Large Sinkhole opens up in Otford, Kent

[j1m]

A large Sinkhole was recently discovered by council workers mowing a disused limestone quarry on the edge of the south downs in Otford Kent.

They almost lost their tractor in it.

This sinkhole sits roughly in the path of underground waterways that bubble to the surface about 200m away.

I've always wondered for years if there were caves around Otford, and this sinkhole presents some major issues for the village, as half of it is built on the limestone hill itself.

This has put a lot of the village at risk, but as I understand it, the council have so far done nothing about it.

I wondered if anyone here knows the best step to take to identify risk in the area?

 

[Lankyman]

I wondered if anyone here knows the best step to take to identify risk in the area?

Very light ones?

 

[j1m]

Very light ones?

Won't be me then at 16st

 

[Andy Farrant]

Most probably a crown hole over an old chalk mine, but as you have not provided any details of where it is, or size it is not possible to give an assessment of the hazard. I might add the risk depends on what you are doing; a small crown hole in an open field may be a low risk, but a major risk if on a construction site. Not likely to be many natural caves in the area, any that do occur are likely to be very small.

If it relates to the old chalk pit just east of the railway station, then the council have done quite a bit to investigate the hazard including geophysical surveys. This might be crown hole over a chalk mine or shaft, or settlement of any back-filled material in the pit.
See: https://www.sevenoaks.gov.uk/news/article/459/otford_chalk_pit_closes_following_new_safety_concerns

 

[j1m]

Most probably a crown hole over an old chalk mine, but as you have not provided any details of where it is, or size it is not possible to give an assessment of the hazard. I might add the risk depends on what you are doing; a small crown hole in an open field may be a low risk, but a major risk if on a construction site. Not likely to be many natural caves in the area, any that do occur are likely to be very small.

If it relates to the old chalk pit just east of the railway station, then the council have done quite a bit to investigate the hazard including geophysical surveys. This might be crown hole over a chalk mine or shaft, or settlement of any back-filled material in the pit.
See: https://www.sevenoaks.gov.uk/news/article/459/otford_chalk_pit_closes_following_new_safety_concerns

Ah thanks for this, and yes it's the chalk pit sorry.

Was aware council have investigated the pit itself, but it's the village that everyone's concerned about, there's a major road directly above the edge of the chalk pit and a lot of homes too.

I was more interested to know if anyone here has any case studies of similar incidents and whether or not this kind of activity poses a risk for nearby infrastructure.

 

[Cantclimbtom]

Ah thanks for this, and yes it's the chalk pit sorry.

Was aware council have investigated the pit itself, but it's the village that everyone's concerned about, there's a major road directly above the edge of the chalk pit and a lot of homes too.

I was more interested to know if anyone here has any case studies of similar incidents and whether or not this kind of activity poses a risk for nearby infrastructure.

Similar incidents...? well a collapsing sand mine, OK not chalk but a historic mine does spring to mind...

But Andy did state above that there have been geophys surveys etc in Otford to see if anything nearby is at risk. His link above seems to imply the risk is likely in the pit area not beyond?

Maybe you could go to the National library of Scotland who have online historical maps (including England) and take a look for reference to other mines/pits etc nearer the village?

Edit: online maps, don't mean going to Scotland for this

 

[Cantclimbtom]

Not seeing anything worrying on the 1840s map

Georeferenced Maps viewer - Map images - National Library of Scotland

Not anything worrying on the 1940s one either

Georeferenced Maps viewer - Map images - National Library of Scotland

 

[PeteHall]

I was more interested to know if anyone here has any case studies of similar incidents and whether or not this kind of activity poses a risk for nearby infrastructure.

Not directly relevant, as the geology is very different, but there was a massive project undertaken by Hydrock, where I used to work, to stabilise the old stone mines at Combe Down.

There is a book about the project, available here: https://www.museumofbathstone.org/v.../the-stabilisation-of-combe-down-stone-mines/

A DVD is included with a film produced by Mendip caver Rob Franklin.

This is possibly one of the more comprehensive case studies that you are likely to find.

A Google search will bring up a lot of relevant info.

 

[Fulk]

This sinkhole sits roughly in the path of underground waterways that bubble to the surface about 200m away.

I'm sure I've read somewhere that the longest proved sink-to-resurgence underground route in the UK is located somewhere in the chalk of the southern part of England, and that it had quite a rapid flow-through time (suggesting open passage of some sort for most of the way); the figure I have in mind is something like ten miles in a straight line. But I've never heard of any follow-up work on this. Does anybody know anything about this, and the potential for caves in the chalk/limestone of Southern England? By the way, I'm aware that some folk say that 'chalk is mechanically incompetent and is incapable of supporting 'proper caves''; they should go and have a look at the caves of Flamborough Head.

 

[alanw]

I'm sure I've read somewhere that the longest proved sink-to-resurgence underground route in the UK is located somewhere in the chalk of the southern part of England, and that it had quite a rapid flow-through time (suggesting open passage of some sort for most of the way);

These might be of interest:

Karst hydrogeology of the Chalk and implications for groundwater protection

OR/15/042 Chalk karst and groundwater flow in the Pang and Lambourn Catchments - MediaWiki

earthwise.bgs.ac.uk

 

[Fulk]

Thanks for that alan.

 

[j1m]

These might be of interest:

Karst hydrogeology of the Chalk and implications for groundwater protection

OR/15/042 Chalk karst and groundwater flow in the Pang and Lambourn Catchments - MediaWiki

earthwise.bgs.ac.uk

This was very interesting thank you

 

[Cantclimbtom]

Luckily for the residents of Otford, according to BGS well records the Greensand is (in the case of the one I looked at: TQ55NW53) 301 feet deep. So if there are any minor karst (cave type stuff) features at the boundary of chalk and sandstone, they'll be 100m below the village and I can't see it affecting anything on the surface

GeoIndex - British Geological Survey

A map–based index of a wealth of information that BGS has collected or obtained from other sources.

mapapps2.bgs.ac.uk

 

[Cantclimbtom]

Lots of water in Otford village if you're willing to dig for it. No natural caves and can't see much history of underground chalk workings in the village although not too further along the Darenth valley there are some notable dene holes and chalk wells so a chalk well is possible for the region but any seem extinct and obscure. Unlikely any in the village itself, as groundwater is hit at only about ~1m or so down

Chalk pit

Excursion to Otford and Darent Valley. Saturday, July 3rd, 1909 - Geologists' Association excursion - MediaWiki

earthwise.bgs.ac.uk

 

[mikem]

Previous discussions:

Thread 'New sinkhole in Hemel Hempsted'

Feb 15, 2014

http://www.bbc.co.uk/news/uk-england-beds-bucks-herts-26206764
BBC item about a shakehole under a house. Brief web search shows the area is on chalk, and has underground workings for the same which have collapsed in the past giving other shakeholes. There are also natural chalk swallow holes in the vicinity...

• Brains
• Replies: 3
• Forum: Idle Chat

& From 2nd comment:

R

Thread 'Sink Holes'

Jan 29, 2014

Heads Up.
Horizon - BBC 2 Monday 3rd Feb at 21:00hrs

"In February 2013, a hole opened up beneath a home in Florida, and swallowed a man.
Jeff Bush was asleep when a sinkhole opened up beneath his bedroom. Despite the efforts of his brother to rescue him, Jeff was never seen again and his body was never recovered.
Professor Iain Stewart travels to Florida to try and understand what killed Jeff, and why the geology of this state makes it the sinkhole capital of the world."

• Rich West
• Replies: 33
• Forum: Caving Chat

 

[mikem]

Plus, more recently & closer:

Thread 'Huge 'sinkhole' emerges in Bexleyheath'

Jun 22, 2022

This is only about 1000m from my home.

Kent Live link

It is certainly a large collapse, but not a sinkhole. It is on unpromising geology of Eocene sediments near the boundary of the Harwich Formation and Lambeth group. The site used to be part of the grounds of a large house called Marten's Grove, so some buried trace of human activity is possible. I think a small tributary of the River Cray once flowed near here and left some fluvial sands behind. A nearby house, long demolished, did once have an ornamental water feature. Old maps offer no obvious explanation. This area of...

• Graigwen
• Replies: 31
• Forum: Rest of The UK

• 
• 

Also:

https://ukcaving.com/board/index.php?direct-messages/bexleyheath.66604/

 

[mikem]

The council have provided more details of their investigations. The quarry was landfilled, so no surprise that it is subsiding, but nothing to suggest other areas might be affected:

Otford Chalk Pit | Sevenoaks District Council

The former chalk pit in Otford is temporarily closed after large and potentially dangerous cavities we uncovered throughout the site.

www.sevenoaks.gov.uk