Monday, 7 November 2011

The Legacy of Extraction

When materials extracted from the earth we are creating a dangerous legacy for future generations.  Although not unique, Coal is the biggest culprit over large areas of the UK.  With several new proposals for adopting a new, relatively untried extraction process, it is worth thinking about the legacy of mining. There are many fundamental differences between hydraulic fracturing or fracking and any of the deep coal mining techniques but that does not mean that similar problems could not occur.

Problems such as subsidence and pollution may not be evident for years, decades or even centuries.  The older mines are the most troublesome in terms of subsidence as there are little or no remaining records of precisely where coal was worked and they are often at shallower depth, where the effect of collapsing mines is more severely felt at the surface.  Information technology should allow decent records of the boreholes used for fracking and the approximate extents of land affected as long as the digital records don't become obsolete - imagine they were stored on 5.25" floppy disks!

When old mines were first abandoned, there would be props supporting the roof and it would be fairly stable but over time parts of the roof will give way in successive layers, eventually reaching the surface as indicated on the sketch below.

The progressive nature of mine collapse can take many years.

It is often the case that a small change on the surface could trigger the final collapse. For example, in Summer 2002, a hole opened up in Riddrie Park Cemetery during a period of very heavy rain.  A teenager was swept into the extensive network of underground tunnels.  The heavy rain,which also caused extensive flooding in the area,  that was thought to be the trigger. In another more recent example, in 2008, construction works to the south Pollokshields East Station caused a collapse of an old mine closing the Cathcart Circle line and five busy commuter railway stations for a month.

There are many more areas with significant risks of mining subsidence and, while loss of life is thankfully very rare, the cost to infrastructure and property is considerable.  Most of Scotland's major infrastructure projects have had to account for the risk of unexpected mine workings in some form or other, including the M74 extension, the M80 and the Stirling to Alloa Railway.

While fracking does not involve excavating large quantities of solid material from the ground, it does change the structure of the rock.  The rock containing the gas is porous but impermeable: it contains lots of bubbles but they aren't joined together, kind of like a honeycomb.  The hydraulic fracturing process simply bursts those bubbles which also reduces the strength of the rock mass and may result in larger voids below the ground.  Although this occurs at a significant depth, I would not be surprised if some signs of ground surface settlement are not evident, given the large areas proposed for gas extraction.

Another concern is that if fracking is undertaken in an area previously mined, perhaps at a different depths, then the pressures could trigger collapses in nearby abandoned mines.  The recent report which indicates that it is highly likely that fracking caused earth tremors near Blackpool and the claim that up to 50 tremors were caused by fracking suggests that mine collapses could well be triggered.

While the earth tremors in Lancashire so far have been too small to cause any damage to above ground structures, I believe that the risks of more severe problems can not be eliminated.

Anothee big concern with fracking, as with historical mine workings, is the risk to ground water aquifers.  This has been widely discussed elsewhere, especially in the US, as has leakage of methane directly to the atmosphere so I won't say any more on these topics for now.

We need to consider the problems we have caused for ourselves in the past, whether it is the ground collapsing from under our feet or run away global warming, before we decide to commit ourselves to more of the same folly.

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