20Nov / 2019
The last twelve months has again been busy for the ground gas sector with new and revised guidance, research reports and equipment developments. The question is has all of this made ground gas risk assessment easier?
The year started with the publication of CL:AIRE TB 18 ‘Continuous Ground gas Monitoring and the lines of Evidence Approach to Risk Assessment’. This was written using plain language to explain the ground gas risk in terms of the source-pathway-receptor pollutant linkage and the lessons learnt from 12 years of continuous ground gas monitoring in an attempt to demystify the subject.
We also saw the publication of a revision to British Standard BS 8485:2015+A1:2019 which contains only a few minor additions and amendments. These were associated with the points scores for higher gas regime sites, clarification of structural barriers in respect of grades of waterproofing, and thickness requirements and tests associated with gas resistant membranes. This document remains the cornerstone of gas protection design for new buildings.
In September the Scottish Government’s commissioned research into the prevalence of CO2 from disused mineral mines was published. This followed on from the Gorebridge incident court cases where a number of residents of a local authority estate were affected by mines gas, resulting in the demolition of sixty four properties. While focussing on the implications for residential buildings in Scotland, I believe that this incident and research report identify a number of important lessons that apply to the whole of the UK.
Most recently we have had the two papers published in Ground Engineering by Geoff Card et al to mark the 20th anniversary of the 1999 Wilson and Card paper that stressed the importance of both borehole gas flow rates and concentrations to the assessment of ground gas risk.
Finally, we have had the continued development of the next generation of continuous monitoring devices, such as the GGS Gas Sentinel®, which includes both continuous flow and continuous concentration monitoring. As a result, better quality monitoring data and gas regime interpretation is now widely available at a reasonable cost for most gas contaminated sites.
I draw a number of observations from the above. The first is that while ground gas contamination is largely a geological problem – the solutions are engineered and need to be designed. Therefore, there needs to be clear communication and understanding between two, very different disciplines.
An understanding of geological principles and the variability of geological units (including made ground) is fundamental to the development of a Conceptual Site Model. The level of uncertainty is inversely proportional to the amount and quality of site investigation data that is obtained.
Engineering design involves the choice and deployment of man-made materials with known properties and performance to provide a quantifiable result. These materials are often standardised with a high degree of uniformity. However, the geological uncertainty must be reflected in the factors of safety that are built into the designed solution.
To my mind, high quality site data, including continuous ground gas monitoring data, will always be preferable to modelled risk assessments using assumed values for the ground gas and geological properties that were inadequately measured.
My second observation is that not enough thought is given to how a site may be modified. Are land levels going to be changed? Will ground improvement works be carried out? Will there be deep structures that intercept gas bearing geological units? Are there abandoned mine workings that will be drilled and grouted? Any or all of these have the potential to profoundly change the gas regime at a site. As in Gorebridge, if the implications of such engineering works are not considered and understood, serious ground gas hazards may remain.
In conclusion, has ground gas risk assessment got easier? I’d answer with a definite yes. With higher quality data and a sounder understanding of ground gas behaviour, I believe we are better able to reduce uncertainty and focus attention on those sites that need gas protection. Conversely, we can also identify those sites that don’t need special measures.
GGS’ webinars will review some of the latest ground gas guidance and the Scottish Government’s research in order to explore the implications for the sector in more detail.