• Black Sheep Diggers presentation - March 29th 7pm

    In the Crown Hotel Middlesmoor the Black Sheep Diggers are going to provide an evening presentation to locals and other cavers.

    We will be highlighting with slides and explanations the explorations we have been doing over the years and that of cave divers plus research of the fascinating world of nearby lead mines.

    Click here for more details

Large Sinkhole opens up in Otford, Kent

j1m

New member
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?
 
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
 
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.
 
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
 
  • Like
Reactions: j1m
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.
 
  • Like
Reactions: j1m
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.
 
  • Like
Reactions: j1m
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

 
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

 
  • Like
Reactions: j1m
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
1000000975.png
 
  • Like
Reactions: j1m
Previous discussions:

& From 2nd comment:
 
Plus, more recently & closer:
Also:
 
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:
 
  • Like
Reactions: j1m
Back
Top