We might be blowing our own trumpet, but Ground & Water prides itself on the quality of its Ground-gas Risk Assessments and we regularly save our clients money…

In this case study it was around £70,000! Forensic investigation obviates gas measures

If you take nothing else away from this guide, please remember this.

CREATING A CONCEPTUAL SITE MODEL (CSM) IS THE VITAL ELEMENT OF A GROUND-GAS RISK ASSESSMENT AND THE DESK STUDY MUST ASSESS BOTH PATHWAY AND RECEPTOR.

Too often we come across lazy risk assessments conducted by others.

“Yes there is a potential source, therefore we must test for it” Errr… NO!

Examples to justify the above we have seen are:

1. “A pond was present 50 metres away which was infilled in 1920s. Lateral gas migration could occur.” This is not likely to happen.
2. “Earthworks for the roads in the adjacent housing site could cause lateral gas migration.” This is not likely to happen.
3. “Gas can migrate from Alluvium to the adjacent site.” This is totally incorrect.
4. “Driven piles will increase the Characteristic Situation.” This incorrect for most sites.

Ground & Water uses the Multiple Lines of Evidence Approach for Ground-gas.

As explained in CIRIA C795:

• Each line of evidence is independently assessed on its own merits
• The degree of agreement and disagreement between relevant lines of evidence is established
• If possible, differences in outcome are explained
• The lines of evidence are weighted – quantitatively or qualitatively
• A conclusion is drawn from all the relevant lines of evidence and a level of confidence or certainty is assigned.

As part of this it is important to get the number of data points and period of monitoring right:

A further consideration in this is getting the well installation correct:

• The wrong installation could mean voiding all results
• The analysis of gassing regimes from flooded wells is a common error.

Its never quite as simple as 1.0m of plain and the rest slotted.

Our Do’s and Don’ts

The top five errors still being seen in Ground-gas Risk Assessments:

1. Calculated GSV from maximum concentration and flow rate regardless of the CSM and response zone.
2. No data quality assessment – I.e. Using high flow rates and concentrations from flooded wells.
3. Increasing from CS1 – CS2 on the basis of elevated levels of CH4 and CO2 when there is no credible source.
4. Not taking into account foundation construction and effect on gas risk.
5. Response zones that span strata.

Developers: Watch out for the above. Ground & Water has moved away from a pre-occupation with gas concentrations towards a more risk-based approach, based on likely flows from the ground and a thorough understanding of the ground model – Source, Pathway and Receptor.

This approach will save you time and money, by cutting out assumptions, unnecessary testing, poor data quality and potentially expensive gas protection measures you do not need.