Many schemes, particularly those on brownfield sites, are faced with the challenge of dealing with ground gases such as methane and carbon dioxide, which can be hazardous to human health and, in extreme cases, may pose an explosion hazard.
As with all site investigations, the assessment of ground gas risk should be based on a thorough understanding of both the ground conditions and site history. In the UK, the investigation, assessment and protection against ground gas is primarily guided by BS 8485, Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings.
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The code, which was first introduced in 2007, underwent a major revision in June 2015. It has been improved significantly, now integrating the investigation, assessment and protection design and verification processes and providing a checklist for those commissioning investigations to ensure they are appropriate for the site conditions and development.
A particularly useful revision within the code is the provision of an empirical method of assessing the ground gas regime, without the need for gas monitoring. Where a limited source of ground gas has been identified, testing of the organic content of soil – which is typically carried out anyway during brownfield investigations – can be used to demonstrate whether gas monitoring is necessary. This is a much more cost-effective – and faster – approach than installing monitoring wells and is less risky than relying on an engineer’s judgement, as was the case in the past.
Additionally, when gas monitoring is needed, the code allows for zoning of risk, rather than assuming a worst case scenario (the maximum concentrations and flow rates of gas identified during monitoring) across the entire site. This should mean protection measures can be designed more appropriately for the level of risk and, as a result, be far less conservative, and perhaps result in further cost savings.
Not only is the revised BS 8485 a useful primary reference for a wide range of professionals but it is also a timely reminder of the need to fully assess the potential risks that ground gases can pose. Its pragmatic approach, with simplified assessment processes and tailored protection design, should help lower costs and reduce the risk of delays from unforeseen ground gas issues, while continuing to ensure that construction workers and future occupants are safe from harm.
Jon Archer is principal engineer at Harrison Group Environmental, managing the geotechnical engineering department
