Coal Mine Gas Risk Assessment may be required if your development site is in a coal mining risk area. This assessmnent will identify if complex mine gas is a risk on your site and steps that need to be taken to minimise the risk.
Is Your Development at risk of Coal Mine Gas ingress?
If your development site is in a coal mining or mining risk area you need to ensure that steps have been taken to assess if complex mine gas risks are present. If there is the potential of mine gas on your development site a mine gas risk assessment should be completed as part of the planning process.
Gas ingress to residential buildings from mine workings can occur and potentially pose risks to human health. Indeed there are several well documented incidents where the gas egress resulted in health issues and action needed to be taken.
The best course of action is to ensure the mine gas risk has been assessed and all necessary remedial measures have been taken before any construction begins. It is in everyone’s best interests, developers, buyers and property owners to ensure that any potential risk to health from gas ingress is identified, and dealt with before it is an issue.
In 2013/14, a highly-publicised incident unfolded in Gorebridge, situated in Midlothian, Scotland. This incident brought to light the potential risks associated with the ingress of gas into residential buildings from underground mine workings, raising concerns about human health (NHS Lothian, 2017). The most common complaints were headaches, dry coughs, dizziness and anxiety.
The town is in a former mining area and the development was underlain by old mine workings with shafts present nearby. Further investigations showed there was mine gas ingress which resulted in the relocation of residents and demolition of 64 homes.
In Northumberland a similar situation also occurred with reports of illness that were symptomatic of exposure to high levels of carbon dioxide. Again further investigations revealed that mine gas was seeping into the properties causing illness. A detailed site investigation followed which highlighted the exact points of gas ingress. The investigations revealed that the gas was only entering through unsealed open ducts containing water pipes. Fortunately once the openings were correctly sealed the properties were safe to return to.
Following these incident, the Scottish Government commissioned research to investigate the broader prevalence of similar cases. Their findings revealed a limited number of incidents occurring in Scotland, the wider UK, and even internationally since the 1950s. This research also underscored the inadequacy of existing standards and guidance on ground gas, particularly concerning factors related to mine gas risks. As a result, it recommended the supplementation of these standards and guidelines. The purpose of this guidance is to shed light on the aspects related to mine gas risk assessment for development, direct stakeholders to existing resources, and advocate for best practices.
The term ‘mine gas’ used in this report specifically refers to ‘coal mine gas’ with the principal components being methane, carbon dioxide, carbon monoxide, hydrogen sulfide and deoxygenated air.
The Gorebridge incident further emphasised the significance of incorporating reasonably foreseeable changes into the mine gas risk assessment process. For sites where there is a potential presence of significant mine gas sources, the risk assessment should remain a dynamic document until the final design is confirmed.
It is also important to ensure that external factors e.g. climate change and regional groundwater conditions should also be thoroughly understood whenever possible.
It’s also worth understanding that whilst the predominant risk in the UK is from Coal Mining activities, other types of mines, such as oil shale, metalliferous, and rock mining (CIRIA, 2019), could potentially lead to gas emissions from the ground.
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The intended audience of this document is those involved in commissioning and undertaking mine gas risk assessments to support proposed development, and local authority officers involved in regulating development management through the planning or building control regimes.
1. Reference should be made to the main information sources that are available to assist assessors carrying out site investigations for mine gas and mine gas risk assessments.
2. Mine gas risk assessments and mitigation design should be carried out by ‘competent persons’ as defined in the National Planning Policy Framework in England and equivalents in the devolved governments.
3. It is important to understand the different methods of working coal, the effect this has on the likely presence of a mine gas hazard being present below a site and the corresponding risk of emissions into buildings. At the very least, CIRIA Report C758D (CIRIA, 2019) is considered to be essential reading for anyone undertaking mine gas risk assessments.
4. Development of a CSM is a vital part of any mine gas risk assessment as well as its use in the interpretation of any gas monitoring data.
5. Risk assessment using the gas screening values and the ‘points system’ in BS 8485 on its own is not likely to be appropriate where there is a risk of mine gas emissions on sites with complex CSMs or where mass advection of soil gas 43 could occur. Detailed quantitative assessment of gas emissions may be necessary.
6. It is important to consider the effect of foundation construction and other below ground infrastructure on migration pathways and the integrity of any barrier layers, and the implication for the mine gas risk assessment.
7. It is vital that after the site investigation or gas monitoring is complete that any boreholes, probeholes or wells are decommissioned and sealed in a manner that prevents them acting as migration pathways for mine gas.
8. Potential areas of uncertainty in mine gas risk assessment and the effects of future changes should be considered. It is important to recognise that this requires site specific investigation and assessment for relevance and credibility. Generic statements are not appropriate (see Appendix 4: Case Study 3). Although a precautionary approach should be applied to any risk assessment, this is not an excuse to invent hypothetical or extremely unlikely hazards or consequences and thus to recommend the mitigation of impossible risks.
9. One of the most significant issues at the Gorebridge incident in Scotland was the disconnect between the gas risk assessors and the development designers. Mine gas risk assessment reports should be transparent as to the foundation (and other) design features that are assumed or included in the assessment. The reports should make it clear that if final designs change, then the gas risk assessment will require updating (even if the risks do not change significantly).
Anyone responsible for carrying out a mine gas risk assessment and mitigation design should be suitably qualified and experienced as required under the definition of ‘competent persons’ in the National Planning Policy Framework (MHCLG, 2021) in England and equivalents in the devolved governments. Land Contamination: Risk Management (LCRM) (Environment Agency, 2020) also requires that for ‘land contamination and planning you must use and meet the National Planning Policy Framework definition of a competent person’.
They should ideally be a Chartered professional member of an appropriate organisation (or be able to demonstrate equivalence) and have competence in geology, understanding of mining and the processes that can cause gas to be produced in mines, how it can migrate to the surface and into buildings, as well as gas risk assessment techniques. Additional accreditation (e.g. SoBRA fully accredited risk assessor for permanent gases or a similar level of qualification such as SQP, SiLC, or RoGEP – is one way of demonstrating professional competence.
