Gap beneath the whole concrete basement

[Alan CA]

Hi everyone,

I encountered this under a basement of a house, kver 40 years old. There is a gap that appears to be consistent under the whole concrete basement slab. This is kne foot deep under the slab soffit. The grade under it is almost flat. Concrete footing surface along on the perimeter is clean and doesn't say it was in contact with substrate (no stains). The bottom surface of the slab is rough as you can see. Not sure if the gap really extends under the whole basement area, but it looks so. The opening made in the basement slab was too small to check.
Do you think this is due to settlement?

 

[HTURKAK]

This is due to settlement of backfill inside perimeter grade beams . Apparently , it is not overall settlement of the bldg..

My opinion..



Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[LittleInch]

Curious.

Settlement of that amount more than that of the supporting foundations with no load seems unfeasible.

The side walls were clearly cast using formwork but the slab not. The side walls show no evidence of settlement happening under the slab.

Without more information on how the slab was actually constructed it's difficult to speculate more.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IRstuff]

What's the stuff at the back of this gap?

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[EireChch]

I had a similar in issue on my own house. The walls were supported on strip footings going down to natural ground, my backgarden and area beneath my house was raised about 1m. The kitchen slab had settled about 50mm.

This was about 5 years ago, I broke out the slab and cast a new 150mm slab with mesh and then dowled it into the perimeter walls. Hasnt moved since thankfully.

 

[LittleInch]

Eire,

50mm I can kind of believe, 300mm I can't.

Also was yours the whole slab which had dropped? This one has no load on it to force any settlement.

Unless he's had a river running under his house settlement seems unfeasible.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Expansive clay soil may shrink 10%, but this doesn't appear to be that because the ground does not have shrinkage cracks.

The structure must have been erected on top of an old landfill or mine.

 

[LittleInch]

But then surely the whole house would have settled, not just the fill between the piers?

Something quite odd going on here.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[coloeng]

When I was working as a plumber in the late 70s early 80s in the Denver area, there were some builders that would place cardboard support forms under the slabs where there was highly expansive soils. My understanding at the time was the cardboard would prevent the expansion of the soil from cracking the slab. If this was done in this case, could insects, ie termites, have eaten all the cardboard so that there is no residue left? Just going out on a limb here.

 

[JAE]

coloeng - there'd probably be some debris or remaining material left to some extent I'd think.

 

[SwinnyGG]

We use paper void form all the time. It's completely untreated, chemical free cardboard designed to rapidly decompose. It is completely gone in about 10 years.

In this case though, it doesn't look like a void formed slab; the bottom of void formed slabs are smooth. The void form behaves exactly like any other bottom form does, and leaves a smooth finish.