Q&A with Tom Henman, RSK
Tom Henman, Director - Geosciences, RSK gave an insightful webinar in March and an online presentation on the Gorebridge incident at the Brownfield Summit this May. Below are his answers to some of your questions you had on the day...
Q. Sometimes a development site will not be flagged as a contaminated land risk but a high risk mining site. The coal authority seem to focus a lot on stability and not ground gas. What is the process the coal authority would go through to appropriately assess ground gas? A. The Coal Authority defines a Development High Risk Area where coal mining risks are present at shallow depth which are likely to affect new development. The focus of this is on mine stability issues rather than gas risk but the latter is typically higher where shallow workings are known or suspected to be present. However, ground gas risks should be considered for all sites in a Coal Mining Reporting area irrespective of whether or not they are in a Development High Risk Area. The Coal Authority also hold some data on gas emissions from coal mines which can be obtained through one of their standard reports.
Q. What is the basis for suspecting under reporting of CO2 impacts? A. Low level exposure to CO2 shares symptoms with many other conditions. Taking into account the small number of reported incidents and the many areas of the UK affecting by coal mining where development has subsequently occurred, under-reporting of incidents seems likely on the balance of probabilities.
Q. Should all properties in coal mining districts have mitigation measures, as a precautionary measure? A. This is a contentious subject as highlighted in my talk and one we felt needed further consideration as detailed in RSK's research report.
Q. Any assessment undertaken of gas risks to existing housing stock, particualrly Gorebridge? A. This would be a matter for the local authority although we understand that there are some other on-going cases.
Q. Were the effects of piled foundations at the properties looked at? A. This was one of the key questions looked at in RSK's Scottish Government project. Our full report can be found here: https://www.gov.scot/publications/research-project-investigate-prevalence-co2-disused-mineral-mines-implications-residential-buildings/pages
Q. Is there a difference in gas risk from undeveloped coal seams as well as mined? A. The risk tends to be greater for mineworkings due to changes in geochemistry and the creation of pathways, such as shafts/ adits and shallow workings. Natural geological strata unaffected by mining may also represent ground gas sources and should be considered in the CSM.
Q. Do you have any suggestions for the regulators re how to adequately regulate development near mine gas issues in the interim until specific guidance is produced? A. Regulators should ensure gas risk assessments are based on a robust CSM, which applies to ground gas as well as mine gas issues. For mining areas a more detailed assessment is likely to be needed than six rounds of gas monitoring and empirical assessment, using a series of lines of evidence. As detailed in RSK's research report, experience of applying standards and guidance on the risks associated with coal mine gas varies within local authorities, and we recommended they should upskill or use external specialists to peer review submissions from developers.
Q. What role do you see the Coal Authority having, rather than Local Authorities, in dealing with mines gas? A. The Coal Authority is an important consultee and data holder, with regulatory role limited to works directly affecting mine workings. Local authorities are the lead regulator under planning and building control regimes.
Q. Were the elevated levels of CO2 in any of the residential homes not detectable by CO2 detectors people often have installed for their boilers? A. Use of CO2 monitors at home is uncommon, the questioner may be thinking about CO monitors?
Q. Please could you touch on the ground conditions and how these contributed to the incident? A. The scope of the research project I was discussing did not consider the causes of the incident at Gorebridge. Please refer to the published Incident Management Team report for more information on this subject: https://www.hps.scot.nhs.uk/publications/hps-weekly-report/volume-52/issue-1/co2-incident-in-midlothian-final-report/
Q. Is the improved compliance, supp guidance already in place/where can we find it and how will improved knowledge share be implemented? A. Not yet but I am working on a CL:AIRE technical paper with Steve Wilson to be published in 2020.
Q. If there is the potential for the risk posed by ground gas in an area to increase over time given rising ground water and climate change, how would you suggest to best mitigate this risks or include them in a RA? Do you know of any guidance? Following on from this, do you think there is the potential to see an increase in the amount of ground gas related incidents in the future on existing developments historically assessed as not being at risk? A. These are some of the key uncertainties in the mine gas risk assessment process considered in RSK's research report and in our view, a key gap in the current guidance. The best approach in assessing these types of sites is to develop a robust CSM and use of either a multiple lines of evidence approach or quantitative assessment of the risks. Uncertainties should be considered in the CSM and assessment, along with their implications for ground gas mitigation design.
Q. Isn’t carbon dioxide toxic in its own right regardless of oxygen depletion? A. Yes