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Wigan, Bolton, Oldham, Bury, Rochdale, Salford and Tameside sit over centuries of coal mining. What that legacy means for your foundations — and the coal-mining check you may need before you build.
The north and west of Greater Manchester was built on coal. The same seams that powered the mills left a legacy underground — shallow abandoned workings, old shafts, and made ground — that still shapes how you have to build. On this ground, the ground investigation isn’t just about strength; it’s about what was taken out from under it.
The bedrock is Carboniferous: the Pennine Coal Measures Group — interbedded mudstone, siltstone and sandstone with the coal seams, up to around 2,000 m thick — resting on the older Millstone Grit. Coal was worked mainly from the Middle and Lower Coal Measures. The rock is heavily faulted, which is part of why the mining record is so complex.
Over the bedrock, glacial till (boulder clay) is the most widespread deposit, with spreads of glaciofluvial sand and gravel (up to ~20 m thick around Rochdale) and deep buried valleys holding 20–30 m of soft fill at Trafford and Salford. Made ground is extensive across the old industrial belt — typically 1–2 m, locally up to 7 m — and often construction or industrial waste.
The defining risk on the coalfield isn’t the soil — it’s shallow abandoned mine workings, old mine entries (shafts and adits) and mine gas. The Mining Remediation Authority (the renamed Coal Authority) maps a “Development High Risk Area” across the parts of the coalfield with recorded mining features that could affect surface stability. A large share of the north and west of Greater Manchester falls within one — and where it does, a planning application for most development needs a Coal Mining Risk Assessment (CMRA). Deep mining here only ended in 1991 (Agecroft Colliery, Pendlebury); the workings it left don’t go away. The first thing to establish on any plot in this belt is its coal-mining position — a coal-mining report answers it.
| Risk | Where / why |
|---|---|
| Shallow coal workings & mine entries | Across the Development High Risk Area — recorded and probable workings, shafts and adits near the surface. |
| Mine gas | Recorded mine-gas sites within the reporting area. |
| Colliery spoil | Old waste heaps — may contain coal and pyrite, which can mean combustion, settlement and aggressive ground chemistry. |
| Made / brownfield ground | The old industrial corridor — variable fill and possible contamination. |
| Weathered upper till | The top few metres of boulder clay can be softer, fissured and weaker when wet. |
Away from mining influence, competent glacial till usually gives good strip or trench-fill conditions — though the weathered upper metres may need to be dug through. The mining legacy is what changes the design: over shallow workings, the answer (driven by the CMRA) may be grouting the voids, drilling to prove and treat old workings, or piling below the zone of influence. Variable made ground and potential subsidence often point to a raft.
Where colliery spoil, made ground or brownfield fill is present, the ground can be chemically aggressive to buried concrete — pyrite in spoil can oxidise to generate sulfates. Under BS 8500 and BRE Special Digest 1, a soil test assigns the sulfate/ACEC class and sets the concrete’s DC class, which may call for sulfate-resisting cement. This varies pit by pit, so it has to come from site testing — not a rule of thumb.
We deliver concrete right across the Manchester coalfield — Wigan, Bolton, Bury, Rochdale, Oldham, Salford, Tameside and the towns between. More town guides are being added to the Atlas.
• Ready-mix for foundations & footings
• Work out how much concrete you need
• Back to the Ground Conditions Atlas
Very possibly. If your site falls within the Mining Remediation Authority’s Development High Risk Area — which covers a large part of the north and west of Greater Manchester — the local planning authority normally requires a Coal Mining Risk Assessment to support most development applications. A coal-mining report tells you where you stand.
Usually yes, once they’re dealt with. Depending on what the risk assessment finds, the workings may be grouted, drilled and treated, or the foundations designed to pile below the zone of influence. It’s a known, solvable engineering problem — the key is establishing the position before you design.
It can. Spoil and old fill may carry pyrite and sulfates that are aggressive to buried concrete, raising the required DC class under BS 8500 / BRE Special Digest 1 and sometimes calling for sulfate-resisting cement. A soil test decides it — send us the class and we’ll match the mix.
Sources: British Geological Survey, Geological ground model for planning and development for Greater Manchester (Open Report OR/20/033); BGS Earthwise (Pennine Coal Measures; ground engineering, Northern England); the Mining Remediation Authority (formerly the Coal Authority) — Development High Risk Area and Coal Mining Risk Assessment guidance; GOV.UK planning guidance; BS 8500-1 and BRE Special Digest 1. General guidance only — not a substitute for a Coal Mining Risk Assessment or a site-specific ground investigation.
Foundations, rafts and slabs, mixed fresh on site to the spec your ground — and your risk assessment — call for.