coal-authority-transparent.png

Mersey Gateway, The Mersey Gateway, Halton Borough, Northwest England by
The Coal Authority

Shortlisted Brownfield Awards Category 4 - Best Application of Remediation Technologies, 
Category 7 - Best Re-use of Materials and Category 11 - Best Infrastructure Project

1. Introduction


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. This entry 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.

The services RSK provided to the client included contaminated land consultancy (with a named Contamination Specialist seconded into the CJV’s environment team), site investigation, remediation 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.


To meet the exacting project requirements, 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.
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.
 

Mersey Gateway Bridge towards Widnes

2. Project background

The Mersey Gateway scheme comprised the construction of a new six-lane tolled bridge crossing the River Mersey and Manchester Ship Canal between the towns of Widnes and Runcorn in Halton Borough, in the northwest of England. The bridge opened in October 2017, but the project included extensive earthworks and remediation that weren’t substantially completed until October 2019. The project was funded by a Public Private Partnership between Halton BC and three sponsors: Macquarie Capital, FCC Construction S. A. and BBGI.


Kier Infrastructure & Overseas Ltd, Samsung C&T Corporation and FCC Construction formed a Construction Joint Venture (CJV) to deliver the project. The works included the construction of the 1-mile-long  cable-stayed crossing and associated new and upgraded linking infrastructure including; embankments, highways, junctions and landscaping works north and south of the river.

The scheme involved mass earthworks comprising a total of approximately 670,000 m3 (1,070,656 t) cut and fill. 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 286,8350 m3 (458,936 t) or approximately 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 Materials Management Plan (MMP).


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
     

3. Innovation and Best Practice

Stabilisation and solidification of arsenic-impacted soils


The objective of the treatment was to remediate approximately 9,000 m3 of arsenic impacted soils (maximum concentration c. 85,000 mg/ kg) which failed the acceptability criteria. These materials originated from a former scrapyard area located within the footprint of a major new road embankment. The aim was to make them suitable for reuse and deposition at another location within the scheme. A CLR11-compliant Remediation Options Appraisal was completed which determined that stabilisation and solidification techniques would be the most appropriate when considering the Conceptual Site Model (CSM), to ensure that all potential contaminant linkages to controlled waters were managed and mitigated effectively upon completion of the treatment process.


The proposed treatment and deposition location for the soils was as a continuous ‘block’ of treated material, located beneath the asphalt surface of an area of hardstanding, forming a maintenance area, located adjacent to one of the main bridge approach road embankments. The detailed CSM identified the main receptor at risk from leachate from placed materials, as being perched groundwater within the made ground/ alluvium at c. 3 mbgl.


Due to the ‘super-hazardous’ status of these soils, off-site disposal at hazardous landfill facilities was discounted on the grounds of feasibility and cost. The Remediation Method Statement (RMS) for this element was informed by a series of laboratory trials undertaken by CE Geochem Ltd. Following an initial suite of mix design investigations,  it became apparent that the heavily impacted  arsenic-contaminated materials were not suitable for treatment using conventional hydraulic binders such as lime and cement.
 
Further mix design studies considered a suite of more innovative binders (HFO and MgO-based) and environmental sorbents to retard arsenic leachability. Stabilisation trials for a range of material types were completed by CE Geochem to identify the most effective solution. These mix design studies provided different optimum mix design formulations for arsenic, TPH and heavy metal (predominately lead and zinc) impacted soils that also contained asbestos fibres.

The final stabilisation design had the effect of reducing contaminant flux through geochemical processes and physically reducing permeability, thereby reducing overall leachability. The resultant stabilised soils produced a high bearing capacity hydraulically bound engineered fill, satisfying both the geochemical and geotechnical requirements for the re-use of these soils.


The development of mix design formulations was based on an initial geochemical and mineralogical study to identify the key geochemical controls on arsenic leachability, allowing a novel mix design strategy to be developed.


Extensive laboratory trials were undertaken to determine the optimum mix design for successfully treating the arsenic- contaminated soils. A total of 80 different mix designs were trialled including the novel use of Layered Double Hydroxide binder phases combined with HFO sorbents.

The successful demonstration of consistent performance at laboratory scale led to the sourcing of waste ochre  HFO produced from several mine water treatment sites operated by the Coal Authority. Final optimisation phases were used to determine precise mix ratios and treatability windows based on arsenic concentrations within the soils.


The optimised mix design had never previously been applied to the treatment of contaminated soils and is unique to this project. Process optimisation and scale-up was achieved through field trials, leading to the development of an ex situ batch mixing process. The requisite control procedures were integrated into the remedial design to ensure materials compliance.

Implementation and Verification

Field-scale trials used well-homogenised soils characterised by portable XRF to confirm that the geochemical performance observed through laboratory investigations could be achieved on-site through optimisation of the treatment process.

Process control parameters used in the construction phase set the upper-bound treatability window for arsenic concentrations at 17,000 mg/kg.

Therefore, following removal of oversize materials greater than 125mm, stockpiles were tested on-site using the site-specific calibrated portable XRF method, allowing materials with source term concentrations exceeding the defined treatability threshold to be re-blended prior to ex-situ stabilisation.

Ex-situ mixing was undertaken through accurate gravimetric addition of binders, sorbent and soil in batches using segregated mixing bays. Quality assurance/ control (QA/ QC) protocols were implemented to record the relative proportions, average source term concentration of arsenic and terminal MCV prior to placement and compaction.

Batches were deemed suitable for placement where MCV > 8 was achieved, which occasionally required extended mellowing periods of up to 24 hours prior to placement to ensure materials would sustain adequate compactive effort to achieve the required in situ density.

Treated arsenic-impacted soils were placed in 250mm layers (commencing at 1m above existing ground- level), systematically bladed out and depth checked prior to compaction using a vibratory roller.

Compaction density was monitored through in situ core cutting at a frequency of 1 location per 500 m2 with Dynamic Cone Penetrometry (DCP) probe holing conducted to assess the rate of strength gain from deeper (older) curing layers as successive layers were constructed.

Field-scale trials: placement and compaction

4. Robust, sustainable and defensible solution

 

Validation testing comprised CBR and UCS testing for geotechnical validation in addition to 16-day semi-dynamic diffusion-based tank testing to assess geochemical compliance. Results from verification semi-dynamic tank testing were compared with the leaching trajectories from optimised mix designs investigated during bench-scale trials (design phase). Verification results demonstrated that remedial objectives had been satisfied in line with regulatory approval.
 
Confidence in the long-term efficacy of the stabilisation/ solidification treatment was provided by the laboratory and on-site geotechnical testing results, which demonstrated the structural integrity of the monolith. The hardstanding overlying the monolith and associated drainage were designed to limit infiltration and hence leaching, so the long-term effectiveness is dependent on the integrity of the hardstanding and drainage, which there is a long-term commitment to maintain as part of the overall scheme.

Technology Verification Model

Diffusive Flux Verification Model Demonstrating Remedial Compliance

5. Cost-effectiveness

Significant cost savings in the order of £2 million resulted from this pioneering project taking into account minimising the cost of off-site disposal due to reduced quantities and reduced requirements for import of materials. It is estimated that the overall treatment cost was in the order of £120/tonne when compared with potential off-site shipping and disposal costs of £300/tonne. The costs of purchasing and delivering alternative non-wastes for the construction of the operation and maintenance area were also avoided.

In addition, re-use of 5,120 tonnes of HFO achieved a further reduction in the otherwise necessary disposal of wastes from Coal Authority mine water treatment sites.

6. Effective public/stakeholder engagement

The project team provided regular updates to the local community through the construction phase of the project and beyond, including through website, social media, newsletters and hosting of a visitor centre.

A collaborative working relationship was established with both Halton Borough Council and the Environment Agency with regular progress meetings and close contact throughout the works. This provided confidence in this innovative new stabilisation technology, with the works completed in full compliance with the CL:AIRE DoWCoP and the environmental permit.

At planning stage, it was not anticipated that the arsenic-contaminated soil would require off-site disposal, although this then needed to be considered at various stages due to the difficulty in identifying a viable treatment process for the arsenic-contaminated soils. Commitments made to stakeholders at the outset of the project regarding managing the legacy of industrial contamination in the area and minimising off-site disposal were therefore able to be fulfilled.

7. Real environmental/economic/social benefits

The construction of the bridge and associated highways (of which the remediation works formed a significant part) have brought much needed improvements to local infrastructure. The legacy of industrial pollution in Widnes from late Victorian times onwards has caused significant concern to regulators and to local residents for generations and a commitment to improve this was inherent to the scheme.

A live project area Contamination Management Plan was produced and all known contamination issues and proposed remedial works were covered by this document, which formed part of the overarching Construction  Environmental Management Plan through which site stakeholder consultation was instigated.

Retention of the arsenic-contaminated soils following treatment on-site resulted in a significant reduction in vehicle movements – estimated to be around 1600 – and associated reduced nuisance (noise, traffic congestion etc.), fuel consumption and carbon emissions. It also avoided the use of sparse landfill capacity. Further sustainability benefits were gained from finding a beneficial use for a waste product derived from mine water treatment, which would otherwise have gone to landfill.

  • Social media-04
  • Social media-01

All content © 2020 Environment Analyst