The risk of carbon dioxide from shallow mine workings
Beginning in September 2018 a study was commissioned by the Scottish Government, and undertaken by consultancy RSK Stirling, to investigate the prevalence of carbon dioxide from disused mineral mines and the implications for residential buildings1. On 5th September 2019 the Scottish Government made the results of the study available. The investigation considered past incidents involving mine gas and carbon dioxide, a comprehensive literature review, and undertook consultations with relevant stakeholders and industry experts with the findings undergoing detailed analysis.
With the analysis complete, the report proposed the following eight points be considered further2:
- “Use and enforcement of model planning conditions and/or changes to Scottish Planning and Building Standards and guidance to ensure adequate risk assessment of mine gas and design, construction and verification of gas protection measures.
- Further research and preparation of supplementary technical guidance relating to the assessment of risks to new and existing developments from mine gas.
- Consideration of mine gas issues and constraints at local development planning stage, especially related to large-scale developments and cumulative effects.
- Improved coordination and communication between Planning, Building Standards and the Environmental Health/Contaminated Land staff in some local authorities.
- Further research to assess the long-term effectiveness of granular fill and perforated pipe ventilation below slab construction.
- Further consideration of the implementation of mandatory mitigation measures in former coal/oil shale mining areas.
- Additional liaison between Scottish Government, Scottish Local Authorities and Northumberland County Council may be mutually beneficial.
- Validation of risk assessment and mitigation design experience of consultants within the developers’ procurement process.”
Why research carbon dioxide
Despite being in the very air we breathe, at higher concentrations carbon dioxide can pose a serious risk to human health and is even classed as a ‘substance hazardous to health’ under the Control of Substances Hazardous to Health Regulations 2002 (COSHH)3. Exposure to higher concentrations can lead to headaches, dizziness and confusion, with extreme cases causing loss of consciousness and asphyxiation.
The Health and Safety Executive’s (HSE) publication “EH40/2005 Workplace exposure limits” outlines several exposure limits of airborne hazardous substance concentrations in the workplace (WELs), calculated as an average over a specified time, including one for carbon dioxide3:
- Long-term exposure limit (8-hr reference period) of 5000 ppm (parts per million)
- Short-term exposure limit (15-minute reference period) of 15000 ppm
In context, the global average concentration of carbon dioxide in the atmosphere for 2018 was 407.4 ppm4.
The inciting incident – Gorebridge Estate
While the risks associated with carbon dioxide are known, it took a substantial incident to prompt the Scottish Government to commission the research. The incident occurred at a recent development of residential dwellings, developed between 2007 and 2009, however the issues didn’t become apparent until between September 2013 and September 2014. During this time 22 of the 165 residents had contacted healthcare services complaining of headaches, dry coughs, dizziness and anxiety. In response to this, NHS Lothian set up an Incident Management Team (IMT) to carry out an investigation to determine the underlying cause5. The IMT carried out continuous monitoring and identified up to 25% v/v, associated with periods of falling atmospheric pressure6, in the cupboard under the stairs in one of the houses.
The subsequent report identified that the carbon dioxide had seeped up from old coal mine workings via several pathways including un-grouted disused mine shafts, natural cracks in the ground and human made interventions over time such as investigation drill holes, grouting work for the Borders rail link, the installation of deep drainage and the presence of vibro-stone columns (compacted gravel columns), which were introduced during construction to stabilise the ground for house building5. These preferential pathways allowed the carbon dioxide to migrate into spaces below the foundation slabs and up into the properties.
The investigation also revealed that the initial pre-development mine gas risk assessment had classified the site as low risk, meaning that no gas mitigation measures would be required.
As part of this investigation, the IMT had to consider potential remedial options to protect the homes from long term gas exposure and render the homes suitable for use. However, it was determined the options available would not offer the necessary protection. Therefore, to safeguard human health Midlothian Council decided to demolish the 64 properties on the site and re-house all the residents5.
The final report made a total of 26 recommendations that needed to be addressed by Local Authorities, NHS Boards, the Scottish Environmental Protection Agency, The Coal Authority and the Scottish Government, and revolved around the following points5:
- The Gorebridge incident and future site management
- The management of future incidents
- National policy and guidance on mine gas risk assessment and risk management
- Future research needs in relation to the problem of mine gas migration
A second incident occurred in September 2016, During the investigation and production of the final report, whereby carbon dioxide affected recently built residential properties close to the development affected previously. This prompted Midlothian Council Building Standards to implement measures to mitigate the risks. The mitigation came in the form of a policy change requiring developments (new and pre-construction at the time) in the Gorebridge area to provide gas protective measures to a characteristic situation 2* as a minimum, even where an investigation indicated gas migration is unlikely5.
*A characteristic situation in terms of gas risk refers to an assumed ground gas regime for designing gas protective measures, ranging from CS1 (very low hazard potential) to CS6 (very high hazard potential).
As these results apply only in Scotland, this does beg the question, will we see that same kind of policy change across the rest of the UK? Thus ensuring a safe and standardised approach to avoid similar events in the future?
Groundsure’s Official Coal Mining Search offers clear advice and recommendations relating to previous coal mining activity at the site of any potential residential property purchase, as well as extra protection via a comprehensive £100k Coal Search Report Insurance Policy.
1. RSK (2019). A mine of information (online). UK, https://www.rsk.co.uk/item/1395-a-mine-of-information.html, accessed 10th October 2019.
2. RSK Stirling (2019). Research project to investigate prevalence of CO2 from disused mineral mines and the implications for residential buildings. Crown Copyright 2019.
3. The Health & Safety Executive (2019). General hazards of Carbon Dioxide (online). UK, http://www.hse.gov.uk/carboncapture/carbondioxide.htm, accessed 10th October 2019.
4. Climate.gov (2019). Climate Change: Atmospheric Carbon Dioxide (online). USA, https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide, accessed 11th October 2019.
5. Lawrie K (2017). Title of Report: Publication of the Final Report of the Incident Management Team; “Carbon Dioxide Incident in Gorebridge Midlothian April 2014”. UK, https://midlothian.cmis.uk.com/live/Document.ashx?czJKcaeAi5tUFL1DTL2UE4zNRBcoShgo=CwvM%2FbHA%2Bbmu0Ct8V2OlK8w3v%2BYS2rEdJUafYNXdfPn2t2gDhbb29A%3D%3D&rUzwRPf%2BZ3zd4E7Ikn8Lyw%3D%3D=pwRE6AGJFLDNlh225F5QMaQWCtPHwdhUfCZ%2FLUQzgA2uL5jNRG4jdQ%3D%3D&mCTIbCubSFfXsDGW9IXnlg%3D%3D=hFflUdN3100%3D&kCx1AnS9%2FpWZQ40DXFvdEw%3D%3D=hFflUdN3100%3D&uJovDxwdjMPoYv%2BAJvYtyA%3D%3D=ctNJFf55vVA%3D&FgPlIEJYlotS%2BYGoBi5olA%3D%3D=NHdURQburHA%3D&d9Qjj0ag1Pd993jsyOJqFvmyB7X0CSQK=ctNJFf55vVA%3D&WGewmoAfeNR9xqBux0r1Q8Za60lavYmz=ctNJFf55vVA%3D&WGewmoAfeNQ16B2MHuCpMRKZMwaG1PaO=ctNJFf55vVA%3D, accessed 11th October 2019.
6. Talbot S, and Card G, (2019). Continuous Ground-Gas Monitoring and the Lines of Evidence Approach to Risk Assessment. CL:AIRE Technical Bulletin TB18. CL:AIRE, London, UK.