Background
The internally important Merseylink Gateway Project was designed to provide additional road access across the Mersey, linking the towns of Widnes and Runcorn unlocking the economic potential of an area in need of regeneration.
Following the breaking of ground, the project encountered significant challenges due to the presence of extensive contamination to land and water across the project.
Sanctus Limited were therefore instructed to undertake remediation and dewatering works to facilitate the development.
Works carried out in Widnes during 2017/18 involved processing, treatment, and disposal of ex-situ asbestos contaminated soils prior to the initiation of main infrastructure works, and the construction of one of three attenuation ponds for the newly created Merseylink Gateway Project.
The site, immediately to the north of the River Mersey had been subject to significant historical industrial use, having been previously occupied by a chemical plant, and adjacent to the Marley Eternit / Turner Brothers asbestos factory. Site investigation undertaken by the client determined that the site was heavily contaminated, and would require significant remediation works prior to the scheme bring approved by the local planning authority.
Several stockpiles of unidentified waste, contaminated strata of Made Ground, and underlying natural material, and a number of local, unrecorded depository sites for asbestos waste were found across the site. The Made Ground, and stockpiles were found to contain considerable quantities of asbestos containing materials (including roof sheeting, asbestos lagging, AIB, & millboard); ‘galligu’ waste (an unstable by-product of soda-ash manufacture in the region, containing high levels of arsenic and other heavy metals and often dumped as spoil heaps or used for infilling); and highly elevated levels of hydrogen sulphide contamination. Given the frequently high water table in the area, hydrogen sulphide contamination had leached into groundwater beneath and within the vicinity of the site.
Innovative Thinking and Exemplary Best Practise
As one of the country’s leading HSE licensed asbestos contractors Sanctus were employed to deliver the required remediation. Following the notification period, Sanctus deployed a unique 32-foot 4-stage decontamination unit that had been purposely designed for external asbestos works; to accommodate a larger workforce, to be able to withstand heavy footfall and the rigors of groundworks.
Sanctus were tasked with processing the contaminated excavated materials to remove all visible asbestos, deleterious materials, and oversized materials, to allow for reuse of the materials within the wider Gateway project, following laboratory testing to determine suitability.
Due to the type of asbestos identified, volume discovered on site and the condition in which it was discovered; Sanctus determined that the contract would have to be undertaken under the conditions of Sanctus’ Licence to work with Asbestos, granted by the HSE. In line with best practice, these works were subject to a statutory 14-day notification period prior to works.
The excavation across the site was undertaken in layers and in accordance with the strata, to better segregate contaminated soils from clean materials. Potentially contaminated materials, and those where asbestos containing materials were clearly visible were stockpiled for screening, using a specialised picking station. The picking station was modified to ensure it was kept under negative pressure and that any air passing through was filtered to remove potential asbestos fibres.
The picking bay was manned by trained and qualified licensed asbestos operatives, who removed and segregated all asbestos containing materials and other materials unsuitable for re-use on site.
Sanctus deployed a bespoke mobile groundwater treatment system for the works at the site, comprising a clarifier, oil-water separator, carbon pods and air-stripping towers under the conditions of our Mobile Treatment Licence. All abstracted groundwater was first passed through Sanctus’ bespoke water treatment plant to reduce the suspended solid and hydrocarbon content, and reduce the disposal cost to the client and impact on the environment. The remaining water was stored and collected by tankers, and sent for treatment at a local, licensed treatment facility.
Once the groundwater level was under control and the excavation of the pond complete, Sanctus excavated a small anchor trench around the perimeter of the pond, and installed a multi-faceted pond lining system comprising a 1.5mm thick HDPE layer, with an underlying protective fleece geotextile to protect the HDPE. The HDPE layer was machine-welded in-situ, with the welds air pressure tested for integrity. Bespoke sections of pond liner system were cut and hand-welded around headwalls into the pond, and subsequently welded to the main lining system.
Cost Effectiveness
Sanctus are committed to reducing environmental impact of our works, including the movement and disposal of materials prioritising reuse on site where possible. Materials were excavated under controlled conditions, including the use of both perimeter dust suppression and localised dust suppression.
In total, over 27,000m3 of site won material was processed for re-use, not only saving the project several million pounds with a significant volume diverted from landfill. This achieved a significant reduction in our environmental impact through the carbon footprint of road traffic – calculated as of over 3,300 lorry movements, or 26,300kg of carbon to dispose of the material – without the associated cost and movement of the importation of replacement material.
Our commitment to supporting the local economy and our extensive national network of contacts allowed us to secure disposal options for contaminated materials and groundwater locally significant costs and environmental impact.
Compliance with Legislation, Codes and Guidance
All processed materials were stockpiled, labelled and sampled for laboratory analysis. The samples were subject to an asbestos screen to confirm the success of the screening process; and following this, chemical and geotechnical analysis suite to confirm suitability for re-use across the Merseylink Project.
Soils containing asbestos were sent to be re-processed. Hand-picked asbestos containing materials arising from the processing were dampened and double-bagged, prior to being disposed of at the nearest economically viable and suitably licensed facility.
Given the nature and condition of the asbestos, full asbestos controls measures were in place throughout the works. This included the use of a perimeter dust suppression system, and a mobile dust suppression system capable of targeting specific works tasks.
Air quality monitoring was undertaken on a daily basis, using pumps positioned around works activities and also placed upon operatives themselves, to ensure the health, safety and wellbeing of our operatives as well as to a number of potential highly sensitive receptors adjacent to site. Air monitoring was analysed on site by an independent, UKAS-accredited mobile laboratory to ensure impartiality of results. The control limit of 0.1-fibres per ml of air was not breached indicated that the control measures deployed throughout the works were effective.
Groundwater was found to lie approximately 2-metres above the base of the attenuation pond, which sat in saturated organic material.
We designed and installed an innovative abstraction system to both control and remove the elevated contaminated groundwater which continually recharged. Sanctus installed a sub-formation 1m x 2m stone filled grip, lined with geomembrane, running the whole length of the attenuation pond base, to allow for efficient drainage of the attenuation pond footprint and the lowering of the overall groundwater table.
Abstraction wells were placed outside of the attenuation footprint, with a subterranean link to the grip; allowing for significant groundwater abstraction on a 24-hour basis, maintaining the groundwater low enough to fully excavate the pond extents. Due to the design of the pond, the nature of the ground conditions and lack of space, we used specialised plant to excavate the attenuation pond. This included the use of a ‘long reach’ excavator from the top of the bank (so as to reach the base), coupled with a ‘Bogmaster’ low ground pressure excavator with extra wide tracks, for working within the base excavation.
A distinct layer of galligu waste was encountered during the excavation. Being a by-product of the former industrial processing, this material was highly unstable and toxic, with the potential to react when in contact with water.
The galligu strata was ‘chased out’ and excavated, before being transported to a mobile treatment facility located on site. The galligu waste was subject to a cement-stabilisation process to produce a stable, non-reactive material that could be re-used as engineered fill where required, providing a fitting legacy for the site.
As part of the infrastructure scheme, Sanctus were instructed to create an attenuation pond. Works for the creation of the attenuation pond involved the setting out, excavation, and lining of the pond; including the segregation, processing, classification and disposal of all excavated materials arising. Particular difficulties were posed by the small site footprint for the excavation, processing and stockpiling of materials; and the requirement for control of the highly contaminated groundwater.
Constraints encountered during the creation of the pond included ground saturated with H2S contaminated water; a footprint with only 1.5m to work around the top of the pond; steeply designed batter inclines due to lack of space; and the movement of heavily contaminated materials from the excavation to the processing area under the Control of Asbestos Regulations (CAR 2012).
Due to the additional hazards identified on site, staff were required to wear not only asbestos specific RPE and PPE, but waders and gauntlets when working in proximity the contaminated groundwater.
We designed and installed an innovative abstraction system to both control and remove the elevated contaminated groundwater which was continually recharging. Sanctus installed a sub-formation 1m x 2m stone filled grip, lined with geomembrane, running the whole length of the attenuation pond base, to allow for efficient drainage of the attenuation pond footprint and the lowering of the overall water table. The abstraction wells were placed outside of the attenuation footprint, with a subterranean link to the grip; allowing for significant groundwater abstraction on a 24-hour basis, thus keeping the groundwater table low enough to fully excavate the pond extents.
Economic, Environmental and Social Benefit
Work on the Mersey Gateway Project has generated 4,640 new jobs through direct employment, regeneration activity and inward investment generating £61.9m a year in gross added value to the region by 2030.
The overall Mersey Gateway enabled as a result of our ground works package, delivering integrated public transport, cycle and pedestrian links across the region benefit people and the environment reducing both congestion and carbon emissions and improving connectivity across the North West.
We believe our projects should benefit people and the environment therefore seek to work as sustainability as possible and take every opportunity to benefit the local economy. Aligned to these values, we recruited local plant and labour for the project, not only to reduce costs but to support the local economy and develop new business relationships with local companies. In accordance with CAR2012, all operatives involved with the works were required to be suitably trained for working with licensed asbestos materials.
As well as employing licensed asbestos operatives, Sanctus undertook a bespoke training and development programme to deliver the required training, mentoring and upskilling of plant operatives to be able to undertake the works, leaving a skills legacy to further benefit the North West into the future.
We take pride in our commitment to public engagement and inclusion believing that our projects are ultimately designed to benefit people and the environment.
The legacy of this project is beginning to be realised for both people and the environment. Journeys along the gateways exceeded 6 million in the last quarter of 2018 showing that more people are benefiting from quicker, easier and more reliable journeys.
The Mersey Gateway Project has also won a prestigious BIG Biodiversity Champion 2017 for regeneration of the saltmarsh and estuary across the site and the introduction of grazing cattle has helped stimulate the return of insects, pollinators and birds to the site. The Mersey Gateway Environmental Trust have also noted that salmon are once again migrating up the Mersey to spawn.
Credit to www.merseygateway.co.uk for the above information.
