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Breaking new ground

soil science

Environmental laboratory and soil science consultancy service SESL Australia uses science to unlock the potential of excavation  waste and turn it into topsoil.

When a layperson looks at an excavation site or tunnel spoil, they generally see a pile of dirt destined for landfill or on-site burial. The team at SESL sees potential.

For more than 30 years SESL has used science to transform previously “unusable” waste into topsoil and subsoil for multiple end-use landscapes. By identifying the physical and chemical properties of site-won soil material, the team can blend (characterise, ameliorate and reformulate) a formula to recover and reuse them to support landscaping aims. Soils can also be “designed” from waste materials including excavation spoil, crushed sandstone, and other organic or mineral waste materials.

“We are trained to look at soil, to feel it, to describe it,” says Simon Leake, SESL Principal Scientist and company founder. “We know the chemistry of soil and can assess the risk of any contaminants on vegetation. We recognise the important indicators in soils. We ask why? What is it? Where does it come from?”

Simon studied agricultural science at university, majoring in soil science. He established his own laboratory and consultancy firm with the encouragement of his father-in-law, fully expecting to be working within the farm and agriculture sector.

He says he quickly became involved in urban soil site projects. His first big job was rehabilitating an ex-gas works site at Manly Point. Following that, he says landscape architects “realised very quickly the value of having trained input on some of the more complicated projects to get the soils right”.

Natural topsoil within Australia is a finite resource with decreasing availability of quarried soil. SESL’s remediation process unlocks the potential of soil and fill already on site.

Sydney Olympic Park (SOP), in 1995, was one of SESL, and Simon’s, ground-breaking projects. 

Large quantities of waste clay soil were on site but because they were heavy, poorly drained and sodic they gave poor results in initial landscape work so alternatives had to be found.

Huge amounts of topsoil were needed so Simon went searching for waste destined for landfill that could be rediverted to make topsoil to cover the clay materials. He used crushed sandstone from other projects to design a copy of a podzolic soil that originally occurred on site, saving millions in costs.


Environmental challenges 

The Woo-la-ra landfill reburial at SOP is one of the largest revegetation sites SESL has worked on.

A clay cap was used to entomb the landfill waste collected from various parts of the site. Land-capping clay is unsuitable for plant establishment, so SESL worked with Waste Services NSW to make topsoil from crushed sandstone collected from all over Sydney. Blended with compost and gypsum they formed soils that were cost effective and successful in rehabilitating the site. 

The concept was developed further at Barangaroo in Sydney, using crushed sandstone VENM (Virgin Excavated Natural Material) from various Lend Lease excavations. 

Chantal Milner has an environmental biology background and heads SESL’s Soil Science team. She says the Barangaroo project presented new challenges.

“There wasn’t enough usable sandstone on site, so we brought in sandstone from other projects,” she says. “One of the big things we noticed was the variability – the sandstone we were getting was quite different from one place to the next. The soil formulation had to keep changing.”

Sandstone with low silt and clay content was blended with crushed glass sand to make subsoils and clean sand to make topsoils. Compost was added to provide the plant nutrient requirements. 

Chantal says the Barangaroo project has been successful, with less than one per cent plant failure in the remediated soil. (Anything with less than 10 per cent plant failure is considered a successful project within the industry.)

She says it’s satisfying to be able to reuse waste in an environmentally sound way, which is also economical for clients.

“We’re using a waste product and keeping it onsite, rather than it being disposed of and going to a tip. Typically, people are saying you can’t reuse subsoil, but we’re showing people how they can and turn it into topsoil products.”

SESL has applied its knowledge to numerous other projects including the tunnel spoil for the Western Rail line, using spoil to make the soils needed for the extensive landscaping works over the Lilyfield tunnel exit. 

Some commercial suppliers such as Australian Native Landscapes are now also commercialising these types of soils.

Looking ahead

Simon says a shortage of quality soil, particularly in Sydney, and a growing demand for urban soil presents plenty of opportunities. He is encouraging the industry to look at sandstone as a higher valued raw material and consider crushing and washing plants to create clean sand. 

“Premium quality sportsfield sand is getting harder to find, and more expensive,” he says. “The opportunity for washing plants is quite exciting.”

Simon is also advocating for Environment Protection Authority resource recovery exemptions to include waste topsoil. Currently waste topsoil can only be redirected for use as engineering fill. Simon says topsoil taken from a recent racecourse resurfacing project would have been ideal for local council or school ovals. Under current regulations the reuse would have required a specific exemption, which can add months and costs to a project.

“The specific exemption system is too slow to allow this,” says Simon. “Topsoil is a gaping hole in the system at the moment. I very much hope we can fix that. I hope people like us at the coalface can be heard.

“We know our waste, we know our soils and we know the needs of plants. There is no such thing as waste, it’s just an inappropriate allocation of a resource. It’s our job to find it a home, or at least an innocuous place for it.” 

For more information, visit: www.sesl.com.au

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