Characteristic situation: classifying ground gas risk

A ground gas characteristic situation is a risk band, from CS1 to CS6, that places a site on a scale according to how much hazardous gas it can deliver to a building. The band is derived from monitoring data rather than from a single reading, and it determines the level of gas protection the building needs. The classification originates in the CIRIA guidance on assessing risks posed by hazardous ground gases to buildings and is carried through into the British Standard on protecting structures against ground gas.

For a reviewer, the classification is the pinch point of the whole assessment. Get the band wrong and either the building is under-protected or the developer pays for a membrane and ventilation system it never needed.

Why ground gas is classified rather than measured once

Ground gas behaviour is driven by pressure. Methane and carbon dioxide generated by degrading organic material, or carbon dioxide from natural sources such as chalk or peat, sit in the ground at concentrations and pressures that vary with rainfall, groundwater level and atmospheric pressure.

A single visit on a dry day with high barometric pressure can return almost nothing from a site that vents freely when the pressure drops. So a one-off measurement is not evidence of anything much.

The classification approach solves this by combining the worst observed concentration with the worst observed flow rate over a monitoring programme, producing a single number for the site's capacity to deliver gas. It is deliberately conservative, and it is designed to be used with a conceptual site model rather than instead of one.

Gas screening values and how they are derived

The gas screening value is the product of the maximum gas concentration and the maximum borehole flow rate recorded during monitoring, expressed in litres per hour.

In arithmetic terms, the concentration as a percentage by volume is converted to a fraction and multiplied by the flow rate in litres per hour. So a borehole showing a maximum of 20 per cent methane by volume and a maximum flow of 2 litres per hour gives a screening value of 0.4 litres per hour. That is an illustrative calculation to show the method rather than a reading from any particular site.

Two features of that calculation attract legitimate criticism and need handling in the report.

The maxima do not have to be from the same visit. The conventional approach takes the highest concentration and the highest flow recorded at any point in the programme, which can combine two unrelated events. Some assessors present both the conventional worst case and a paired calculation using values measured together, and explain the difference.

Flow rate measurement is also sensitive to instrument and borehole construction. A low flow reading on a poorly sealed borehole or one with a water-logged response zone tells you about the installation, not the ground.

The classification bands and what each implies

The bands are not the whole answer. The guidance is explicit that the screening value is a starting point and that the classification has to be considered alongside the gas source, the pathway, the receptor and the building type.

A CS2 site under a block of flats with a basement car park is a different proposition from a CS2 site under a single storey warehouse with a ventilated void. Residential development with private gardens, where occupants dig and build sheds, sits at the sensitive end of the scale, and the standard treats building types differently for that reason.

Monitoring rounds and weather conditions

The number of monitoring visits should be justified by the conceptual model rather than taken from a rule of thumb, and an assessment that does not explain its programme is incomplete.

What a reviewer should look for:

• a monitoring period long enough to capture seasonal variation, with more visits where a landfill or made ground source is present
• at least some rounds taken during falling atmospheric pressure, and ideally when pressure is below about 1,000 millibars, because that is when gas emission peaks
• barometric pressure, pressure trend, groundwater level and weather recorded at every visit
• the borehole response zone, its depth and whether it was submerged at the time of reading
• consistent instrumentation, calibrated, with the detection limits stated
• oxygen and carbon dioxide recorded alongside methane, because depleted oxygen with elevated carbon dioxide indicates a different source and a different risk

A programme of six visits taken in one dry fortnight is not equivalent to six visits spread across three months including two low pressure events, even though both report six rounds.

Protection measures tied to each class

The British Standard assigns protection by scoring the contribution of each element of the system: the structural barrier such as a reinforced concrete slab, the gas resistant membrane, and the ventilation layer, whether a passive ventilated void, a subfloor venting system or a positive pressurisation system.

The required total score rises with the characteristic situation and with the sensitivity of the building type. So a low band may need little more than a well detailed slab with a membrane, while a higher band on housing will need a ventilated void, a verified membrane and often a monitoring or alarm provision.

The practical point for a developer is that the band changes the slab design, not just the membrane specification. Discovering a higher band after the structural design is fixed means revisiting foundations, service entries and the ground floor build-up.

Where volatile organic compounds are present as well as permanent gases, the membrane specification changes materially, because many gas membranes are not suitable for hydrocarbon vapours. That has to be identified at the investigation stage.

Verification on site

Specification is not protection. The CIRIA guidance on testing and verification of ground gas protection systems sets out what a verification programme covers: an independent verifier, inspection of the membrane before it is covered, testing of joints and service penetrations, photographic records, and a verification report that the authority can accept in discharge of the planning condition.

Service penetrations are where systems fail. A perfectly installed membrane with an unsealed soil vent pipe or an incoming electricity duct provides a direct pathway, and these are found on inspection far more often than anyone would like.

Build the verification into the programme and the budget. A condition requiring a verification report before occupation is standard, and it cannot be satisfied retrospectively once the floor slab is down.

Where the classification is contested

Disagreements between a developer's consultant and a council's contaminated land officer usually turn on one of four things: whether the monitoring programme was long enough and captured low pressure conditions, whether a single anomalous flow reading should drive the classification, whether the borehole installations were suitable, and whether the conceptual model justifies treating part of the site differently from the rest.

The resolution is usually more monitoring rather than more argument, which is why an early start on the gas monitoring programme is worth more than a well written discussion section. Where the site history suggests a gas source, commission the monitoring at the same time as the Phase 1 work rather than waiting for the Phase 2 scope.

Land contamination assessment is a report type where a suitably qualified and experienced professional is legally expected: the national policy places responsibility on the developer and authorities expect work carried out in accordance with the relevant British Standards by competent persons. An AI-assisted Contaminated Land Assessment draft is a starting point for that professional and not a substitute for them, and the same applies to the ground investigation behind a Geotechnical Assessment. If you need a reviewer with the right standing, the professional directory is the route.

This is general guidance, not legal advice. For specialist technical reports, qualified professional input may be required. Check the position with your local planning authority or a qualified professional before relying on it.

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