Evidence base for a site in a coalfield referral area

Coalfield referral area development needs one document that most other sites do not: a coal mining risk assessment, submitted with the planning application. Sites falling within the mapped development high risk areas are referred to the national coal mining authority as a statutory consultee, and many local planning authorities list the risk assessment on their own validation checklist, so an application without one can be rejected before it reaches an officer.

The assessment is a desk study, not a ground investigation. Its job is to identify what mining legacy may be present and to say what intrusive work is needed. Confusing the two is the most common error.

Checking whether the site is in a referral area

The mapping is published by the national authority responsible for coal mining legacy, formerly the Coal Authority and now the Mining Remediation Authority, and it distinguishes development high risk areas from low risk areas. High risk means recorded or likely shallow workings, mine entries, outcrops or other features close enough to the surface to affect development.

Do three checks and record the date of each.

1. The published development high risk area mapping, to establish whether the site is inside one.
2. A coal mining search report for the site, which returns recorded mine entries, recorded workings, licensed areas, subsidence claim history and surface hazards.
3. The minerals and waste local plan, to see whether the site also sits in a mineral safeguarding area, which can bring a separate requirement for a prior extraction assessment.

A site can be outside a high risk area and still have coal mining legacy, particularly where workings are deep or where a tip or an unrecorded shaft is present. Absence from the mapping is a reason to keep the assessment short, not a reason to skip the history.

The coal mining risk assessment and its content

The assessment should follow a consistent structure and reach a conclusion an officer can act on.

It sets out the site and the proposed development, the sources consulted with their dates, and the coal seams recorded beneath the site with their depth and thickness. It identifies recorded and probable shallow workings, recorded mine entries with their positions and the accuracy of those positions, coal outcrops crossing the site, past and present licensed areas, mine gas potential, recorded surface hazards and any history of mining related subsidence.

Then it does the part that matters. It assesses the risk to the proposed development, feature by feature, and recommends the intrusive investigation needed to resolve each one. Recommendations should be specific: the number and depth of rotary open hole probes, their location relative to the footprint, the treatment expected if workings are confirmed, and the gas monitoring regime.

An assessment that ends by recommending a watching brief has not done its job. Officers are looking for a conclusion that the site can be developed safely, subject to defined investigation and remediation, and that conclusion has to be reasoned from the records rather than asserted.

Shallow workings, shafts and adits

Three features drive the engineering response.

Shallow workings, meaning coal removed at a depth where collapse can reach the surface, are usually treated by drilling and grouting the voids, or by designing foundations to span or found below the affected zone. The extent of grouting is set by the probe results, so the cost is not known until the intrusive work is done. Budget a range rather than a figure.

Mine entries, being shafts and adits, are the serious ones. A recorded shaft has a zone of influence around it calculated from the shaft diameter and the depth of superficial deposits, and development inside that zone is resisted unless the entry has been located, treated, capped and recorded. The recorded position of a shaft is often approximate, sometimes by tens of metres, so locating it can require trial trenching across a search area. Plan the layout so that no dwelling sits over an uncertain shaft position, because moving a house on a layout costs less than treating a shaft under it.

Adits and outcrop workings behave differently again, following the seam rather than sitting at a point, and they can act as pathways for gas and water.

Intrusive investigation and its permits

Any intrusive work that will intersect coal, coal mine workings or a mine entry needs a permit from the authority before it starts. Applying for it is straightforward, but it takes time, and it has to be in place before the rig arrives.

That permit requirement sits on the critical path twice over: once for the investigation, and again for any grouting or shaft treatment, which is itself an entry into coal. On a scheme where the ground investigation is being run alongside the planning application, allow for the permit in the programme and start the application as soon as the investigation scope is settled.

The investigation itself is normally combined with the geotechnical and contamination work, since the same rig and the same boreholes can serve all three. A Geotechnical Assessment prepared to Eurocode 7, and where the site also has a contamination history a Land Contamination assessment prepared in accordance with BS 10175, should be scoped together with the coal work rather than commissioned separately. Land contamination reporting requires a suitably qualified and experienced author, and the local planning authority will expect one, so PlanScope's draft is a starting point for that professional rather than a substitute.

Ground gas from mine workings

Mine gas is a distinct hazard from landfill gas and from naturally occurring ground gas, and it behaves differently. Methane and carbon dioxide can migrate along workings and up old shafts, and concentrations vary with atmospheric pressure, so a single round of monitoring taken on a settled high pressure day proves very little.

What assessors expect is monitoring across a period long enough to capture falling atmospheric pressure, with the pressure recorded at each visit, and a characteristic situation derived from the results using the recognised approach to gas risk assessment. The gas protection measures that follow, if any, are designed to the relevant standard for gas protective measures in buildings and verified during construction.

Two practical notes. Gas monitoring wells need to be installed early, because the monitoring period is measured in months on a site with a high gas risk. And a scheme with basements, deep services or a large ground floor slab has a different gas exposure from one on a raft, so the building design and the gas assessment have to talk to each other.

Foundation design implications at planning stage

You are not expected to submit a foundation design with a planning application. You are expected to show that a viable foundation solution exists and that the layout does not depend on building over something that cannot be treated.

In practice that means three things. The layout should keep structures clear of mine entry zones of influence and should not rely on treating a shaft whose position is unknown. The indicative foundation approach should be stated, whether reinforced rafts, piles or conventional footings after grouting. And the drainage strategy should acknowledge the ground conditions, because infiltration into treated or untreated workings is usually unacceptable and soakaways near a mine entry certainly are.

Expect conditions requiring the intrusive investigation, a remediation scheme approved before development starts, and a verification report before occupation. Those are standard and should be anticipated rather than resisted.

Programme and the order of work

The sequence that works:

1. Coal mining search and high risk area check at the point of site appraisal, before any offer.
2. Coal mining risk assessment while the layout is still flexible.
3. Permit application for entry into coal, and installation of gas monitoring wells.
4. Intrusive investigation and gas monitoring, with the layout adjusted if workings or entries are confirmed in unexpected places.
5. Planning application with the risk assessment, the investigation results where available, and the indicative foundation and remediation approach.

Running the assessment after the layout is fixed is the expensive way round. A shaft discovered under plot seven at the intrusive stage costs a redesign, a fresh drainage layout and sometimes a unit.

The data sources and how to interrogate them are covered in the note on checking a site against coal mining risk data, and the wider brownfield evidence pattern is set out in the note on which reports a 12 flat brownfield scheme needs.

This is general guidance, not legal advice; check the position with your local planning authority, the mining authority or a qualified professional before relying on it. 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.