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How county faces slumping into the past with sinkhole explosion

PUBLISHED: 06:30 05 April 2015

Collapse of four garages into sinkhole after dissolution beneath it Credit: British Geological Survey © NERC

Collapse of four garages into sinkhole after dissolution beneath it Credit: British Geological Survey © NERC

British Geological Survey © NERC

Prolonged wet winters are washing away our landscapes leaving some parts of the county literally slipping into a big black hole. But just how are these holes created and what are we falling into?

They’ve brought traffic to a stand-still, swallowed people’s garden sheds, even uncovered long buried graves.

The sinkhole has become a rather familiar sight in the county over recent years.

Just this year, a shed in Swanley was consumed as the ground below it literally opened up beneath it. Homes and businesses arond a square in Gravesend had to be evacuated after a gaping hole appeared just last month, while perhaps the best known of them all, was when a 15ft-deep hole suddenly emerged on the central reservation of a busy stretch of the M2 near Faversham this time last year, forcing a partial closure for nine days.

So just what are these sinkholes - or in some cases Deneholes (deep. ancient, man-made chalk mines) and how are they being created?

The M2 hole, which was some 15ft deep and required more than 40 tonnes of pea shingle to fill in, was caused by the wet weather wearing down the soil and weakening the roof of an old man-made chalk mine, according to experts, who found evidence of the activity on old maps.

Initially, however, there was uncertainty, as the entire area rests on chalk bedrock, called the Seaford Chalk Formation. This mineral is partially soluble by weakly acidic water. If it’s swamped with water, either because of heavy rainfall or a burst water main, it can erode very fast. Large swathes of the county’s geological formation has chalk underlying it, and this has led to fears that sinkholes could crop up with greater regularity. Especially given climate change making us more vulnerable of heavy, prolonged periods of rainfall.

Dr Vanessa Banks is the team leader for shallow geohazards and risks at the British Geological Survey – the leading geoscience research unit in the country.

She told KoS that there was scope to see more occurrences of such holes in Kent in the future.

“There are a number of issues to be concerned with in the future. High rainfall in Kent could lead to more sinkholes appearing, as water is normally the trigger for them to form,” explains Dr Banks.

She said the county was an area which, historically, had a greater number of small chalk mines. These ancient chalk mines - which normally only comprise one shaft, and a single chamber are the Deneholes.

Dr Banks said: “In medieval times, chalk would be dug up and used on the fields to get them ready and primed for crop planting. Kent also has a number of historic brickworks which would have made used of mined chalk. Where it was taken from the ground, once the reserves were exhausted, the Deneholes would be capped, usually with organic material like wood.

“This organic matter can, over time, give way, leading to deneholes appearing suddenly. Water, again, can exacerbate this degradation of material and speed the whole process up.”

Liquid penetration, it seems, is key to the sinkhole issue. During 2014, a large number of sinkholes appeared, particularly during the early part of the year.

Dr Banks and the BGS attributed this to what was described by the Met Office as an ‘exceptional period of winter rainfall’. Between December 2013 and January 2014, the south east witnessed 372.2mm of rain. That was more than any other two month period since 1910.

Dr Banks told KoS: “Those storms saw a powerful jet stream weather pushing low pressure across the Atlantic which hit the UK. If we saw more of this weather over Kent, then we would almost certainly see more sinkholes and Deneholes likely to emerge.”

According to environmental consultant Alice Roper, based on Sheppey, that could be a problem.

Ms Roper said: “Our climate is changing in the UK, and most projections are that we will start to see milder, but wetter weather across the country. This will obviously lead to higher levels of rainfall, which will impact the stability of the ground in places which have lots of chalk.

“Sudden bursts of rain can overwhelm the drainage system already, and this is likely to get worse with more of it across the county. In areas which are underlain by chalk, this could lead to water erosion which could make them unstable.”

While sinkholes are an issue, Deneholes could be the big issue in the future. because of the abundance of chalk mining in the county.

Secretary of the Kent Underground Research Group, Hugh Farrar, said that the extent of the chalk mining holes is unknown in the county – according to him, there could be hundreds which are currently lying undiscovered.

He told KoS: “They’re quite widespread across the county, with the peak chalk mining activity around the 13th and 14th centuries. Some people will tell you that there are thousands, though I’d be a bit more conservative with my estimate and say it’s more likely to be in the hundreds. The reality is though, we don’t actually know really, and we don’t have any idea where they are.

“They are only really likely to be discovered when eventually their cap fails, or someone stumbles across one.”

However, according to Mr Farrar, there is good news for people panicking about their house sinking into one of the county’s Deneholes.

“There isn’t a real reason to worry too much about Deneholes, as the vast majority of them were dug in very agricultural areas. Because of the chalk’s primary use on the fields, you’re unlikely to find many, if any in urban areas.”

And the BGS say that they are working hard to make sure that no other unexpected chasms open up across the county.

Dr Banks said: “Currently, the BGS is going through historical maps and archives to database as many Deneholes as possible. Many locations are laid out on historical maps, but they have fallen off current maps. That means we are trying to update all known locations so that we can make sure they are protected.

“Sinkholes are also being investigated, although as they form naturally, they take a bit of modelling. We are currently working on susceptibility mapping to try and predict the areas which are likely to be most prone to them. In Kent, there are likely to be a number of areas which are susceptible to sinkholes forming because of the underlying geology.”

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