From biopolymers to recycled tyres — research could pave the way for better roads

Scorching hot summers and cold, wet winters are contributing to the poor quality of many Australian roads – a double whammy for engineers who have to deal with expansive clay soils that move as the seasons change.

A Federation University researcher hopes to improve the quality and longevity of Australian roads by introducing innovative sustainable materials – including some that are edible – during construction.

Civil Engineering lecturer Dr Amin Soltani, whose research focuses on geotechnical engineering, has been trialling products to minimise the reliance on cement and lime for soil stabilisation applications – mainstays of road construction for decades.

“Generally speaking, soils are poor construction materials and need to be engineered to make them suitable. It’s a serious problem throughout Australia, particularly here in southern Australia because our expansive clay soils are highly susceptible to seasonal changes,” Dr Soltani said.

“During winter, the soil absorbs moisture and they expand in volume and during dry seasons they lose their moisture and shrink. The challenge here is that the amount of expansion and contraction is not the same. When you have this cyclic uneven increase in volume and decrease in volume, the structure which is resting on the soil also moves.

“This movement can crack the structure, in this case, the road or pavement. Whenever you see all those cracks on the asphalt surface or even the potholes, it is at least in part to do with the soil. There are other issues, including the quality of asphalt and the way that we construct our roads, but the soil is really important in Australia and it's causing a lot of problems.”

To address these issues, engineers mix lime – and sometimes cement - in the road subgrade to create a stable surface for the subbase, base and asphalt layers to sit on, but Dr Soltani says this process doesn’t always work and isn’t environmentally sustainable. In most cases, this process will work, but if the soil has organic matter, high salt concentrations or compounds like sulphates, the roads are more likely to fail earlier than expected.

For the past five years, Dr Soltani’s research focused on trialling sustainable materials that could either replace lime or minimise reliance on it. This has included waste materials like recycled tyres and plastics, chemicals including synthetic polymers and biopolymers, and sulfonated oils.

Research is underway worldwide into the stabilisation of soils using different products, but Dr Soltani, who collaborates with researchers from around the world, says it is difficult to translate research results in other countries to Australian practice because each soil in each area has its own unique properties and problems. A unique climate is another factor in doing research tailored for Australia.

His studies include a promising field trial in Adelaide using recycled tyres, working with colleagues from the University of Adelaide. His most recent study has been trialling biopolymers in laboratory simulations.

“Most biopolymers are completely edible, you can even purchase these products in a supermarket. They're usually used in the food industry and for biomedical applications as a thickening agent and are derived from either plants, animals or microorganisms,” Dr Soltani said.

“Typical examples are xanthan gum and sodium alginate, edible polysaccharide biopolymers that can be diluted at small amounts in water and used to mix and compact the subgrade soil in situ.

“Requiring a small portion means we are also reducing lime/cement transportation costs as well. That study revealed that we can reduce lime consumption by up to 50 per cent.”

The lab tests are to monitor the performance of the soil mix over time and Dr Soltani says the early results are promising but field trials are needed to see if the research would translate to a real-world solution. He hopes to start a trial in a suburban area of Ballarat.

“We claim in Australia that we're designing our roads to last 20-25 years, but many often don’t last this long before roadwork is needed. For a project like this, because we don't know exactly what the outcomes would be, we would need to be cautious with selecting a site – for instance, a suburban road with a speed limit of around 50 to 60 kilometres per hour to minimise safety issues.” Dr Amin Soltani

He said a trial would likely include a small section of road with the traditional treatment of cement and lime, a section with the biopolymer-based product, and a third section with the biopolymer mixed with cement and lime.

“Three different sections and they would be instrumented with optic fibres, strain gauges and moisture sensors to monitor the ground movement and moisture/suction changes in the ground over time,” Dr Soltani said.

“It is an interesting and exciting prospect. Biopolymers are a relatively new area and the idea of using products like these is becoming more important because of the sustainability issue and governments striving for net zero emissions and better waste management,” Dr Soltani said.

“Another reason I do this research is because it's cross-disciplinary. I get the chance to work with chemists because I don't know how to modify biopolymers – that's not my area – and I get to work with experts in recycling, geotechnical and pavement engineers, and water engineers to develop innovative solutions.”

Dr Soltani hopes his research into using biopolymers to transform soils will go beyond solving road engineering problems. Biopolymers could also be applied to serve other needs, including supporting vegetation growth through increasing water retention of soils, groundwater control, dewatering and thickening mine tailings and controlling dust and erosion control for unsealed haul roads.