Stabilization of Lateritic Soil Using PET Bottle Shreds and Periwinkle Shell Ash for Subgrade Layer of Road Pavement

Every road begins with the ground beneath it. Before a single layer of asphalt is laid, engineers must ensure the soil below can support the weight of cars and trucks without sinking or cracking. This bottom layer, known as the subgrade, is the foundation of the entire structure. In many parts of the world, the available soil is laterite, a reddish earth rich in iron and clay. While plentiful, this soil often struggles under pressure, especially when wet, making it a poor candidate for building roads on its own. To fix this, engineers traditionally mix in other materials to strengthen the soil, but these additives can be expensive and energy-intensive to produce. At the same time, the world is grappling with a growing mountain of waste. Plastic bottles and discarded shells from consumed seafood are two such waste streams that clog landfills and harm the environment. The question driving recent research is whether these two very different problems—weak soil and excessive waste—can be solved together by turning trash into a construction tool.

A team of researchers at Covenant University in Nigeria set out to test this idea using two specific types of waste: shredded plastic from polyethylene terephthalate, or PET, bottles, and ash made from burning periwinkle shells. Periwinkles are a type of sea snail widely eaten in West Africa, and their shells are often thrown away in large quantities. The researchers collected lateritic soil from a site in Ogun State and began a series of laboratory experiments to see how mixing in these waste materials changed the soil's behavior. They first prepared the soil by drying it and crushing it into a uniform powder. They then took used plastic bottles, washed them, and shredded them into small strips roughly the size of a fingernail. For the shells, they cleaned them, burned them in an open fire, and then heated the resulting ash in a furnace to ensure it was chemically active before grinding it into a fine powder.

The team approached the problem in two stages. First, they tested how much shredded plastic the soil could handle on its own. They mixed the soil with small amounts of plastic shreds, ranging from two percent to ten percent of the total weight. They found that adding plastic made the soil stronger, but only up to a point. When they added six percent plastic by weight, the soil reached its peak strength. Adding more than that actually made it weaker, as the excess plastic began to get in the way of the soil particles packing tightly together. With the optimal amount of plastic identified, the researchers moved to the second stage, keeping the plastic content fixed at six percent and varying the amount of periwinkle shell ash. They tested the mixture with ash added in amounts from two percent up to ten percent.

The results showed a clear pattern of improvement. The natural, untreated soil had a very low ability to bear weight, registering a value of 5.5 percent on a standard test used to measure soil strength for roads. When the researchers combined the six percent plastic shreds with six percent periwinkle shell ash, the soil's strength more than tripled, reaching a value of 19.5 percent. This specific combination also significantly increased the soil's ability to resist being squashed, a property measured by its compressive strength, which rose from 120.5 units in the natural soil to 273.3 units in the treated mix. The researchers also observed that the treated soil became less permeable, meaning water could not pass through it as easily as it did through the untreated dirt. This is a crucial benefit for road foundations, as water infiltration often leads to softening and failure.

Despite these gains, the researchers noted that the treated soil, while much stronger than before, still did not reach the high strength levels required for the heavy layers of a road that carries constant traffic. The best mix they created is suitable for the subgrade layer of roads with light traffic, but it is not yet a replacement for the base layers of major highways. The study suggests that the strength comes from a dual mechanism: the plastic shreds act like tiny fibers that hold the soil together, while the ash from the shells reacts chemically with the soil to form cement-like bonds. However, the team also found that adding too much ash, specifically beyond six percent, caused the strength to drop again, likely because the extra fine powder disrupted the bond between the soil and the plastic.

This work demonstrates a practical way to improve the quality of local soil while simultaneously reducing the volume of plastic and shell waste in the environment. The researchers concluded that using these specific waste materials is a viable path forward for road construction in regions where lateritic soil is common. They emphasized that while the results are promising, further testing is needed to see how well this soil holds up over many years of wet and dry cycles. For now, the study offers a concrete example of how engineering can turn two environmental burdens into a single, useful resource, provided the mixture is carefully balanced to avoid the point where too much of a good thing becomes a problem.