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Contaminated land: New guidance offers pragmatic approach

Revisions to standards for designing protective measures for ground gases has resulted in a practical approach to ground gas investigations that could save both time and money.

Many developers are faced with the challenge of dealing with ground gases which can be hazardous to human health and in extreme cases pose an explosive hazard. Recent updates to the code of practice used on sites to manage the risks from methane and carbon dioxides mean that a more practical approach is now possible.

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New guidance

Jon Archer believes that the new Code of practice for the design of protective measures for methane and carbon dioxide ground gases will save clients time and money

The major revision to BS8485, Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings was the first update since its launch in 2007. The update has delivered significant improvements by integrating the investigation and risk assessment of ground gas with design of protection measures and their verification.

The revision also has a checklist for contaminated land practitioners, building designers, regulators and those commissioning investigations, to help them ensure the investigation and assessment approach is appropriate for the site conditions and development.

Additionally, BS8485 now gives general guidelines for conceptual site modelling, helping practitioners to decide whether to use an empirical, semi-quantitative or detailed quantitative risk assessment approach to assess the potential impact of ground gases on new buildings.

Empirical approach

A particularly useful addition to the revised standard is an empirical method that can be used to assess the ground gas regime on sites where risk is perceived to be low. These sites typically have limited sources of ground gas, with less than 3m of made ground.

The empirical method relies upon the analysis of the organic content of made ground to demonstrate whether or not monitoring is required. If total organic carbon (TOC) is less than 1%, then monitoring is not needed, as this limited organic content is unlikely to generate ground gases.

Clearly, this approach is much more cost-effective – and faster – than installing monitoring wells, with the added benefit that, as TOC is ordinarily measured as part of ground contamination investigations, data will be readily available. It also provides more certainty than relying on engineering judgement, as was the case in the past.

Where TOC is greater than 1%, BS8485 advises gas monitoring using the standard accepted practice of a semi-quantitative approach to screening ground gases. Gas concentrations and flow rates are used to calculate a gas screening value (GSV) and a characteristic gas situation (CS).

The higher the GSV, the higher the CS, so CS1 represents a very low risk and requires no protection; CS6 represents a very high risk and development may not be appropriate or would require the detailed design of extremely robust protection measures.

“Early application of the new code suggests that the integrated approach could reduce investigation costs by 15%.”

Jon Archer

The CS determines the minimum score that must be achieved by a combination of gas protection measures, such as structural barriers, ventilation and membranes, incorporated into the design of new buildings. BS8485 provides scores for standard protection measures, in different building types: for example, CS2 requires a minimum score of 3.5 for private dwellings.

Points are gained by installing protection measures, for example a reinforced cast insitu suspended floor slab with minimal penetrations (up to 1.5 points) and an appropriate gas resistant membrane (up to 2 points).

BS8485 also gives guidance for detailed quantitative risk assessment, which is more appropriate when semi-quantitative assessment results in measures that may be overly-conservative; on sites with moderate or high ground gas risk; or where building structures and foundations are complex.

Zoning

Another major change is the ability to zone ground gas risk when monitoring is needed. So, rather than assuming a worst case scenario, where the maximum concentrations and flow rates identified during monitoring exist across the entire site, protection measures can be designed for buildings based on the level of risk for individual areas. This should result in less conservative, more cost-effective and more appropriate levels of protection measures being specified.

For example, where a specific source is identified on site, such as a backfilled pond, monitoring in and around the feature can be used to zone protection measures. Buildings deemed to be at risk can be suitably protected and those further away can be built safely, without the need for gas protection.

Assessing ground gas risk as part of an overall site investigation is important for new developments, particularly those on brownfield sites, but the scale and detail of the investigation are often open to interpretation.

Early application of the new code suggests that the integrated approach could reduce investigation costs by 15%; specifically when ground gas monitoring identifies a slightly elevated CO2 concentration of more than 5%.

It has long been the accepted approach to consider raising the level of risk in these situations to apply a characteristic situation of CS2, therefore increasing the gas protection measures on site. However, classifying the site based on soil organic content; removing the need for monitoring; applying gas risk zoning or using modelling can justify a characteristic situation of CS1, so protection measures are not needed, reducing costs.

Overall, the comprehensive revision of BS8485 reinforces the importance of a thorough understanding of both ground conditions and site history in assessing the risk from ground gas. As such, it provides a much clearer and integrated approach to assessing ground gas risk and the protection measures required for new buildings, giving an approach that practitioners can rely upon and could save clients time and money on new developments.

Jon Archer is principal engineer with Harrison Group Environmental

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