Kinwood Vale – Landfill Regeneration by CampbellReith
Shortlisted for Best Sustainable Brownfield/Urban Regeneration/Infrastructure Scheme Award
Summary
The Kilnwood Vale project, located near Gatwick, is a significant sustainable regeneration by Crest Nicholson. CampbellReith led the remediation of the 205 Ha site which included a former 60 Ha landfill, and transformed this into a thriving community, including 2,750 residential units, 744 affordable homes (27%), a two-form entry primary school (with room for expansion), retail and employment space, various community facilities (elderly care facility and community centre), and extensive green spaces (including a brook and naturalised SuDS features). It has also involved the design of railway bridges to enable the future adjoining development.
The site was previously a deep landfill and was not developable because of land instability (settlement), high ground gas risk and irregular topography. The regeneration has been possible because of the notable innovations in the approach to landfill remediation that CampbellReith implemented at the site to facilitate the reuse of excavated soils whilst addressing the technical concerns associated with ground gas risk, soil contamination, ground settlement and civil engineering challenges.
CampbellReith have been the lead engineer [geotechnical, land quality, civil and structural] and have designed and overseen the whole remediation and strategic infrastructure construction.
The whole redevelopment adopted sustainable development principles, but the remediation specifically contributed significantly to environmental and socially aligned UN Sustainable Development Goals (SDGs) and the government’s Social Value commitments.
Notable Innovation or Exemplary Best Practice (Remediation)
The earthworks strategy used exemplary best practice note RB17 and forensic soils testing (fTOC) to develop a revised Conceptual Site Model to control gas risk. This approach removed the previous constraint of land-based gas protection that impeded the future development / commercial strategy of the Client and allowed each completed earthwork phase to be assigned Characteristic Situation 1-3 with provisions limited to passive gas protection for buildings.
The fTOC testing of soils (after excavation, sorting and processing) was used in conjunction with drum tests and surface gas emission surveys to compare predicted and actual gas emission rates. This removed the requirement for ground monitoring after completion, which significantly reduced risk of delays to developers. An innovation was applied to RB17 to also use the fTOC results to also control residual creep expanding the geotechnical remit of this.
A combination of surcharge and earthworks (innovatively adapted from highway work standards) was employed to achieve predefined post-treatment ground settlement criteria.
These techniques were combined after detailed quantitative modelling to determine residual ground settlement (consolidation, collapse and creep) and the gas emission rate of the placed and treated soils. The remediation balanced cut and fill to create a re-contoured, development ready, site with resilient strategic infrastructure. The digital ground model anticipated the soils recovery rates from processing, allowed for bulking and compaction and the recycling of soils between surcharge and placement. This process eliminated the vast majority haulage traffic movements from the site.
The ground treatment integrated with the design of roads and drainage with contingencies for residual settlement and future constraints (for example future applied loads) including steepened pipe gradients, mattress beds for roads and drainage, placement of a low permeability layer to mitigate gas migration and water inundation and rocker pipes at transitions. The risk assessment and remediation design also considered the masterplan, with the most sensitive end uses (e.g. a school) planned for areas of lowest residual ground risk. In doing so this also assisted programme.
Robust, Cost Effective, and Defensible Solution
Design ‘performance criteria’ and residual risks were established at the outset, and this provided a fundamental clarity that all parties were able to understand and communicate. This allowed the Client to meet its commercial objectives, delivering a longer term process of site remediation and placement of strategic infrastructure (with regulatory approval) and allow individual land parcels to be de -risked, verified and sold to ‘on plot’ housebuilders.
Compliance with Legislation, Codes, and Guidance, and Adherence to Diversity Practice
The project complied with legislation, codes, and guidance by integrating available standards in an informed and innovative manner to develop a large landfill site for housing. Detailed discussions with regulatory authorities agreed the performance criteria and risk mitigation provisions for onward construction before the remediation started. The detailed design process was then split into phases, with specific design documents for each phase, including geotechnical reports, environmental reviews, earthworks specifications, and ground gas interpretation reports and verification reports in line with LCRM and [at the time] Eurocode guidance, with planning approval received for the remediation as each earthworks phase was complete.
Effective Public / Stakeholder Engagement
The project involved extensive discussions with a broad platform of stakeholders including housing warranty bodies (NHBC), statutory authorities for roads (West Sussex County Council), statutory authorities for sewers (Thames Water), and the local Planning Authority (Horsham District Council). Engagement to consider the perspectives of the local community were primarily managed by the Client. Recovered soils have been reused on site utilising a Materials Management Plan that was agreed with both the Environment Agency and Contaminated Land Officer at the commencement of the scheme.
The development has taken place over many years and has become inevitably complicated with numerous planning boundaries, earthwork phases, housing plots and technical approval boundaries. To allow effective engagement, CampbellReith developed an open-source GIS system (GISSMo) to share the key reports associated the ground treatment and its validation and to communicate the ongoing residual design requirements. This has allowed the Client, developer teams and regulators to rapidly access and interrogate georeferenced information on an ongoing basis, considering their specific boundary of interest.
Real environmental and social benefits, clearly linked to SDGs and demonstrating Social Value.
The development has taken place over many years and has become inevitably complicated with numerous planning boundaries, earthwork phases, housing plots and technical approval boundaries. To allow effective engagement, CampbellReith developed an open-source GIS system (GISSMo) to share the key reports associated the ground treatment and its validation and to communicate the ongoing residual design requirements. This has allowed the Client, developer teams and regulators to rapidly access and interrogate georeferenced information on an ongoing basis, considering their specific boundary of interest.
Environmental Benefits
The project has reused soils on-site which enhances sustainability and minimizes waste. The material removed from site was restricted to Unacceptable Material (for example occasional asbestos containing materials and off-site recyclables such as metal and wood) that could not become Engineering Fill.
The whole site masterplan includes 48 hectares devoted to open spaces, promoting biodiversity through the creation of native habitats connected by green corridors. These spaces provide recreational areas for residents and enhance the local ecosystem. The provision of these spaces was integrated with the remedial design, with the treatment reduced in areas that were not sensitive to gas risk or settlement.
In relation to Social Value, the remediation has supported Model Award Criteria (MAC) 4.1 (Deliver additional environmental benefits in the performance of the contract including working towards net zero greenhouse gas emissions) via direct focus on reducing emissions associated with adherence to the waste hierarchy, removal of off-site vehicle emissions, and the contribution to environmental benefits through water as a result of the realignment of the brook and provision of naturalised SuDS infrastructure.
Social and Economic Benefits
In consideration of MAC 1.1 - 1.4 the project provides significant wider social value, via the provision of affordable housing in a walkable accessible neighbourhood, educational/healthcare and community facilities, employment opportunities and improved transport and green space infrastructure.
From a construction perspective, the project has provided longer term employment and training opportunities for lower skilled workers, including both local workers, those from a disadvantaged social background and those recently migrated to the UK.
In relation to CampbellReith, it has fulfilled MAC 2.1 - 2.3 associated with creating opportunities, training and education. It has been an exemplar opportunity to train geotechnical engineers, contaminated land specialists and civil engineers on the technical aspects of the scheme, site supervision, health and safety, environmental protection and contract management. All the team members have experienced the Client and Contractor interface and the complexities of running an exceptionally long term and complicated project with many lessons learnt. One of the lasting benefits of this project will be the career advancement and training it has afforded those engaged in it.
The project supported an internship and MSc for the University of Reading associated with ground gas and an undergraduate training seminar at Brighton University and has been used to provide earthworks training in-house (and to subsidise external ICE training). In addition it has been the topic of several companywide CPDs, used in graduate recruitment, and our involvement in STEM at school and college events.
In terms of providing employment, CampbellReith retrained a contract supervisor and employed them on the consultancy team after they were made redundant by the Contractor. In addition, the current site engineer has been retrained to allow them to revert to a career in geotechnics after initially studying this at university but becoming employed in finance after graduation. The wider scheme also provides a school.
Work on the project has brought together a number of specialist SMEs, like CampbellReith, creating opportunities for entrepreneurship, growth and business creation (MAC 3.1, 3.4, 3.5). In terms of the site works, the project has resulted in the significant employment of site staff, involving a collective workforce of approximately 20 full time workers operating over an 8-year construction programme and will also have contributed indirectly to jobs created through the supply chain and increased local spending.
Conclusion
Over 10 years a landfill has been turned into social housing, community space and blue green infrastructure. The design work by CampbellReiths’ engineers and scientists has enabled this and, in the process, contributed to environmental sustainability and social value. It is an exemplar scheme bringing notable Social Value and demonstrable environmental, social and economic benefits.
A video and overview of the scheme can be seen here: Kilnwood Vale – Landfill Remediation – Shortlisted for Best Sustainable Brownfield Regeneration Project - CampbellReith | Consulting Engineers