St Erth Multi-Modal Hub, 2019
The site had a significant Japanese Knotweed problem owing to its close proximity to the railway which is renowned for spreading this invasive species. In total around 4600m3 of soils were deemed to be contaminated with knotweed or at risk of contamination and therefore unsuitable for general disposal offsite. A series of methods were employed to deal with the knotweed which included on site encapsulated burial – used for around 30% of the total. This was limited as this had to take place outside the footprint of the car park and at significant depth (4m cover desired) and the groundwater restricted where this could be done. The mining works involved extensive excavation to fill hollows and 60% of the soils were processed into an engineering fill through a process called lime improvement. This was utilised for some of the lower risk soils which did not initially meet the required specification. Lime was added to the soils at a controlled rate and mixed into the soils before being compacted in layers to improve the strength and stiffness of the material. The final method employed was screening of the worst contaminated soils to hand pick the knotweed rhizomes using a series hoppers and conveyor belts. The rhizomes were then disposed of and the resulting soils were incorporated into the works with a long-term monitoring plan in place for any regrowth.
This significantly reduced the need for export – the soils would have had to had gone to a tip in Cullompton, the nearest facility which could take it and around 120 miles from the site.
Hydrocarbon & Heavy Metal Contaminated Soils
The St Erth Multi Modal Hub site had varied historical uses which included mining as well as more recent industrial use. As a result, there were a variety a land contamination issues on the site. One of these issues related to the previous use of part of the site as a car scrap yard. A disused railway siding had at some point over the years been turned into a waste trench for a variety of materials arising from the car scrapyard works. This meant that the soils in this area were heavily contaminated with hydrocarbons (oils) and heavy metals such as lead.
The soils had to be removed from the site as part of the Planning Conditions for the scheme, in order to put a halt to ongoing leaching into an aquifer below the site. Removing the contaminated soils directly to landfill would have meant transporting them hundreds of miles to the nearest suitable waste facility. To mitigate the commercial and carbon implications of such a transport operation, Wills Bros employed the use of bioremediation to treat the soils in situ. This allowed the levels of contaminants in the soils to be reduced to such an extent that they could then be disposed of as non-hazardous material and at a waste facility much closer to the site.