Characteristic situations in a ground gas assessment
The gas screening value decides the characteristic situation, and the characteristic situation decides the protection. Here is what has to sit behind that number for a reviewer to accept it.
A characteristic situation is the classification that decides what gas protection a building needs, and it is derived from a single number: the gas screening value. That value is the maximum borehole gas flow rate multiplied by the maximum gas concentration recorded in that borehole, expressed as a volume of methane or carbon dioxide per hour. Everything else in a ground gas assessment exists to make that number defensible.
Reviewers who reject these reports rarely disagree with the classification. They disagree with the monitoring behind it.
Why gas is assessed separately from soil
Soil contamination and ground gas share a conceptual site model but not a method. Soil risk is assessed by comparing concentrations against assessment criteria for a given land use. Gas risk is assessed by rate of emission into a confined space, which is why flow rate matters as much as concentration.
A borehole reading 60 per cent methane with no measurable flow presents a very different risk from one reading 5 per cent with a steady 2 litres per hour. Report the second and ignore the first and you will classify the site wrongly in both directions.
The sources differ too. Made ground and landfill are the obvious ones. Alluvium, peat and other organic natural deposits generate methane without any history of tipping, and in coalfield areas mine gas arrives through a different pathway with different controls. State which source the conceptual model identifies, because the monitoring design follows from it.
Monitoring rounds, duration and atmospheric conditions
This is where most reports fall down. The published industry guidance sets expectations for the number of monitoring visits and the period they span, scaled to the gas regime suspected: a small number of rounds over a short period for a low-risk site with a thin made ground, considerably more over several months where a landfill is involved.
Counting visits is not enough. The visits have to sample the conditions under which emissions peak, which means periods of low and falling atmospheric pressure. A monitoring set collected across a settled high-pressure summer will understate flow, and a competent reviewer will say so.
Every round should record:
- Atmospheric pressure and the pressure trend over the preceding 24 hours
- Borehole response, including whether the standpipe was capped and for how long
- Peak and steady-state concentrations of methane, carbon dioxide and oxygen
- Flow rate, with the instrument and its detection limit stated
- Water level in the standpipe, since a flooded response zone reports nothing useful
- Weather, ground conditions and any site activity that could disturb the readings
Where a report gives a table of concentrations with no pressure data, the classification cannot be checked and the reviewer has no choice but to ask for more.
Calculating the gas screening value
The calculation is simple and the judgement is not. The screening value is the maximum flow rate recorded in a borehole, in litres per hour, multiplied by the maximum concentration recorded in that same borehole as a percentage, divided by one hundred.
Two points cause argument. The first is whether to use the maximum values recorded at any time, or the maximum coincident values from a single visit. The conservative approach, and the one most regulators expect, uses the worst recorded flow and the worst recorded concentration even if they occurred on different days.
The second is the treatment of outliers. A single anomalous flow reading taken while a standpipe was being uncapped is not evidence of a gas regime, and a report should say plainly which readings were discounted and why. Silently dropping the high value is what triggers a request for the raw field sheets.
Calculate a screening value for each borehole and report them individually before you report a site-wide figure. Averaging across a site with a filled pond in one corner hides the only part of the site that mattered.
Assigning a characteristic situation
The classification scheme runs from situation one, very low risk, to situation six, very high risk, with the bands set by ranges of the gas screening value. The commonly used bands place very low risk below roughly 0.07 litres per hour, low risk up to about 0.7, and then progressively higher bands to the upper end above about 70 litres per hour.
The classification is not arithmetic alone. The guidance expects the assessor to consider the source, whether the gas regime is likely to change over time, the presence of oxygen depletion, the sensitivity of the proposed development and the reliability of the data set. A site at the top of a band with a thin data set should be classified in the band above, and the report should say so explicitly rather than leaving the reader to infer caution.
Residential development with gardens, and any building with a basement or a suspended floor void, raises sensitivity. So does a building with a large footprint where a membrane defect would be hard to find.
Protection measures by situation
The design of protective measures for new buildings follows a points-based approach in the relevant British Standard, where the characteristic situation and the building type determine the protection score required, and individual measures contribute points towards it.
The measures are familiar: a well-ventilated underfloor void or a passive venting layer, a gas-resistant membrane with properly formed joints and service penetrations, and in the higher situations, active systems with monitoring and alarms. Structural elements contribute too, since a reinforced concrete raft with a low permeability performs differently from a beam and block floor.
The point reviewers press on is that the score has to be achieved by measures that will actually be built and can be verified. A design relying on a perfectly installed membrane in a building with dozens of service penetrations and a tight programme is a design that will not achieve its score on site. Note that this sits alongside, not instead of, any geotechnical requirements, which belong in the Geotechnical Assessment.
Verification of what is installed
Verification is a separate exercise with its own published good practice, and the planning condition usually depends on it.
It needs an inspection regime agreed before work starts, a named competent inspector attending during installation rather than afterwards, photographic records of joints, penetrations, laps and repairs, integrity testing where specified, and an as-built record showing the extent of the membrane against the structural drawings.
Where a basement is proposed, the gas design and the waterproofing design interact and should be coordinated by the same team, which is a point the Basement Impact Assessment should pick up.
Reporting so a regulator can audit it
Write the gas chapter so that a reviewer can reproduce the classification without ringing you.
- State the conceptual site model and the gas sources it identifies.
- Give the monitoring design, the number of rounds, the period covered and the rationale against published guidance.
- Tabulate every round with pressure, trend, concentrations, flow and water level.
- Show the screening value calculation for each borehole, with the values used and any discounted readings identified.
- State the characteristic situation, the band boundaries applied and any professional judgement that moved the classification.
- Set out the required protection score, the measures proposed and how each contributes.
- Specify the verification regime and who will carry it out.
Land contamination work is a report type where a suitably qualified and experienced professional is legally expected, and a draft is a starting point for that person rather than a substitute. Arrange the review through the professional directory once a Contaminated Land Assessment draft exists.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.
Frequently asked questions
How is the gas screening value calculated?
Multiply the maximum borehole gas flow rate in litres per hour by the maximum gas concentration recorded in that borehole as a percentage, then divide by one hundred. The result is a volume of methane or carbon dioxide per hour. Calculate it for each borehole individually before any site-wide figure is reported.
How many monitoring rounds are needed?
It depends on the gas regime suspected. Published guidance scales the number of visits and the monitoring period from a handful over weeks for thin made ground to many more over several months where landfill is involved. What matters as much as the count is whether the visits captured periods of low and falling atmospheric pressure.
Should maximum flow and maximum concentration come from the same visit?
Most regulators expect the conservative approach, using the worst recorded flow and the worst recorded concentration from a borehole even where they were measured on different days. Where an assessor uses coincident values instead, the report should say so and justify it, because the resulting classification can differ by a whole band.
Can professional judgement change the characteristic situation?
Yes, and it should where the evidence is thin. The guidance expects the source, the stability of the gas regime, oxygen depletion, the sensitivity of the proposed development and data reliability to be weighed alongside the screening value. A site near the top of a band with few rounds should normally be classified in the band above.
What does verification of gas protection involve?
An inspection regime agreed before installation, a competent inspector attending while the membrane is laid, photographic records of joints, laps, penetrations and repairs, integrity testing where specified, and an as-built record against the structural drawings. It cannot be reconstructed after the floor is finished.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.