That sinking feeling


29 August 2014 by Sue Nelson

February 2014 saw the appearance of at least three times more sinkholes in the UK than is normal for a whole year. Sue Nelson talks to experts from the British Geological Survey to find out what's making these huge holes open up.

Sue Nelson: One five-metre hole on the M2 in Kent, another in a High Wycombe driveway, a seven-and-a-half-metre hole that caused three homes to be evacuated in Yorkshire, and 20 houses evacuated around a giant hole in Hemel Hempstead...

Principal geologist Tony Cooper and engineering geologist Vanessa Banks - I know sinkholes are holes in the ground but what's the technical definition?

Tony Cooper: Strictly speaking a sinkhole is a natural phenomenon caused when rock dissolves underground and the surface above collapses. But a lot of collapses are due to mining and ancient workings - including so-called deneholes in the south of England. The result is the same and in places it is very difficult to tell the difference between a man-made sinkhole and a natural one.

Sue Nelson: So a denehole is one of these man-made holes?

Tony Cooper: A denehole is a shaft, usually dug to extract chalk to fertilise the ground in medieval times. They would dig through weathered rock to reach chalk that contained more minerals. Then that area would be enlarged and have more tunnels dug out from it. When they had finished they would just block the top of the shaft and cover it, often leaving no sign at the surface.

Sue Nelson: I gave a few examples of where these sinkholes have been occurring over the UK. Is there something that connects all these places?

Tony Cooper: There is a connection in that they are all areas of soluble rock. We've basically got four types of soluble rocks in the UK that give us sinkholes: chalk, limestone, gypsum, which is the raw material for plaster and dissolves very quickly - if you took a block the size of a large van and put it in a river it would dissolve in about 18 months - and salt.

Sue Nelson: What sort of research do you do here that helps you know which areas are more likely to have sinkholes?

Vanessa Banks: We are interested in how water moves into and through these soluble rocks, how its chemistry changes as it moves through the ground, and how it emerges from the ground as springs. We put a lot of effort into understanding these flow paths and one of our specialists does 'dye tracing tests'.

Sue Nelson: So you put some dye in at the top end and see where it comes out to work out which route it has taken?

Vanessa Banks: Absolutely - either a dye or another inert substance that doesn't affect water quality. But the water may take more than one pathway so we're monitoring lots of springs.

Sue Nelson: The wettest January in centuries and a pretty soggy February sound like ingredients for a perfect storm for sinkholes.

Vanessa Banks: Well there is this strong association between the December-January rainfall for the south and south-east of England and the number of collapsed features, though possibly that's not so much due to increased dissolution as disturbance to the surface materials that cover the voids.

Sue Nelson: How do you reassure people who live above soluble rocks that a sinkhole isn't going to appear in their back garden anytime soon?

Tony Cooper: Compared to the amount of people affected by storm damage or flooding, the number of people affected by sinkholes actually is very small. Even in parts of the country with lots of sinkholes you are more likely to suffer damage from other things. It's just a more unusual sort of thing to happen and it is very drastic if it does happen under your house - but not that many houses are actually affected by them.

Sue Nelson: When sinkholes do strike, Vanessa, do you effectively act as a hit squad - do you go out to study them as and where they occur?

Vanessa Banks: We don't actually send a response team to all of these events but when we can we do visit to collect information for scientific purposes.

Sue Nelson: What about the future? With climate change and the possibility of wetter weather ahead for the UK, does this mean that sinkholes are likely to happen more frequently?

Vanessa Banks: They have occurred in the past in response to prolonged rainfall, but it will very much depend on where the increases in storms and rainfall occur.

Sue Nelson: You've got all this research here and all this knowledge about the UK - I suspect the insurance and building industries would be very interested in the research you do?

Tony Cooper: Yes - we've actually looked at the whole country to see the distribution of the soluble rocks and the places that are more prone to dissolution and we've created a product called 'Geosure' for the insurance industry. It doesn't really affect people's insurance premiums but it gives the insurance business a handle on where their liabilities lie.

We also feed our research into the building industry and in certain places they do have to take account of the sinkhole situation. Some places have special planning routines in place, which involve detailed investigation and require appropriate mitigation measures to be put in place. The problem comes in actually interpreting the geology and whether mitigation measures are robust enough - so there is some discussion going on about what are suitable mitigation measures, and whether some sites should be built on at all.

This Q&A is adapted from the Planet Earth Podcast, 4 March 2014.