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

1) Sub-question/ discussion points

A) 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?

Geotechnical Specialist

Existing standards and guidance don't require a holistic approach, they just imply it. The problem is that it should be done but is not required. Risk assessment, design and planning, and construction all done separately.

Insurers

Process is adequate, but implementation is of variable quality. Variable quality of conceptual site model by consultants and developers. Monitoring variable quality as well. Too much guidance on risk assessments. Recommendation to clarify guidance in a short paper, training of local authorities and consultants, or big game change.

Geotechnical Specialist

Process is adequate, but in mine gas R.A. further lines of evidence need to be considered. Further guidance around application of conceptual models and SPR approach and an addendum on guidance on mine gas would be useful.

Developer

Much guidance published (which is clear about the conceptual model and having lines of evidence), but it is applied and interpreted of varying quality. From a developer’s perspective, need to check experience of consultants. Often monitoring done over too short of a period of time. Spot monitoring misses the atmospheric event. Mining areas should be flagged to make sure quality monitoring data collected there. The risk assessment should be done knowing what type of building is being constructed because building type will affect the risk.

Developer

Yes, but the expertise with which to use the guidance sometimes is not adequate. If CIRIA C665 & Wardell Armstrong guidance is followed, it is unlikely that an incident will occur. Deviation and lack of understanding leads to incidents.

Consultant 2

BS8576, BS8485, CIRIA C665 etc. are not specifically about mine gas, they’re about all ground gas. The process is adequate if you follow the guidance, but the guidance is not being applied. It's been difficult (especially in last 10-15 years) to get everybody to understand the importance of the conceptual site model. If you look at the conceptual site model, then the process is adequate because you are considering the whole potential for ground gas generation.

The guidance does lead one to consider the potential for gas to be generated and/or to move in ways specific to that particular site, but too often people will put in shallow wells, measure nothing, and report everything is all right without thinking about how gas might be generated or move. Assessment must consider what will happen if flow rates increase, if monitoring did not measure the maximum. Sensitivity analysis should always be carried out.

People should not rely solely on GSV’s. In CIRIA C665 there are examples of GSV being wrong. Steve Wilson and Geoff Card have stressed to not rely on GSV but consider the conceptual site model.

• Sometimes the local authority officer does not understand the guidance and applies it incorrectly.
• The guidance is there if one chooses to use it. In the original BS8485, in one of the appendices, there was a list of qualifying questions to be considered when deciding on protection measures including the complexity of the subfloor and the liability of the data. The flow chart in section 7.1 of CIRIA 665 captures the important questions to consider.

Recommendation:

There would be benefit in supplementary guidance being developed to look specifically at mine gas sources and pathways.

When outside of competence, consider third party review. 3rd party review has been very valuable to use on multiple occasions.

Consultant 1

No real guidance on the risk assessment process. The only true guidance is the NHBC document traffic light system. Having a conceptual side model is very important. In terms of the SPR approach, there is no adequate methodology on how to do the process. Assessments must be site specific. Baseline monitoring is not effective unless you have designed it specific to the site. Worst case scenarios must be considered (like the NHBC and CIRIA guidance say), a 1-in-50-year event atmosphere condition. More than one line of evidence must be used, and the source needs to be well understood. CIRIA 152 has quite a lot of discussion about the sources of gas, not exclusive to landfill, and has a conceptual model which includes mines.

Local Authority

Local authorities vary in approach to the risk assessment. Risk assessment can be challenging because guidance is lacking on mine gas.

1) Sub-questions/ discussion points

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

Geotechnical Specialist

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Insurers

Should consider chronic risks.

Geotechnical Specialist

Should measure both CO2 and methane in the risk assessment.

Developer

Chronic risks are considered for standard contaminated land issues but not for ground gas.

Consultant 1

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Consultant 2

The approach was developed initially for methane and expanded to include CO2. But the lower level chronic risks are not adequately considered. The subsequent guidance has tried to address that where it talks about VOC. People might need to consider third party reviews when outside of scope.

1) Sub-questions/ discussion points

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

Geotechnical Specialist

The guidance is very good, it's the implementation that gives us problems. Mine gas doesn’t feature strongly in much of the guidance. The guidance tends to focus on ground gas in relation to landfills. However, mines are unique and can create a massive volume of gas very quickly.

The way in which the industry calculates GSVs has never been proven to be correct. Site investigation and gas protection design are not absolute because calculating GSV is not absolute.

Insurers

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Geotechnical Specialist

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Developer

Further guidance about GSV values and how they are applied would be beneficial. The risk assessment must incorporate the design and construction techniques.

Consultant 1

Source must be understood.

Consultant 2

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1) Sub-questions/ discussion points

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

Geotechnical Specialist

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Insurers

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Geotechnical Specialist

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Developer

If you are building new houses, you must consider how new developments will affect old ones (typically not done but should be). Gas risk assessment for existing properties is very disruptive. A different approach will be required in the future to make it successful.

Consultant 1

Very little guidance for existing properties, just have to demonstrate that there is not a risk.

Consultant 2

Guidance is completely adequate for existing development as opposed to a new development. The approach is the same.

Question 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?

2) Sub-question/ discussion points

A) 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?

Geotechnical Specialist

Risk assessments don't consider the worst-case scenario. It should be considered earlier in the planning process. With regards to climate change, greater levels of groundwater, rate of change is relatively modest compared to instantaneous changes you get from low pressure weather events, but it’s all worth noting, and climate change is especially going to increase the risks.

Geotechnical Supplier

The guidance is appropriate, it just comes to accuracy of reports on the site. Often uncertainty is not addressed. Former coal mining sites must be considered differently because there are more uncertainties (groundwater, stabilization, etc). There are lots of uncertainties and you must put in proper mitigation to account for those uncertainties. If the risk cannot be rules out, then you need to be precautionary. It's probably quite common to not consider future uncertainties and just do spot monitoring in a shallow well and have a bad CSMl.

Insurer

The guidance is appropriate, it just comes to accuracy of reports on the site. Often uncertainty is not addressed. Former coal mining sites must be considered differently because there are more uncertainties (groundwater, stabilization, etc). There are lots of uncertainties and you must put in proper mitigation to account for those uncertainties. If the risk cannot be rules out, then you need to be precautionary. It's probably quite common to not consider future uncertainties and just do spot monitoring in a shallow well and have a bad site model.

Geotechnical Specialist

Grouting of shafts for geotechnical purposes should include a vent pipe.

Rising groundwater levels and consequences of climate change need to be considered, possibly by the coal authority. They are not considered at present.

Developer

Current Guidance does consider future changes within red line boundary, but outside of this is less robust. Potential climate change impacts or rising groundwater not really considered. The local authority should be the one considering implications of turning off the pumps of mines. Limited knowledge in how information the groundwater levels in a mine is acquired.

Developer

The ground gas risk assessment guidance does not ask you to look forward to the future. It asks you to consider the site at the time of assessment. The conditions that will be represented post-construction may be much different to those present during the risk assessment (vibrostone columns, etc). The current guidance doesn’t ask you to potentially consider the design life of the property or if changes to the property occur.

Recommendations:

• Guidance should take into account your risk assessment site being a preferential pathway for ground gas migration.
• Should take into account the design life of the property or if changes to the property occur.
• Foresight: if building council housing with design life of 60-100 years, the RA should consider, are there any sources at the site in the 60-100-year period that are going to change? With current guidance, RA considers a 10-year plan.

Consultant

In old mines where there has been regional pumping for decades where the water table has been depressed 100m plus and the pumping stops, then the water table comes up and at one mining site for example, the water table came up 60m in a few years. Implications for the ground gas regime must be considered.

Consultant

Someone assessing a site can only deal with what’s in front of them. It’s the responsibility of people creating new developments to assess that their construction won’t affect existing developments.

It is unreasonable to consider how your development might change in 50 or 100 years, it is too uncertain. People should just consider what is foreseeable. If there is specific knowledge such as evidence for rising groundwater levels, that should be taken into account as part of the assessment and conceptual site model. Foreseeable issues should be taken into account, but implications which are speculative should be left along.

The gas risk assessment originally should be conditional and based upon X, and if X changes, then the risk assessment must be re-done.

For the developer to appoint someone right through the process to keep an overview of these types of issues would be impractical and likely ignored. Half the time a site ends up getting developed out by somebody different than it started with, and all with different teams.

Building sciences consultant

Seal should last at least 25 years if not exposed to weather (but

may need checking periodically).

Expanding foam quality sealant for penetrations of slab to emplace services with life time guarantee, UV will destroy the foam.

Construction techniques may decrease quality.

Damp proof membrane will provide sealant for gas migration. Degree of sealing will vary with emplacement of membrane.

CO2 would flow out rather than up so increased levels of ventilation have no impact.

Local authority

A national strategy is required for mine water recharge.

2) Sub-question/ discussion points

B) What additional guidance could be provided on this?

Geotechnical Specialist

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Geotechnical Supplier

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Insurer

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Geotechnical Specialist

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Developer

Knew of recent paper on climate change and remediation that would be relevant.

Developer

See 2A

Consultant

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Consultant

See 2A

Building sciences consultant

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Local authority

There is a lack of guidance specific to mine gas.

2) Sub-question/ discussion points

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

Geotechnical Specialist

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Geotechnical Supplier

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Insurer

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Geotechnical Specialist

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Developer

There are gaps in current knowledge.

Developer

See 2A

Consultant

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Consultant

See 2A

Building sciences consultant

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Local authority

-

2) Sub-question/ discussion points

D) 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).

Geotechnical Specialist

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Geotechnical Supplier

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Insurer

If you have many developments over a coal mining area, there might be pathway over pathway. This is covered in the guidance. The whole site must be considered.

Geotechnical Specialist

Cumulative risk of multiple developments should be assessed using a precautionary approach

Developer

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Developer

See 2A

Consultant

Must look at cumulative effects of developments

Consultant

See 2A

Building sciences consultant

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Local authority

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Question 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?

3) Sub-question/ discussion points

A. 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 considered in the risk assessment process?

Geotechnical Specialist

Too often the structural design is done in isolation. Vibrostone columns are a good structural engineering solution, but massively increase the risk. All of the professionals involved in the construction of a site must understand the risk of ground gas and consider it in their design.

Insurer

Vibro-stone columns increase risk. This is definitely covered in ground gas risk assessment, even in CS1. Consultants need to be more aware. There is currently variable quality in awareness.

Geotechnical Supplier

In foundation design, mines should flag as higher risk. There is a lack of awareness and training. Consultants need to be more aware. Planning conditions would require a developer to assess foundation design to consider how it will be impacted by ground gas.

Developer

Do need to consider foundation design. Use of vibrostone columns might not be taken into account in ground gas assessment. There's a disconnect between ground gas and geotechnical/ structural assessments. More linkage is required

Developer

Problems occur when risk assessment done far before development—miss that maybe vibrostone columns might be added or that additional sites will be added which then elevates risk.

Recommendations:

• The conceptual model needs to consider the final design
• RA needs to be reviewed throughout the process to see if it’s still applicable
• Would be useful to have geo-environmental consultant and environmental consultant collaborating
• Responsibility lies with the developer to ensure site is suitable for use, not the local authority.

Consultant

In mining areas, there has to be a checklist or flowchart of things that have to be considered.

3) Sub-question/ discussion points

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

Geotechnical Specialist

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Insurer

See 3A

Geotechnical Supplier

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Developer

NHBC should cover this as part of the 10-year insurance cover.

Developer

See 3A

Consultant

See 3A

3) Sub-question/ discussion points

C. What additional guidance could be provided on this?

Geotechnical Specialist

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Insurer

See 3A

Geotechnical Supplier

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Developer

LA Guidance in Scotland varies (although supplementary planning guidance in Central Belt).

Developer

See 3A

Consultant

See 3A

Question 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?

4a) Sub-question/ discussion points

Is the risk assessment process proportional?

Geotechnical Specialist

SPR model not applied properly and not looked at early enough in the design process.

Geotechnical Supplier

Yes

Insurer

Where uncertainty in site, you must adopt a precautionary approach and adopt mitigation measures.

Geotechnical Specialist

Should adopt the same approach as in volatile organics.

Developer

A precautionary approach with conservative metrics is in the guidance. Scoring system is effective. Consultants adopt a tick box approach.

Developer

Interpretation of the guidance is open to developers. If guidance applied fully and appropriately by experienced individuals, there would not be problems.

Consultant

Yes, if they are followed. The broad guidelines in BS8485 for ground gas conditions do not apply in difficult situations. On routine sites, a simple and straightforward system is easy to follow, but nearly impossible at a complex site. A good risk assessment and a good site conceptual model are important. At high risk sites, a risk-based approach is needed which must be quantified.

Consultant

Up to interpretation. The point system of BS8485 was meant to help people reach a competent system for gas protection. The scope for interpretation is there and it is a good thing, it just needs to be improved properly (e.g. there are some words in there about being unnecessarily conservative).

Local Authority

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4a) Sub-question/ discussion points

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

Geotechnical Specialist

The approach should be precautionary. If guidance was used appropriately, it would be sufficiently precautionary. Ground gas management protection often seen as over precautionary by solutions providers. The regulations are good but should be managed and policed better.

A problem we've seen since we produced BS8485 is that if people are less confident in their risk assessment, then they resort to a more over engineered mitigation method. It all relies on the risk assessment process. And a risk assessment is site specific and dependent upon what is being built (residential or non-residential building).

Geotechnical Supplier

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Insurer

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Geotechnical Specialist

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Developer

Often more cost-effective to install mitigation measures. Membranes, if installed, require verification.

Developer

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Consultant

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Consultant

People often take a precautionary approach and install gas protection systems they don’t need. Then they take no care at installation and do not get any gas protection. When a site actually needs a gas protection measure, it should be installed and verified properly.

Local Authority

Two methods of precaution are required. The quality of mitigation can vary. There is a two-part planning development standard approach in CS2 and validation and verification in CIRIA.

Question 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)?

4b) Sub-question/ discussion points

Is the gas RA and need for mitigation revisited when a change is made to building construction or foundation design?

Geotechnical Specialist

There is a lack of holistic thinking in maintaining the source pathway receptor model and applying it to various types of construction.

Problems in risk assessments tend to be picked up at the last minute. If it is picked up at the last minute, then decisions have often already been made in the design process. Again, too often the design of the building structure is done in isolation. The industry should go back to how it was done in the 1980s, where engineers designed, they constructed, and the contract was built. It’s been a progressive problem of the design-build scenario of the work being isolated across different groups of professionals.

The highest cost of membrane solutions is the cost of sealing them around penetrations and awkward corners and changes in levels etc. A simple flat membrane that runs across a foundation and goes to the outside of the building, BRE414, is relatively inexpensive to install and can be done fairly well. At buildings with very complex designs, the installation of the membrane is very complex and almost impossible to achieve without highly experienced people.

Geotechnical Supplier

Different method of construction (standard block floors) in Scotland than in the rest of the UK. In England use a lot of concrete beam and block floors with a ventilated void below. In Scotland there are some floor slabs being constructed above granular fill. It is fairly common for membranes to be damaged through the installation and construction process. The industry should be looking at the construction process when designing gas mitigation measures.

With granular fill with 10mm plastic pipes, ventilation capacity on vented solum is much improved. With granular fill, angular material will block up all of the voids whereas rounded won't. Some of the pipes don’t go from one side of the building to the other which could create a negative pathway that it draws gas into the pipe itself. Sometimes we have seen the interweaving of the perforated pipes - so air has to pass through the granular fill to get to the other side of the building. We need to confirm what is the most efficient system for protection. In Scotland builders are still building with pipes and granular fill and expert not confident in its performance

Insurer

Sites often sold on using consultants reports with many caveats covering risk and their own liabilities.

Developer

Information can be lost over development lifecycle. A mandatory reports register would help here. Quite a lot of the time contamination assessment is done by one party, the geotechnical assessment is done by another party, and the foundation design is completed at the end. This is a potential problem and it is the developer’s job to understand the linkages.

Consultant

A risk assessment has to asses that the proposed mitigation methods will work. Any time there is a change, there should be a re-evaluation of the risk. The local authority could make their approval conditional, approved unless something changes.

Consultant

How you will be constructing your building should be an integral part of the risk assessment project. If how the building will be built changes, then the RA and mitigation measures must be reconsidered.

Recommendation:

When reports talk about the gas protection system, they should be in general principal terms, so they are understandable for the structural engineer and architect.

Coordination, discussion, and cooperation between consultants, structural engineers, and architects on gas protection systems, and the ultimate design of what the building looks like and how it will work.

Building sciences consultant

Slabs and timber frame provide greater thermal mass. Timber frame construction has been around a long time in Scotland. They heat and cool quickly. Solid slab floors stabilise heat flow.

This construction type has been driven by cost efficiency and regulation (e.g. disabled access). Timber frames provide more insulation: higher thermal standards Section 6 of Building Standards Regs (Part L in England & Wales).

Block and beam with raised timber, provided all are sealed, with a ventilated void will enable gas to be driven out of structure. A continuous membrane under slab should also do this, but penetrations for services may not be sealed so may be a problem in high risk areas.

There should be more education/training as there is a lack of understanding by guys on site. A problem in high risk area School at Cumbernauld provides examples of best practice: piles onto peat bog, gas membranes sealed and welded.

Manufacturers of membrane provide validation–very rigorous as they provide a 10-year guarantee.

Small housing developers might take more risks.

Developer

Our company uses 100% manufactured timber kit construction, driven by mobility access issues which requires external and internal ground levels to be similar. This lends itself to slab construction, partly for aesthetics e.g. otherwise ramps required.

Our company uses granular fill and perforated pipes, with 3 m centres below the slab is typical. The granular fill is typically 20mm gravel.

An active system of ground gas dispersal has been used but is not the ideal solution as there is a requirement for long term maintenance with associated liabilities.

Mobility access has been a big regulatory driver since 2000, albeit not using slab at ground level would avoid a number of flooding issues. The weight of the structure is not an issue, there is a marginal decrease in weight from using a timber frame which is not significant in terms of overall loadings.

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

5) Sub-question/ discussion points

A) Is the standards and guidance on mitigation measures prescriptive enough?

Geotechnical

It’s more the application. The regulations are adequate. BS8485 is a relatively slim document and was designed to provide guidance. A lot of engineering practices have varying levels of knowledge and experience. More commercially oriented groups frequently found gaps/loopholes in the guidance, rather than using it to assist them in the process. 51 was made to close some of the gaps, but it still leaves some room for interpretation.

Geotechnical

There is room for developers and designers to take a flexible approach to their design. I have seen on many occasions developers stretch the boundaries of the guidance to meet their own requirements. Not necessarily to the benefit of the best outcome for the project itself. Driven by commercial considerations.

Insurer

Guidance is at an adequate level. However, there's no recourse for developers not following rules (unless something goes extremely wrong). There's a problem with having the right information at the right time. Government does not provide information early enough in the process, which is of no consequence to the developers except that there may be a delay later down the line. Devising points of verification is important.

Geotechnical Specialist

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Developer

The mitigation measures are open to interpretation and are not very prescriptive. It might be a precautionary approach to put in gas protection measures, but then they are put in very badly. If a site is deemed as high risk, that should trigger there being an extra building control and the gas membranes being further verified. A lot of councils require robust risk assessments and request precautionary measures, but it is not clear what membranes should look like.

Developer

BS8485 is comprehensive, robust conservative assessment. A certain amount of points has to be achieved to balance the risk with the points system, and sometimes developers will limit protection for cost. Improvements in longevity and in the application of the mitigation measures could be made.

Consultant

The points-based system does not address specifics. It is a generic solution for a generic problem. GS3 doesn't take into account pathways.

Pilings are a risk and they are tested to see if they will work. Why can’t you apply the same principals to gas migration? Verification is hard to do. Mitigation should be sufficiently rigorous so that the added development of post-installation verification is unnecessary.

Developer

The risk assessment is carried out by experienced consultants, OUR COMPANY tend to use the same pool of consultants for this work over a long time scale. Attempting to influence the outcome of a risk assessment would entail significant financial risk. Modern management structures in for developers have technical directors at same level as land directors and therefore technical issues and risks are taken into consideration more in the land acquisition process.

Building Control will require verification of the membrane and the inspection is carried out by experienced consultants. This verification included a range from visual inspection to tests. OUR COMPANY requires photographic records and a certificate from the consultants. The certificate will be included in the habitation certificate for a dwelling. OUR COMPANY use third party review to get around quality issues.

Membranes are not popular with groundworks contractors as they add to the complexity of completing a project.

5) Sub-question/ discussion points

B) Are consultants/ developers interpreting the standards appropriately?

Geotechnical

The standards are broad, which is the point. It leaves room to interpret based on the specifics of a site. There are some problems due to a lack of understanding. Also, generalist engineers or consultants may not encounter ground gas issues often, so they lack experience. It's the policing and the maintenance of the standards that should be improved.

Geotechnical

In fully understanding the how the membrane is installed onsite, one needs to understand the difficulties the installers have to overcome. The local authority's capability to critically review design is based on their own personal experience. Those doing verification need the technical knowledge in building and construction methods.

Insurer

There is a problem with consultants with a contaminated land background not being appropriately experienced or qualified to assess mine gas risk

Geotechnical Specialist

There needs to be more awareness/ training.

Developer

The problem is that ground gas is often part of the contaminated land assessment. Developers tend to rely on consultants and consultancies are generalists that do heavy metals, water assessment and then ground gas.

Developer

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Consultant

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Developer

See 5A

5) Sub-question/ discussion points

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

Geotechnical

The person doing the ground gas assessment should have a concept of what's being built. Each expert has to have a knowledge of the other areas. CLR11 is less rigorously enforced, which is an excellent approach on contaminated land not just the proportionality of the risk, but also the flow chart. We need to invest more in the experience of the checker and the provider.

Geotechnical

Contaminated land professionals not necessarily trained in ground gas mitigation measure installation. Need more training on construction methods.

Insurer

There is a lack of experience amongst consultant and architects. They may not understand amber 1 and 2.

Geotechnical Specialist

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Developer

Implications of design not fully considered. In an environmental investigation, you don't go into much foundation discussion. We should be focusing on geomonitoring of foundations. Vibrostone columns might not be considered by consultants as leading to contamination.

Developer

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Consultant

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Developer

See 5A

5) Sub-question/ discussion points

D) To what extent are poor installation/ inadequate verification factors?

Geotechnical

Currently, gas membranes performance is in excess of the construction industry's ability to install them.

Geotechnical

Ventilated void is always the first level of protection.

Insurer

NVQ for installers has raised standards of installation but will take time to work through system to fully improve standards. Verification is very variable. Over the years it has generally improved. There is little push back on developers to improve practice.

Geotechnical Specialist

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Developer

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Developer

After installation, gas membranes should undergo integrity testing to ensure they are working.

Consultant

How do you do a gas risk assessment without understanding the receptor (the building)? There is a training gap. You can now get SoBRA qualifications for human health, and r ground gas risk assessment.

Developer

Work is carried out by ground works contractors who now use sub-contractors to install membranes, previously ground works contractors used to install membranes themselves. The use of specialist companies that use bespoke seals and membrane pieces around the foundation has resulted in improved practice. Two to three years ago quality was more mixed e.g. the use of tapes to provide sealing was not effective compared to the plastic weld which is used now.

Question 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?

6) Sub-question/ discussion points

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

Geotechnical Specialist

Liability of certain construction methods is up to interpretation. But, unequivocally the case that a fully ventilated solum is a very good mechanism for reducing the potential impact of ground gases to the building above. The best method of gas protection is up for debate. Ventilation is variable over time whereas a barrier is either there or it's not so is more reliable. Ventilation is dependent on things like wind speed.

Geotechnical Supplier

Beam and Block floor with ventilated void is the first line of defence. In England, most houses built on a beam and block floor. This design incorporates ventilation - 1500 mm sq open vented capacity per linear meter of wall. Any gas mitigation measures required for a site would need to be incorporated into that system.

Construction methods in the rest of the UK by default have a ventilated void. The industry should be looking to change the method of construction to incorporate a ventilated void which provides a very good level of defence against any gasses which may get into the building to start with.

Insurer

Ventilation is the first line of defence; unventilated voids are a risk. In England 90% houses modernized. In Scotland it is very rare that you have ventilators because it is more cost effective to put in a ventilated void rather than a slab.

Geotechnical Specialist

Sub floor ventilation important, granular fill and perforated pipe can reduce sub floor ventilation.

Developer

Most dwellings in England use a ventilated void, and major housebuilders follow standard designs where possible, e.g. Redrow has 3 types of houses. Timber framed construction used in Scotland. The use of a wooden frame facilitates use of concrete slab. 'Modern Methods of Construction' have been adopted more widely in Scotland than England.

Developer

Ventilated solums have been constructed in mining areas for hundreds of years. Yet it is rare to get mine gas within constructed property in the UK. Houses from the 1920s, 30s, 40s are not impacted by ground gas because they had a ventilated sub floor, air control and were not as tightly sealed.

Consultant

Ventilated solum reliability depends on floor type. If there is a ventilated sub floor with a structural slab that’s not cracking, and is sealed properly on the outside, then a ventilated solum wouldn't change things. Basements also offer a lot of protection.

The ventilated sub-floor void is a first line of protection, and by taking that away it places more emphasis on adequate construction maintenance of gas protection measures, including a membrane, service entries, etc.

It all comes back to having a good conceptual site model.

It's adequately covered in the guidance and standards. It’s all related to having a gas protection system, as opposed to one gas protection measure and building in an appropriate level of redundancy.

Sometimes the RA will be done, then the design progresses, and the foundation method might change or ground improvements that will be used, by a third party. And then that gas RA isn’t re-assessed.

There is definitely a gap between people at the investigation stage and the development stage (the developer, structural engineers and architects). The more information that moves between organisations, the higher the potential for recommendation to get lost.

Building sciences consultant

Yes, a solum reduces risk. Risk will depend on construction technique. Gas migration through slabs will be slow/ non-existent so migration not through slab itself but from around the perimeter and from services entries.

6) Sub-question/ discussion points

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

Geotechnical Specialist

The revision of BS8485 in 2015 put far more detail about foundation design and risks with different slab constructions than the 2007 document. The 2007 document didn’t discuss any of that, it advised to find structural engineering knowledge elsewhere. Changes in building standards in the last 5-10 years to provide more disabled access has placed more reliance on gas mitigation design and moved away from ventilated solum. There needs to be more research on resistivity of concrete slabs to the passage of gas. The guidance doesn't tell you about that.

Geotechnical Supplier

Guidance from CIRIA etc. has certainly made a difference because there is a lot more verification and validation being done on site. But the person doing the validation must have the knowledge and understanding of ground gas issues. Everyone needs to know what they are doing and why to get a solution that is fit for purpose. Ackroyd going through Assessor Qualification process just now. That qualification has just been released by the British verification council. Verifiers should be standardised and validation without a qualification should not be accepted.

Insurer

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Geotechnical Specialist

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Developer

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Developer

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Consultant

See comments above

Building sciences consultant

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Question 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?

7) Sub-question/ discussion points

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?

Geotechnical Specialist

It’s overwhelming. If the gas kept out, it’s a positive. If gas gets in and can’t get out, it’s a negative. The most research that has been done about this has been about radon because radon is a gas that only causes a risk in increasing concentrations over extended periods of time. In research by Melansis back in 2014 medical professionals said that the improvements we are making for thermal energy efficiency is potentially creating an increased number of deaths per year for radon by about 15-20%.

Consultant

No evidence of whether it is or not, there may be an NHBC report on air quality in housing to be commissioned

Insurer

Biggest driver for gas entry to a house is the heat exiting the property. If increased insulation and little temperature variation in a house, that should reduce the suction effect.

Consultant

Yes, if you have a draughty building, the gas getting into it is going to escape out. If you have a building that is airtight, any gas getting into it is going to escape.

You’ve got to make sure your gas protection system works. If it works, then gas can’t get into the building.

Building sciences consultant

Measurement of CO2 increasing indoors due to lack of ventilation in the whole house. Houses in Scandinavia typically have whole house heat recovery system which provides high air quality.

• Air quality is a big issue in residential properties because of more efficient energy use and sealing of windows. (Air tightness on dwelling, Trickle vents – 3 m3/hr per m2 at 50 pascals, Assumes at 3m3/h/m2)
• Numbers for air quality are distributed throughout the whole house equally, but living rooms and bedrooms are especially under-ventilated
• There was a study in Finland on the impact on human health that is still being evaluated. Further research to consider impact of ventilation on human health would be useful.

7) Sub-question/ discussion points

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

Geotechnical Specialist

The guidance has been written by people who are experts on the topic and didn't foresee people interpreting the guidance differently or actively seeking loopholes.

Consultant

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Insurer

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Consultant

See comments above

Building sciences consultant

See comments above

7) Sub-question/ discussion points

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

Geotechnical Specialist

Gas ovens and heaters create background levels of CO2 is a problem. Also creating sealed homes and then creating slots for service entries for utilities is a source of risk. Sources should come in through the side.

Consultant

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Insurer

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Consultant

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Building sciences consultant

See comments above

Question 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?

8) Sub-question/ discussion points

If you agree with this proposed approach, how should “mandatory gas risk mitigation measures” be interpreted in accordance with BS845? If you disagree with the approach, please explain why.

Geotechnical Specialist

No. It’s very tempting to take a blanket approach to a problem. The problem is that every single site will have a different level of risk according to geology, gas potential, etc. If you standardise the risk, you standardise the solution. Every single site that is defined by the CA as a former coal field, or that is sitting near a landfill site or has any other reasonable cause for ground gas contamination should have an appropriate risk assessment.

CIRIA C659 and the NHBC traffic light system to me is saying, if it’s a really risky site, you install gas mitigation measures properly, and if it’s a less risky site, you don’t do it properly.

Geotechnical Supplier

Ideally, yes. However, the construction industry would not be very happy with the cost implications of doing that. Problems will be stopped with proper risk assessment and mitigation design. Blanket requirements of gas mitigation for every site is unnecessary, only some will need it. It should come down to having a robust gas risk assessment which should consider constructability. Design practices and risk assessments can be robust, but construction methodology is not taken seriously enough.

Insurer

It needs to be clarified further. Ventilated void is preferred over a membrane in coal mining areas. Mandatory gas protection may lead to complacency in construction techniques. You have to make sure that the membrane is working and that it has been verified.

Geotechnical Specialist

Can be over precautionary; however, sometimes mitigation is cheaper than understanding the source.

Developer

What would a mandatory gas protection measure look like? May not be straightforward to implement.

Developer

The current Coal Authority guidance on construction of dwellings on former coalfields is that gas mitigation measures should be used or at least strongly considered, that was in about 2016. A Wardell Armstrong report mentioned that when doing an investigation of a site in a former mining area, pathways may become opened to deeper coal and mines and increase risk. Placing blanket ground gas mitigation measures where they are not required is a problem. There needs to be consistency in application. A blanket mitigation measure shouldn’t remove the need for a proper risk assessment and geotechnical investigation.

Compliance from consultants might diminish if robust mitigation was required where it was not needed. Developers will not want to spend money mitigation.

Consultant

No, mandatory measures are generic and don’t necessarily resolve the problem. They are overly precautionary for some sites, and insufficiently precautionary at the high-risk sites. They will also will cost a lot of money.

Consultant

Mitigation measures must be site specific, considering the conceptual site model, the gas regime, and the RA, which all determine the necessary ground gas mitigation measures, which must then be installed properly.

Building sciences consultant

Yes, mandatory gas mitigation is the most sensible solution moving forward.

8) Sub-question/ discussion points

How should such an approach be applied e.g. limited to CA defined high risk development areas?

Geotechnical Specialist

See comments above

Geotechnical Supplier

See comments above

Insurer

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Geotechnical Specialist

Could require ventilated sub-floor voids in in areas underlain by old coal workings.

Developer

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Developer

See comments above

Consultant

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Consultant

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Building sciences consultant

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Question 9: Can retrofitting be carried out effectively for existing properties affected by mine gas as an alternative to demolition?

Geotechnical Specialist

Retrofitting is very difficult and very expensive. The major pathway will be cracks in the screed. You can retroactively put a membrane in, seal up the walls, and put a screed down. Another potential pathway is up through the cavities. You can try cavity filling using close cell foam, or you can jack the building up and try to put a continuous DPC across the whole thing which is massively expensive. Another way of doing it is to actually use positive pressurization, ground negative, ventilation system. There’s a real reticence towards using managed activated systems on private residential buildings because the householder has to keep them going.

Geotechnical supplier

Retrofitting is doable and definitely has advantages over demolition.

Techniques are to either put membrane on top of slab or remove the slab and incorporate the new membrane under the new slab.

Insurer

Retrofitting can be done successfully provided there is a good design, and experienced personnel carry it out. Retrofits also need verification.

Geotechnical Specialist

Not difficult to do but leaves a duty of care and difficult to maintain in long term. E.g. retrofitting at Western Quarried Runcorn where elevated mine gas present.

Developer

Retrofitting is feasible e.g. Northwich where retrofitting was cheaper than demolition and rebuilding. However, retrofitting can get very disruptive.?

Developer

Geoff Card, who has offered lots of guidance, has the approach is that in a high-risk area, retrofitting cannot be relied on. Traditional retrofitting usually involves adding a membrane and a floating floor, and the membrane would be taped and lapped behind the plaster boards and ventilation. Installing retrofitting can be really intrusive ad require removing stairwells and bathrooms.

Consultant

Yes, retrofitting can work.

Consultant

Retrofitting can work – example where we retrofitted gas protection into a housing estate and the risks were mitigated.

Retrofitting can work but is disrupting to the residents. When you do anything to an existing property, it requires lots of communication with residents.

The guidance says and it’s the practice, that your gas protection system is designed so that it is effective and lasts for the lifetime of the development and does not therefore require ongoing monitoring. How would you do ongoing monitoring inside someone's home?

Building sciences consultant

Sealing of penetrations into a void can be carried out, and top hats used for pipes can be fully sealed. No reason why retrofitting shouldn't work, except for variability of workmanship in the house building sector.

Retrofitting can be done easily. Pressurize the house to find leakage pathways and use tracers to get pathways out of building. It is worth attempting to seal a house and assess impact.

Developer

Retrofitting is expensive, and gaining access to private properties is particularly problematic e.g. the cost of retrofit might be £10k, but the overall cost might be much greater because of ‘distress’ to occupants etc. As an example of the cost benefit assessment, you mentioned the Penicuik development of 450 units where it was considered quicker and less of a risk to put in place membranes rather than risk a retrospective fix. Installing a gas membrane costs £1500 to £1800 per unit, but the cost of retrofit would dwarf this.