Former Gasworks

Essex

The Problem

BIOGENIE Remediation UK Limited was commissioned by National Grid Property Holdings Ltd (NG) to remediate the former gasworks at the small market town of Great Dunmow, Essex. The works were undertaken voluntarily by the client as part of a national programme of works to address potential statutory liability issues and render the site suitable for a commercial/open storage end use.

The site historically was part of a larger town gasworks, but portions of the former works had been sold over the years. The site comprised three different landowners, NG, Cadent Gas (who have live gas distribution infrastructure including a Pressure Reduction Station onsite) and Uttlesford District Council (UDC) who own the adjacent car park.

Site investigations had identified gasworks contamination and below ground structures including a cross boundary tar tank and shallow basements backfilled with demolition rubble and asbestos. The tank was one of the original gasholders that was later converted into a tar tank and sat cross-boundary partially between the NG a council landownership. The historical industrial legacy of the site had left coal tar in the former tank, made ground containing elevated PAHs, TPH, metals and asbestos and DNAPL in the groundwater.

The site itself was small (~0.18ha) and had difficult and confined access due to a narrow one way road system and immediately surrounded by sensitive residential properties, a Council and Co-op supermarket car parks, church, BT depot and small museum. The small size of the site, limited space and live gas infrastructure and above ground telecommunications cable (running across the top of the tar tank) posed a real challenge to the works.

A pre-works site inspection highlighted structural concerns with two retaining walls, one between the site and neighbouring properties and the second on the frontage of the council car park. The walls were in poor condition, partly exacerbated by vegetation growth and at risk of potential risk of collapse.

The Solution

Sustainable, practicable and sympathetic design

The remediation design developed a sustainable, practicable and sympathetic methodology supported by a dedicated human health and controlled waters Detailed Quantitative Risk Assessment. Works were designed with particular respect to mitigating potential nuisance to the local community such as vehicle movements, noise, vibration, dust and odour from the remediation activities.

The design was submitted to the Local Planning Authority and gained the agreements of the Council’s Pollution Control Officer and the Environment Agency. The works were also undertaken in-line with the CL:AIRE Definition of Waste Code of Practice.

Communication Strategy

A communications strategy was developed alongside the wider client team with key consultees such as the local residents, businesses and community groups notified of the works. An open day was held prior to the works starting, articles in the local press and also various letter drops. A STEM Educational ‘Mad-Science’ session was organised with the local Scout group and presentation given on the importance of safety on construction sites.

Local meeting room facilities were hired through a local charity running the local Maltings Museum. Assistance was also provided for them to apply for a Local Community Funding Programme ran by the Client. The bid was successful with funds being used to build new disabled access facilities for the museum.

Considerate site setup, enabling and working methods

BIOGENIE UK took temporary possession and responsibility for the live gas Cadent site and a portion of the adjacent Council car park throughout the works. Party Wall Agreements were put in place with the neighbouring residential properties for access into their gardens.

Works began with a detailed utilities survey and vacuum excavation of all identified services. Several anomalies were encountered and further investigated and surveyed to update Cadent’s records. The enabling works also included vegetation clearance and the removal and demolition of site boundary fences and walls.

Once exposed, the tar tank was stabilised in-situ with a pre-defined site specific cement and PFA mix that had been pre-mixed off-site to mitigate the potential generation of dust and minimise site deliveries. A total volume of 121 m3 was stabilised with clean, uncontaminated material segregated for re-use onsite or taken offsite for recycling at a local waste management facility.

Other excavations and turnover included the removal of below ground structures including three small basements backfilled with asbestos impacted rubble. The asbestos impacted rubble was carefully segregated and sent to a Soil Treatment Facility. Localised surface asbestos fragments were also hand picked and disposed off-site to a licensed facility.

General brick and concrete encountered within excavations were segregated from soils and taken off-site for recycling at a local waste treatment facility. A total of 925 m3 of soil was excavated and validated before being retained for re-use onsite. Manual DNAPL recovery from existing groundwater boreholes was undertaken during the works with minimal recharge. Approximately 1 m3 of DNAPL was collected and disposed off-site to a licensed facility.

Excavation areas were all sampled and verified before being backfilled and compacted prior to the installation of a geotextile layer and 300 mm imported clean flint gravel cover from a local quarry.

Works were managed sensitively at all times, with particular respect to mitigating potential nuisance to the local community by minimising vehicle movements, noise, vibration, dust and odour. Daily monitoring was undertaken and mitigation methods employed.

The Result

The project was awarded the NG ‘Best Remediation Project’ at the 2023 National Grid Property Awards at the Oval Cricket Ground. Chris Taylor of National Grid who judged the category commented that despite it’s small size, the project had numerous different elements and challenges and it was clear that the team had carefully and sympathetically considered and overcome these to deliver a successful project to a very high standard.

The works have led to a much improved site appearance and security that has gained the approval of the client and local community. At the time of writing BIOGENIE UK are awaiting Regulatory sign-off in-line with the planning conditions.

The demolition and sympathetic rebuild of retaining walls alongside the boundary with residential properties and public pavement were incorporated into the works with the agreements of the residents. The car park was also fully reinstated in agreement with the Council with replacement tarmac surface, car park spaces re-painted and crash barrier installed. An upgraded palisade boundary fence and a pedestrian emergency access gate for Cadent personnel were also installed.