Geotechnical characterization of plastic waste materials in pavement subgrade applications

Scarcity of traditional construction materials has motivated researchers to explore alternatives, and besides crushed glass, reclaimed asphalt pavement, and scrap tires, to name a few, plastic waste (unwanted or unusable plastic objects) has also gained attention in recent years. Plastic waste is traditionally re-used or recycled, but it often ends up as trash on curbsides, in landfills, or in our seas and oceans. The substantial amount of plastic waste produced annually worldwide, and its environmental repercussions are the rationale for exploring alternatives in order to recycle plastic waste into construction materials. This exploration can also benefit many industries and would help minimising adverse environmental impacts associated with dumping tones of plastic waste in landfills. Using plastic waste material with soil for soil reinforcement purposes has revealed some improvements in terms of strengths of materials, but nevertheless, this potentiality has not been fully assessed for different types and forms of plastic waste with natural sub-grade soil in the road industry. In this paper, ground, flaky, and pelleted shapes of four sorted types of plastic waste from a recycling market were combined with silty or clayey gravel and sand soil of the A-2-7 AASHTO type. These plastic types are: low density polyethylene (LUPE), high density polyethylene (HDPE), polyethylene terephthalate (PETE), and polypropylene (PP) resins. Their various geotechnical properties have been assessed thoroughly. The investigation process entails assessing compaction, Californian Bearing Ratio (CBR), strength, resilient modulus, and permeability properties for both natural subgrade soil and modified subgrade soil with the aforementioned types of plastic waste. The results obtained show that the addition of plastic wastes decrease the maximum dry densities of the subgrade soils because of the lower relative density of the plastic material compared to the soil particles. It is also found that the addition of plastic wastes can increase or decrease the CBR and MR values of the subgrade. The nature of change (increase or decrease) and its magnitude are a function of the plastic content, shape and type. Permeability values of many subgrade soil samples increased with the addition of plastic waste, whereas the hydraulic conductivity of some soils modified with plastic remained unchanged. Subgrade soils with plastic had higher friction angle and lower compressive strength than plastic-free soils. The results of this research suggest that partial replacement of subgrade soil material with plastic waste may prove useful in mad subgrade applications.