Darwen Housing Site: When the Risk Isn’t Coal

A crater, estimated to be approximately 30 meters in depth, emerged close to newly built homes, causing understandable concern for residents and leading to an emergency response and investigation. Early assumptions often point to coal mining in cases like this – however, this wasn’t coal.

The ground failure was caused by historic mineral extraction, specifically a sandstone known locally as flagrock.

What is flagrock and why does it matter?
Flagrock is a sandstone that splits naturally into flat, regular slabs. In the 1800s it was widely quarried for paving and building stone across parts of Lancashire. Extraction was often shallow, irregular and poorly recorded by modern standards.

Unlike deep coal workings, these old stone workings don’t behave predictably. They can leave shallow voids, weakened rock and unsupported ground that may remain stable for decades – until it doesn’t.

That’s what we’re seeing here.

Why wasn’t this picked up?
This is most likely due to people still often equating “mining risk” with coal. A standard coal mining report only looks for coal-related hazards. It does not assess historic stone quarrying, flagstone extraction or other non-coal minerals. If that’s all you order, you will miss risks like this.

In Darwen, the issue wasn’t subsidence from coal. It was collapse into old sandstone workings. A coal report would come back clear and give false reassurance.

What a minerals report would have shown
A non-coal minerals report would have flagged:

• Historic sandstone and flagrock workings in the area
• The likelihood of shallow extraction rather than deep mining
• Potential for voids close to the surface
• Risk of sudden collapse rather than gradual settlement.

It doesn’t automatically stop development but what it does tell you is where further site investigation may well be needed. Boreholes, probing or redesign at the right stage could have identified the weakness before houses were built on top of it.

I think the hard truth is that this wasn’t an unforeseeable event. The risk existed long before the first brick was laid. The problem is not a lack of data but perhaps joined-up due diligence. Too many decisions are still made on the basis that a coal search equals a mining search, which is not the case.

Coal is just one part of the picture. In many parts of the country, non-coal minerals are the bigger risk.  Without understanding what was taken out of the ground in the past, you’re gambling with what happens in the future.