Is our mining legacy coming back to haunt us?

Towards the end of last year, it was reported that more than 170,000 Bristol houses were at risk of ground movement. Groundsure, which specialises in environmental risks like ground stability, recently released research that identified ground-breaking numbers of endangered properties across the UK. Significantly, those with foundations built close to disused mine shafts – the entries to old mines – have an increased likelihood of subsidence.

With anticipation that more than eight million properties could see resultant ground movement with consequential effects in the coming years, is there a way to counteract the negative effects of our mining history?

An extensive mining legacy

Ex-mining towns and cities once dripping in rich resources now face increasingly unstable futures. To excavate natural minerals from deep underground, long and narrow vertical holes were dug to provide access to fuels like coal and precious metals like silver. Once resources were accessible, mining was carried out in a manner of ways.

For coal, room and pillar was the most common method, with the rock being extracted across a horizontal plane. “Cross-galleries” were extended out into the coal seam leaving unworked coal to form a network of pillars that provided “permanent” roof support. However, coal was also sometimes mined from these pillars, decreasing the overall stability of the mine and increasing the potential for roof collapse.

The longwall extraction method opened up similar vulnerabilities, too. Coal was almost completely extracted, with only some pillars of coal left intact to support the roof. It goes without saying that the weaker the remaining pillars, the more likely the occurrence of ground movement. Such incidents can lead to a whole host of present-day infrastructure issues, including full property collapses.

Water, a subsidence catalyst

Generally speaking, ground movement associated with past mining activities is rather well understood. However, there are a number of observations that remain quite “grey” by nature. For example, where coal seams interact across different levels, it’s more difficult to predict the magnitude and extent of ground movement.

Likewise, in the event of a roof collapse above an open gallery, sinkholes can be formed. In the UK, 32,000 sinkholes occur naturally, even before disused mines and extreme weather occurrences like heatwaves and freezes are factored in.

Add to the puzzle the effects of water, and you begin to see how much mines are – quite frankly – mining their own problems. Where some water is present in room and pillar models, further subsidence is likely. Soil foundations expand in the wet and contract on drying, decreasing their stability. Yet where a mine is completely flooded, a force supportive to the mine roof is provided, counteracting instability. Further, the lack of oxygen prevents chemical breakdowns of coal and its supports, causing pillars to retain valuable strength.

Though we often talk about ground movement in terms of subsidence – downwards movement – researchers have also discovered a new phenomenon in which the surface moves upwards. It’s been hypothesised that water, once again, plays the integral role; it causes clay minerals to swell, subsequently making the ground spike upwards.

Looking to a more stable future

Whichever direction ground movement occurs in, the precarity surrounding ex-mine shafts is concerning to homeowners, construction companies and insurers. With UK’s population expected to grow 3.2% by 2030, demand for housing is becoming more urgent. Developers are gradually becoming forced to consider locations near to ex-mining sites which are likely to have weaker, unstable foundations. As such, it’s imperative that buyers are informed of, and understand, related riss.

Environmental authorities are calling for more in-depth surveys and research to be carried out to keep house owners and business holders aware of relevant activities relating to the ground. Importantly, changes to how we actually monitor ground movement is fast becoming a phenomenon of its own, with traditional methods being replaced with models more reflective of real-time data. Decisions about sites become more informed and counteractions more proactive by nature.

Knowledge is, indeed, power. The more we understand ground behaviour, the more effective the processes we put in place to minimise the chances of catastrophic events.

• Matthew Bray is the CEO of SatSense.