By converting waste materials into durable backfill, this collaborative trial is helping to preserve precious natural crushed rock.
Backfilling excavated trenches for services located under trafficable pavements requires substantial natural resources.
Virgin high-strength rock is sourced from quarries, requiring extensive crushing and manufacturing processes to develop high grade crushed rock.
This process, depleting finite resources and contributing to greenhouse gas emissions through extraction and transportation, is unsustainable.
According to Ehsan Yaghoubi, Associate Professor in Civil Engineering at Victoria University (VU) this scarcity of sands and gravels has become a significant issue for construction projects.
“Many projects have faced delays because companies have to source materials from far outside urban areas, which adds time and cost,” he said.
Aggregates should be prioritised for essential construction materials such as concrete and asphalt.
“Trenches don’t need to be filled with such valuable resources. Finding alternatives helps conserve these natural materials.”
A sustainable solution
To solve this challenge, VU partnered with Greater Western Water (GWW) and Ground Science on the Sunbury Backfill Trial to test the feasibility of using recycled materials in lieu of crushed rock.
Funded by Sustainability Victoria, this trial builds on the success of earlier projects in 2021 and 2023, which involved extensive laboratory tests of these materials in non-trafficable areas, such as parks and backyards.
“Since these areas don’t bear the load of traffic, it was a suitable starting point,” Yaghoubi said.
Following these initial trials, identifying the best mix designs for traffic-bearing applications took another two years. The material mixes contain no natural aggregates, instead comprising recycled concrete, waste plastic, glass and tyre fragments in specific proportions.
This method both conserves natural resources and repurposes materials that are otherwise destined for landfill.
“We created different mix designs, tested them under simulated traffic loads, and selected the top two,” Yaghoubi said. “Our lab tests showed that the strength of these recycled mixtures are comparable to natural crushed rock.”
Put to the test
To test the durability of the mix designs in trafficable areas, a road section with three trenches was backfilled at GWW’s Sunbury Depot – one with conventional crushed rock, and two with the recycled mix (one bound with cement, the other unbound).
To support the trial, GWW provided staff, machinery and access to roads at their Sunbury site, with Ground Science providing technical expertise, and field testing. Over the next year, VU, GWW and Ground Science will be monitoring the performance of the backfilled trenches.
“We have developed a tool with a camera to take monthly overhead photos of these sections, tracking crack development over time to determine which section was performing better,” Yaghoubi said.
A walking profiler device is also being used monthly to measure surface deformation from passing trucks.
Future implementation
Following another year of monitoring, findings will be published – with VU, GWW and Ground Science working together to explore how industry uptake may be encouraged more widely. There is an opportunity to support the use of this recycled mix for trench backfill in utilities infrastructure, particularly in areas with heavy traffic loads.
“We’re already coordinating with a batching plant to integrate this process, with our implementation framework also being tested in the trial,” Yaghoubi said.
“We can share with contractors and stakeholders where to [develop] the mix, deliver it to site, and how to back-fill trenches – so hopefully when they put out a tender for a job, they will consider this as an option as a construction material.”
By using this new backfill material in a 10 m trench section, approximately 30 to 40 tonnes of crushed rock can be saved.
This approach also reduces costs, with landfill fees and levies avoided. “Producing this backfill mix at scale can be cheaper than using crushed rock,” Yaghoubi said.
Ernie Gmehling, Managing Director of Ground Science, who has been involved in the project since the early stages, describes the material as “the next best thing since sliced bread”.
“It has sparked so much interest,” he said. “I’ve presented it at conferences and to private businesses, and we’ve had meetings with prospective manufacturers – everybody can see the benefit of it.”
Collaborate to win
Collaboration with industry partners is key to the success of applied research, overcoming challenges in implementation and bringing the products to use in the real world, Yaghoubi explained.
“Ground Science completes quality control testing of trench backfill when water authorities backfill their trenches and GWW are asset owners of trenches and pipelines, so they know the everyday challenges that exist and how to resolve them,” he said.
“We’re talking about innovative green construction materials and techniques to build our infrastructure and transport network that could be stronger, cheaper and greener,” he said.
It takes a lot of partners to get research work through the process of validation and then into the market space.
“Without collaboration by partners, these things don’t occur,” Gmehling said. “We’re trying to do a lot better for the planet, and this is a way forward.”
GWW’s General Manager of Operations and Delivery Solutions, Jodie Hallam said projects like this pave the way for long-term environmental benefits by potentially turning recycled materials into viable products, supporting a circular economy.
“It’s an innovative approach to address landfill waste and offers a cost-effective, sustainable and efficient way to backfill trenches in critical areas while still responding to urgent access requests promptly,” she said.
“We see potential to share this technology with our peers across the water industry, expanding these benefits beyond our service region.”
Find out more about the Great Western Water and Victoria University Partnership, and explore the capabilities of VU’s Institute for Sustainable Industries & Liveable Cities.
