The ground gas assessment section of a Phase 2 report

The ground gas section of a Phase 2 report fails review for one of three reasons: too few monitoring visits to support the conclusion, flow data recorded at a resolution that cannot distinguish a characteristic situation, or a characteristic situation assigned from gas screening values alone with no conceptual site model behind it. The gas work is where a contaminated land assessment most often has to go back out to site, and the cost of that is a delayed start on groundworks.

What a competent reviewer is looking for is a chain of reasoning: source, pathway, receptor, measured data, screening, classification, protection measures, verification. Break any link and the rest carries no weight.

Monitoring regime, visits and atmospheric pressure

The number of visits is a risk-based judgement, and it has to be justified in the report rather than asserted. A made ground site with a thin fill layer and no organic content supports a shorter regime than a site over a former landfill, and the report should say which it is and why.

Two conditions matter more than the raw count. Monitoring should cover periods of low absolute atmospheric pressure, conventionally taken as below 1,000 mbar, and periods of falling pressure, because gas emission from the ground increases as pressure drops. A regime of eight visits taken entirely in a settled high-pressure spell tells you very little about the worst case.

Record barometric pressure at each visit, together with the trend over the preceding 24 hours, temperature, recent rainfall and ground conditions. Where a visit was taken in the wrong conditions, keep the data and say so rather than quietly presenting it as representative.

Monitoring wells should be fully developed and purged before readings, with the response zone depth and the gas tap arrangement recorded. A well screened across both the fill and the underlying stratum produces a blended result that is hard to interpret and easy to attack.

Gas screening values and limiting concentrations

The gas screening value combines the borehole flow rate with the gas concentration, giving a figure in litres per hour for the rate at which the gas of concern is emerging. The characteristic situation bands in the widely used classification run from CS1 at less than 0.07 l/h, through CS2 at 0.07 to 0.7, CS3 at 0.7 to 3.5, CS4 at 3.5 to 15 and CS5 at 15 to 70, to CS6 above 70 l/h.

Flow is not the whole test. The classification tables also set limiting concentrations, so a very low flow carrying a high methane concentration does not automatically fall into the lowest band. Work from the current published table rather than from memory or from a spreadsheet inherited from a previous job, because the version in circulation matters.

Use the maximum values rather than averages for screening. Averaging across visits smooths out exactly the event the assessment exists to capture. Where a single outlier dominates, present both the maximum and the distribution and explain your reasoning for the value carried forward.

Characteristic situations and how they are assigned

Assigning a characteristic situation is an act of professional judgement informed by the screening values, not an output of them. The guidance expects a lines-of-evidence approach: the conceptual site model, the ground conditions, the gas regime observed, the nature and thickness of any gas-generating material, and the proposed development form.

A site returning CS2 screening values but sitting over 4 m of domestic refuse deserves a sceptical eye, because the gas generation potential is there even if the monitoring period happened to miss it. Conversely, carbon dioxide at modest concentrations in a chalk or limestone setting may be entirely natural and have no landfill-style source behind it.

Say in the report what the source is understood to be. A classification with no identified source is a number without a story, and the reviewer has no basis on which to agree with it.

Borehole flow rates and data quality

This is where most reports are weakest. Low flow rates sit at the edge of what field instruments resolve, and a flow meter reading 0.0 l/h may mean no flow or may mean flow below the instrument's detection limit. Those are different conclusions.

The report should state the instrument used, its stated resolution and range, the calibration record, and how negative or zero readings were treated. Where readings sit at the resolution limit, the honest approach is to use the limit of detection as the value and show that the classification still holds, rather than treating zero as zero.

Readings should be taken over a long enough period for the measurement to stabilise, with the duration recorded. Spot readings taken within seconds of opening the tap capture the headspace rather than the flow.

Protection measures and the points score approach

Where protection is needed, the design route for methane and carbon dioxide in new buildings is a scored approach: points are allocated to the structural barrier, to ventilation measures beneath the building and to any gas-resistant membrane, and the total must meet the score required for the characteristic situation and building type.

The important design point is that the structural barrier does real work. A well-detailed reinforced concrete slab of defined thickness and crack width contributes more than a membrane laid over a beam-and-block floor, and a design that leans almost entirely on a membrane is fragile because membranes get damaged during construction.

Ventilation is the other dependable element, and passive subfloor ventilation with properly sized and positioned vents is preferred over anything requiring maintenance by future occupiers. State the assumed building type, because the required score differs between a dwelling with a sealed floor void and a commercial unit with a ventilated underfloor space.

Set out the scoring explicitly, item by item, with the total. A reviewer should be able to check the arithmetic without reconstructing your assumptions.

Verification of installed measures

A protection design is worth nothing without verification on site, and the planning condition will usually require a verification report before occupation. Plan it into the construction programme at the design stage.

Verification covers membrane integrity testing, inspection of joints, penetrations and upstands, photographic records referenced to a drawing, confirmation of vent positions and free areas, and sign-off by someone independent of the installer. Good practice guidance on testing and verification of ground gas protection systems sets out the expectations, and discharging the condition without that evidence is difficult.

Penetrations are where systems fail: service entries, pile heads, column bases and the junction between the membrane and the cavity tray. Photograph each one.

Where monitoring data is insufficient

Sometimes the data will not support a conclusion. The defensible responses are to extend the monitoring, to adopt a precautionary classification and design for it, or to install protection suitable for the worst credible case and verify it.

What is not defensible is presenting a thin dataset as adequate. If the conclusion depends on six visits in settled weather, say so, and state what further monitoring would resolve the uncertainty. A reviewer who can see the gap will usually accept a precautionary design. A reviewer who finds the gap hidden will reject everything.

Where ground conditions also drive foundation design, coordinate with the Geotechnical Assessment so that the same borehole records, strata descriptions and groundwater levels appear in both documents. Inconsistency between the two is a routine reason for a consultee to ask for clarification.

Reporting the conclusion defensibly

Three sentences should carry the conclusion: what the characteristic situation is, what protection is therefore required, and how it will be verified. Everything before that exists to support those three statements.

Keep the conceptual site model at the front and update it to reflect the Phase 2 findings rather than reproducing the Phase 1 version unchanged. Where the findings have changed the model, say what changed. That is the single clearest signal to a reviewer that the assessment was carried out rather than assembled.

Ground contamination and ground gas reports must be prepared in accordance with BS 10175 by suitably qualified and experienced persons, and for Phase 1 and Phase 2 land contamination work a competent professional is required rather than preferred. PlanScope's draft is a starting point for that professional, not a substitute, and the review route is the professional directory. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.