Background
The South Down Road Site in Lewes covers an area of ~1.2Ha close to the town centre, in one of the most desirable areas of East Sussex. This site had previously been viewed as unviable for development due to its former use as a domestic and inert waste landfill between 1960 and 1979. The site is adjacent to vital services on the eastern boundary including a Royal Mail Distribution Centre, and an NHS Fleet Maintenance Centre, residential areas to the North East and public open space to the west. The landfill extends over the adjacent industrial estate, public open space and areas of flood defence close to the River Ouse only 200m to the South East. There is an ephemeral stream on the boundary to the east of the site draining toward the Ouse.
A unique opportunity presented itself as Sanctus were engaged to deliver a sustainable, safe and efficient development for 80 individual residential plots consisting of three storey apartment blocks and two storey modern townhouses with undercroft parking. Supplying much needed housing for 80 families as well as commercial units to add to the local economy and community. The old landfill was unlined and constructed without mitigation/control measures other than a relic 0.6m deep gravel ground gas ventilation trench adjacent to the residential boundary to the North East. The site was nominally ‘capped’ with material which at the time of site works was found to be between 200-1000mm of a subsoil type material. Under this capping layer various landfill materials had been identified, extending to 3.9mbgl. Under which a peaty alluvium layer between 3.6-13.0m deep, graded into a sandy clayey gravel alluvium. The Holywell Nodular Chalk Formation bedrock strata is a principal aquifer and was encountered beyond 13mbgl. Additionally, the site was located near a Source Protection Zone 1, a few hundred meters offsite to the South.
Technical Challenges & Solutions
The strategic goals had always focused on the minimisation of offsite disposal. The client was therefore open to potential design and engineering solutions to reduce the need for disposal, whilst providing a safe and sustainable development. As the works progressed, it became clear that the ground conditions required highly specialised and innovative skills and design solutions. Sanctus was able to deploy its mobile treatment licence as licensable asbestos specialists and engage directly with the regulators throughout the 26-week programme.
Sanctus’s client supported the Definition of Waste Code of Practice and associated Materials Management Plan to demonstrate the need to retain materials and treat soils on site under its bespoke environmental permit where needed.
There were very limited records and characterisation of the wastes ahead of site works, however previous investigations had identified free fibres of asbestos within the samples and elevated lead and benzo(a)pyrene within the soils. Groundwater in shallow units was impacted by hydrocarbons and polyaromatic hydrocarbons.
Planning Constraints
Due to the volume of residential plots significant undercroft car parking spaces had to be made available along with specific drainage goals using Sustainable Drainage systems (SuDs) to reduce loading on local sewers.
Enabling Works
During vegetation removal and early works two stands of Japanese Knotweed were discovered in the capping material. This material was delineated and removed from site for suitably licensed disposal prior to the commencement of works.
Waste Composition
Although site investigation had previously been undertaken, some asbestos free fibres were identified below quantification limits and no bulk asbestos materials were identified in exploratory holes or samples. The landfill material was heterogenous and containing variable sands, clay, brick, fabric, glass, metal, timber and plastic. Various obstructions and oversize items such as containers, oil drums and gas canisters were frequently encountered.
During the enabling earthworks, Sanctus discovered bulk Asbestos Insulation Board (AIB) dispersed throughout sections of the landfill and works in these areas became notifiable to the HSE under the Control of Asbestos Regulations 2012. Following this discovery and implementation of enhanced asbestos control measures along with the difficult waste characterisation of landfill materials, designs were modified to raise design levels of softscape areas where possible to severe future exposure pathways.
The abundant AIB and risk assessment required us to adapt our working practices to incorporate handpicking and reduced reliance on mechanical screening. The site was found to have ground gas emissions of CO2 commonly above 10% although due to the age of the waste limited methanogenesis was occurring. However, Made Ground on site was classified CS3/Amber 2 for both Methane and CO2.
Water Control
Although groundwater was recorded from previous investigations, the groundwater levels that were encountered were much higher than anticipated. Three hydrogeological units were identified during works. Firstly, the surface water due to the works being carried out during one of the wettest winters on record, the second at 4.00mbgl in alluvium and waste interface and at 10.00mbgl from the gravel alluvium
After works began ponding of the surface was frequent and exacerbated by the cohesive areas of the landfill material. As a result of the abnormally high water table Sanctus over dug and installed temporary groundwater abstraction wells. Sanctus then deployed its water treatment plant and the site was dewatered under a trade discharge consent to allow for the discharge of treated water into the sewer network. These surface water issues were compounded by a leaking offsite land drain. Due to high levels of sediment and dissolved contaminants, Sanctus passed the water through a clarifier, oil water separator and granular activated carbon vessel prior to discharge to the foul sewer. The system was actively monitored on and offsite by telemetry and third-party lab testing. In order to manage water levels across the site Sanctus sunk a series of sumps and drainage channels across which the groundwater could be abstracted. In total some 1500m3 of ground water was removed and treated over the duration of the project.
Material Volumes and Engineering
The main challenge was the processing and engineering of the available site won materials to the required geotechnical properties for reuse and reinstatement. The target was to reuse material on site, recover and improve less suitable materials, screen, sort and reduce volumes where reuse or recycling was not possible.
To accommodate the planning requirements the development included an area of undercroft parking. This excavation, up to 2.5m below the original site levels was undertaken to create the required void space by reengineering material. By adapting site levels to allow for permeable paving and lined SuDs this material was accommodated elsewhere. In areas of the site under the central carpark and lined suds, ‘clean’ cover capping materials were over-dug to win additional materials, to allow the further treatment and engineering of materials that would overwise have required disposal.
Levels were then raised with imported recycled aggregate to form the pile mat therefore removing the need for virgin quarried aggregates.
Treatment of Soil Materials
Screening and sorting of materials used a large grate riddle attachment on an excavator, a low intensity handpicking line and 3-way screening system to maximise the usable material retained on site. This method allowed processing of ~3000m3 landfill waste.
Due to the heterogenous nature of the landfill and the compressibility of the underlying Alluvium, buildings at the site were designed with piled foundations. Sanctus developed the piling risk assessment recommending CFA piles enabling the retention of materials unsuitable to the geotechnical requirements of other foundation solutions whilst protecting the underlying aquifer. To enable this, piles were designed with aggressive materials in mind with the BRE SD1 rating of DS-3 and AC-3.
Following this a cost-effective solution to negate the requirement for piling across the road, reducing concrete and piling options was also developed (saving up to 10t of concrete per pile). The on-site capping material was stabilised using lime to form three 300mm layers of geotechnically suitable material. Following each stabilisation run, 8 passes with a 16t single drum roller compacted the material, producing the firm stable platform required for the construction of the new access road. Reducing excavation for import and placement of subbase.
At formation level a rapid high efficiency impact compaction rig was used to compact the road on a 2m grid system. The compaction had a cone of influence up to 3mbgl at each location. CBR testing was carried out across the formation with 100% CBR values returning >5% exceeding the geotechnical specification.
Once reinstatement of other areas began, the unrecoverable (unrecyclable) wastes were placed first into the fill area and compacted. The site levels were then made up with the site won soils and aggregates that had been screened from the wastes and placed and compacted then finished with cover soils. This produced a significantly more stable platform than the original landfill, which was further reinforced by a geotextile and a demarcation high visibility layer followed by recycled aggregates to form the piling mat.
The application of the three-layer multi geogrid solution reduced the volumes of imported recycled 6F2 by 33%, whilst also reducing the requirement for further excavation to accommodate additional pile mat thickness. Service lines were pre-dug and backfilled with site won treated and engineered material which was validated by an independent laboratory to reduce future legacy issues. The access road was required to be a free draining permeable area. 1.3m deep, 0.1m wide holes were augered on a 1m grid system across the road. The resulting holes were lined then backfilled with 20- 40mm processed gravel, providing vertical drainage bypassing the landfill and shallow groundwater unit and dispersing within the deeper Alluvium. The combination of screening and various treatments enabled the betterment of 18,675m3 of otherwise unsuitable material.
Demonstrating Best Practice
Benefits of Re-use
The approach of re-engineering in excess of 11,900m3 of mixed materials had the net effect of removing almost 3,000 HGV movements to landfill and subsequent import of material.
The net carbon saving from the reuse of material, avoiding virgin aggregates and disposal offset the carbon footprint of this scheme by approximately 547,000kg. This is approximately equivalent to the amount of carbon sequestered by 100,000 trees over the duration of the project, whilst providing much needed homes for 80 families near the town centre of Lewes.
Environmental Net Gain & Betterment
The scheme included lined SuDs to reduce infiltration through the landfill material retained, protecting the adjacent stream. The SuDs included permeable paving, block paving and free draining lined systems to the deeper aquifer unit on the main access road.
The works themselves removed licensable asbestos products, treated impacted groundwater and improved materials to form a low permeability barrier system for the site, whilst removing only 309m3 of gross contaminants not suitable or safe to remain on site. This material includes the disposal of 27m3 of Japanese knotweed impacted soils treated for reuse offsite, and the removal and disposal of 25m3 asbestos. The change in site levels and re-engineered site won materials also allowed for an ecological refuge with clean site won soils to be included in the optimised design.
Conclusion
Sanctus re-engineered this former landfill to allow the development of much needed housing for 80 families. This was achieved while redefining what’s possible in the reuse of material on site, significantly reducing transport and import of materials (and the associated carbon emissions) whilst removing legacy environmental risks for the future.
The strategic and sustainability goals of this projects were far exceeded with minimal amount of offsite disposal, and new and innovative approach used to allow onsite re-use of material.
In addition to the successful remediation of a ~1.2Ha site, the improvement and reengineering of 12,000m3, the treatment of 1500m3 of contaminated water and the saving of over 500 tons of carbon, our approach achieved cost savings of at least £3.3m based on local disposal rates alone. We hope this project helps inspire others to approach challenging remediation projects by adopting a reuse first approach, applying the principles of the circular economy at scale to practical site-based developments.
