## 4 Results
### Oedometric Compression Tests  
The variation in specific deformations (\(\varepsilon\)) for every vertical stress applied (\(\sigma^{\prime}\)) is shown in Table 7. Analysing the results in Table 7 and the compression-deformation curve (Figure 8), it can be concluded that the most efficient admixture is Cl2 PET. The lowest specific deformation was obtained in the case of the clay + 2% PET admixture, with values ranging from 1.040% for a vertical stress of 20 kPa to 13.580% for a vertical stress of 1000 kPa. Throughout the range of applied vertical loads (20-1000 kPa), the specific deformations have a lower value in the case of the Cl2 PET mixture than in the case of clay unimproved with waste plastic materials (Cl). As the percentage of PET added to the clay increased to 4% and 6%, the specific deformations also increased compared to the Cl2 PET admixture and to the clay in its natural state. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & & \multicolumn{8}{c}{**Shearing Strength Parameters for**} \\ \hline  & **Cl** & **Cl2 PET** & **Cl2 PET** & **Cl2 ECT** & **Cl4 PET** & **Cl4 PET** & **Cl4 PET** & **Cl4 CEM** & **Cl6 PET** & **Cl6 PET** & **Cl6 CEM** \\  & **17** & **23** & **17** & **23** & **17** & **17** & **23** & **17** & **17** & **23** & **17** \\ \hline \multicolumn{10}{c}{Peak Parameters} \\ \hline \(\Phi^{\prime}\) & (\({}^{\circ}\)) & 21.49 & 18.83 & 24.47 & 20.93 & 22.29 & 21.53 & 20.20 & 35.01 & 27.79 & 24.40 & 30.66 \\ \(c^{\prime}\) & (kPa) & 30.96 & 10.37 & 19.60 & 19.79 & 66.44 & 31.41 & 29.96 & 47.77 & 5.97 & 18.53 & 80.62 \\ \hline \multicolumn{10}{c}{Residual Parameters} \\ \hline \(\Phi_{\mathrm{ns}}\) & (\({}^{\circ}\)) & 10.61 & 12.37 & 14.19 & 12.21 & 11.83 & 9.62 & 11.33 & 34.14 & 19.94 & 11.92 & 20.48 \\ \(c_{\mathrm{res}}\) & (kPa) & 20.17 & 0.50 & 1.12 & 9.70 & 24.97 & 18.13 & 15.49 & 12.63 & 0.00 & 13.15 & 12.57 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison for shearing parameters of unimproved and improved clay with PET and cement [9]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{4}{c}{\(\varepsilon\) (\%)} \\ \hline  & & **Cl** & **Cl2 PET** & **Cl4 PET** & **Cl6 PET** \\ \hline  & 0 & 0.000 & 0.000 & 0.000 & 0.000 \\  & 20 & 1.055 & 1.040 & 0.795 & 1.050 \\  & 50 & 1.805 & 1.795 & 2.140 & 2.940 \\ \(\sigma^{\prime}\) (kPa) & 100 & 2.935 & 2.710 & 4.005 & 5.380 \\  & 200 & 5.475 & 5.165 & 6.710 & 8.480 \\  & 300 & 7.555 & 7.145 & 8.875 & 10.485 \\  & 500 & 9.760 & 9.265 & 11.650 & 13.275 \\  & 1000 & 14.435 & 13.580 & 15.425 & 17.455 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Variation in specific deformations (\(\varepsilon\)) for every applied vertical stress (\(\sigma^{\prime}\)). The lowest void ratio values were obtained in the case of the clay + 2% PET admixture, with values between 0.539 for a vertical stress of 20 kPa and 0.330 for a vertical stress of 1000 kPa. The lowest voids ratio values throughout the range of applied vertical loads (20-1000 kPa) were recorded for the Cl2 PET admixture. Also, for the Cl4 PET admixture, the voids ratio values were lower than in the case of natural clay, but higher than in the case of the Cl2 PET admixture. The variation in the oedometer deformation modulus (E\({}_{\text{oed}}\)) in relation to the effective stress (\(\sigma^{\prime}\)) is shown in Table 9 and Figure 10. It can be observed that for an addition of 2% PET, the oedometer deformation modulus (E\({}_{\text{oed}}\)) decreases slightly for the vertical stress range 20-50 kPa. For the rest of the applied vertical loads, the values of the oedometer deformation modulus were higher for the Cl2 PET admixture than for the unimproved clay. The best results were obtained for the Cl2 PET admixture; for the other two types of admixtures (Cl4 PET and Cl6 PET), the values of the oedometer deformation modulus were generally lower than those of the initial unimproved clay. Thus, the results indicate an important improvement in the soil stiffness, obtained by a small addition of shredded PET (2%).