Sustainable shear behavior of clayey sand reinforced with recycled PET strips under moisture variation

Moisture-sensitive granular soils containing fines, such as sand-clay (SC) mixtures, lose their shear strength after cycles of wetting and drying, compromising the stability of shallow foundations, embankments, and sub-base layers. Improving performance using conventional stabilisers such as cement and lime comes at a cost to the environment and the economy. This study examines recycled polyethylene terephthalate (PET) strips as a green reinforcement for a sand-kaolin mixture (65% sand, 35% kaolin; SC according to the unified soil classification system unified soil classification system (USCS)). Direct shear tests were performed conducted at normal stresses of 100, 200, and 300 kPa and moisture contents of ranging from 0 to 12%. The unreinforced samples suffered a loss of strength of up to 50% at 12% moisture content, while the PET-reinforced soils achieved a maximum shear stress that was 25-30% higher, 40% greater cohesion, and a 35% reduction in vertical deformation. The friction angle was slightly better (+ 1.3° at 0-4% moisture content) but decreased at higher water contents due to lubrication. Even at 12% moisture content, the reinforced soils had a cohesion of 19 kPa compared to 15 kPa for the unreinforced soils and a shear stress of 90 kPa compared to 145 kPa for the dry strength. These results confirm that PET strips act as traction elements, resisting softening due to moisture and offering a sustainable and cost-effective alternative to traditional stabilisers, while contributing to circular economy initiatives.