## 2 Literature Review  
In the literature, different types of soils improved with different waste plastic materials, in the form of fibres, granules, strips, have been studied. The main objective of these studies was to improve the geotechnical parameters of the soils.  
[PERSON] et al. [29] studied the behaviour of a clay of intermediate compressibility from Kerala district, India, improved with different percentages of plastic granules. The plastic granules were added in a proportion of 0.25%, 0.5% and 0.75% by the weight of dry clay. The investigation determined that the improvement method proved to be a proficient technique for improving the geotechnical characteristics of the clay. An improvement in the compressibility of the clay was found, as well as an increase in the shear strength. Optimum results were obtained for 0.25% plastic granule addition by the weight of dry clay.  
A study conducted by [PERSON] and [PERSON] [30] showed the effect that PET waste in the form of strips has had in reinforcing a clay of low compressibility used in the subgrade of a pavement. After a series of California bearing ratio (CBR) and Proctor compaction tests an improvement in the dry density and CBR values of the reinforced samples was found. Optimum results were found for the samples formed from clay and 6% PET strips(\(25\,\mathrm{mm}\times 5\,\mathrm{mm}\)) added by the dry weight of the soil. It was noted that the optimum percentage of PET waste used for clay reinforcement caused a reduction in the thickness of pavement, which led to a reduction in the cost of the project.  
[PERSON] et al. [31] conducted a standard Proctor test, Direct Shear Test and CBR tests on samples made from a silty sand reinforced with PET wastes strips. Tests results indicated a substantial increase in the maximum dry unit weight, shear strength parameters, and CBR value when PET waste reinforcement is added to the silty sand. The level of improvement in silty sand's geotechnical proprieties varies depending on factors such as the soil type, plastic material content, and strip size. For these tests, the optimum results were obtained when using a PET waste content of 0.4% along with a strip size of (\(15\,\mathrm{mm}\times 15\,\mathrm{mm}\)).  
Using polyethylene plastic waste bags in soil improvement and stabilization has been investigated in various geotechnical infrastructure applications, such as slope stabilization, embankments, and road bases. Tests have demonstrated that the incorporation of plastic waste materials into soil improves the bearing capacity and shear strength of weak soils, diminishes soil settlement and increases resistance against liquefaction while improving durability [32; 33; 34; 35].  
[PERSON] et al. [36] studied the proprieties of a soft clay from Pahang, Malaysia, improved with different percentages (0%, 5%, 10% and 20%) of polyethylene terephthalate and polypropylene (PP) in random distributions. A total of 22 samples were prepared and subjected to unconfined compression tests (UCT). Test results showed that the unconfined compressive strength (UCS) increased progressively as the percentage of PET fibre reinforcement in the soil increased, until it reached its peak at an optimal content of 10% fibre reinforcement. In the case of polypropylene fibres, the soil reached its maximum strength when it contained a 20% fibre content.  
The encouraging results found in the literature illustrate that the addition of plastic waste materials in soil improvement has a beneficial effect on the geotechnical characteristics of soils. The research carried out on this topic shows that the field studied is of great interest all over the world. This field offers solutions to two current problems: reducing plastic pollution by reusing materials considered to be waste and increasing the mechanical properties of soils with poor characteristics to support the loads from engineering structures.