OT: Filling a sinkhole

[Part Timer]

I happened to turn my TV on this morning (BBC1 I think) and it was one of those have a moan about insurance not paying out type programmes. I woman had a sinkhole open up earlier this year in her back garden, somewhere near London or SE IIRC. Her buildings insurer refused to pay out as it made no difference to the insured property (sufficiently far away to not cause structural damage to the house).

They gave the dimensions as 8ft diameter and 20ft deep. She ended up paying 11 grand to get it filled in! So I did a bit of googling. I make the volume to be around 28.479 cubic metres. Crushed concrete "type 1" evidently weighs 2.1 tonnes per cubic metre. This job would therefore need around 59.8 tonnes. A supplier found at random wants £384 inc VAT for a 8 wheeled tipper full of "18-20 tonnes":

Worst case scenario 59.8/18 = 3.32 loads £384*3.32=£1275

Ignoring whether the insurer's smallprint or the householder were in the right, 11 grand! Plenty left over for some topsoil and grass seed! Am I missing something? Someone will probably find even cheaper filling materials.

[Bill Wright]

Lots of people are paying to dispose of poor quality mixed hardcore.

Bill

[Robin]

Not my thing but from seeing farmers long ago try to plug sinkholes a few thoughts:

a. access? I doub't the tipper driver would want to get that close to the hole even if he could. And if the material has to be 'barrowed through a terrace house.... b. compacting? If it starts off 20 feet deep you'll be needing to top up for quite some time to come if it's not compacted. And AIUI the days are gone when you get lower an apprentice down on a rope with a whacker ;) c. short-term cover-up or long-term cure? If there's an underground cavern you need more of an "engineered" fix with a concrete plug at the bottom and then a layer of clay to stop water doing its thing on the plug. And if the work is being done 20 feet down it ain't gonna be cheap

[Nightjar]

From the videos I've seen of filling in sink holes, it is done from a distance, using an excavator with a very long reach. Hiring that, with an operator, for a few days probably accounts for a fair bit of the 11k.

[Jabba]

Part Timer wrote

Before fill one in, you'd need to know what caused it. Can't just dump

30 tons of s**te into a hole that was created by an underground stream.

[polygonum]

Walter's Ash.

The hole was one metre from the house/bungalow so making sure that was safe definitely an issue - not as simple as a hole in the middle of a field. (Though clearly even those can have difficulties that are not immediately apparent.)

[Tim Lamb]

Field holes tend to be small at the surface and much larger at the bottom. Sign of an underground stream here (Herts.)

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[dennis]

fill it with two part foam!

[Tim Watts]

Would have thought large grade concrete rubble would be idea - will let the stream flow and will fill the hole.

[Part Timer]

Surprised nobody's mentioned £11k of car body filler yet!

[Jabba]

Tim Watts wrote

You don't know a lot about water. The holes between the large stones will fill with smaller rubble when the flow is low, that restricts the flow when flooding occurs. Starting the process all over again.

[Tim Watts]

OK - so what was there before the sinkhole? Was it really that permeable?

[Harry Bloomfield]

Part Timer was thinking very hard :

Maybe the expense might have been the hire of a crane to get it over to the hole, if the hole were inaccessible to a lorry.

Had the hole been left unfilled, no doubt it would eventually affect the house structure, I'm surprised the insurer refused to pay out to avoid further damage.

[Jabba]

Tim Watts wrote

Do you know how sinkholes are formed and how long they take before they fall in ?

[Tim Watts]

Gypsum in one case - and quite quickly (years).

Now, if you can get out of condecending mode for 5 minutes, please explain how filling the hole with loose rubble can be a bad thing?

[Tim Lamb]

ISTR the photo showed a rainwater down pipe coinciding with at least one of the recent holes. I don't see a problem with rubble fill but it may not meet property insurers requirements.

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[Brian Gaff]

Its the survey costs that are high, as these sort of holes seldom just occur, they are usually caused by flowing water. If you bung a big pile of concrete in the way, you then divert the water and it all starts again somewhere different. I am not sure what they do, but there was a recent programme on BBC 2 about them and some had to have larger holes made to investigate what the issues were. Brian

[Gazz]

Prolly cheaper to hae got it lined with concrete and tiled, swimming pool in the garden should increase the value a bit.... mind, an 8 foot wide by 20 foot deep pool may be of more use to high divers or scubadivers.