease. In the case of Cl2 PET admixture, the shear strength increased with a value of 8.91 kPa for \(\sigma=100\) kPa, 8.63 kPa for \(\sigma=200\) kPa and 8.35 kPa for \(\sigma=300\) kPa, compared to the shear strength of the unimproved clay (Cl). For the Cl4 PET admixture, the shear strength increased more, with a value of 4.19 kPa for \(\sigma=100\) kPa, 2.59 kPa for \(\sigma=200\) kPa, and 0.99 kPa for \(\sigma=300\) kPa, compared to the shear strength of the Cl2 PET admixture. For the Cl6 PET admixture, the shear strength started to decrease, with a value of 1.28 kPa for \(\sigma=100\) kPa and with a value of 1.28 kPa for \(\sigma=10  
of 0.22 kPa for \(\sigma\) = 200 kPa, compared to the shear strength of the Cl4 PET admixture. For \(\sigma\) = 300 kPa, the shear strength increased with a value of 0.84 kPa, compared to the shear strength of the Cl4 PET admixture. Analysing the variation in the clay shear strength, in relation to the percentage of PET added, it can be stated that a small addition of PET waste for improving a clay has a beneficial effect on the shear strength of the studied soil.