The project
This project focused on a site being developed to accommodate a large residential estate in the West Lothian area.
Our client brought us in following several phases of site investigations had identified a number of potential ground gas sources - including both the natural geology and as a result of former land uses.
We were asked to investigate the ground gas regime, both at depth and within the shallow deposits.
We were also tasked with identifying any vertical migration pathways present, confirm which ground gas sources were actually present and complete a ground gas risk assessment (GGRA).
Our monitoring
We carried out extensive ground gas monitoring, employing multiple techniques to build a robust understanding of the ground gas risks present.
Our monitoring included:
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continuous ground gas monitoring of bulk and trace gases to understand the gas regime, identify ground gas driving mechanisms and develop an accurate conceptual site model (CSM).
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dissolved gas analysis to identify dissolved gases present and their likely origin.
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purge and recovery tests to indicate gas generation potential, gas behaviour and gas volumes.
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periodic monitoring to provide greater spatial coverage and additional data and observations for our 'lines of evidence' approach.
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surface walkover and flux box surveys to identify any potential vertical migration and surface emissions.
The results
The monitoring results confirmed that the elevated ground gases were associated with the Upper Oil-Shale Group at depth. We also identified that the underlying sandstone was acting as a gas reservoir.
We completed a detailed ground gas risk assessment
(in-line with BS 8485:2015 + A1:2019). We were also able to produce a detailed, accurate Conceptual Site Model (CSM) to help understand and communicate the ground gas regime and any credible migration pathways.
Overall, our GGRA classified the site as CS3 (Characteristic Situation - level 3) which is deemed a moderate risk.
The outcomes
Our comprehensive continuous monitoring approach enabled us to help our client understand accurately the complex ground gas regime at their project site.
Identifying the relevant risk level present accurately meant that our client could incorporate appropriate ground gas protection measures into their designs at an early stage.
Ensuring ground gas protection measures appropriate to the risk present during early stages of design process can safe avoidable complications later in the project.
And most importantly, by implementing appropriate protection measures, help keep the future occupants of this residential development safe.