Former William Blythe Chemical Works, Restoration of landfill site and creation of a country park
Shortlisted for Brownfield Awards Category 13 - Best Biodiversity Enhancement
Organisations: RSK Environment Ltd., RSK Geosciences, Synthomer plc., Gleeson Homes, Buckingham Group Contracting Ltd.
Summary
The site was a former chemical works near Burnley including a hazardous waste landfill which was restored over the last ten years in accordance with an RSK design enabling sale of part of the factory site by Synthomer plc. to Gleeson Homes for housing development. Gleeson Homes are mid-way through construction of the housing development and are currently creating an access footpath within the landfill (retained by Synthomer) to enable recreational public access which was an intended benefit of the landfill restoration.
The restoration has made safe an area that represented a potential risk to public health whilst significantly enhancing biodiversity. It also created a site of clear amenity value to the local community both within the new housing development and the wider village through the following:
▪ Providing a solution to combine landfill closure best practice with the creation of an accessible public open space on the doorstep of the new housing development realising an opportunity to enhance the health and wellbeing of new residents.
▪ Competitive tendering and re-measurement were selected to best apportion the risks and to achieve the most cost-effective, and durable solution. Selection of a quarry fines by-product for the restoration soils, provided a sustainable, nutrient poor subsoil producing optimal conditions for biodiversity enhancement
▪ Applying Environment Agency (EA) Construction Quality Assurance (CQA) guidance to provide a confident solution, in conjunction with planning conditions, landscaping best practice and responding to the requirements of the Lancashire Biodiversity Action Plan.
▪ Removing a source of concern, the site has been transformed both aesthetically and as an amenity into an asset for the local community. The project included extensive consultation and contribution from the Local Parish Council and the Burnley and District Civic Trust who awarded a certificate to the project team in recognition of the community benefits generated by the work undertaken
▪ Restoring a chemical works landfill previously producing leachate, which could only be disposed to foul sewer, to create a surface water environment providing new habitats for wildlife, safe spaces for public amenity, and unlocking adjacent brownfield land for residential redevelopment.
▪ Selecting design, structure and landscaping to provide a durable and adaptable solution, evolving over the last decade to meet the requirements of the EA, residential developer and local community, the ultimate objective being that the site is not constrained by any long-term licence, permit or maintenance requirements.
Timeline and Key Roles
Former landfill and chemical works
Background
The restored landfill (5 ha) is within the former William Blythe chemical works (total 15 ha) located on the outskirts of Hapton, Lancashire, operating on the site adjacent to the landfill since the mid-1800s. Waste materials generated from the production of arsenic, lead, tin and zinc and sulphur-based compounds on site were progressively deposited in the landfill over a period in excess of 150 years.
The remainder of the land (10 ha) was occupied by the former factory buildings (now under development by Gleeson) and an area of undeveloped grassland and woodland, designated as greenbelt. The waste deposited has a thickness of up to 10m. Due to its age, no controls such as an engineered liner or capping were in place, with the waste placed directly over the natural clay.
Over time, as only parts of the landfill were in use, it had progressively become overgrown. It comprised some established mature woodland and a plantation (silver birch) added during a phase of partial restoration in the 1990s to screen the site from the M65
Investigation and Risk Assessment
The landfill remained in use until 2005. Following the issue of a Closure Notice by the EA in 2006, RSK was appointed by the site owner (Synthomer plc) to undertake intrusive investigations and risk assessments across both the landfill and the wider William Blythe site. This work developed the site conceptual model and ultimately the most appropriate landfill restoration strategy.
The landfill contents include very high concentrations of metals, arsenic and asbestos which prior to full restoration posed a potential risk to human health preventing access and potential redevelopment of the remainder of the former factory site.
Risk to groundwater (Secondary aquifer within Coal Measures) were largely mitigated by the presence of a thick layer of low permeability Till. However, in its unrestored state, the landfill leachate represented a significant risk to surface waters. The EA were consulted throughout the process of investigation and risk assessment to ensure the work undertaken was adequate to inform a future restoration and monitoring strategy.
Placement of mineral layer and drainage stone
Restoration
Planning consent for the landfill restoration scheme was granted in 2010 and RSK supported the Client in the selection of a remediation contractor, Buckingham Group Contracting Ltd. RSK developed an Earthworks Specification for CQA documents and managed the tender process. The project risks were assessed with the client and used to develop suitable contract terms. After tender assessment, RSK recommended a contractor and the contract was prepared and signed. RSK administered the contract as the Engineer, with the ICE 7th edition remeasurement contract being selected to obtain best value.
Various options were explored for a capping material including a bentonite matting layer. The chosen solution, on the grounds of both cost-efficiency and proven effectiveness and durability was a 0.5m low-permeability clay layer, 0.3m gravel drainage and 0.2m restoration soils comprising a subsoil layer to create a 1.0m thick barrier. The capping design was established taking account of EA Technical Guidance on Capping and Restoration of Landfills.
The capping materials were sourced from local quarries and the selected ‘subsoil’ was a quarry fines material. This was a by-product of aggregate manufacture and therefore a more sustainable construction product. It had an added benefit of being well-draining and nutrient poor which was ideal for supporting wildflower meadow species and curbing the growth of grasses. However, it was found that a balance was needed between enhancing biodiversity and achieving slope stability: more grass seed needed to be applied during later re-seeding events for the steeper gradients to help bind the fine surface material and prevent formation of runoff gullies.
A soil analyst explored the options for manufacturing soil using paper production waste or reservoir silt, as a means of reducing topsoil import costs. Detailed discussions held with the planning officer confirmed the methods for achieving acceptable soil depths and conditions for future planting.
Once the waste had been reprofiled, the mineral layer was placed and rolled sequentially onto the surface, and keyed into the underlying natural clay. It was necessary to construct a deep in ground clay barrier along some boundaries to completely tie-in the cap to the underlying clay and prevent water in adjoining made ground and historic drainage systems from flowing under the landfill.
The selected landform removed existing very steep gradients, is most efficient at shedding rainwater and takes the form of an irregular dome surrounded by a ‘V’ shaped drainage ditch, collecting in an attenuation lagoon prior to passing to foul sewer. Residual flows of leachate are also collected at two locations and flow passively to foul sewer. RSK supervised and validated the restoration work which was completed in 2011/12. The Landfill Closure Report and validation reports prepared by RSK were referenced in an updated Environmental Permit.
Biodiversity Enhancement
A Phase 1 Habitat Survey was undertaken by RSK as part of the planning application for the redevelopment. This identified the site to be in an ecological setting which includes areas of common moorland to the east; Pollard Moor, Hapton Common and Bentley Wood Green. These sites comprise a complex mosaic of habitats including neutral grassland, acid grassland, modified bog, flushes and scattered scrub and trees.
The survey indicated habitats present on site to contain common and widespread species. They were identified as being easily replicated and of negligible nature-conservation value from a botanical perspective. Even those existing habitats that were more structurally diverse such as plantation, scrub, reed-swamp, and rush pasture were described as common in the landscape, typical of former industrial sites and ‘unexceptional’ in their extent or diversity.
RSK prepared the landscape design for the landfill, taking into account the findings of the Phase 1 Habitat Survey and provided a detailed specification for planting areas and tree and wildflower and grass species within meadow areas, which was agreed with Lancashire County Council taking into account the priorities of the Lancashire Biodiversity Action Plan (BAP).
The BAP lists several habitats to be considered in the planning process, which was used by RSK in developing species lists. Overall, the proposed landscaping and planting has significantly increased the diversity of habitats and number of plant species and overall ecological value of the site. The range of habitats created was also designed to attract several invertebrate and vertebrate species to the site in the medium term.
Landscape Design
The restoration required the removal of the existing poor-quality trees from within the landfill area, and the planting of over 3,500 new broadleaf native tree species across both the landfill and surrounding greenbelt area covering a total of 19,000m2. A donation of trees was also made by Synthomer to the Parish Council to improve existing amenity areas within the village of Hapton.
The three main landscape features and habitats identified in the landscape plan were:
Tree and Shrub Planting
Proposed structural tree and shrub planting to screen detracting features (including the M65 and electricity pylons) and to improve biodiversity.
Wetland Habitat
Improvement upon existing open ditches, to include a flood balancing attenuation lagoon and widened ditches, to complement existing wetland and moor habitats, when combined with targeted planting, encouraging the diversity of wetland plant species in the medium to long term.
Wildflower Meadow
Most of the landfill capping layer was seeded as a species-rich wildflower meadow which now provides seasonal interest and colour, as well as supporting insects and birds.
Habitat management measures were incorporated to mitigate for the loss of habitat due to the clearance of the site including;
enhancement of habitats and areas of transition (including woodland and wet woodland, ditches, reedbed, hedgerows, grasslands, ponds; bat foraging and bird nesting habitats)
Damaged perimeter hedgerows were replaced and the incorporation of attenuation for rainwater runoff in the form of
widened ditches and lagoons provided opportunity to create 1,900m2 of wetland habitat and marginal reed swamp habitat (500m2).
RSK continues to manage a landscape management contract which has ensured the successful establishment of tree and grass planting on both the landfill and surrounding areas (see images).
Enhanced planting within greenbelt land and landfill beyond
Monitoring and Aftercare
Since the restoration completed in 2011, RSK has undertaken aftercare monitoring of the landfill and its environment. RSK provides annual reports, providing records to the EA of monitoring of environmental media, slope stability and landform settlement. RSK is also in the process of assisting Synthomer in securing surrender of the Environmental Permit for the landfill.
Gleeson Redevelopment; fulfilment of long-term planning requirements
Investment in restoration of the landfill by the client, Synthomer which, as well as responding to regulatory requirements, enabled the sale and redevelopment of the adjoining former factory plot. The area was sold in 2016 to Gleeson Homes who proposed the construction of over 200 homes.
The planning determination for this development included a Section 106 agreement stipulating the creation of access to the restored landfill, so that the area could be used by the residents of the housing development as a public open space amenity. This was agreed by Synthomer and the EA and a licence granted to enable formation of a series of footpaths across the landfill.
This has taken effect in 2021, the work carefully observed by RSK to ensure integrity of the landfill cap and monitoring infrastructure is maintained.
Attenuation lagoon with coir netting reinforced banks (top image taken in 2011, bottom image taken in 2021)
Access footpaths being placed by Gleeson
Conclusion
In conclusion, restoration of the former industrial site and landfill at Hapton is considered an exemplary case study of best practice in long-term planning and delivery of site restoration to achieve biodiversity enhancement and social/ amenity value.