The Ground & Water approach – The original desk study highlighted that there was a potential ground-gas (Methane and Carbon Dioxide) risk because of a historical, and local authority recorded, landfill site ~150m to the south-west of the development. This source being possible buried putrescible material within the landfill.
Having reviewed the original desk study and risk assessment, in October 2016, Ground and Water constructed three Terrier Windowless Sampler Boreholes to a depth of 4.45m bgl around the development. Combined groundwater and ground-gas monitoring wells were installed to 4.00m bgl in all trial holes. Ground and groundwater conditions were also evaluated and monitored. Ground-gas monitoring was carried out on six occasions between November – December 2016. A detailed risk assessment followed.
The outcome – The results from the gas monitoring – relatively low Carbon Dioxide levels, no Methane and depleted Oxygen – and risk assessment revealed the site fell into a Characteristic Situation 1, where no precautions against the ingress of ground-gas were necessary.
No further action was considered necessary and the development could move ahead to completion.