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Combining best practice & innovative data science analysis methods on a former chalk quarry

 

Client

Arup, Eastern Quarry Ltd

…………………………

Our Solutions

  • Ambilytics: Environmental data analytics
  • Continuous monitoring of gas in a borehole
  • Flux chamber testing

Challenge

A large former quarry was being developed with a residential-led masterplan. Land forming earthworks using millions of cubic metres of original overburden materials are ongoing.

On the first phase of development, elevated carbon dioxide and flow rates were measured in monitoring wells and gas membranes were installed above the traditional vented void for residential housing. The site would have classified as characteristic situation CS3 or NHBC classification amber 2 using a generic assessment approach, with a maximum methane concentration of 44% in deep wells and maximum carbon dioxide concentration of 18%.

The challenge was to demonstrate that the gas in the wells did not represent a risk to the proposed buildings and that it was being generated at a very low rate by natural processes and thus would not result in any significant surface emission.

Approach

A ”multiple lines of evidence” approach was used to investigate the ground gas risk on the site. Continuous monitoring in the monitoring wells at different depths was completed. A key element of the Ambisense data is that each reading is linked to weather data from a local station. This allows the consultant to consider the relationships between rainfall, atmospheric pressure, wind speed, etc and the gas parameters. The continuous monitoring data collected by the Ambisense units was analysed using the Ambilytics platform and provided evidence that the gas in wells was caused by infiltrating rainfall during heavier rainfall events that resulted in biological oxidation of small quantities of organic material that are present in almost all soils.

Ambilytics provided correlation plots, heat maps and 1D partial dependency models. This was combined with extensive verification data of the earthworks fill produced by Arup and other information sets to demonstrate that the source of the gas was well understood and would not be a risk to new housing.

Methane present in deeper wells was shown to be partitioning from groundwater and was not migrating through the fill material to the ground surface.

Further evidence was provided by correlating gas monitoring in wells with surface emissions tests carried out by Ambisense using flux chambers.

The result was that it was possible to show the elevated gas concentrations and flow rates in wells were not representative of the actual gas risk. This was approved by the NHBC and the development was able to proceed without the need to fit costly gas protection measures. The cost of the Ambisense units and the Ambilytics analysis was tiny compared to the savings achieved on the project.

The end result was approved by the local authority and the facility was able to open on time and without the need to retrofit costly gas protection measures.  Thus, the Ambisense units and the Ambilytics analysis paid for itself.

Results

  • Customer saving of £1 million
  • The analysis demonstrated that there was no risk of gas emissions to buildings.
  • This removed the need to install gas protection membranes in the development, which provided a significant cost saving.
  • Significantly reduced ongoing investigation costs compared to traditional methods
  • Rationalised the verification gas monitoring in future phases thus avoiding delays between completion of earthworks and acceptance by regulators.
  • Provided confidence to NHBC in signing off proposals

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