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
9 Data Analysis

9 Data Analysis

9.1 Analysis Methodology

Following the stakeholder engagement and interviews with expert consultees, our meeting transcripts were collected and reviewed, and an issues/findings log was created. For stakeholder responses, see Annex 3 – Stakeholder Engagement

3(a) Issues log from Stakeholder Engagement workshop and 3(b) Stakeholder responses to questions. For industry expert responses, see Annex 4 the Findings log from the Consultation with Industry Experts is presented.

Our aim here was to distill all the issues and perceived solutions obtained on the day and identify the main patterns.

Additional reviews were completed by the RSK research team leaders carrying out a deductive approach to collate these themes. The responses were coded and assigned where possible to one of the eight issues relating to the IMT building standards questions.

The collation and analysis from each population type i.e. stakeholders and industry expert consultees were completed separately. Our overall process is shown in Figure 9-1 Qualitative Data Analysis below and the results of fact finding presented in Sections 9.2 and 9.3. Both the issues and findings were then compiled to generate a shortlist to assess the feasibility and subsequently the options for consideration.

Figure 9‑1 Qualitative Data Analysis

Figure 9‑1 Qualitative Data Analysis

9.2 Stakeholder Analysis

The issues identified in the stakeholder analysis are summarized in Table 9-1.

Table 9‑1 A summary of findings from the stakeholder analysis

9.2.1 Risk Assessments

RA 1

There is a lack of experience/expertise in LAs to fully understand site risks and risk assessments

RA 2

There is lack of communication between planners, LA assessors and building standards departments

RA 3

There is no guidance on safe/unsafe levels for CO2

RA 4

Gas monitoring on sites is insufficient

RA 5

The risk assessment is not ‘future proof’

RA 6

The risk assessment does not include cumulative impacts

RA 7

Coal Authority Operations

RA8

Mitigation Measures for Existing Developments

RA 9

Site investigation boreholes are creating new pathways for CO2

RA 10

Current construction techniques are increasing the risk of CO2 ingress or accumulation.

RA 11

Lack of information on CO2 sources and pathways related to colliery spoil, former open cast sites and peat.

RA 12

There needs to be more emphasis on mine gas information provided at planning permission stage

RA 13

Monitoring of gases after structural work/ground stabilisation had taken place are not included within risk assessments.

9.2.2 Energy Efficiency

EE 1

The ‘chimney effect’ is increased within an airtight property

9.2.3 Mitigation Measures

MM 1

Fixing a mandatory level of mitigation may cause further unintended issues.

MM 2

Gas Membrane Risk

MM3

Ventilated Sumps CO2 parallel with Radon mitigation

MM4

Gas Ventilation subsequent blocking due to poor maintenance or lack of notification to owners

9.2.1 Risk Assessments

There was a general consensus from stakeholders that the current gas risk assessment is valid, but that it is frequently implemented poorly or not well understood. Submissions to LAs are noted to be highly variable and frequently suggesting mitigation measures which cannot be implemented until the development has begun. Although adhering to current guidance, the risk assessment fails to fully consider temporal change and/or cumulative effects.

RA 1 There is a lack of experience/expertise in LAs to fully understand site risks and risk assessments.

Feedback from stakeholders in some LAs indicated that there has been some loss of experience over the last decade. The main concern is that environmental health officer/contaminated land officer (EHO/CLO) has a good understanding of contaminated land practices and procedures and landfill assessment but not CO2 mine gas in relation to former coal mining activity. In particular, a robust understanding of the source in the subsurface and the conceptual model.

Solution Option 1 - Support relevant LAs with compulsory training days on coal mine gas risk. The format of the training could involve expert consultants and the CA creating a workshop to improve understanding of the environmental risk and improvements on the guidance and assessment process.

Solution Option 2 - BGS, CA and former miners to assist LA with further information. Collaborate to create dedicated information about each coalfield and access to site-specific data, where available. Possibly culminating in a short document (2-4 pages) detailing the source material (type of coal), geology and style of mining including potential pathways, water levels and possible temporal changes. This could support more site-specific risk assessments and support regional planning by defining the risk areas with more precision and understanding. This could also be regarded as a peer review process.

RA 2 There is lack of communication between planners, LA assessors and building standards departments.

Stakeholders raised concerns that in some LAs there is not enough communication between the various departments involved in residential property development. The risk assessments are sometimes not being adhered to correctly because the development plans are not taking into account the gas risk correctly (although sometimes this is unavailable at an early stage). The responsibility for assessing the adequacy of the gas risk assessment and mitigation design is split between environmental health and the building standards personnel, who may or may not be fully aware of the level of gas risk and/or thus not authorising the correct level of mitigation. In some cases, relevant planning conditions recommended by EHO/CLO staff have not been acted on, or site assessment reports have been submitted. On other occasions the risk is acknowledged but too late in the process and either the permission has been approved or development is already under construction.

Solution Option – Departmental liaison early and GIS risk mapping, particularly with regard to large scale residential developments. Building standards, planners, developers and the environmental health teams of an LA should have a meeting prior to planning applications being submitted in order to understand the gas risks. This would benefit all parties and pre-emptively reduce risk by being able to adapt building plans and potential site investigations from generating new pathways or constructing buildings in the more high-risk areas with insufficient mitigation.

Fife Council has taken a very proactive approach and is working with the Coal Authority to create advanced risk maps and candidate sites that are safer for new developments on brownfield areas (such as old coal fields). In addition to this they have built up a GIS database with this information and have retained a log of those properties constructed with gas mitigation measures. This approach should be a consideration for other local authorities.

This could be scaled to larger sites where an intermediate level of risk ‘zoning’ could take place with link to local development plans.

RA 3 There is no guidance on safe/unsafe levels for CO2.

Current guidance does not specify a threshold of CO2 to initiate mitigation measures or what is regarded as safe/unsafe for domestic properties. Standards and guidance tend to focus on short term acute risks from CO2 and neglect longer term chronic risks of exposure.

Solution Option – Establish a nationwide threshold of CO2/O2 deficiency.

HSE currently provides workplace exposure limits (WELs) for CO2 (EH40/2005). However, this is directed towards the working environment and enclosed spaces, not domestic dwellings. They provide a formula for the conversion of eight-hour time weighted average WEL to a continuous (24 hour) equivalent. EH40 provides guidance on standards for situations where occupational exposure is essentially continuous. In these cases, a continuous exposure limit is derived by dividing the eight-hour TWA exposure limit by a factor of five. Based on the eight-hour TWA for carbon dioxide (5000 ppm, 9150 mg/m3), this would give a continuous exposure concentration of 1000 ppm or 1830 mg/m3.

Dr Colin N Ramsay Health Protection Scotland has used this calculation with the Gorebridge IMT report to derive an outline acceptable domestic exposure threshold. This level of 1000ppm could easily be reached is some domestic circumstances i.e. within an occupied bedroom overnight. However, if levels above this threshold were being recorded within an unoccupied room it should trigger further investigation. Whilst some limitations exist around this approach, it gives the most robust, scientifically-derived standard to use for continuous exposure.

Further research is needed. However, this would delineate a benchmark in Scotland, and possibly across the rest of the UK from which authorities would operate. A general blanket threshold value may help to isolate more chronic issues that may arise from persistent levels above a certain threshold exposure. The issue will still remain that a threshold value will not capture or help prevent sudden chronic events such as those in Gorebridge.

RA 4 Monitoring of sites is insufficient.

There was significant feedback that gas monitoring takes place over an inadequate time period(s), with insufficient frequency or limited coverage.

Solution Option 1 – Increased frequency or continuous monitoring (to include ‘critical pressure drops’).

BS8756: 2013 states sufficient monitoring should be carried out over a sufficient period to allow ‘prediction’ of worst-case scenario conditions. In order to capture a more robust dataset, monitoring should be continuous and cover a period that encapsulates three to four low pressure events (critical pressure drops CL:AIRE, 2018).

It can be difficult to capture a ‘worst-case’ scenario if such an event did not occur over the monitoring period. An example was cited that in the summer of 2018 there was a long dry spell of six to eight weeks of consistently high pressure. Any monitoring over this period would be inadequate because the conditions were unusual and would not have captured any rapid falling pressure event.

The solution is that the existing standards and guidance should be enforced, and site investigation and monitoring should commence earlier in the planning process.

Solution Option 2 – Introduce model planning conditions for areas affected by mine gas (see e.g. example of NCC). Model planning conditions, including covering aspects of mine gas risk assessment, for example as a pre-commencement condition.

RA 5 The risk assessment is not ‘future proof’.

Several stakeholders were concerned that previous risk assessments may now be invalid as mine water levels rise and climate changes. These are factors which may increase the upward movement of CO2. This is referenced to in standards and guidance, but no detail is provided on methods of assessment.

Solution Option – Temporal changes need to be logged and/or assessed.

Further research is required into the temporal changes within the former coal workings to understand any trends. These trends could be noted and added to support the conceptual site model by indicating an increased or lowered gas risk. The information could then be included in environmental risk assessments as part of the LDP process, and also used to supplement existing standards and guidance.

RA 6 The risk assessment does not include cumulative impacts.

It was identified in the Gorebridge IMT Report and confirmed by stakeholder engagement that cumulative impacts are not being documented or assessed. This includes the risk posed by a number of developments within the same former coalfield area but are assessed on an individual basis.

Solution Option – Cumulative risk assessment used at planning stage. Stakeholders suggested a cumulative impact assessment for specific former coal mining areas may be required. This could also be implemented in a similar way to the above issue by incorporating a cumulative assessment into the LDP process.

RA 7 Coal Authority operations

A significant number of stakeholder responses related to Coal Authority operations. These comprised the following:

  • No risk is logged in a CA report if there is no monitoring or no past incident has occurred in the area
  • There are not enough gas and groundwater monitoring points across former coal fields
  • Coal mine maps are not detailed enough (therefore the risk source is poorly understood)
  • The organisation is ‘reactive’ and not very proactive and will assist with data on request but is limited in its provision of data and scope of work.

Solution Option - The Coal Authority is a statutory body and have stated that it would require more financial support to begin to solve the above issues. At present the CA use the data they hold to inform their site-specific reports and the zoning of development in high risk areas. Site-specific data is not available to third parties. It was noted that Coal Authority representatives attending the workshop recognise the current limitations of their approach and are keen to explore opportunities for greater collaboration with LAs regarding mine gas issues.

RA 8 Mitigation measures for existing developments.

It is important to note that preventative mitigation installed at the time of construction is much more feasible than retrofitting. To quote from Sizer et al. (1996), “it is generally recognized that effective gas precautionary measures for developments are achieved by attention to structural details, which cannot be readily addressed post-construction.”

Stakeholders present from The Coal Authority identified that the organisation infrequently adopts retrospective mitigation measures such as membranes. The standard protocol was to demolish the properties to eliminate the risk, and therefore the liability, to the properties in question. On a large scale, however, this is costly and unsustainable and involves significant upheaval for the residents involved. The CA does passively and actively vent shafts and adits and is currently examining alternative methods to address this issue for residential properties.

Solution Option – Mitigation measures designed and fitted in new properties are much more effective than retrofitting similar measures. Where there are high risks to existing residential properties, there is uncertainty. It was stressed that one of the best overall mitigation measures would be more effective planning ensuring only suitable sites were selected for development. This would include more detailed risk mapping to prevent high risk sites being selected and requiring significant site investigation and mitigation measures which ultimately costs the LA and developer significant resources.

RA 9 Site investigation boreholes are creating new pathways for CO2.

A ground investigation undertaken to understand the subsurface structures, land contamination (including ground gas), geotechnical/mining issues or other ground conditions can create preferential pathways between the surface and workings for gas migration. These can be lost or damaged over time or during site demolition and redevelopment.

Solution Option - All boreholes must be decommissioned properly. Boreholes drilled in any former coal mining area need to be decommissioned properly and in accordance with SEPA guidance. This could be included as a relevant planning condition. They would require proper capping and grouting if they are no longer being used for monitoring. In addition, a log of the location should be kept ensuring no properties are located directly over a potential pathway especially if the boreholes have reached shallow workings.

RA 10 Current construction techniques are increasing the risk of CO2 ingress or accumulation.

The construction of concrete slab foundations with no ventilation, gas membranes which are subsequently punctured by service ducts, and also the limited verification for correct installation of membranes are increasing the risk of CO2 ingress and/or accumulation.

Solution Option 1 – Construct properties with a ventilated solum. Stakeholders favoured the construction of a ventilated void solum. Although this does not eradicate the risk, it is easier to verify and is believed to be more effective overall than a gas membrane or sump.

Solution Option 2 - It was broadly accepted that developers consistently construct to minimum standards. Raising the minimum standards would be a simple solution although this may cause standards to be too prescriptive and thus restrict development.

RA 11 Lack of information on CO2 sources and pathways related to former mining areas including colliery spoil and former open cast sites.

Solution Option – Further research required to update the risk assessment process in coalfield areas to distinguish between areas of high, medium and low risk. This work should include a detailed review of BGS and Coal Authority datasets.

RA 12 There needs to be more emphasis on mine gas information provided at planning permission stage and for LAs in general.

This issue was highlighted in CIRIA Report 149. Coal mine risks that are assessed in support of planning applications regularly relate more to structural issues rather than gas.

Solution Option – Include mine gas issues at LDP stage. 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. Additional secondary legislation and guidance following on from this work would be required to strengthen the risk assessment process for new developments.

RA 13 Monitoring of ground gas after undertaking structural work/ground stabilisation is not being included within risk assessments.

Solution Option - Developers should be required to carry this out especially if groundworks involve piling or stabilisation of shallow workings. Such works are undertaken under a Coal Authority permit.

9.2.2 Energy Efficiency

It was generally regarded by stakeholders that air tightness is a potential contributing factor to potential CO2 issues. Additional points were raised that suggested some properties are not built to the specified air tightness requirements or that where CO2 (or ventilation) monitoring is to take place in the property then monitoring should be in specific rooms.

EE 1 The ‘chimney effect’ within properties.

The ‘chimney effect’ is a useful analogy to describe the vacuum effect where a small pressure gradient is created within a property where an upper level window is opened, and air is drawn up from beneath to replace the warm air discharged. A leaky property will exacerbate this effect.

If ground gas is getting into a property but the property is otherwise airtight, the gas will accumulate within the property which is a serious concern. However, if gas cannot get in in the first place, then having an airtight property is fine.

Generally, one of the poorest performing area within any airtight property’s envelope are the service intrusions (soil pipes) in the slab within the ground floor toilet and kitchen. Therefore, if there has been significant accumulation of CO2 beneath the subsurface of a house, then negative pressure here and reduced unintentional ventilation elsewhere could draw significant volumes into these areas of the property. This is particularly important if gas has built up below a membrane that has been punctured with service ducts.

It was noted that kitchens and bathrooms are also areas of the building which are fitted with mechanical extractor systems to remove warm moist air. These systems will induce a negative pressure within these rooms which may also draw in any CO2 gases via the service ducting.

In addition, the lower levels of unintended background ventilation within airtight properties can allow CO2 to accumulate.

Solution Option – Construct New Properties with a Ventilated Void Solum. Stakeholders suggested that air tightness should not be reduced because this is counter-productive in trying to improve energy efficiency. The main, albeit indirect solution, would be to construct a ventilated void solum as opposed to a gas membrane to mitigate the gas risk. The reason for this is that a ventilated void solum provides a buffer between the property and the ground, and gas at ground level will be dispersed. When a window is opened in the overlying property and the ‘chimney effect’ begins to establish, the air drawn up into the property will be less concentrated with CO2.

It was also discussed that ventilation standards may require reexamination to ensure they are producing the correct results for the building design. This research has recently been undertaken highlighting that information dissemination on these issues might be the issue. A Scottish Government Report; ‘Investigation of Occupier Influence on indoor Air Quality in Dwellings’ was produced in 2014 (Sharpe et al.).

9.2.3 Mitigation Measures

MM 1 Fixing a mandatory level of mitigation may cause further unintended issues.

The issues raised by stakeholders relating to mandatory mitigation were:

  • Having a minimum mandatory mitigation requirement may not be an adequate solution and insufficiently precautionary at some sites
  • By fixing a mandatory level of mitigation it may give the developers a perception that the risk is mitigated, and emphasis will be reduced for the risk assessment and also the verification of gas protection measures
  • There is already regular conflict between developers and LAs about the choice of mitigation, this regularly occurs when developer and LA timelines become poorly aligned
  • Prescriptive standards may restrict development and/or cause blight
  • The use of mandatory measures would still need to be accompanied by use of appropriate site-specific investigation and risk assessment, with additional mitigation being implemented where the minimum mandatory requirement is deemed to be insufficient.

Solution Option - There is no single solution that addresses the issue of mandatory mitigation. Although the suggestion of having a mandatory gas membrane fitted to properties with a high risk of CO2 ingress was supported by some LAs, the fact remains that this would create uncertainty over whether or not the risk has in fact been mitigated.

In the Gorebridge area there is a requirement from the Local Authority for mandatory mitigation via a membrane on new properties. Other LAs noted they were not aware of this requirement or the authority used to request such a measure.

The general feeling of stakeholders is that if there were to be mandatory measures these should be a minimum for a particular area or site and the mitigation should possibly be a ventilated void solum as opposed to a membrane.

MM 2 Gas membrane risk

There was a general consensus that gas membranes have numerous risks relating to improper use, incorrect installation, damage after installation, insufficient verification and no sensible record to prevent future damage.

In addition to the current gas membrane issue, there are also legacy issues where properties claim to have mitigation fitted but have no paperwork or log of it happening. Fitting a gas membrane should also not be a substitute for a gas risk assessment.

Solution Option - Training and qualifications for installers are reported to be improving for the verification of membrane installations and LAs are focusing on ensuring verification reports are provided for all properties fitted with membranes. However, the stakeholders generally had a negative view of the quality and verification of membrane installation on site.

MM 3 Ventilated sumps CO2 parallel with radon mitigation.

It was raised during the workshop that sumps have been used to mitigate against radon ingress and (along with membranes) used heavily in high risk radon areas such as Aberdeenshire.

Sumps/ventilation systems have been fitted to uninhabited domestic properties in Gorebridge and they are currently under testing. However, there is limited practical experience and evaluation of efficacy for this technique in conjunction with coal mine gas. Sumps by themselves in passive form can simply collect CO2 and unless actively ventilated (which is not approved for domestic premises) by a fan would provide ineffective mitigation. Radon is also radioactive and therefore decays over time reducing the potential levels collected. In addition to this a sump may not mitigate against a sudden drop in pressure.

Solution Option - There was no singular issue regarding sumps that shone through from stakeholder feedback. There was a general belief that sumps work to a degree. In addition, the issues were more generalised around their effectiveness, for example, air needs to be ventilated to prevent the sumps filling up, however, an electric fan cannot be used in case methane has ingressed due to the explosive risk. If sumps are to be suggested as solutions for CO2 ingress, then further evaluation and research would be required.

MM 4 Gas ventilation subsequent blocking due to poor maintenance or lack of notification to owners

There are issues surrounding ventilation regarded the subsequent blocking or lack of owner awareness. Various vents or trenches can be a good mitigation method for CO2, however, these get blocked with plant material, objects or debris rendering them ineffective.

Solution Option - The house deeds or tenancy agreement should highlight the importance of the gas vents requiring the owner to keep them clear and maintained.

The caveat to this solution is that highlighting a gas issue at a property may reduce its market value or deter people from living in the property. Developers, agents and landlords may be reluctant to advertise such issues within the documentation.

9.2.4 Considerations for existing developments

It is important to note that preventative mitigation installed at the time of construction is much more feasible than retrofitting. To quote from Sizer et al (1996): “it is generally recognized that effective gas precautionary measures for developments are achieved by attention to structural details, which cannot be readily addressed post-construction.”

Stakeholders present from The Coal Authority identified that the organisation infrequently adopts mitigation measures such as retro-fitting membranes. The standard protocol was to demolish the properties to eliminate the risk, and therefore the liability, to the properties in question. This is, however, costly and unsustainable and involves significant upheaval for the residents involved. The CA does passively and actively vent shafts and adits and is currently examining alternative methods to address the issue for residential properties.

9.2.5 Additional Issues Raised

There were several issues raised by stakeholders which were outwith the scope of the eight building standards related questions. The full list is documented in Annex 3(b). These key issues, in summary, include the following:

  • In relation to ground gas risk and mitigation requirements within buildings, which department within the LAs takes overall responsibility of the issue is muddled between building standards, contaminated land, environmental health etc.
  • Process timelines are not communicated well enough causing stages of development and mitigation decisions to overlap which brings unnecessary delays to developments because information around gas risk is picked up too late in the process. This includes planning permission being granted in unsuitable areas
  • Gas issues are only likely to be picked up if they are on the scale of Gorebridge
  • No ground gas monitoring occurs for smaller properties or extensions
  • More research is required into the understanding of CO2 pathways to improve risk assessments and to supplement standards/guidance on ground gas assessment and mitigation
  • Local Authorities are under serious pressure with time and resources.

9.3 Analysis of Expert Consultation Findings

9.3.1 Risk Assessments

In relation to the risk assessment process, most expert consultees highlighted the importance of developing a robust conceptual site model and of applying the Source- Pathway-Receptor (SPR) approach to risk assessment. Most consultees felt that adequate guidance was provided in BS8485, CIRIA C665 and other documents about applying this approach, but several noted that it was not always done effectively and that sometimes those developing the conceptual model might not have appropriate skills and experience.

The consultees were asked the eight questions highlighted in Table 7‑2 IMT Questions to Building Standards Stakeholders. These responses are recorded in Table 9-2 to 9-8, and the questions are repeated above each table. One additional question about retrofitting was also added.

1. Consultees were asked to consider if the current mine gas risk assessment process is adequate at correctly determining the level of risk, especially in relation to assessing mine gas levels at sub-surface depths likely to be representative of gas migration potential from underground mine sources?

Table 9‑2 Topics discussed with consultees (Question 1)

Topics discussed Responses

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?

General consensus amongst the experts consulted was that the existing standards and guidance is robust for assessment of ground gas. However, those experts with greater experience of mining-related sites felt that the existing guidance should be supplemented with additional considerations relating to mine gas sources and pathways.

What are the perceived gaps in the current mine gas risk assessment process? Is there too much reliance placed on measurements of gas concentrations and flow over a short time period?

Often spot gas monitoring may be limited and can miss the worst-case falling pressure events. A multiple lines of evidence approach should be used to inform the CSM and gas RA.

Should there be differences in approach to CO2 versus methane e.g. to consider chronic risks?

Most consultees responded that both chronic and acute risks should be considered in the risk assessment, with the focus of current standards and guidance being on assessment of acute risks. Chronic risks of CO2 exposure need further consideration.

Should further guidance be provided specifically on how to assess mine gas sources and pathways?

Several consultees considered more specific guidance around mine gas risk assessment would be useful as mentioned above.

What about for existing properties e.g. under Part IIA?

It was noted that there was little guidance available for assessment of gas risks to existing properties, where the GSV approach is not suitable. Often gas monitoring is required in properties which is invasive/disruptive for residents.

2. Is the current risk assessment process fit for purpose particularly in terms of taking account of 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?

Table 9‑3 Topics discussed with consultees (Question 2)

Topics discussed Responses

Is the current risk assessment process fit for purpose particularly in terms of taking account of 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?

General agreement amongst the experts consulted was that this is referred to in current standards and guidance, but mixed views as to whether or not it is adequately considered in gas risk assessments.

To what extent do current standards and guidance consider the effect of future potential changes in the ground and the implications for future ground gas risk assessments? E.g. climate change, rising groundwater levels, mine grouting?

Current guidance does consider future changes within the red line boundary. There was a diversity of opinion about taking into account developments outwith the red line. Some consultees stated it was impossible, others noted that the responsibility lay with future developers of adjacent sites to consider the cumulative impacts.

Consultees also mentioned the need to be precautionary and account for uncertainties. Some discussion about whether the use of the Quality Mark Scheme (NQMS) could be beneficial for such sites since this specifically requires consideration of uncertainties in risk assessments and their implications.

What additional guidance could be provided on this?

Recent papers related to climate change and ground remediation were mentioned which could be extended to provide guidance on rising groundwater levels and climate change.

Is research needed in this area to better understand the effects?

Gaps in current knowledge could be filled by further research.

How should the cumulative risk from multiple developments in mining areas be assessed? E.g. 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 site or buildings to prevent gas migration)?

Consultees noted that a precautionary approach should be adopted, and the cumulative risk of all developments should be considered when creating a new development. This could be linked to the planning process. The need for a ‘responsible person’ oversight of a development over time was mentioned.

3. Is there sufficient emphasis in the current mine gas risk assessment process on the potential for other interventions affecting the soils or substructures 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?

Table 9‑4 Topics discussed with consultees (Question 3)

Topics discussed Responses

Is there sufficient emphasis in the current mine gas risk assessment process on the potential for other interventions affecting the soils or substructures 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?

General agreement amongst the experts consulted was that consideration of these issues is a requirement of current standards and guidance, but responses were mixed as to whether it is adequately considered in mine gas risk assessments. There can be a disconnect between ground gas and geotechnical/ structural assessments for building design.

Site specific mine gas risk assessments are based upon the current condition of the site. To what extent do potential changes to the site, related to the development, need to be considered in the risk assessment process?

Potential changes to the site, e.g. use of foundation methods which might alter pathways should be considered. There was a recommendation for the LA’s approval of a risk assessment to be conditional, approved unless the site conditions change, then the risk assessment must be redone.

How can activities associated with the development that can affect the gas CSM, e.g. ground improvement, stabilisation, ground source heat pumps, be incorporated better into risk assessments?

The foundation design should consider the mine gas risk assessment. Sometimes the foundation design will be carried out sometime after the risk assessment which increases the risk to that development, and the gas RA should be revisited where a foundation design has changed.

What additional guidance could be provided on this?

In mining areas additional guidance is useful such as LA supplementary planning guidance. A flow chart or checklist was suggested for mining areas.

9.3.2 Mitigation measures

In relation to mitigation measures, two broad questions were asked of consultees and the discussions focused around several topics including the effectiveness of the available guidance, implementation and verification of mitigation measures.

4a. 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?

4b. 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)?

Table 9‑5 Topics discussed with consultees (Question 4)

Topics discussed Responses

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?

There was consensus amongst all the experts consulted that the mitigation design process in BS8485:2015 is precautionary if used correctly, however, this is reliant on a robust CSM and gas RA.

Is the risk assessment process precautionary and proportional?

The risk assessments are adequate where consultants have the experience and understanding of coal mine gas risk to include appropriate monitoring and assess the risks appropriately within the conceptual model.

To what extent is the existing process precautionary? Does the precautionary approach extend to design of mitigation measures?

The process should be precautionary if the guidance is followed. There is sometimes a question mark over the experience of the designers in understanding mine gas issues.

Does the process adequately emphasise the need to take account 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)? Is the gas Risk Assessment and need for mitigation revisited when a change is made to building construction or foundation design?

Consultees agreed that the source-pathway-receptor (SPR) model should be updated right through the design and construction of the development and any changes to the design and construction reviewed in the light of the SPR model.

However, consultees noted that this does not always occur. Problems are encountered when maintaining documents where a site is sold on for development by a third party and particularly when this takes place some time after the design was first completed.

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

Table 9‑6 Topics discussed with consultees (Question 5)

Topics discussed Responses

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

BS 8485:2015+A1:2019 and other guidance is robust but also generic. The guidance generally does not address specifics and therefore is not sufficiently prescriptive and sometimes open to interpretation/manipulation. Also, the lack of recourse for those not following the guidance was mentioned.

Is the standards and guidance on mitigation measures prescriptive enough?

Are consultants/ developers interpreting the standards appropriately?

There was a consensus that developers rely heavily on consultants and other professionals such as architects for interpreting guidance. Issues identified with respect to consultants included a lack of experience in building and structural design. It was recommended that there needs to be more awareness/training.

To what extent are limitations in knowledge e.g. of building design from land quality professionals a constraint?

Concerns were reported about the failure to appreciate the implications of design in relation to the risks related to ground gas generally and mine gas in particular. Consultees stressed the importance of understanding the building/foundation design as it is often the pathway in the source pathway receptor model.

To what extent are poor installation of membranes and inadequate verification factors?

A ventilated void is seen as a first line of protection for mitigating risks associated with mine gas.

In Scotland, recent housing developments use a slab construction with granular fill and perforated pipes to provide sub-slab ventilation. NVQ training and qualifications for installers is reported to be improving verification of membrane installations. Sealing techniques and technology for membranes have improved in recent years.

9.3.3 Construction techniques

The question posed to consultees in relation to construction techniques was:

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?

Table 9‑7 Topics discussed with consultees (Question 6)

Topics discussed Responses

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?

Most consultees agreed that a ventilated layer is the first line of defence. Several remarked on the contrasts between construction practice in England and Wales, where a ventilated void solum is often utilised, and in Scotland where slabs underlain by granular fill and a perforated pipe are used. It was noted that where ventilated void solums were in place that it was rare for mine gas to gain access to a building.

What factors are driving the reduced use of a ventilated solum beneath domestic properties?

In Scotland since 2000 accessibility requirements have resulted in dwellings being designed with external ground levels at a similar level to the internal floor level. This lends itself to slab construction with granular fill and perforated pipes to provide sub-slab ventilation.

The change also appears to be related to 'Modern Methods of Construction', which have been adopted more widely in Scotland than England.

To what extent is the issue addressed in existing standards and guidance e.g. BS8485?

The revision of BS 8485 in 2015 provided more detail about foundation design and particularly slab constructions and provision of ventilation.

9.3.4 Energy Efficiency

The question posed to consultees in relation to energy efficiency in modern buildings was:

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?

Three topics were covered in relation to this issue as shown in Table 9.8 Topics discussed with consultees.

Table 9‑8 Topics discussed with consultees (Question 7)

Topics discussed Summary of Responses

To what extent is the assumption correct that the drive towards air tightness and improved insulation contributing to the retention of mine gases within a property?

There was general consensus amongst the experts that this is correct although the best-informed opinions in this area indicated that there was a general problem with indoor air quality in Scotland and the UK as a result of insufficient ventilation. This problem would be exacerbated by entry of mine gas into a dwelling. However, it was noted by some that where there was less suction e.g. in an airtight house, there would be less gas drawn into the building through the slab and service entries. Several consultees mentioned that further research was required and that lessons could be drawn from Scandinavia.

To what extent is this considered in the standards and guidance e.g. BS8485?

BS8485:2015 doesn’t cover this issue specifically; however, if gas protection measures have been designed, installed and verified correctly in accordance with the standard, then gas shouldn’t be able to get into the building and the problem is avoided.

Are ambient levels of CO2 from household sources understood as a ‘baseline’ to which mine gas emissions may increase?

Consultees indicated that ambient levels of CO2 as a baseline appear to be poorly understood and that further research is required.

9.3.5 Mandatory Mitigation Measures

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?

There were a variety of views expressed by consultees ranging from those supportive of mandatory measures to those who considered that such a step would be counterproductive as it could lead to complacency amongst developers and their consultants. There was some discussion as to how ‘mandatory gas risk mitigation measures’ should be defined in the context of BS8485:2015 e.g. CS2 or CS3. However, most consultees agreed that regardless of whether there was mandatory gas mitigation or not, a robust risk assessment and appropriate design, construction and verification of gas protection measures must still be carried out.

A view held by several consultees was that if the risk assessment process was followed correctly, then mandatory mitigation would not be necessary. There was also concern about mandatory mitigation being overly precautionary and overly expensive in some sites but not robust enough for other sites.

Other concerns related to how mandatory measures would be specified and implemented and whether or not mandatory measures could lead to inconsistencies in the implementation of mitigation measures.

There was also some contradictory feedback around the cost impact of mandatory measures. A housing developer consulted on this issue remarked that the key aspect was the establishment of a consistent ‘level playing’ field so that the cost of development was essentially the same for all potential developers of a site. As noted earlier, one local authority in Scotland has trialed mandatory gas protection measures (CS2) in a former mining area, and Northumbrian County Council have adopted this measure as part of their planning conditions.

A decision as to whether or not compulsory mitigation measures may be required could be linked to environmental risk assessment as part of the development of Local Development Plans.

9.3.6 Retrofitting

Views on the effectiveness of retrofitting of mitigation measures were sought from Consultees and particularly as to whether or not this can be carried out effectively as an alternative to demolition where existing properties have been found to adversely affected by ingress of mine gas.

Most responses provided on this issue indicated that retrofitting is technically feasible and dependent on building construction details and the level of gas risk, i.e. more suitable for low-moderate rather than high risk gassing sites. There is wide experience of undertaking this.

It was commented that retrofitting can be expensive, although this may still be cheaper relative to the costs of demolition and rebuilding. It also entails substantial disruption to residents if a property is occupied, for example temporary relocation. The same levels of workmanship and verification should be applied as for new build properties, i.e. adherence to BS8485:2015 and CIRIA C735 requirements. Neither document covers retrofitting specifically and as mentioned in section 6.4, CIRIA are due to commence a research project imminently to prepare good practice guidance on retrofitting.

A concern expressed by some consultees about retrofitting was the on-going duty of care it can place on the developer.


Contact

Email: sarah.waugh@gov.scot