Metalliferous mining legacy in the south west

In the coalfields, a single national body holds abandonment plans and issues a standard mining report, and a developer knows within days whether the site sits in a high risk area. Nothing equivalent exists for tin, copper, arsenic, lead or tungsten mining in Cornwall and west Devon. The records are scattered across archives, many workings were never recorded at all, and the risk assessment starts from a blank sheet.

That is the single most important thing to understand about a metalliferous site. Absence of evidence is not evidence of absence, and a desk study that finds nothing has told you almost nothing.

Why non-coal legacy is handled differently

Coal mining has been subject to statutory record-keeping and a national reporting service for decades. Metal mining in the south west largely predates any comparable requirement. Working was intermittent, undertaken by numerous small ventures over centuries, and abandoned without deposit of plans in many cases.

The result is an asymmetry that catches developers from outside the region. There is no authoritative map of workings, no standard risk classification, and no statutory consultee who will tell you whether the site is affected. The local planning authority relies on its own constraint layers and on the applicant's investigation.

Cornwall and west Devon also carry a heritage overlay that a coalfield site does not. Much of the former mining area lies within a World Heritage Site, and engine houses, counthouses, stacks and dressing floors are frequently listed or fall within the curtilage of a listed structure. The interaction between structure and setting is covered further in the notes on listed building curtilage and what the listing captures, and it applies to more mine sites than people expect.

Shafts, adits and shallow workings

Four hazard types account for most of the ground risk.

Shafts. Vertical, often unlined in rock below a short collar, sometimes several hundred metres deep. Historic capping was frequently timber or a rail-and-rubble arrangement that has since decayed. A shaft can be entirely invisible at the surface and still collapse under load.

Adits. Near-horizontal drainage and access levels driven from valley sides, usually at shallow depth and often unrecorded. They can run under a site from a portal some distance away.

Shallow stopes and openworks. Where the lode was worked from the surface downwards, leaving narrow, deep voids following the vein. These are the hardest to find because they are linear rather than circular and do not show as a neat depression.

Mine waste. Burrows, dressing floor residues and tailings, which combine poor bearing capacity with high metal concentrations. They are a geotechnical and a contamination issue at the same time.

Lodes run in consistent directions across a district, so the orientation of known workings on adjacent ground is a useful predictor of where unrecorded ones will lie on your site.

The records that exist and what they miss

Assemble the desk study from all of the following rather than from one record.

• Historic Ordnance Survey editions, which mark many shafts as "Old Shaft" or "Shaft (dis)" and show tramways, leats and dressing floors.
• Tithe maps and estate plans, which often record mineral setts and sometimes shaft positions not on the OS.
• Abandoned mine plans held in national and county archives and by the British Geological Survey, which are incomplete for metal mines and vary greatly in accuracy.
• The county historic environment record, which maps recorded mining features as heritage assets.
• Aerial photography and airborne laser survey, which pick up collapse hollows, spoil and linear depressions under vegetation.
• Geological mapping showing lode and fault orientation.

The limitations are as important as the content. Plan accuracy is often poor by modern standards, surveyed positions can be tens of metres out, and depth information is frequently absent. Say all of this in the report. A desk study that presents historic plan positions as though they were surveyed coordinates misleads the reader who uses it later.

Intrusive investigation and probing

Desk work narrows the search. Only intrusive work resolves it.

A typical sequence runs walkover survey first, looking for depressions, changes in vegetation, spoil and masonry. Then geophysics across the site, commonly a combination of ground conductivity, magnetometry and microgravity, chosen for the target geometry rather than out of habit. Then rotary open-hole probe drilling on a grid across the areas of proposed development, tightened over suspected lode positions and over any anomaly the geophysics identified. Then trial trenching or excavation to prove and expose any shaft collar found.

Probe spacing is a judgement call, and it should be stated and justified rather than defaulted. A grid wide enough to miss a two metre shaft has not answered the question the council asked. Where a structure will sit over a suspected working, the probing has to be dense enough that the structure's footprint is demonstrably clear.

Report all of this in a Geotechnical Assessment that sets out the conceptual ground model, the investigation rationale, what was found, and the residual uncertainty. Eurocode 7 governs the design, and the interpretation should be signed by someone with experience of mining legacy specifically, because the failure mechanisms differ from ordinary ground engineering.

Capping, grouting and what it costs

Remediation is standard in principle and expensive in practice.

Two cost drivers dominate. The first is that a shaft found late forces a layout change, because a cap plus a standoff can sterilise a plot. The second is grout volume, which cannot be known until the workings are proved and which can run well beyond the allowance in a fixed price contract. Price the investigation properly and it becomes possible to price the remediation. Skip the investigation and the remediation becomes an open-ended liability.

Radon deserves separate mention. South west England is a radon affected area independently of mining, and Building Regulations Approved Document C requires protective measures where the property sits in an affected area. Check the radon status of the site alongside the mining work, since both influence floor construction.

Arsenic and metals in the soil

Mineralisation in the region means naturally high arsenic levels, and centuries of processing, calcining and arsenic production mean anthropogenic enrichment on top of it. Lead, cadmium, copper and zinc follow the same pattern.

This is a land contamination question in its own right, handled through a Phase 1 desk study and a Phase 2 investigation under the framework in the Planning Practice Guidance, with the investigation designed and reported in accordance with BS 10175 by suitably qualified and experienced persons. It is not a matter a geotechnical engineer can absorb into the ground investigation report.

Two regional points. Normal background concentrations published for English soils identify a distinct mineralisation domain in the south west with arsenic values above the national picture, and that domain is relevant to how risk is assessed rather than a reason to stop assessing. And garden soils in a residential end use drive the risk assessment, so cover systems, imported topsoil and verification of the cover become the practical remedy.

Commission the contaminated land assessment alongside the mining investigation. The same boreholes and trial pits can serve both if the scope is written that way, and a single mobilisation costs materially less than two.

What the council will condition

Expect a conditions package along familiar lines: a phased ground investigation and risk assessment, a remediation strategy for both stability and contamination, a verification report before occupation, a reporting of unexpected contamination clause, and measures for radon and ground gas.

Two things reduce friction. Agree the investigation scope with the council's contaminated land officer before you start, in writing, because a Phase 2 designed without that conversation frequently has to be repeated. And be explicit about residual risk in the closing section of the report. On a metalliferous site nobody can promise there is no unrecorded working, and a report that claims otherwise invites a reviewer to test it.

Browse the full list of report types to see which documents a constrained site needs, or enter your postcode on PlanScope to start from your own site.

This is general guidance, not legal advice. Check the position with your local planning 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.