## 1 Introduction  
Over 300 million tons of plastic waste materials are generated annually, with a significant amount ending up in the environment, where they persist for decades [1]. Each year, at least 8 tons of plastic waste materials make their way into the ocean, which is equivalent to a garbage truck dumped into the ocean every minute. If no measures are taken, it is estimated that by the year 2030, the quantity will increase to two trucks per minute, and by 2050, it will reach four trucks per minute [2,3]. Finding solutions for the elimination or environmentally friendly reuse of these waste materials is necessary [4]. Recent studies have shown a new approach to reuse plastic waste materials in geotechnical engineering in soil improvement and stabilization [5, 6, 7, 8, 9, 10]. Soil improvement and soil stabilization are terms used to describe any technique, be it physical, chemical, biological, or a combination of these, that alters the properties of natural soil to meet specific requirements for geotechnical engineering. If the available soil is inadequate to support the loads of engineering structures, traditional methods of improvement and stabilization of soil using hydraulic binders are already known [11; 12]. However, this method often proves to be costly due to the high price of additives. The concept of soil reinforcement with fibres was introduced by [PERSON] in 1969, who observed an increase in the shear parameters of reinforced samples [13]. Since then, research into improving soils with fibres has advanced, and nowadays both natural and waste fibres are used in the process. Research has been performed on improving soils with fibres from bamboo [14; 15], jute [16; 17], cori [18; 19], palm [20; 21], sugar cane [22], waste rubber [23; 24], and waste plastics [25; 26; 27]. In most cases, the results showed an increase in the geotechnical parameters considered in the studies such as an increase in the internal friction angle and cohesion, an increase in the shear strength of the soil, improved compressibility, and improved behaviour of the studied soil to moisture variations. The use of plastic waste materials in soil improvement can serve as a means of recycling and reusing these waste materials, providing an efficient soil improvement method at reduced costs. By collaborating with local plastic waste repositories, it is possible to acquire the raw material for improving and stabilizing soils. Soil improvement with plastic waste materials might raise ecological discussion, but further research must be conducted to determine the viability of this solution. The civil engineering field includes various applications of plastic waste materials, such as using them for soil stabilization in base and subbase layers of pavements, reinforcing earthen embankments, and mitigating soil settlement in foundations. Waste plastic materials can also serve as a cost-effective substitute for other expensive admixtures like cement and lime in soil improvement processes. The purpose of this paper is to study the geotechnical parameters of a clayey soil improved with shredded polyethylene terephthalate (PET) waste. This paper will highlight the beneficial effect of PET waste on clayey soil by analysing the variation in the internal friction angle, cohesion, shear strength, and compressibility, as the percentage of added PET in the test samples increases. The use of PET waste from water, soft drinks and beer bottles in soil improvement offers a solution to two major current problems: reducing pollution through recycling and reusing plastic waste materials (PET), and improving the mechanical properties of soils for adequate load-bearing capacity during engineering constructions. Polyethylene terephthalate (C\({}_{10}\)H\({}_{8}\)O\({}_{4}\))\({}_{h}\), commonly referred to as PET, is a polymer synthesized from petroleum hydrocarbons by combining ethylene glycol and terephthalate acid.