Publication - Research and analysis

Prevalence of CO2 from disused mineral mines and the implications for residential buildings: research

In 2017 the NHS Lothian Incident Management Team investigated reported cases of ill health affecting residents of a recently built local authority housing estate. This research is looking for similar incidents and considers implications for building standards.

Prevalence of CO2 from disused mineral mines and the implications for residential buildings: research
11 Conclusions & Options for Consideration

11 Conclusions & Options for Consideration

The general aims of this research project were to consider a range of issues relating to the assessment and mitigation of mine gas issues for new and existing developments in Scotland. The project has comprised a fact-finding exercise involving background research on previous incidents involving mine gas and CO2, consultation with relevant stakeholders and industry experts, and detailed analysis of the findings.

11.1 Summary of Building Standards related questions

A specific objective of this project was also to explore the eight recommendations in the Gorebridge IMT report which related to Building Standards. Responses to these questions were tabled in Annex 3b Stakeholders and Annex 4 Expert Consultees.

In Table 11-1 we set out the summary findings from the responses to these eight Building Standards related issues.

Table 11‑1 Gorebridge IMT report questions which relate to Building Standards

IMT Building Standards Questions

Response Area

1/ Is the current mine gas risk assessment process adequate to correctly determine the level of risk, especially in relation to the requirements for assessing mine gas levels at sub‐surface depths likely to be representative of gas migration potential from underground mine sources?

Stakeholder Engagement – Indicated that the risk assessment process should be adequate, however, it can be poorly implemented.

Expert Consultation – Agreement with stakeholders that guidance is adequate if followed but is not always complied with or enforced. Recommended additional guidance and clarification specific to mine gas risk would improve the situation.

Analysis – Better enforcement of current standards, development of supplementary guidance to enforce risk assessment requirements and further research is required in key areas.

Options – For suggestions for improvement refer to Option 1 in Section 11.2.

2/ Is the current risk assessment process fit for purpose particularly in terms of taking account for future potential changes in mine gas dynamics and migration risk factors (e.g. due to ground stabilisation measures, additional developments, etc.) that could lead to an increased risk of gas migration into properties over the long term?

Stakeholder Engagement – Indicated that the risk assessment process should take account for these issues, however, it can be poorly implemented. Assessment of the impact of adjacent future developments is difficult to include in conceptual models.

Expert Consultation – Risk assessments should make an allowance for future change in the design life of the property but note that changes in groundwater levels due to climate change or cumulative development are uncertain and may be speculative.

Analysis – Better enforcement of current standards, development of supplementary guidance to enforce risk assessment requirements and further research is required in key areas.

Options – For suggestions for improvement refer to Option 1 and 3 in Section 11.2.

3/ Is there sufficient emphasis in the current mine gas risk assessment process on the potential for other interventions affecting the soils or sub structure underpinning any building development, to alter the risk of mine gas migration and consequently to render any pre‐development assessment redundant and inadequately precautionary to protect public health?

Stakeholder Engagement - Indicated that the risk assessment process should take account of these issues, however it can be poorly implemented.

Expert Consultation – The standards and guidance are clear on the need to consider interventions that could affect the CSM. Activities such as mine grouting are regulated by the Coal Authority and should consider risks of impact to existing development.

Analysis – Better enforcement of current standards and development of supplementary guidance to enforce risk assessment requirements

Options – For suggestions for improvement refer to Option 1 and 2 in Section 11.2.

4/ Are the current criteria used for deciding what constitutes a sufficiently precautionary approach to mitigation appropriate; particularly where there is known to be a potential risk of mine gas migration? Does the process adequately emphasise the need to take account of construction methods that may add to that risk (e.g. the use of vibro-stone underpinnings or solid slab floors that are not separately vented to the outside atmosphere)?

Stakeholder Engagement – Indicated that the mitigation design is sufficiently precautionary, however it can be poorly implemented. A precautionary approach with conservative metrics is included in BS 8485:2015. It was noted that the gas risk assessment is usually carried out well ahead of final foundation design, so risk assessments should be revised any time there is a change.

Expert Consultation - Interpretation of the guidance is to some extent open to developers. If guidance applied fully and appropriately by experienced individuals, and suitably enforced there would not be problems.

Analysis – Better enforcement of current standards and development of supplementary guidance to enforce risk assessment and mitigation design requirements.

Options – For suggestions for improvement refer to Option 1 and 2 in Section 11.2.

5/ In determining the need for mitigation measures, is the current scope for interpretation of the guidance open to developers at present appropriate?

Stakeholder Engagement – Views differed over the points-based system within the BS 8485:2015. Noted it is intended to be precautionary but is not designed to be prescriptive. Feedback was that some developers look for loopholes in the guidance rather than use them to assist in the risk mitigation process

Expert Consultation – The points based system in BS8485:2015 seen as a generic solution. Developers tend to look a reducing cost via interpretation of the guidance on a site-specific basis.

Analysis – Better enforcement of current standards and development of supplementary guidance to enforce risk assessment and mitigation design requirements.

Options – For suggestions for improvement refer to O Option 1 and 2 in Section 11.2.

6/ Are construction methods that do not involve creating a ventilated solum beneath the ground floor of a property, inherently more liable to permit the transmission of mine gases to the inside of these properties compared to a traditional ventilated solum construction type?

Stakeholder Engagement – General response was that a ventilated void under the floor was a very good defence against gas migration. However, there was debate over the long-term performance over the perforated pipe and granular fill solum used in Scotland over vented open voids.

Expert Consultation - a ventilated solum is seen as the 1st line of protection in reducing the risk of gas ingress.

Analysis – Indicated that further research was required to provide an evidence base for this view.

Options – For suggestions for improvement refer to Option 5 in Section 11.2.

7/ Is the drive to improve the energy efficiency of modern properties by increasing the levels of insulation and ensuring they are less prone to uncontrolled air movement (draughts) and are consequently more air tight, a potential factor contributing to the retention of mine gas emissions
that manage to penetrate a property?

Stakeholder Engagement – Response was mixed - airtightness will impact the retention of any gases able to enter a property but should also reduce gas ingress. Stakeholders suggested that air tightness should not be reduced because this is counter-productive in trying to improve energy efficiency.

Expert Consultation - Agree that a draughty building will allow gas to escape out the building. Experts gave varied views on whether airtight buildings were riskier than older buildings. Agreement that best option is not to allow the gasses into the building.

Analysis – Noted that mitigation measures or ventilation under the property were the best defence.

Options – For suggestions for improvement refer to Option 5 and 6 in Section 11.2.

8/ Would the simplest and most appropriately precautionary solution to the problems highlighted by the Gorebridge incident be to require mandatory gas risk mitigation measures in all new residential and similar developments in areas of Scotland defined by the Coal Authority as former coalfields?

Stakeholder Engagement – This would be overly precautionary for some sites, and insufficiently precautionary for the high risk sites. Much debate over what mitigation would be mandatory.

Expert Consultation – Views varied with most disagreeing but some agreeing with the statement. Many noted that generic mandatory measures would not necessarily resolve the issue and risk assessment and mitigation should be site specific.

Analysis – Requires clear definitions and consultation on impact.

Options – For suggestions for improvement refer to Option 6 in Section 11.2.

The consultees responses summarized in Table 11-1 above were reviewed in full within Section 9 of this report. This ensured the related issues and the potential improvements highlighted were fed into our options assessment, the output of which is contained in Section 11.2.

From our review of the eight IMT Gorebridge Building Standards related questions, options to consider for improvement are addressed in Section 11.2 under options 1,3,5 and 6. Options 2, 4, 7 and 8 did not come directly from the eight IMT building standards related questions, but came out of stakeholder engagement and expert consultation, of which recommendations were made beyond the scope of the eight IMT questions.

11.2 Options to Consider

The options to consider in addressing mine gas issues identified in the Gorebridge IMT Report have been identified through the assessment conducted in Section 10. The data from the Stakeholder Engagement and Expert Consultation were analyzed and integrated to provide a summary of the issues in Table 10-2 along with a potential solution for each. Potential solutions were assessed and scored based on their feasibility, impact and timescale of implementation which are ranked in Table 10-2.

The 23 potential solutions were assessed in Section 10 as having either

  • high impact and high feasibility (upper right quadrant of Figure 10-1),
  • high impact and low feasibility (lower right quadrant of Figure 10-1),
  • low impact and high feasibility (upper left quadrant of Figure 10-1), or
  • low impact and low feasibility (lower left quadrant of Figure 10-1).

Potential solutions are also colour coded in the traffic light system to show the timescale of implementation.

The eleven potential solutions within the upper right quadrant of Figure 10-1 should be taken to action first which is why they are included as ‘options to consider’ below, see Table 11-2 Options to consider with corresponding potential solutions from which they were derived . These potential solutions taken forward were identified as having a high impact and high feasibility. Where there are similarities in terms of improvement actions, we have grouped solutions together to create the numbered options to consider below. Of note, potential solutions coded with a green traffic light will be the quickest to implement and may be chosen for prioritisation.

Table 11-2 Options to consider with corresponding potential solutions from which they were derived.

Option to Consider

Outcomes from Section 10 Analysis Fig 10.1

Option 1

5, 11, 17

Option 2

4, 6,11, 10

Option 3

2

Option 4

1A, 2

Option 5

16

Option 6

20

Option 7

2

Option 8

9

Solutions in the upper left, bottom left, and bottom right quadrants of Figure 10-1 were not put forth as ‘options to consider’. These potential solutions are significant recommendations for improvement, but they may not reduce the occurrence of acute events. We recommend the potential solutions in the upper left and lower right quadrants be reviewed and taken to action after all of the ‘options to consider’ have been addressed. We recommend that the potential solutions with low impact and low feasibility not be taken forward.

The eight final options to consider are set out below:

Option 1: The use and enforcement of model planning conditions as well as changes to Scottish Planning and Building Standards and guidance to cover adequate assessment of mine gas (including worst-case conditions) should be considered.

In accordance with standards, guidance and the NQMS, to remove uncertainties and provide future-proofing, the decommissioning of boreholes and other preferential pathways should be considered in the gas risk assessment, mitigation design and integrated within foundation and building design.

Risks and uncertainties created by additional pathways should be reassessed when there are significant changes to the foundation or building design.

What occurs at present:

Scottish Building Standards and guidance, e.g. ‘Technical Handbook – Domestic’, do not explicitly cover the assessment and mitigation of ground gas issues. Planning guidance, i.e. PAN33 and local authority supplementary planning guidance, contains reference to ground gas but not specifically to issues relating to mine gas. Planning conditions are imposed on a case by case basis with variations in approaches within and between different local authorities leading to a lack of consistency and potential for insufficient mitigation of mine gas related risks, as happened at Gorebridge.

Evidence of need for change:

Consultations with stakeholders and particularly experts (see section 9.3) have identified that many of the shortcomings in mine gas risk assessments, mitigation design and verification relate to a lack of compliance with existing standards and guidance relating to ground gas. In addition, the requirements of these documents have not been incorporated into relevant Scottish planning and building standards guidance. Model planning conditions recommended by the Scottish Government for developments in areas of former mine workings would be valuable in ensuring all relevant issues have been addressed and achieving a more consistent approach across all relevant local authorities.

Next steps or where good practice can be found:

  • Update PAN 33 and/or local authority supplementary planning guidance to make reference to key issues relating to mine gas and relevant standards and guidance to be followed. This could also refer to the role of the NQMS in such assessments to make sure all legislative requirements and necessary standards connected to the management of land contamination are met and uncertainties are adequately considered.
  • Update relevant Scottish Building Standards and guidance, e.g. ‘Technical Handbook – Domestic’, to make reference to key issues relating to mine gas and relevant standards and guidance to be followed. Approved Document C applicable in England provides an example of this approach.
  • Develop draft model planning conditions in consultation with CoSLA/relevant local authorities to cover:
    • adequate assessment of mine gas (including worst-case conditions) in accordance with relevant standards and guidance
    • gas risk assessment and mitigation design integrated with foundation and build design, and reassessment when there are significant changes
    • consideration of uncertainties/future-proofing
    • decommissioning of boreholes and other preferential pathways, and
    • verification of gas protection systems in accordance with BS 8484 and CIRIA C735.

Option 2: Further research and preparation of supplementary technical guidance relating to the assessment of risks to new and existing developments, specifically from mine gas.

What occurs at present:

Standards and guidance relating to ground gas reference mine gas as an issue of concern. However, the risk assessment approach does not consider explicitly some aspects relevant for mine workings, such as assessing the gas source, relevant pathways, changes over time and effect of cumulative development.

Evidence of need for change:

Several specialists consulted in the Expert Consultation phase of the project referred to a lack of specific guidance available relating to mine gas issues and relevant factors to consider in such assessments. This lack of specific guidance was also mentioned by Northumberland County Council. Within Scotland, several Local Authority staff consulted in the Stakeholder Engagement process commented on a lack of expertise and understanding around mine gas, which would be partially mitigated by specific guidance around the risk assessment process.

Next steps:

One immediate solution might be to develop Supplementary Planning Guidance or a similar document which provides a clear explanation of the issues. For example, similar documents have been prepared by various local authorities, supported by Environmental Protection Scotland, in relation to contaminated land assessment. Inclusion of additional emphasis that the CSM developed in the risk assessment process needs to be reviewed when there is a change to the design would be key in these guidance documents.

More generally there seems to be a decline in research activity into the issues arising from abandonment of mines. There was substantial activity from the 1990s onwards into the impacts of mine abandonment on the water environment; however, this is an issue that is now well understood. More recently research has addressed the impact of mine gas emissions as greenhouse gases in the context of climate change. The Gorebridge incident and the stakeholder engagement and expert consultation process, carried out as part of this project, suggests that there is a requirement for additional research into the sources of mine gas and CO2 particularly, in the context of the Source-Pathway-Receptor model. This research should be geared towards improving the understanding of how emissions from former coalfields may change over time in response to issues such as mine-water rebound, mine collapse and climate change effects. A clear understanding of the source of mine gas and how that is likely to develop over time will enable a much clearer understanding of the current and future risks, as well as associated uncertainties.

Option 3: Include consideration of mine gas issues/constraints at local development planning stage, especially with regard to large-scale developments and consideration of cumulative effects.

What occurs at present:

Stakeholder consultation has indicated that mine gas issues are not considered as a constraint at local development planning stage. There is also no mechanism for considering cumulative effects, particularly where adjacent developments may be occurring in parallel.

Evidence of need for change:

Some areas of Scotland affected by former mine workings are currently undergoing rapid development, for example parts of Midlothian and Lanarkshire. There is a risk that such rapid development on a large scale could affect ground gas emissions and the effectiveness of designed mitigation measures due to cumulative effects and unintended interactions between developments. Such issues would be best considered at the time of updating LDPs where areas are zoned for development, such as residential use.

Next steps:

A major update of planning legislation in Scotland is pending (Section 3.3). The Planning (Scotland) Bill was introduced to Parliament on 4 December 2017. The Bill is intended to strengthen the planning system's contribution to inclusive growth and empowering communities. The Bill sets out proposed high level changes to the overall framework under which planning operates; in particular the Bill proposes a major update of the way local development plans are taken with a 10-year cycle as opposed to the current 5 years.

In section 7.3, we highlighted consultation with the BGS focused on the use of environmental data and supporting information to inform planning policy and, in particular, the issue of providing site investigation and other environmental data early in the planning process. Work has been carried out by a sub-group of key agencies in Scotland to examine what environmental information is potentially relevant to inform preparation of Local Development Plans (LDP) and how this information can be made more accessible. Liaison and GIS identification would help identify within LDPs areas that may be of high risk of CO2 emissions and potential cumulative impacts.

Option 4: Improve co-ordination and communication between planning, Building Standards and EHO/CLO staff in some local authorities and provide additional budget for training or external specialist support where needed.

What occurs at present:

Within section 7.1 stakeholder engagement, some local authorities noted that on complex sites there was insufficient communication and handover between departments verifying assessments and compliance with regulations. Experience of the guidance and risks associated with coal mine gas varies within local authorities as does the use of external expert peer review submissions from developers.

Evidence of the need for change:

These issues could lead to insufficient checking and enforcement of standards and guidance through the mine gas risk assessment, mitigation design and verification stages.

Next Steps:

  • Good practice was highlighted within Fife and Midlothian Councils and it was noted that The City of Edinburgh Council’s Planning and Environmental Health protocol cited as an example of best practice.
  • Using existing lines of communication between Scottish local authorities’ good practice should be shared. Local authorities should consider reviewing their internal handover processes and lines of communication to improve consistency in the governance of regulations applied to development of sites identified as at risk.
  • Provide additional budgets for training or use of external specialist support, where needed.

Option 5: Further research to assess the long-term effectiveness of granular fill and perforated pipe ventilation below slab construction as utilised in modern housing construction in areas where there is a high likelihood of mine gas emissions.

What occurs at present:

In Scotland, housing developments have tended to utilise a slab construction with granular fill and perforated pipes to provide sub-slab ventilation since 2000 in order to comply with mobility access regulations whilst avoiding the raising of buildings and to ensure that ramp access is not required.

Evidence of the need for change:

Several consultees questioned whether or not this construction technique may increase the level of risk in relation to mine gas particularly if a gas membrane was not emplaced correctly or if ventilation was subsequently blocked accidentally by operatives during construction or at a later date by home owners.

Next Steps:

Further research is required to test the effectiveness of granular fill and perforated pipe in both design and construction techniques and to consider if further regulation is required to ensure the operational effectiveness of the design as a mitigation measure in the long term.

Option 6: Consider further the implementation of mandatory mitigation measures in former coal/oil shale mining areas.

What occurs at present:

The application of mandatory mitigation measures in areas of former mine workings has been trialled by one local authority in Scotland on a limited basis and by NCC. Both have adopted an equivalent to Characteristic Situation 2 (CS2) as defined in BS 8485:2015, i.e. ‘low risk’ sites. The experts consulted in this project were not typically in favour of this option whereas views amongst the stakeholders were widely variable.

Evidence of the need for change:

If a mandatory approach was to be adopted, it is likely that the level of protection would be similar to CS2 to avoid an overly precautionary and costly approach that could form a barrier to development. All consultees in favour of this approach accepted that ground gas assessments, mitigation design and other considerations specific to mine gas would still need to be undertaken on behalf of developers and reviewed for adequacy by local authority staff. Therefore, while this appears a relatively straightforward option, the benefits appear to be more limited.

The next steps would involve:

  • wider stakeholder consultation
  • more detailed evaluation of how to identify areas of risk and the level of gas protection measures to implement.

Option 7: Liaise with NCC.

In parallel to the main stakeholder consultation we identified in section 7.3, our additional stakeholder liaison revealed that Northumberland County Council has extensive experience of dealing with mine gas related issues similar to those recorded in the Gorebridge IMT Report since at least the 1950s. This has led to increased vigilance regarding the risks posed by CO2 particularly for new developments.

We recommend further liaison between Scottish Government, Scottish Local Authorities and NCC environmental health officers may be mutually beneficial, particularly in relation to progressing options 1-4 above.

Option 8: Validate consultants.

Validation of the experience and qualifications of those designing mine gas mitigations measures is required by developers during the procurement process to appoint consultants.

Evidence of the need for change:

Stakeholder at various levels within the industry have flagged up the lack of awareness within developers and consultants on mine gas issues and the requirements for risk assessing these issues. Examples are sighted by consultees of non-experts being appointed to carry out specialist work for which they were inexperienced.

Next Steps:

As with option one above, specific guidance in the form of Supplementary Planning Guidance or similar document which provides a clear explanation of the issues would be an initial step.

Requesting that developers require experience with mine gas assessment and mitigation during the procurement process to appoint consultants will assist in ensuring guidance and standards are being adhered to.

11.3 Conclusion

The fact finding research carried out for this project to investigate the prevalence of CO2 from disused mineral mines and the implications for residential buildings has involved a very detailed process of literature review, stakeholder engagement, primarily with Local Authority staff in Scotland, and consultation with key technical experts with detailed knowledge of the issues. In addressing the eight building standards related questions in the Gorebridge IMT report, we have collated the published literature and the opinions of stakeholders and technical experts to provide solutions to the issues raised.

A number of options for further consideration have been presented above based on their effectiveness in improving knowledge in relation to the risks posed by CO2 from abandoned mine workings. More robust enforcement, risk assessment and mitigation, will help to prevent further instances of ill-health in residential buildings from CO2 associated with abandoned mine workings.


Contact

Email: sarah.waugh@gov.scot