Stabilization of Low Plastic Clay Soil with Recycled Pet Strips

Abstract

Expansive and low plasticity clay soils pose significant challenges in geotechnical engineering due to their low bearing capacity, low strength, and high volumetric instability under moisture variations, often leading to excessive settlements, cracking, and premature failures of pavements and shallow foundations. Conventional stabilization methods such as cement and lime are also effective but are associated with high cost, carbon emissions, and environmental concerns. Simultaneously, the accumulation of Polyethylene Terephthalate (PET) waste has become a critical environmental issue particularly in developing countries like Nepal, highlighting the importance of sustainable reuse solutions. This study presents an experimental investigation on the stabilization of low plastic clay (CL) soil using waste PET bottle strips. Soil samples were collected from the Kalimati area of Kathmandu at a depth of 1.5 m and subjected to standard laboratory tests including moisture content, Standard Proctor compaction, Atterberg limits, and specific gravity. The untreated soil exhibited an Optimum Moisture Content (OMC) of 28% and Maximum Dry Density (MDD) of 1.54 g/cm3. With increasing PET content (0-1.25%), MDD decreased to 1.45 g/cm3 while OMC reduced to 22%, indicating a reduction in water affinity and density variation. Unconfined Compressive Strength (UCS) improved significantly from 37.79 kPa (0% PET) to a maximum of 58.22 kPa at 1% PET, representing a strength increment of 54.82%. Beyond the optimum content, UCS decreased to 52.11 kPa at 1.25% PET due to reduction of soil-strip interaction efficiency. The results confirm that 1% PET content provides optimum reinforcement, improving strength, ductility and overall soil performance. The study demonstrates that waste PET bottle strips can serve as an effective, low cost and environmentally sustainable alternative for soil stabilization in pavement subgrades and shallow foundation systems, while simultaneously addressing plastic waste management challenges.