## 4 Results
### Oedometric Compression Tests
Correlating all the results obtained using the oedometer tests, it can be stated that a small percentage of addition of shredded PET can improve the mechanical characteristics of a clay by decreasing specific deformations and the void ratio variation and increasing the oedometer deformation modulus. In terms of compressibility, the best results were obtained in the case of clay improved with 2% PET waste. For this admixture, the specific deformations and voids ratio variations had the lowest values, and the oedometer deformation modulus values had the highest increase. [PERSON] et al. [42] carried out a series of oedometer tests in 2016 on a clay like the one used in this study, improved with different percentages of polyethylene (2%, 4%, 6%, and 8%). It was found that the most suitable percentage of PET for improving the compression parameters of the clay was 4%. The results from that study cannot be compared with those in this paper, because the oedometer tests were different. Vertical axial stresses applied for the tests in 2016 were from 50 kPa to 300 kPa, while in this study the vertical stresses applied were from 20 kPa to 1000 kPa. However, it can be stated that clays from the Cluj-Napoca area can be improved with different percentages of PET, their geotechnical parameters being positively affected by the addition of plastic waste materials. In another study, this clay improved with 2%, 4% and 6% PET was tested in a climatic cabinet. To simulate the in situ behaviour of the PET-improved clay, four cylindrical samples (Cl, Cl2 PET, Cl4 PET, Cl6 PET), with a diameter and height of 10 cm, were subjected to fifteen freeze-thaw cycles. The temperature varied in the climate cabinet from \(-10\)\({}^{\circ}\)C to \(+40\)\({}^{\circ}\)C, and the humidity increased up to 90%. It was found that the clay-PET admixtures behaved well in terms of shrinkage cracking. As the percentage of PET in the samples increased, shrinkage cracks decreased. The best results were obtained with a percentage of 6% added PET, the shrinkage cracks being almost inexistent [10]. Analysing the results of the tests carried out on clays from the Cluj-Napoca area, it can be noted that a small percentage of addition of plastic waste materials has positive effects on the mechanical characteristics of a clay. Increases in internal friction angle, cohesion, shear strength, and stiffness and decreases in the deformation and void ratio were observed, for a small addition of polyethylene terephthalate (2% and 4%). This study, corelated with the literature research, has shown that these results are influenced by the amount of PET added to the soil, its shape and its distribution in the studied samples. PET also had a beneficial effect on clay-shrinkage cracks, reducing their number and thickness. These results are encouraging, even if the level of improvement is not as significant as using hydraulic binders. It should be kept in mind that the costs of hydraulic binders are much higher compared to the extremely low costs of plastic waste materials. In addition to the low-cost aspect, it should be considered that recycling and reusing plastic waste materials has a favourable effect on the environment, by reducing pollution and automatically reducing the risks it causes.