16 Dec 2015
December 16, 2015

Ground gas precautions

It has always been required that buildings are designed and constructed to provide a safe structure.  Many years ago, issues of ground gas hazards were left entirely to the judgement of the design team and, where considered necessary, were addressed using basic engineering principles (influenced by the mining industry and associated ground gas experience).   In 1986, an explosion demolished a house at Loscoe.  It was apparent that the explosion was due to landfill gas migration from a nearby domestic refuse site.  As a consequence, ground gas became a material planning issue and a specific item for building control approvals, alongside gas controls around landfills.

Over the years, ground gas assessment and the design/installation of precautionary measures to buildings have become progressively more detailed and also more prescriptive.  Unlike other elements of contaminated land assessment, there is a presumption that ground gas measurements on a limited scale is a “good idea” and also the use of over-designed (and not necessarily rational) solutions are widespread in the industry.  It is perceived that this approach provides cautious/conservative solutions.  However, if the investigation and designs/precautions are not prepared on the basis of a valid ground model, and using a rational approach, there is a significant risk of unforeseen consequences and/or inefficient and costly solutions.

At Crossfield Consulting, we have a long track record of successfully undertaking assessments of ground gas regimes and, where necessary, implementing appropriate and cost effective solutions.  In-house expertise is extensive, with the benefit of senior staff with over 25 years experience of ground gas assessment and authorship of key documents in this sector.  We also use the latest published guidance, including BS 8485:2015, BS 8576:2013, NHBC TE7:2012 etc, as it is important to justify any assessment/design both to third parties/stakeholders and to regulatory authorities.

It should be noted that this year’s update to BS 8485 effectively precludes the use of standard polyethylene membranes as a recognised gas protection measure.  By no means does this reflect a significant risk to completed developments that use this material.  The change in membrane requirements is largely part of the drive to improve installation controls and quality assurance/verification procedures.  Hence, for compliance with published requirements, it may now be necessary to use a propriety gas-resistant membrane (ie. tested and with designated gas resistance/transmissivity).  It should be remembered that gas precaution measures require to comply with a points scoring system given in BS 8485 and, providing the correct points score is achieved to meet the requirements for a site’s characteristic gas situation, it may not be necessary to use a membrane as one of the precaution measures.  The installation of ground gas protection details has suffered in the past from issues of poor workmanship and/or site control.  This is now addressed by requirements for independent inspection/verification, sometimes with in situ testing to demonstrate a “gas-tight” membrane has been successfully installed.

In view of the above requirements, the costs relating to the construction of ground gas precautions are increasing.  Hence, it is more important than ever to ensure that the investigation and assessment stages are carried out properly, to minimise any tendency for unnecessary over-design (which is common in this area).  However, it can be challenging to accommodate the required ground gas monitoring periods (sometimes extending to many months) within a site acquisition and development programme.  This is where skill and a thorough understanding of the ground conditions, ground gas regime and regulatory requirements are particularly important, so as to achieve an economical solution within a reasonable timescale.  This approach is recognised in all the projects undertaken at Crossfield Consulting with successful developments throughout the UK.