## 5 Conclusions  
In this study, the variations in the shear and compression parameters of a clay improved with different percentages of PET were analysed. Based on the experimental results, the conclusions of this study are presented below. A small addition of PET had a beneficial effect for the shearing characteristics of the soil. The peak shear strength increased with the percentage of PET added to the samples, the most significant variation being registered for 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). The highest increase in the internal friction angle was recorded for the Cl6 PET admixture, the value increasing from 18.83\({}^{\circ}\) for Cl to 24.40\({}^{\circ}\) for Cl6 PET (an increase of 29.58%). For cohesion, the best result was recorded for the Cl4 PET admixture, with a value of 29.96 kPa, an increase of 19.59 kPa compared to the value of Cl (10.37 kPa). For the residual shear parameters, the best result was registered for the cohesion value of the Cl4 PET admixture. The cohesion increased from a value of 0.50 kPa for Cl to a value of 15.49 kPa for Cl4 PET. For the internal friction angle, the values for the three clay + PET admixtures were slightly lower than the value for unimproved clay. A small percentage of 2% PET added to the clay improved its compressibility parameters by decreasing specific deformations. Throughout the entire range of vertical stresses applied (\(\sigma^{\prime}\) = 20-1000 kPa), the specific deformations decreased for the Cl2 PET admixture, with an average percentage of 4.53%, compared to the deformations for the unimproved clay. For an addition of 2% PET, the void ratio value decreased by an average percentage of 10.25% throughout the entire range of vertical stresses applied (\(\sigma^{\prime}\) = 20-1000 kPa), compared with the values of unimproved clay. The clay stiffness was improved for the vertical stresses applied, \(\sigma^{\prime}\) = 50-1000 kPa, with an average percentage of 7.39%, at an addition of 2% PET. The following areas can be explored in future research to complete the study's results: a microscopic analysis of the contacts between clay particles and PET in the studied samples, as well as an analysis of how PET size and its distribution in the samples affects the clay's mechanical parameters. The research in this paper demonstrates that for a slight and moderate increase in the geotechnical parameters of a clay, it can be improved by adding a small percentage of plastic waste materials (2% or 4%). The use of plastic waste materials in soil improvement has a great advantage over the use of hydraulic binders: a very low cost for the procurement of the material with which the improvement can be made. This method offers solutions to two current global problems: improving soils with weak geotechnical properties and recycling and reusing plastic waste materials in an innovative way, thus contributing to reducing pollution. Although this method of improving soils with plastic waste materials has begun to be increasingly studied by researchers, showing encouraging results, there are few studies that have analysed the behaviour of plastic materials in the subterranean ecosystem. Under varying temperature and humidity conditions, or under the effect of UV radiation, plastic materials can emit certain chemical compounds into the soil. These chemical compounds need to be analysed to find out if they are harmful or not to the ecosystem. [PERSON] et al. [43] conducted an analysis of the chemical compounds that PET from the clay-PET admixtures releases into the soil, after the application of rapid ageing cycles. It was found that the percentage of PET has an extremely small or no influence on the chemical composition of the clay. Various chemical compounds were identified in clay-PET mixtures, such as lubricants, and aromatic carboxylic acids.