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

Description of the issue identified

Description of the potential solution identified

Comment on potential impact and score:

0 = limited change

1 = driver for change

Comment on feasibility and score:

0 = complex to implement or requiring additional budgets

1 = Easy to implement, no major budgetary implications

Traffic Light Symbol

Green underway or effective in 2 years

Amber effective in 2 to 5 years

Red Possibly 5 years or greater to develop

0 to 2

(Low to High)

‘Impact’ was scored in terms of how effective the solution would be as an agent for change. If a solution would provide limited change, it would score a ‘0’. If it would provide major change, for example by promoting best practice or reducing risk, then it would score a ‘1’.

‘Feasibility’ was scored by the complexity of implementation. For example, a complex solution might involve interaction with many organisations, a lack of obvious ownership, or a requirement for legislative changes and would score ‘0’. A solution that might involve one or two organisations, has obvious ownership and/or changes to business or regulatory practice would score a ‘1’.

‘Timescale’ is not scored within the overall ranking. We have used a traffic light system to indicate a timescale to implement the solutions. Items were flagged as red (long term, greater than five years), amber (two to five years) and green (on-going or less than two years).

1/ Lack of experience and expertise in some Local Authorities (LAs) was indicated in both stakeholder engagement and expert consultation, limiting their ability to effectively peer review assessments against standards and guidance.

1A/ Support LAs with additional training and/or access to peer review support from external specialists.

1: This would help ensure that reports being submitted under planning and Building Standards are compliant with existing standards and guidance.

1: Feasible and is already being applied by some LAs. Funding constraints within local and central government departments may be a limiting factor.

Already underway for some LAs and could be rapidly procured by others through existing frameworks e.g. Scotland Excel.

2

1B/ Greater assistance/ engagement with other agencies e.g. CA, BGS, industry experts.

0: This could provide useful additional data for LAs in some areas but would not address the underlying issues.

1: Feasible. Low cost for LAs although funding and the availability of personnel in CA or BGS may be a limiting factor.

On-going process already initiated with LAs collaborating on this project and the Gorebridge IMT investigation.

1

2/ Lack of consideration of the cumulative effects of multiple developments on former mining areas.

Departmental liaison within and between LAs at LDP stage. Stakeholders flagged up planning requires GIS risk mapping of former coal field areas into areas of high, medium and low risk based on parameters such as depth, age and type of mining or degree of groundwater rebound. Especially relevant with regard to large-scale developments and consideration of cumulative effects.

Further consideration is required to assess where responsibilities should be assigned for cumulative impacts outwith a redline development boundary.

1: Liaison and GIS identification would help identify areas of potential cumulative impact by stakeholders as these are not being addressed currently.

1: Feasible (subject to political will). Costs associated with internal LA GIS resource or procuring external support.

Clarity required on responsibilities for cumulative impacts outwith a redline development boundary. The guidance does not clearly assign this responsibility to the developer of an existing site as opposed to the developer of a future adjacent site. Also, adjacent developments can occur concurrently.

Can be implemented as part of LDP development.

2

3/ There is no guidance on values for unsafe levels of CO2 in domestic properties to initiate mitigation measures.

Establish a Scotland and or nationwide threshold for CO2 /O2 deficiency.

0: Although impactful, it would not on its own eliminate chronic events.

1: Feasible, low-moderate cost.

A value of 1000 ppm or 1830 mg/m3. has already been indicated based upon workplace exposure limits.

This may require additional research to verify.

Medium term due to time to procure and undertake research.

1

4/ For new developments the guidance around risk assessment is generally adequate but there is scope for more specific detailed guidance around mine gas issues.

Guidance specifically covering relevant sources and pathways for mine gas and uncertainties/ potential for future changes.

This would need to be informed by further research to consider available case studies and data held by CA, stakeholders and experts. Similar Special Purpose Guidance has been provided to LAs in the past e.g. for Contaminated Land.

1: Would help address uncertainties and limitations in mine gas risk assessment process.

Note linkage to solution 1 above.

1: Feasible, low-moderate cost, particularly if led by a single LA with expertise.

Medium term due to time to procure and undertake research.

2

5/ Monitoring of sites to assess gas risk can be insufficient or poorly targeted.

Ensure adherence to guidance is enforced particularly with regard to worst-case scenarios (CL:AIRE 2018) which would capture potential Gorebridge-style events.

Consideration of pre-commencement planning conditions to ensure that data collection for mine gas risk assessment commences early to maximize the ability to include the worst-case falling pressure events.

1: Ensuring sufficient data collected in line with TB12 guidance would help address uncertainties and limitations in mine gas risk assessment results.

Note linkage to solution 1 above.

1: Feasible, at moderate cost.

Practicality - the requirements to monitor through periods of low meteorological pressure drops will bring uncertainty into the data collection schedule.

Implementation could be low to medium term.

Impacted LAs may need to identify high risk areas and include the data requirement for monitoring in pre-commencement planning conditions.

2

6/ There is limited specific guidance around the risk assessment of existing properties.

Detailed specific guidance for the risk assessment of existing properties (under Part IIA) is required.

The planned CIRIA project on retro-fitting may cover this to some extent.

1: High impact as it would reduce uncertainty and may improve outcomes for properties in proximity to CO2 events.

1: Low cost (assuming CIRIA guidance is published).

Short term. CIRIA is currently looking to appoint a research contractor for this project.

2

7/ Future potential changes to sites need to be considered in the risk assessment process.

Adopt a precautionary approach as required by the standards and guidance. This should be linked to the planning process including improved information and enforcement by planning/ Building Standards/EHO staff.

Issue of reports under the NQMS, which specifically requires consideration of uncertainties in the risk assessment process, would assist with improving the quality of reports, if this was requested by LAs.

1: Following best practice guidance would help address uncertainties and limitations in currently submitted mine gas risk assessment.

Note linkage to solutions 1 and 4 above.

0: Guidance is in place, but supplementary guidance on considerations for sites affected by mine gas would be beneficial.

Medium to long term.

Requires revision to guidance documents/ preparation of supplementary guidance.

1

8/ What are the implications for existing properties surrounding a new-build development if gas membranes are deployed (e.g. is there a need for venting trenches surrounding the new development to prevent gas migration)?

Ventilation beneath the membrane should negate the potential for build-up of ground gas beneath a property. Venting should be to air. The guidance is clear that any risks associated with that venting should be considered in the risk assessment and mitigation design for a new development and LAs should require this to be done.

0: Ventilation measures designed in accordance with BS8485: 2015 should negate the potential for build-up beneath a property.

Refer to item 2 re: cumulative development impact.

0: In general, additional venting should not be required.

Not applicable

0

9/ Training and experience of those designing or installing mine gas mitigation measures can be inadequate.

Examples are cited by consultees of non-experts being appointed to carry out specialist work for which they were inexperienced.

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

Note linkage to solution 4 above.

1: Improved compliance with design standards and effectiveness of gas protection measures.

1: Additional awareness/ rigour within the procurement process for specialist consultants.

Design details would require sign off by a validated/qualified engineer confirming the details comply with current guidance.

A short to medium term solution requiring additional guidance to be provided to developers by local authorities.

2

10/ Revisions made in the overall design of a foundation or structure subsequent to the original design do not always consider the impact on mine gas mitigation measures e.g. the creation of preferential pathways during construction.

This may be particularly the case where changes are made sometime after the original design due to a delay in the development or a change of ownership.

The CSM developed in the risk assessment process needs to be reviewed when there is a change to the design. This is already covered by existing ground gas standards and guidance but needs better enforcement by planning/ Building Standards/EHO staff.

A ‘hold point’ is needed to ensure that the gas risk assessment is reviewed and updated whenever design changes are made that could affect the assessment. This could be done through introduction and enforcement of model planning conditions for areas affected by mine gas (see e.g. example of NCC conditions). Planning guidance and Building Standards should also be amended to reference this issue.

1: Enforcement of relevant planning conditions would be an effective way of mitigating this risk.

1: This is an extension to the existing planning and Building Standards system.

Short to medium term to develop. Consult and publish model planning conditions.

2

11/ Guidance related to the application of mitigation measures for ground gas is not specific to mine gas.

Specific guidance related to the design of mine gas mitigation measures should be considered e.g. in the form of a further Supplementary Planning guidance document and/or changes to Scottish Building Regulations and standards.

1: High - Revised technical guidance explicitly stating relevant standards and guidance documents will improve compliance by increasing visibility.

1: Feasible, low cost. Entails updates to already existing documentation within Building Standards and LAs.

Revision cycle for guidance documents vary from annual updates. LA document revision cycles will vary.

2

12/ The verification system for gas membranes is not sufficient.

Independent verification on installation with supporting documentation. This should already be taking place and if not, there is a failure in the construction and possibly regulatory processes. A new NVQ for verifiers is currently being developed by the industry.

0: Should already take place if guidance is followed.

0: Numbers of qualified independent verifiers are low. Would require industry investment to improve the quality and capacity.

Long lead in time to roll out verification qualification to significant enough numbers to improve the situation.

0

13/ Homeowners are unaware of gas ventilation mitigation on their property. Any future maintenance works, extensions etc. could damage the integrity of installed gas protection systems.

Details of installed gas protection systems should be provided when the property is let or sold. Conveyancers should include such information in standard searches.

1: Public awareness and protection of existing mitigation will reduce long term risks.

0: Existing systems and processes used. However, it may be seen as a potential blight issue affecting property sale values.

Medium -– there are existing processes amongst some developers.

1

14/ The ventilation below a foundation is the ‘first line of defence’ in mine gas mitigation measures.

Application of a ventilated solum or granular fill with perforated pipe should be mandatory for future developments in areas of high risk of ingress of mine gas into a property.

N/A: This should already take place in modern developments if standards and guidance are followed (e.g. BS 8485).

N/A: This should already take place.

N/A: This should already take place.

0

Construction Techniques

15/ Developers will always build to the minimum required standards.

Independent verification of membranes required through planning and building regulation.

See Issue 12.

0: Should already take place if guidance is followed and regulations through planning are robust.

0: Should already take place if guidance is followed and regulations through planning are robust.

N/A: Should already take place if guidance is followed and regulations through planning are robust.

0

16/ The use of granular fill with a perforated pipe below a foundation slab to provide ventilation has been used commonly in Scotland for over 20 years and consideration is required as to its effectiveness.

Further research is required to assess the long-term effectiveness of granular fill and perforated pipe ventilation systems in areas at high risk of mine gas emissions.

1: The perforated pipe ventilation system is extensively used in Scotland since 2000. It was primarily driven by mobility access regulations and without specific consideration of the long-term effectiveness in areas of high risk of ingress of mine gas into a property.

1: A research programme including long term monitoring is feasible albeit it would require budget and ownership to take forward.

Could be completed within a 12-month programme.

2

17/ Site investigation boreholes are at risk of creating preferential pathways for mine gas migration to the surface.

Appropriate decommissioning and accurate log of borehole locations. This could be enforced by Planning/Building Standards/EHO staff through application of a standard supplementary planning condition.

See link to item 10 above.

1: Enforcement of relevant planning conditions would be an effective way of mitigating this risk.

1: This is an extension to the existing planning and Building Standards system.

Short to medium term to develop. Consult and publish model planning conditions.

2

18/ It is assumed that CO2 would take the path of least resistance. Is it possible to create paths outwith affected buildings that would result in escape routes for the gas?

Current standards and guidance focus on ventilation and dilution of ground gas ingress into properties to acceptable levels. Venting systems external to buildings are routinely used for management of landfill gas emissions and in some cases for redevelopment of such sites. We are not aware of its uses for managing mine gas emissions and a key issue would be appropriate design of vents to reduce CO2 concentrations to acceptable limits at ground level. It may be an approach to consider for multiple developments in former mining areas, but further research would be needed to consider the viability of such an approach.

1: A venting system may be an effective risk reduction mechanism in existing properties.

0: This may be an approach to consider for multiple developments in former mining areas, but further research would be needed to consider the viability of such an approach.

Would require significant time to carry out a trial and assess results.

1

Energy efficiency and the air tightness of modern dwellings

19/ Two lines of thought have emerged in relation to the impact of air tightness:

1. A general concern was expressed about air quality in modern housing generally and from CO2 levels specifically.

2. The ingress of mine gas may be decreased in an air tight property because there is less suction.

A long-term baseline study of CO2 levels in modern properties may address this issue. A first step is a specialist literature review to map current research in this area which would then indicate what further research may be required.

0: Could eventually provide some evidence on chronic assessment of CO2.

A research programme could be implemented within a 12-month programme to assess and report on the impact of air-tightness of a property.

0: The major requirement for such a project would be either a property impacted by mine gas or a simulation of this.

A 12-month programme would enable long term monitoring to be carried out.

0

Mandatory Mitigation

20/ Mandatory gas mitigation measures including ventilation and membranes in areas at ‘high risk’.

Two lines of thought have emerged in relation to this:

1. This approach will be overly precautionary for some sites but unlikely to be precautionary enough for the higher risk sites, and there are potential risks of undermining the quality of gas risk assessments and design/ construction/ verification of gas protection measures.

2. There are inherent uncertainties in gas risk assessment for mine gas sites, therefore a minimum level of protection (e.g. CS2) should be required to be precautionary.

Mandatory gas mitigation measures. This is being successfully applied by one local authority in Scotland via Building Standards for ‘low risk’ (CS2) sites, albeit on a small scale, without challenge by developers. NCC in England also have a process in place to require conditions in certain developments.

Appropriate risk assessment and design/construction/ verification of gas protection measures in accordance with current standards would still be required and would need to be rigorously enforced. Arguably if this is being done effectively, the use of mandatory conditions should not be required.

It is unlikely that such an approach could be practically adopted for ‘high risk’ sites since these require more detailed evaluation, risk assessment and mitigation design.

It could potentially be applied to low-moderate risk sites in former mining areas. Further work would be needed in collaboration with the CA and LAs to identify and define areas where the mandatory approach could be applied. It is possible that these areas could be further differentiated, e.g. by geospatial studies of environmental risk, within Local Development Plans as will be required in the forthcoming Planning Act. This might be a way of reducing the scale of solution to areas where it is clearly necessary and make the solution more manageable to implement.

1: If such an approach was to be adopted more widely across relevant areas of Scotland, more extensive consultation with stakeholders, particularly developers, would be required as well as more detailed consideration of how to apply this.

1: This is being successfully applied by one LA in Scotland and by NCC in England. It would require clear definition as to how areas would be identified and liaison with industry and developers.

This could be implemented in a fairly short timescale.

2

Retrofitting of existing properties

21/ What retrofitting works (e.g. gas membranes and/or ventilation/ pressurisation) can be carried out to existing properties that would give certainty to successfully ‘managing’ the presence of CO2?

Several case studies have come to light where retrofitting of gas membranes have been successful. Therefore, it may be a viable alternative to demolition in some circumstances.

Supplementary technical guidance is needed in this area, which is already being led by CIRIA.

0: Limited case history to estimate how viable this may be as a mitigation measure.

Previously recorded CO2 mine gas events generally relate to proximity of mine shafts with works carried out by the Coal Authority on venting the shaft were found to be effective.

1: Feasible (assuming CIRIA guidance is published).

Low (assuming CIRIA guidance is published).

1

Regulatory Issues

22/ Lack of communication between LA departments– planning, building standards, environmental health.

Early departmental liaison and geospatial risk mapping linked to Local Development Plans so that information is readily available for analysis. Improved co-ordination and communication is needed between planning, Building Standards and EHO/CLO teams in some local authorities. The City of Edinburgh Council’s Planning and Environmental Health protocol provides an example of this.

Planning conditions recommended by EHO/CLO consultees should be acted on by planning officers.

1: High in selected LAs where coordination on mine/ ground gas issues are required and coordination issues have been noted.

0: Good practice processes already exist and could be shared between authorities; however, this would require additional resources to develop which may not be practical for every local authority.

Low to medium

Requires coordination and time for new processes/systems to become embedded.

1

Other Issues

23/ High levels of CO2 may eventually become exhausted, are there any ways of determining how and when that might happen? Would there be any way of accelerating such a process?

In the context of the SPR approach, mine gas CO2 should be considered as a ubiquitous source because it is created by the interaction of coal (C) and oxygen (O) in air.

Complete submergence or infilling of old mine workings is not a viable option. Accelerating the process by greater ventilation may risk spontaneous combustion.

Not an applicable solution.

Not an applicable solution.

Not an applicable solution.

N/A