6 Standards and Guidance
6.1 Review of Current Standards and Guidance
There are a series of available standards and guidance relating to ground gas assessment and mitigation that have been developed since the late 1980s. The term ‘ground gas’ includes CO2, one of a number of gases occurring and generated within the ground in both made ground or natural deposits, including former mine-workings (BSi, 2015).
A number of high profile incidents occurred in the 1980s, predominantly relating to landfill gas, which caused the displacement of people, severe damage to structures and loss of life. This raised awareness of hazards relating to ground gas and triggered research and the development of best practice approaches, which have fed into development of ground gas guidance over the last 30 years. Much of the early guidance (led by BRE and CIRIA) was driven by the need to understand risks from landfill gas, but this later expanded to include all sources including mine gas. Methane has often been seen as the primary risk driver in relation to mine gas, although CO2 has frequently been considered.
There are now a considerable number of guidance documents available as detailed in Annex 2. These have more recently been consolidated into two British Standards:
- British Standard BS 8485: Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings, first published in 2007 and updated in 2015
- British Standard BS 8576 Guidance on investigations of ground gas, 2013.
BS 8485:2015 covers site investigation and risk assessment methods for ground gas as well as the design of appropriate protection measures. It states that ‘it is anticipated that specialist advice is needed in the assessment of the ground gas data and in the risk assessment phase.’ It also states that ‘the framework is not prescriptive and professional judgement may be made as to the acceptability of risk and whether there might be benefit in undertaking more rigorous site assessment or adopting conservative measures in design’.
BS 8485:2015 states that it is recognised ‘that there are a number of factors requiring consideration which affect the sensitivity of a development to the effects of ground gas and that there are a range of design solutions available for different situations. Emphasis is placed on the justification and recording of risk assessments and design decisions throughout the process. As a code of practice, this British Standard takes the form of guidance and recommendations and it should not be quoted as a specification’.
BS 8576:2013 provides guidance on the monitoring and sampling of ground gases and is intended to be used in conjunction with BS 10175:2011+A2:2017 ‘Investigation of potentially contaminated sites – Code of practice’. It covers volatile organic compounds (VOCs) and permanent gases, including carbon dioxide, methane and oxygen. The focus of the document is on development sites and the risk posed by gassing sites to neighbouring land and developments, although it is also stated to be relevant to Part IIA.
Other key guidance documents that provide useful reference sources for many aspects of ground gas assessment and mitigation include:
- CIRIA, Assessing risks posed by hazardous ground gases to buildings, C665, 2007
- NHBC, Guidance on evaluation of development proposals on sites where methane and carbon dioxide are present, 2007
- Chartered Institute of Environmental Health, The local authority guide to ground gas, 2008.
Further detailed information is provided in Annex 2 - Table of Standards and Guidance.
6.2 Ground Gas Risk Assessment
All the documents referred to above are based on the framework provided in Model procedures for the management of land contamination, CLR11 (EA, 2004). This provides the technical framework for structured decision making about land contamination. The overall approach presented in CLR11 in dealing with past land contamination is one of risk management, including ‘all the processes involved in identifying, assessing and judging risks, taking actions to mitigate or anticipate them, and monitoring and reviewing progress’. Risk assessment for land contamination, including ground gas, in CLR11 is based on the contaminant source-pathway receptor approach. A pollutant linkage is present and hence a risk is realised when all three elements are in place. The conceptual site model represents the characteristics of the site in diagrammatic or written form that shows the possible relationships between contaminants, pathways and receptors.
BS 8485:2015 states that ‘before protective measures for buildings can be designed, an appropriate risk assessment should be carried out to decide whether there is a potentially hazardous situation and what the magnitudes of associated risks are.’ Two approaches are presented:
- An empirical, semi-quantitative approach
- A detailed quantitative assessment approach.
The former, which is most commonly used, involves the use of monitoring data collected from gas monitoring standpipes installed in the ground. This approach leads to derivation of an appropriate gas screening value (GSV) and is also referred to as the Modified Wilson and Card approach. The GSV is defined as the ‘flow rate of a specific hazardous gas representative of a site or zone, derived from assessment of borehole concentration and flow rate measurements and taking account of all other influencing factors, in accordance with a conceptual site model’. The GSV is then used to select an appropriate Characteristic Situation (CS) for selection and design of gas protective measures. Characteristic situations range from CS1 to CS6, with the higher the CS level, the higher the hazard potential.
It is important to note that the GSV thresholds are guideline values and not absolute. The GSV thresholds may be exceeded in certain circumstances, if the site conceptual model indicates it is safe to do so. Similarly, consideration of additional factors such as very high gas concentrations, should lead to consideration of the need to adopt a higher risk classification than the GSV threshold indicates.
Protection measures within new buildings can be prescribed using a point scoring system. A key feature of the GSV approach is that it should include consideration of worst-case conditions, such as a rapid fall in atmospheric pressure.
BS 8485:2015 also states that ‘a detailed quantitative assessment of gas emissions should be carried out in appropriate situations, such as where sites have moderate to high hazards, where buildings have complex foundations, and where the [CS] approach described suggests an over-conservative assessment of risk posed by the presence of gas in the ground’.
An alternative and commonly used method of ground gas risk assessment, referred to as the ‘Traffic Light System’, is contained within the NHBC guidance. This is designed to be applicable to low rise residential development utilising beam and block floor constructions with clear void ventilation.
BS 8576:2013 focuses on appropriate methods for data collection in the site investigation phase rather than risk assessment. It does however state that “in order to complete an assessment of the risks posed by the presence of permanent and other ground gases, it is necessary to understand the potential sources of gas in and around a site. It is important to collect information on the other aspects of the site, including for example the history of the site, the natural and man-made geology of the site and surrounding areas, the hydrogeological regime, and the uses of the site and surrounding land. This information is used to develop a conceptual model of the site and surrounding area. Development of the conceptual model requires an understanding of both the short-term (e.g. explosion or asphyxiation) and long-term hazards posed by the permanent gases and VOCs that might be present.”
Other recent changes in ground gas assessment relate to the availability and increased use of ‘continuous’ ground gas monitoring equipment as an alternative or supplementary to ‘spot’ monitoring.
6.3 Ground Gas Mitigation and Verification
BS 8485:2015, and other guidance referred to above and in Annex 2, detail the approaches to design of gas protection measures. BS 8485:2015 states that it ‘is intended to be used by designers of gas protection measures and by regulators involved in the assessment of design solutions’. It presents a points-based system for the selection of appropriate gas protection measures based on the CS and building type spanning the range of private residential to large commercial/industrial. This approach is intended to be used ‘unless the protective measures are designed using quantitative modelling methods’.
When the minimum gas protection score has been determined for the building as a whole, or for each part of the building, then a combination of two or more of the following three types of protection measures should be used to achieve that score:
- The structural barrier of the floor slab, or of the basement slab and walls if a basement is present
- Ventilation measures
- Gas resistant membrane.
Once the types of protection measures have been decided, the detailed design and specification of the measures should be undertaken and recorded in a design report.
It should be noted that BS 8485:2015 concerns the construction of new buildings only; the retrospective design of protection measures for completed buildings and the design of retrospective protection measures after completion of building construction are not covered.
Following increasing recognition that verification of gas protection measures was an area requiring improvement due to poor construction industry practice, CIRIA produced supplementary guidance C735, ‘Good practice on the testing and verification of protection systems for buildings against hazardous ground gases’ in 2014.
BS 8485 was then updated in 2015 and comments that ‘the installation and subsequent protection of the membrane are key factors in its performance’. A poorly installed membrane cannot perform, however well detailed and irrespective of the performance of the material. Historically, reference has been made to verification and integrity testing without having any referenced documents against which to judge. The verification process is now described in CIRIA C735 and as such, confidence in the installed solution can be measured. The process removes the uncertainty of unqualified or inexperienced installation operatives by requiring a verification plan to be drawn up prior to the installation, with frequency and type of verification being dependent upon the qualifications of the installation operatives, site risk and design criteria. Verification in accordance with C735 therefore now forms part of the points-based system in BS 8485:2015.
A Level 2 non-vocational qualification (NVQ) is available for installers of gas membranes. A proposed NVQ (Level 4) for verifiers of ground gas protection measures led by the British Verification Council is currently going through the approval process, with the Property Care Association (PCA) as the administrating body.
6.4 Research and Emerging Guidance
The outcome of consultation with organisations involved in the production or publication of relevant standards and guidance is detailed below Table 6.1 Guidance Body Consultation findings .
Table 6‑1 Guidance Body Consultation findings
CIRA remain active in this area. About to commission project to develop guidance on retro-fitting of gas protection measures.
CL:AIRE is active in this area with recent publications and ones in preparation. Related publications planned for 2019 include continuous ground gas monitoring and the lines of evidence approach to risk assessment, and a case study publication on the Gorebridge case.
BSI committee EH4 reviewed whether BS8576:2013 required revision earlier this year and decided that it was not necessary at present. The next review will be considered in about 2023. BS 8485:2015+A1:2019 issued in January 2019.
BRE are not currently active in the area of ground gas. They have produced guidance on radon but have no major research currently underway in this area.
NHBC has produced technical guidance previously and is currently planning to review and update their existing ground gas publication in 2019/2020.
The CIEH Local Authority Guide to Ground Gas is planned to be updated with interim issue of Ground Gas Information Sheets by EPG Ltd.