## 3 Materials and Methods  
The flow chart of the procedure followed to obtain the samples for tests can be analysed in Figure 1.  
The soil used in this study is a silty clay with medium swelling and shrinkage activity (classification according to the Romanian Norm NP 126/2010 [37]) (Figure 2a). It was collected from a site in the Cluj-Napoca area, Romania, from a depth of 1-10 m. The physical proprieties and granulometric composition of this clay are shown in Tables 1 and 2. Figure 1: Flow chart of the procedure followed to obtain the samples. The polyethylene terephthalate used in the clay-PET admixtures was provided by a local plastic waste recycling repository. PET came from recycled water, beer and soda bottles and was cleaned using specific methods for cleaning and recycling plastic waste materials: \begin{table}
\begin{tabular}{c c c} \hline
**Properties** & **Parameter** & **Value** \\ \hline \multirow{6}{*}{Physical properties} & Specific weight \(\gamma_{\text{s}}\) (kN/m\({}^{3}\)) & 26.70 \\  & Dry unit weight \(\gamma_{\text{d}}\) (kN/m\({}^{3}\)) & 14.54 \\  & Natural unit weight \(\gamma\) (kN/m\({}^{3}\)) & 18.11 \\ \cline{1-1}  & Natural water content w (\%) & 21.05 \\ \cline{1-1}  & Plastic limit, w\({}_{\text{P}}\) (\%) & 15.95 \\ \cline{1-1}  & Liquid limit, w\({}_{\text{L}}\) (\%) & 47.65 \\ \cline{1-1}  & Plasticity index, Ir (\%) & 31.70 \\ \cline{1-1}  & Consistency index IC [-] & 0.83 \\ \hline \end{tabular}
\end{table} 
Table 1: Physical properties of used clay. \begin{table}
\begin{tabular}{c c c} \hline
**Soil Type** & **Grain Size d (mm)** & **Percentage (\%)** \\ \hline Clay & d \(<\) 0.002 & 47.61 \\ Silt & 0.002 \(<\) d \(<\) 0.063 & 40.20 \\ Sand & 0.063 \(<\) d \(<\) 0.2 & 11.26 \\ Gravel & 0.2 \(<\) d \(<\) 6.3 & 0.93 \\ \hline \end{tabular}
\end{table} 
Table 2: Granulometric composition of used clay. Figure 2: Used materials: (**a**) clay; (**b**) shredded PET; (**c**) clay + 6% PET; and (**d**) clay + OWC + 6% PET. it was washed and the impurities were removed, it was shredded, and it was washed again and dried. The PET used in this study had irregular shapes, with sizes ranging from 3 mm to 12 mm (Figure 2b). The test samples were created in the laboratory by manually mixing clay with different percentages of PET (2%, 4%, 6%), randomly distributed. The amount of PET added was related to the dry mass of the clay. For each test, 4 types of samples were produced (0%, 2%, 4%, 6%), each type containing 3 samples. To produce 3 samples of one type, 2000 gr. of clay sieved through a 2 mm sieve was manually mixed with one percent of PET (i.e., for a percentage of 6% PET, a quantity of 120 gr. of shredded PET was added to the 2000 gr. of clay) (Figure 2c). The admixture was then mixed with the optimum water content (OWC) of 18% (Figure 2d). The optimum water content is the water content at which, because of compaction in the Proctor apparatus, a maximum measurable density is obtained. After compaction in the Proctor apparatus (Frowag, model 2151), the three samples were collected. A total of 12 square samples (6 cm \(\times\) 6 cm \(\times\) 2 cm) were collected from the Proctor apparatus for direct shear tests and 12 round samples (6 cm \(\times\) 2 cm) from oedometric compression tests (Figure 3a-c). The composition of each type of samples, their coding and form is given in Table 3.  
All tests were performed according to Romanian standards, valid at the beginning of the experimental programme. The optimum water content was determined as per standard STAS 1913/13-83 [38]. Direct shear tests were performed according to STAS 8942/2-82 [39] and compression tests were performed according to STAS 8942/1-89 [40]. The equivalent of these standards can be found in SR EN ISO 17892--Geotechnical investigation and testing [41]. Direct shear tests were performed on samples immersed in water under consolidated-drained conditions (CD). The consolidated-drained shear test is performed when the soil is loaded at a slow rate and there are natural conditions for water drainage from the soil pores.