The project
Our client brought us in assist on their project which was part of a large scale, strategic re-development of historically heavily contaminated land. Our work focused on an area of the site being re-developed for residential housing.
Previous uses of the site included industrial activity, a chemical works, soapworks, historical gasometers, railway infrastructure, brickworks and clay and gravel extraction (subsequently infilled with waste).
More recently the site was occupied by high-rise flats and a school - with these being demolished as part of the wider strategic re-development. The site had also been subject to remediation, including significant cut and fill works.
We were asked to carry out a ground gas monitoring programme to provide evidence to update the conceptual site model (CSM), and assist the validation of our client's remediation strategy.
The project timescales were tight for our client, and we needed to complete the entire monitoring programme in just 6 weeks.
Our monitoring
We used various monitoring techniques to generate a robust dataset to help us accurately understand the residual ground gas regime at the site.
Our monitoring included:
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continuous ground gas monitoring of bulk and trace gases at 10 locations across the site for a period of 6 weeks.
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additional periodic ground gas monitoring to provide greater spatial coverage to the dataset and further 'lines of evidence'.
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vapour sampling to assess any residual risk for volatile organic compounds (VOCs) and provide cross-validation of datasets.
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dissolved gas sampling to identify dissolved gas concentrations, migration pathways, likely sources and their significance.
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surface walkover surveys and flux box testing to quantify any potential vertical gas migration and identify any surface emissions.
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weekly site visits to carry out monitoring and provide regular updates to our client.
The results
The outcomes
In a short period of time, our continuous monitoring generated robust evidence which confirmed the effectiveness of the remediation efforts in substantially mitigating the ground gas risks present.
This provided reassurance to our client that their remediation strategy, in combination with implementing appropriate ground gas protection measures, was effective and their site development could proceed safely.
Installing protection measures retrospectively during later stages of a development can be time consuming, costly and cause project delays. Verifying the effectiveness of the remediation strategies and identifying appropriate protection measures at this early stage, helped the client avoid potential complications later in the project.
Using our monitoring programme, we were able to provide a detailed ground gas risk assessment (in-line with BS 8485:2015 + A1:2019).
The monitoring results confirmed that the remediation methods employed had significantly reduced the level of ground gases present in the upper layer of the site's substrate.
We identified moderately elevated levels of some trace gases. However, these were in keeping with the nature of the former use of the site.
Overall, our GGRA classified the site as CS2 (Characteristic Situation - level 2) which is low risk. This classification also helped identify appropriate additional protection measures.