Project: Ground Investigation for a Solar Farm
Timescale: 4 months
A former landfill site in Wolverhampton was earmarked for a solar farm to produce energy for a nearby NHS hospital. ERS was commissioned to conduct an intrusive ground investigation to help inform the design of a foundation solution for the solar farm.
The site in Bowmans Harbour, Wolverhampton, was once mined for coal and later operated as a landfill until it was closed in 1996. The site had an engineered cap with a gas collection system, still in operation at the time of the ground investigation. The cap was assumed to consist of 1m of clay overlaid with 0.5m of restoration soil and the site was underlain with a network of pipes relating to the gas collection system and leachate management system. Some sections of the site were steep and densely vegetated, making access difficult.
The 11.5 hectare vacant brownfield site was earmarked for re-development as a solar farm which would provide up to 6.9MW of renewable energy to power the nearby New Cross Hospital. The development was proposed by a joint venture between the City of Wolverhampton Council and the Royal Wolverhampton NHS Trust as part of a Public Sector Decarbonisation Scheme (PSDS).
ERS was commissioned by a renewable energy solutions specialist to undertake an intrusive ground investigation at the site to confirm the make-up of the landfill cap, obtain soil samples and perform in-situ geotechnical testing which would help inform the foundation design for the new solar farm. Due to the status of the site as a closed landfill with active gas and leachate management systems, ERS agreed a Construction Quality Assurance (CQA) Plan with the Environment Agency in advance of the site investigation works.
Over 50 windowless sampler boreholes were drilled to map the cap thickness, inspect and log the capping materials and collect samples for geotechnical testing.
As the waste material in the landfill was still actively gassing, extra care and additional gas monitoring were required to ensure there was no release of toxic or environmentally damaging gas into the air as a result of the ground investigation activities. On completion of each borehole, concentrations of landfill gases were monitored for 3 minutes in the open borehole prior to sealing with a bentonite and cement mix.
Following the drilling and backfilling of the boreholes, bespoke CQA testing was carried out to show that the works had no impact on gas emissions or infiltration through the cap.
Infiltration testing and “fluxbox” gas emission monitoring was undertaken to prove that the drilling and backfilling hadn’t compromised the landfill cap or the gas collection system. Furthermore, a site wide gas emission survey was undertaken and compared to a similar survey undertaken prior to the drilling works.
The fluxbox gas monitoring and the emissions survey showed no methane emissions from the backfilled boreholes, and the infiltration testing showed the boreholes would not be a preferential pathway for rainwater into the cap. This confirmed that the site investigation did not impact the cap performance but also gave some reassurance that shallow driven piles could possibly be used for the solar array where the cap was sufficiently thick.
Following the ground investigation, our client was able to design a foundation solution comprising a mixture of pad and pile foundations and proceed with the construction of the solar farm. The solar farm is expected to fully power the hospital’s heat pump system for around 288 days per year, potentially saving the NHS Trust £15-20m over the next 20 years.