Introduction

The assessment of ground gas risk can be made using the approach established in Contaminated Land: Applications in Real Environments (CL:AIRE) Research Bulletin 17 (RB17 A pragmatic approach to ground gas risk assessment).  This alternative methodology involves a forensic description of made ground and determination of total organic carbon to assess the amount of degradable material present.  On many low risk gassing sites it will allow gas well installation and monitoring to be avoided, for example where only natural soils are present or similarly made ground with low gas generation risk.  In such cases, RB17 might provide a more reliable and rapid indicator than current approaches that rely on periods of gas monitoring.

The framework is based on a comprehensive desk study, robust conceptual site model (CSM), adequate site investigation and appropriate laboratory testing of soils.  Further information on the application of RB17 is given in BS 8485 (limiting values of thickness and organic content of made ground are specified in Table D.1).  The methodology can also be used alongside gas monitoring as a separate line of evidence in risk assessment.  This could allow the period of monitoring to be reduced from that specified in CIRIA C665 or help to avoid extended monitoring where anomalous results are obtained that are not consistent with the CSM.

Analytical Methodology

The main constituents are divided into seven separate fractions.  The fractions are weighed to determine the proportion of each in the sample.  The total organic carbon content of the fine fraction is then analysed.  The fractions include:

• Coarse inert particles including gravel, concrete, brick >10mm
• Visible wood, branches, tree etc.
• Visible vegetation grass, food waste etc.
• Visible cloth, leather
• Visible metal, glass, ceramics and other inert material
• Other degradable material
• Fine soil including gravel <10mm (original sample weight minus sum of the above)

References

1. BS 8485:2015+A1:2019 Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings
2. Contaminated Land: Application in Real Environments: A Pragmatic Approach to Ground Gas Risk Assessment. RB17. CL:AIRE, 2012
3. Construction Industry Research and Information Association. Wilson S., Oliver S., Mallett H., and Card G. Assessing risks posed by hazardous ground gases to buildings (C665) London: CIRIA 2007

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