Replacing aggregate with subsoil to save carbon and waste on road projects

New Civil Engineer Talk To The Team For An In-Depth Look Into The Ecofill Innovation

Turning subsoil into an aggregate replacement is already cutting carbon, cost and waste on new housing estates. So how have housebuilders stolen a march on big infrastructure?

National Highways has set a plan to achieve net zero carbon for maintenance and construction of roads by 2040. That is now just 16 years away and the industry has much to do if it is to change the way highways are built and maintained. 

“This can’t be done by tweaking how we already work,” explains Ecofill chief technical officer Seb Love.

“Let’s say you’re building a road and you’ve got 1,000 lorries of subsoil to remove from site and another 1,000 lorries of aggregate to bring on site. All those lorries generate CO2 and other pollution that needs reducing, so an old fashioned approach to reducing that would be to reduce distance. You’d try to find local disposal sites for the waste subsoil and local aggregate sources to import to site from.

“That’s not a bad thing to do, but it’s not enough. It’s not going to achieve the targets the industry has been set. We need to think bigger. We need to get rid of the lorries.”

Getting rid of those lorries is one thing Ecofill is helping housebuilders with by turning subsoil into aggregate replacement on site.

Doing this on just four projects with Avove, Redrow, Taylor Wimpey and Barratt Homes, resulted in a 40,484t reduction in material sent to landfill and a 45,953t reduction in aggregate imported to site. As a result, 9,508 lorry journeys – or a cumulative 350,000 road kilometres – were cut from construction, saving 104,036 litres of fuel.

The result of this is that 315t of carbon, 589kg of nitrogen dioxide and 6,646g of PM10 particles have been eliminated from those projects. 

Treating subsoil

To create the aggregate replacement, soil samples were analysed at laboratories so that a bespoke binder could be made to reflect the exact nature of the subsoil at each project site. The analysis included tests for water content, particle size distribution, liquid limit, plastic limit, resistance to fragmentation, sulphates, chloride and organic content.

Although that takes time, once the binder is ready it can be used to turn 1,000t of subsoil into aggregate replacement material on site per day. Very few journeys are needed – one to bring the processing machine in, one to take it away when it has finished, and periodic deliveries of binder brought to site throughout the process. 

Along with environmental benefits, this also saves money on landfill costs and aggregate costs, eases logistical challenges and wins friends with local planners and communities affected by lorries heading to site.

Ecofill stresses that the aggregate replacement is not technically aggregate, although it meets the series 800 national specification requirements and performance levels for highways and the series 600 specification for earthworks in the highways Manual of contract documents for highways works. The product performance is the same as traditional capping and fill materials such as for 6F5, 6F2 and type one subbase.

How housebuilders moved first

That level of performance has made it easy for housebuilders to adopt in full compliance with national and local authority design guidelines. But it raises a question – why have housebuilders led the way, not major infrastructure projects? 

“I think a big part of it is because the housing developers have changed,” explains Ecofill head of marketing Sean Rowland.

“They’re not just throwing up houses nowadays. They’re placemaking and integrating with communities, creating brand new villages over the course of a decade or more. So they think more carefully about how they build and the impact they have on existing communities and the local environment.

“And even though they’re all big names, they’ve got regional offices who are kind of competing with each other all the time to do better and be trailblazers. That makes them open to new ways of working.”

The other side to that is why the same thing has not happened yet with big roads projects. Love thinks it is only a matter of time before they catch up.

“There’s nothing sinister at work. The very big infrastructure companies want to decarbonise. They want to save money. It’s just harder for them to jump first with a new technology.

“They need to hang back and see it proven first before they apply it to a very big project.”

That patience may be well founded because the pressures mounting on the sector all push the same way. According to the government’s Waste Statistics Regulation Returns, waste soil makes up 58% of all landfill waste in the UK.

At the same time, the Mineral Products Association reports that in 2020, only a quarter of aggregate materials were from recycled sources. So, turning subsoil into aggregate replacement could dramatically reduce the UK’s waste and extensive quarrying needs.

And then there is the legacy impact on nature and agriculture. This has led to the process being deployed on temporary roads on one big infrastructure project – the ongoing Vyrnwy Aqueduct modernisation programme. For that project, infrastructure services and engineering company Avove partnered with Ecofill to provide 37 temporary haul roads with a total length of 22.4km across agricultural land. 

Temporary roads are vital to big infrastructure works but can reduce soil fertility, raise pH levels and even cause soil erosion in the long term. 

“Quicklime or cement can increase the pH massively,” says Love. “So crops can’t grow and the nutrients can’t be uptaken. That can have severe detrimental effects on land use. So importing aggregates was normal for temporary roads because we didn’t have a better alternative. But because we formulate very bespoke binders, we had an idea.”

That idea resulted in an inspection test plan being drawn up and subsoil samples collected before work began. Those samples were then sent to a United Kingdom Accreditation Service (UKAS) accredited lab for analysis to determine the binder to be used and provide a baseline for the soil’s pre-disturbance character. That means that when temporary roads are removed, efforts to return the soil to its original state can be verified.

“The outcome achieved the structural requirements and the design life of the temporary structures for temporary access roads, compounds, etcetera,” explains Love.

“And we did it in a way that could revert them back to their original pH range and nutrient status afterwards.”

Love is proud of that achievement, but with the 2040 targets approaching, and as biodiversity net gain requirements for major projects come into force next year, he warns against complacency.

“There’s still a lot more for us to develop. We have a system now where we can do that but we have to keep thinking bigger because the industry still has those big targets to hit.”