Void under foundations

[brycheiniog]

All,

While investigating a couple of small holes that had appeared on the edge of my drive, I discovered a large void which appears to run underneath the ring foundation of my garage block. The area I was looking at is to the LHS of the garage where the car port is. We are out in the fens so ground conditions are not ideal for building so I am slightly concerned the garage is going to start disappearing into a big hole in the ground! The garage is supposed to be on piled foundations born out by my discovery of one of the piles (See photo below..)

The garage:

The area of the drive showing where I stuck the spade in initially. The spade met very little resistance... The two holes looked a bit like rabbit holes but could have been caused by water I guess. There are some cracks.in the clay surface under the gravel..

I thought I should have a look a bit further back so stuck the shovel into some cracks in the soil close to the garage and the spade basically disappeared. The soil seems to have a consistency of sand which I am guessing is imported topsoil as we are out in the fens and the rest of the garden is very peaty. A couple of minutes of digging produced this:

Sticking a camera and light down yielded:

Here is a video clip looking underneath:
https://photos.app.goo.gl/nqNwCfDEtdDzs6Dy7


The video clip was taken from the end of the trench nearest the fence. The void appears to run as far as my torch would illuminate across the back of the garage and obviously the whole way forward to the front of the garage. The void is about 2 feet across and the inner wall seems to be peaty soil. Judging my the cobwebs and so on the void has been there some time.. There is no sign of movement anywhere in the garage structure so I guess the piles and ring are doing their job.

My questions are:

1) Do I need to do something about the void?
2) Assuming I need to get somebody to investigate further, what speciality should I be talking to?
2) What is the likely cause of the void?


Thanks!!

Jonathan

[footnote]Edited by brycheiniog1 on Sunday 30th September 20:56[/footnote]

 

[JohnD]

IME voids under drives, patios and houses have always been caused by long-term plumbing leaks washing away the soil.

However I have no experience of peaty soil so your might be different.

Local structural engineers and builders should be familiar with your conditions.

 

[KenGMac]

brycheiniog, good evening.

We have just come through a very dry hot summer [Sorry obvious statement] but, the lack of rain water could?? have allowed the peat to dry out if you think of it a bit like the London Clay related subsidence problems? most / all insurers are running flat out trying to play catch up with the volume of new Subsidence related claims pouring in to them

I have seen loads of Peat related Insurance claims where the property is built partly or entirely on peat at some point below the founds.

Your problem is IMO different in that you have peat + sand and if the structure was piled, it was piled for a very, very good reason, piling aint cheap

Can you track down the original structural engineer that instructed the design of the foundations? if so they may be able to shed some some light into the rational for piling, and this will assist on the way forward?

I have heard of such problems in a place I recall was called Ernisettle where the ground drops in areas.

Given that the founds have been piled, it is unlikely that there is a sink hole or similar, because the piles if driven into a sink hole would drop like a Tonne of bricks into the void, the operatives would detect this and more investigations would have followed.

The vid clip appears to show another exposed pile?

Placing your insurer on warning may be an option???

It is not "heave" as the ground is receding not lifting.

Ken.

 

[wessex101]

As Ken suggested you can get tremendous shrinkage in peat sub soils. I suspect the ring beam foundation was cast when the peat was wet and the presence of the garage slab and recent dry years has caused the peat to dry out and shrink.

It should not be a problem as the ring beam is sat on top of the piles and the piles themselves are sitting on much deeper more stable ground. If the ground beam and I assume fully suspended floor slab have been designed and constructed correctly you could in theory excavate the entire area under the garage and it would still sit quite happily on the piles.

If you start to see any cracking in the walls or floor slab then it is time to call a structural engineer.

 

[HERTS P&D]

I would be calling in a concrete lorry and fill the void with concrete. (if it was me) Before the garage starts to move or crack.

Andy

 

[SFK]

Daniel,
I was thinking about your suggestion to fill void with concrete.
My gut feel (no experience whatsoever) is that this will be a bad idea if the peat has shrunk because of a dry summer.
If you fill the void with concrete, add then we have a wet winter, I feel that the peat will take up the water, expand and the OP will have 'heave' resulting with the garage breaking its slab.
Again, only a gut feel as I suppose a chance the peat under the concrete will stay dry.
SFK

 

[HERTS P&D]

Daniel,

Who is Daniel?

Andy

 

[SFK]

Andy
AKA Daniel Craig!
Sorry, I forgot that spies prefer not to use their pseudonym.

 

[Notch7]

If the garage is quite a recent construction, I would get the building regs drawings from the local authority, that will tell you the construction.

If the garage is on a piled construction with a reinforced ring beam and concrete beam suspended floor, then the voids are irrelevent. Driven piles could go down 10 metres.

I dont know much about piles but I thought they were always round?

 

[stuart45]

There's loads of square ones.

 

[JohnD]

bored piles are AFAIK always round

But the driven ones I've seen are I think all square. Not sure if there were old round ones.

 

[KenGMac]

brycheiniog, good evening.

OK.

Driven piles placement I have been in charge of are square, no matter how deep.
Bored piles are round and the external surfaces are very rough where the concrete has leached into the surrounding soil.

You need to get the plans and if possible any lodged soils investigation reports, these two things will give you an insite into the rational of the design.

There is a web site that may? be of some use, I use it extensively to find out what the geology is that I am dealing with, the site is on I Pad apps, it is "I Geology" run by the British Geological Institute

As an aside? has there been any building work locally? and if so was there any "De-Watering?" to cope with a peat layer. Likewise, has a leaking underground mains water pipe been located and fixed or indeed a drain repaired?

Personally, i would not infill the void, at least not for a long time. peat tends to be very highly hydroscopic and can act at times like a slow ebbing and rising of a tide OK it does occur over time and in special circumstances?

Ken.