Are humans causing more sinkholes?

It’s a pretty uncontroversial yes this week. Humans do contribute to sinkholes and probably have ever since the invention of irrigation. Anything that has the potential to divert water into weak points beneath the earth will accelerate the creation of the pits into which houses, cars and unfortunately people sometimes fall. The drying out of the ground by abstraction or the soaking from a burst pipe are the most direct examples of human-caused sinkholes.

But it is important to remember that the timescale for many of these events is thousands of years, meaning the majority of sinkholes started undermining the earth long before humans had anything to do with it. These are primarily natural phenomena, which humans can occasionally influence.

The connection to climate change is strong. Greater variability in rainfall will almost certainly create a more volatile earth. But more sinkholes are pretty low on the list of things we should be concerned about because of climate change.

Today’s question is a bit of a ruse. By asking whether these events can be attributed to humans, we really want to know if there is anything we can do to stop them. It is mostly impossible, or at least too expensive, to check whether the house you are buying or building is on top of a giant alka-seltzer. But even in this month of madness, sinkholes are tremendously infrequent events. Their occurrence is likely to increase and there are things we can do to mitigate the risk of them happening, but the interest in them, driven by the media (myself included) harps on the primal fear I mentioned in my intro. The sickened fascination induced by sinkholes makes them seem like a present danger, which they really are not. I’m reminded of George Monbiot’s piece on sharks this week in which he said the extreme domesticity of our lives makes us “believe that any remaining hazards presented by the natural world are far more dangerous than they really are”.

Thanks for the debate today and all the great comments. Watch your step!

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Is there any way to detect cavities in the ground which might cause dangerous sinkholes? Or does so little time elapse between cavity formation and a sinkhole event that it would be impossible unless all vulnerable ground was monitored 24/7? (That sounds like a logistical impossibility to me.)

Sorry if this was mentioned. I just skimmed through the updates because I'm a little busy and using a tiny screen.

fauxtronic20 February 2014 3:50pm

Thanks Fauxtronic for this question. I spoke to John Howell at Aberdeen University about this and he said there are techniques that can identify cavities. These include seismic instruments, ground penetrating radar and microgravity. But they are pretty expensive exercises.

Alan Cripps from the Royal Institute of Chartered Surveyors says none of these measures are common property surveying techniques and even if they were, it would not guarantee your home was safe from sinkholes. The occurrence of a burst pipe or some other undetectable leakage could create a rapid hollowing out under a property that would not be there when you first moved in. The best you can do is avoid buying a house sitting on limestone or gypsum.

Surveyors will often have good local knowledge of an area’s mining history and this should allow them to help you avoid mine shafts, but even then only a fraction of the country’s mines have been recorded.

Thanks to Cripps for the image above.

Nigel Cassidy from Keele University says the occurrence of sinkholes raises questions about water management and urban development. The recent flooding has highlighted the country’s ineffective storm drainage. He said government deregulation of development combined with increasingly temperamental rainfall due to climate change was cause for concern. Sinkholes can occur during drying out after lots of rain has compromised the integrity of the ground.

“Lots of water, then dry could see these type of phenomena become a lot more prevalent and a lot more problematic.

“We don’t manage our storm drainage properly, from a flooding point of view. One of the implications of not having that is the higher risk of sinkholes in urban areas.”

Cassidy said the particular geology of chalk sinkholes in parts of England, such as Hemel Hempstead, might cause issues with drinking water. All the water that creates a sinkhole has to go somewhere, he says, and in the UK it could leak contaminated water into the aquifer from which drinking water is drawn.

As I have said, this month has been an exceptional one for sinkholes. Hence why I’m sitting here writing about them. We know that the primary trigger for these events has been abnormal rainfall. But what were the particular set of circumstances that caused these holes?

Ripon

This area is well known for its sinkholes, caused by a large deposit of highly erodible gypsum. Gypsum is plaster of paris. Its chemistry (CaSO4.2H2O) makes it highly soluble in water. Much more so than limestone. Ripon has the worst gypsum related subsidence in the UK because it is a large deposit, close to the surface.

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Hemel Hempstead and High Wycombe

The rock beneath this area is chalk. Chalk can erode in slightly acidic rainwater over thousands of years and cause sinkholes. But the added complication is the historic practice of chalk mining. Chalk mines typically consisted of a shaft or pit that fell straight down and then spread below in a series of small mines. These were then filled in with unstable material which can eventually leak away leaving a cavity.

This post was amended after reports of a sinkhole in Swansea were found to have been misreported.

I have an answer to this question from Tony Waltham, who literally wrote the book on sinkholes (it’s called Sinkholes and Subsidence). He said the circular shape of sinkholes is caused by equilibrium. Soil and rock escapes through a hole at a certain point underground and the resulting subsidence centres on that point. Like sands through the hourglass.

I have been trying to find out whether there is a possible link between fracking and sinkholes as some activists have suggested. Many of the reports seem to conflate sinkholes and the reports of mini-earthquakes that have been blamed on fracking. But the idea got me wondering about how the massive abstraction of water needed to perform fracking, might affect the occurrence of sinkholes.

David Shilston, president of the Geological Society of London, said: “The abstraction of water from the shallow ground surface can cause a collapse if the ground already has the propensity to collapse.”

He said it is possible that an activity like fracking, if it took water from the wrong place, could cause sinkholes. But there is no one to one correlation between the two.

“Fracking will have to take water from safe sources,” he said, meaning renewable, continuous aquifers and above ground.

Chris Smith, from Cambridge University’s Naked Scientists, says the vast majority of sinkholes are naturally occurring, simply because the area they occur in is vast. 15% of the land mass of Africa, Europe, Britain and Australia and 20% of the US sits above limestone, which is one of the major soluble rocks that can create sinkholes. Most of this land is uninhabited and so there is little direct human impact.

But, there is a link, says Smith between the acidification of rainwater and sinkholes. Sulphurous acid and nitrous acid in rain erode limestone over many, many years. Smith says it is likely we are accelerating this process by pumping sulphur and nitrogen into the air during the burning of fossil fuels.

David Shilston, president of the Geological Society of London, disagrees, saying atmospheric pollution would eventually change the ground chemistry but only after thousands of years. In human time frame and especially in 200 years of industrialisation, we probably haven’t seen a significant effect.

Human activity has little to do directly with the formation of sinkholes.

But there are also situations where mines collapse. Llanrhidian Marsh on the north gower coast had a hole in it one day. Coombe Down in Bath has suffered collapsing of mines, and there have been one or two cases in Dorset too.

If Climate change is leading to more intensive rainfall events, then previous above and below ground drainage systems will become expanded to cope with that change. This would mean more sinkholes, spring lines, landslips and the like as land previously stable is no longer stable. It would also potentially lead to more collapses of mine adits etc.

Proving a link between one specific incident and climate change would be tough, but the physics underlying the geological change is straightforward, as is the link to demonstrate our actions are making a difference.

DavidHodd20 February 2014 11:35am

Almost inevitably, climate change rears its head. Without getting too deep down that particular sinkhole I’d like to refer you to last week’s eco audit in which I looked at the link between the recent storms in the UK and climate change. We know climate change is making it rain more in Britain. It follows that we will see more sinkholes as a result.

There is no particular research that I can find which links sinkholes and climate change. But Harley Means, a geologist at the Florida Geological Survey (FGS), told New Scientist that a correlation was not unlikely.

It is possible, says Means, although there is no direct evidence of this. Torrential rainstorms and long droughts are both predicted to become more common in the next century as the planet warms, particularly in the south-eastern US. “It’s speculation right now,” says Means. He suggests that research that compares climactic conditions with periods of high sinkhole activity could provide evidence.

But Anthony Cooper from the BGS says high incidence rates may only last for a time. There will be a number of locations close to collapse that could be pushed over by a spate of abnormal rainfall. This would see the kind of spike in occurrences we are seeing now. Once these have caved in, he said the rate decrease.

Underwater sinkholes...

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Surveyors Geoinvestigate said recent sinkholes in High Wycombe and Hemel Hempstead were likely a result of natural fissures in the rock or historic chalk mines, which would not have been recorded. Chalk mines were accessed by a vertical shaft which creates dangerous conditions for sinkholes.

The workings date from 1700s – 1900s and have been identified in Nash Mills and Highbarns areas in the south of the town. Chalk was mined for building stone and burnt to form lime to spread over clay soils to improve agricultural land. Typically the workings were small and reached by a vertical shaft in the chalk which would be infilled at the end of the shafts life, the miners moving on to dig another shaft nearby.

Because of water erosion and softening of the ground natural fissures or openings in the chalk or man made mine opening eventually became so large that the clay soil which covers and hides these holes can no longer provide a solid bridge over it. The result is that the ground can collapse either gradually or as at Oatridge Gardens [Hemel Hempstead] catastrophically and without warning. Where the ground failure is progressive and slower it is sometimes possible to see the signs of problems such as building cracks, building subsidence and the formation of saucer shaped depressions in the surfaces of driveways, roads, lawns or agricultural land.

Anthony Cooper, a geologist at the British Geological Survey (BGS), says humans can certainly play a role in the creation of sinkholes, but they can also be avoided.

“I don’t think man in this country is aggravating them to a large degree, although there are exceptions and there are precautions that need to be taken. If people do things properly, there shouldn’t be an increase.”

He says proper drainage is important to avoid sinkholes, as well as the careful monitoring of pipes and runoff.

The BGS website says:

Several things can trigger sinkholes. The simple process of gradual dissolution can cause a sinkhole to form at the surface.

However, other factors, including humans can induce sinkholes to form, such as:

• Heavy rain or surface flooding can initiate the collapse of normally stable cavities, especially those developed within superficial deposits.
• Leaking drainage pipes, burst water mains, irrigation or even the act of emptying a swimming pool are all documented examples of sinkhole triggers.

The commenter below summarises the way in which human activity can channel water into areas and contribute to sinkholes:

There are a number of contributing factors to consider here:

1. The increase in population and the spread of the population means that there is a greater chance of encountering a sinkhole. So the odds go up simply by being more proximal.
2. The increase in impermeable surfaces means that rainwater does not infiltrate direction into the ground and percolate into the water table (again this may be as a result of urbanisation). This means that where rainwater/surface-water does finally make it to a place where is can infiltrate into the ground, this infiltration is concentrated. Combine that with an area where the erosion of sand and gravel results in a sinkhole and you have an increased likelihood (though by no means a certainty).
3. Building, mining, piling etc in prone areas can alter the loading, drainage and groundwater regime, thus increasing the likelihood of a sinkhole forming, or possibly speeding the process up.

As someone else pointed out, the erosion of rock to form a sinkhole is on a geological timescale (albeit a pretty quick one by those standards) not really a human one. However the removal (washing out, collapse etc) of infilling sediment to expose the sinkhole can be abrupt (it's a bit like Kerplunk). It's also worth remembering that some features attributed to sinkholes are a result of historic mining and the collapse of addits. Cornwall (for example) is littered with old shafts and addits, and historically no-one was very good at keeping track of where they were.

munkeyspasm20 February 2014 12:10pm

Sinkholes can also be caused by a dramatic reduction in ground water, says Cooper. “A water filled cavity beneath a bridge of material will actually support that bridge to a certain degree. The physical presence of the water helps to support the cavity. So if you the pump a lot of water out, all of a sudden that material which is water saturated and heavier suddenly collapses in.”

There have been a number of documented examples in Florida and China (and suspected cases in the UK) in which abstraction for agriculture has caused the earth to subside.

“Plant City, pop. 32,000, is Florida’s strawberry capital. During a record cold snap this past winter, farmers pumped millions of gallons of water onto their strawberries to keep them warm. Cavities in the limestone aquifer emptied, weakened and gave way,” said the Wall Street Journal in 2010.

Ian Stewart (the geologist, not the mathematician) covered this quite well in Horizon a few weeks back. He put the blame mostly on urban sprawl, but he also said that increased agriculture in some areas was causing increasing volatility in the local water table, which he said could also be a factor.

Damian Crosby20 February 2014 11:40am

It is still available on iplayer and well worth watching:

http://www.bbc.co.uk/iplayer/episode/b03tz705/Horizon_20132014_Swallowed_by_a_Sinkhole/

giveusaclue20 February 2014 12:01pm

According to the BGS, Lewis Carroll was inspired by the common sinkholes around Ripon, not far from where he lived as a boy, when he imagined the rabbit hole that lead Alice to Wonderland.

Alice fell down- down- down- deep into the earth, following the white rabbit. Curiouser and curiouser she thought, in this underground wonderland the walls are made of sparkly gypsum - CaSO4.2H2O (with apologies to Lewis Carroll). It has been suggested that the author’s vision of Alice falling down a deep vertical hole into an underground land was inspired by natural geological events, notably subsidence at Ripon in North Yorkshire. There is a connection between the author, the city of Ripon, and dramatic subsidence that occurred at Ure Lodge, where the alleged model for the published ‘Alice’ illustrations used to live. Beneath this area, the gypsum has a water-filled cave system within it, but gypsum dissolves quickly so that the caves enlarge and commonly collapse. Collapse at Ure Lodge has continued to the present day. It recently caused the destruction of four modern garages and the evacuation of several houses, including the Lodge itself. Subsidence in Ripon, and many other places underlain by gypsum poses a severe constraint on the development of those areas.

Thanks to Robin Bisson from the Science Media Centre for the tip off.

Art critic Jonathan Jones has written for the Guardian about holes in the earth in art and literature. Rather than fear them, he says, we should allow them to awe us into delight as they give us an insight into the momentous forces that create the ever-changing world.

To build or buy a house, to simply drive to work and expect home to be there when you get back, is actually to take a massive gamble on a turbulent hidden planet. Sinkholes are no surprise - just a sudden revelation of the massive forces that are constantly changing the ground beneath our feet. Catastrophic change and unimaginable metamorphosis are the rule, not the exception, in geology. Sinkholes merely make visible what we choose to ignore.

Another view of Britain’s soluble rocks, this time with the different types of rock. The timescale of erosion of limestone is thousands of years, according to Anthony Cooper of the BGS. Whereas gypsum can disappear in tens of years.

Water. Water. Water. Anthony Cooper from the British Geological Survey (BGS) says the single most important element in any sinkhole event is water, either in absence or abundance. Too much water can cause soluble rocks such as gypsum and chalk to dissolve and erode, creating underground shafts. Conversely, if ground water is removed, through abstraction or prolonged drought, underground rocks can crumble under the pressure from above.

Sinkholes can occur slowly or dramatically. This is dependent on the material that coats the surface. Sand will subside along with the material beneath. Meaning a gradual sinking. But a more robust material like clay can hold together for much longer, leaving a chasm beneath.

The Guardian’s John Henley investigated what causes sinkholes in 2013:

Natural sinkholes – as opposed to manmade tunnel or cave collapses – occur when acidic rainwater seeps down through surface soil and sediment, eventually reaching a soluble bedrock such as sandstone, chalk, salt or gypsum, or (most commonly) a carbonate rock such as limestone beneath. In a process that can last hundreds, sometimes thousands of years, the water gradually dissolves small parts of the rock, enlarging its natural fissures and joints and creating cavities beneath.

As the process continues, the loose, unconsolidated soil and sand above is gradually washed into these cracks and voids. Depending on how thick and strong that top layer is (sand will not last long; clay can hold out for millennia), and how close to the surface the void beneath is, the land may be able to sustain its own weight – and that of whatever we build on top of it. But as the holes grow, there will come a day when the surface layer will simply give way.

“Once those caves start to collapse, the materials above will simply funnel in,” says Dr Anthony Cooper, a principal geologist at the British Geological Survey, which maps the country for rock types susceptible to sinkholes and carries out surveys for developers, builders and individuals worried about the prospects of the land caving in beneath them. “It’s just like an eggtimer, really.”

When does a pothole become a sinkhole?

Britain’s skin has broken out in a rash of sinkholes this winter. Normally, Britain averages one reported sinkhole each month. But in February five significant holes have opened. A woman is in hospital in Swansea after her car fell into a hole in her garden yesterday. Another in High Wycombe swallowed a Volkswagen Lupo.

Scientists from the British Geological Survey (BGS) say this winter’s incessant rain is the major factor in the number of sinkholes. But the other important factor is the type of rock below the surface. Soluble rocks like gypsum and chalk, which abound in parts of England and Wales, can erode over time, leaving a chasm beneath our feet.

In Britain, scientists differentiate between sinkholes caused by erosion of rocks and the collapse of old mine shafts. Mineral extraction has left the country riddled with unmarked holes, which were not gazetted until the 1870s. This means Britain is prone to both sinkholes and mine collapses.

Sinkholes speak to the same fear as earthquakes and volcanos, they are the moment when our benign home reveals its true and violent nature. We see Earth as an immovable object. “Solid ground” is our habitat, our safe, intransigent anchor in a dangerous universe. It’s also a myth.

Earthquakes and volcanos are created by forces that transcend humanity, they are beyond our control. But this is not the case with sinkholes. Some are caused by humans. A yawning pit that swallowed a factory in Guatemala City in 2010 was caused by a leaking sewage pipe.

Britain has seen a spike in the occurrence of sinkholes in the past month. Geologists say water is the primary factor in any sinkhole event, and Britain has certainly had its fair share of that. But what other factors are at play? Are sinkholes becoming more common? What role do humans have in their creation? And can we control them?

Please join in the discussion by contributing in the comments below,tweet me or email me. If you are quoting figures or studies, please provide a link to the original source. Later I will return with my own verdict.