The Mersey Gateway: Innovation through remediation

November 12, 2020

In light of the upcoming Brownfield Awards, for which RSK’s submission for the project ‘The Mersey Gateway, Halton Borough, Northwest England’, has been shortlisted for three categories, Environment Analyst Director and Brownfield Awards Judge Ian Grant spoke to RSK’s Haydn Keen, Project Director and Stephen Pettitt, RSK Technical Director and Contamination Specialist for Merseylink Civil Contractors Joint Venture (MCCJV) for the duration of the project. The resulting interview can be found on the Environment Analyst website.

The project, forming part of the £1.8 billion Mersey Gateway infrastructure scheme, utilised risk assessment and applied a range of remediation techniques to achieve successful remediation and enable reuse of over one million tonnes of soils. The entry, led by RSK Geosciences and RemedX, focuses on a highly innovative solution that was developed and applied to remediate and allow reuse of large quantities of arsenic-contaminated soils within the scheme. It was shortlisted in Category 4 – Best Application of Remediation Technologies. The overall project was also shortlisted for Category 7 – Best Re-use of Materials and Category 11 – Best Infrastructure Project. We will find out if it triumphs in any of these three categories at the virtual awards ceremony on 19 November 2020.

The services RSK Group provided to the client included contaminated land consultancy (with a named contamination specialist seconded into the MCCJV’s environment team), site investigation, remediation design and supervision, archaeology, regulatory liaison and wider environmental services throughout the project. RSK’s specialist remediation contracting division, RemedX, worked in partnership with the main contractor to ensure that the soil remediation remained compliant with all soil, groundwater and surface water treatment completed under its Mobile Treatment Permit and associated deployments. RSK’s in-house laboratory, Envirolab, provided chemical testing throughout the project.

The scheme involved mass earthworks comprising the cut and fill of more than 650,000m3 (over 1 million tonnes) of material. All the soils re-used within the scheme were deemed to have met the approved site-specific Earthworks Acceptability and Validation Criteria (EAVC) and were reused under the CL:AIRE Definition of Waste Code of Practice (DoWCoP) Materials Management Plan (MMP). The legacy of heavy industry, particularly chemical manufacture and processing in the areas adjacent to the river on both sides (predominantly in Widnes) meant that a wide range of ground contamination was present, including heavy metals, petroleum hydrocarbons, volatile organic compounds (VOCs), PAHs, asbestos and cyanide.

A total of approximately 287,000m3, or around 40% of site-won soils, were treated/remediated to enable them to meet the defined earthworks acceptability criteria for re-use and retained on-site under the site’s MMP, set up in accordance with the CL:AIRE DoWCoP. 

The remediation technologies involved comprised: 

  • screening and segregation 
  • stabilisation and solidification of soils impacted with arsenic, other heavy metals, and petroleum hydrocarbons
  • groundwater and surface water treatment comprising filtration and aeration.

During the course of the project, two main types of contaminants requiring remediation were identified: those impacted with TPH, and those with arsenic. Trials indicated that stabilisation using cement, PFA and lime was a feasible option for remediating the TPH. After further comprehensive laboratory and field trials the remediation method was refined and implemented.

The remediation of the arsenic proved more challenging. After comprehensive trials and laboratory testing, including RemedX engaging specialist laboratory and research into alternative stabilisation methods, a novel solidification/stabilisation approach using Hydrous Ferric Oxide (HFO) sorbents, together with Magnesium Oxide as a cementitious binder, was developed and optimised. Waste ochre HFO was then sourced from Coal Authority-operated mine water treatment sites, further increasing the sustainability benefits. The optimised mix design had never previously been applied to the treatment of contaminated soils and is unique to this project, which was one of the largest brownfield infrastructure projects completed in Europe in recent years.

The re-use of approximately 9,000 m3 of waste soil which had been stabilised/solidified under a Mobile Treatment Permit, facilitated the construction of an operational area for use by the bridge and highways maintenance contractors. The remediation targets and engineering specification were fully met and achieved estimated cost savings in the region of £2 million. In addition to the cost savings, RSK’s remediation techniques also had significant benefits on the environment, as less material needed to go to landfill and there was therefore significantly less plant movement on site and on the surrounding road networks, which were already under pressure.

It is anticipated that this innovative technology could be applied on other sites where no other form of remedial technique is suitable and where disposal or off-site treatment options is not practical or economically viable.

You can watch the full interview on the Environment Analyst website. The Brownfield Award winners will be announced on 19 November 2020 at a virtual awards ceremony.

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