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Void Space Beneath Slab

 
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Old 04-29-2017, 10:26 PM   #1
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Void Space Beneath Slab


Im doing some remodeling inside of a 1924 bungalow in NE Portland. After cutting open the slab to reroute some DWV line it was quickly noticed that there was no soil beneath a 3x3' area. The bottom of the hole is about 18" beneath the slab. There will be a spread footing for a column that is getting relocated right where this void is.

Any suggestions on what I should do to help sturdy things up? im a bit worried about having a footing and lot of load on a spot that is settling.

Im assuming this is caused by settlement due to incomplete consolidation of the soil before pouring the slab.
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Old 04-30-2017, 12:21 AM   #2
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Re: Void Space Beneath Slab


I would be concerned that there is more of this scenario going on.

May be best to contact a SE or Soils Engineer to evaluate the situation.

Would relieve you of a lot of liability.

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Old 04-30-2017, 12:52 AM   #3
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Re: Void Space Beneath Slab


Was there a leak? Water can move soil.
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Old 04-30-2017, 09:29 AM   #4
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Re: Void Space Beneath Slab


No leak anywhere that we've found. So it is likely that there is more of this going on beneath other parts of the house?

There are reported issues of water coming up near where one of those drain lines surfaces. Would a poor downspout system cause this kind of erosion?
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Old 04-30-2017, 09:39 AM   #5
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Re: Void Space Beneath Slab


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No leak anywhere that we've found. So it is likely that there is more of this going on beneath other parts of the house?

There are reported issues of water coming up near where one of those drain lines surfaces. Would a poor downspout system cause this kind of erosion?
Yes. As would any number of other things. We have a combined storm and sanitary sewer here. Sometimes the rain drains went under the house. You might try to investigate those. If you can't tell where they go, run a hose into each for a while.

It seems pretty localized if the slab stayed flat with such a big void underneath. You could possibly core into the slab in a few other locations to try and determine if the problem extends throughout.
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Old 04-30-2017, 01:54 PM   #6
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Re: Void Space Beneath Slab


Drag a length of chain across the floor and you will notice a different sound as it crosses a hollow spot. No need to drill unless you find one of those sounds. The change is quite distinct and is easily noticed. If you try it where you know there is a hollow, you will understand.
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Old 04-30-2017, 06:56 PM   #7
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Re: Void Space Beneath Slab


k crete as we call it...or an engineer would call it "flowable fill"
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Old 04-30-2017, 09:20 PM   #8
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Re: Void Space Beneath Slab


If you have to put a column there, have a soils engineer check it out first. Adding weight to a problem soil is a really bad plan.

FWIW, I made my living for more than 20 years correcting these types of problems.

If you would like to discuss your situation in more detail, shoot me a message and I'll get with you by phone or email. There's often more to it than can easily be covered in a post.
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Old 05-03-2017, 08:03 PM   #9
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Re: Void Space Beneath Slab


I'm having trouble finding a residential soil engineer, anyone in the Portland area know of someone they've used?
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Old 05-03-2017, 08:15 PM   #10
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Re: Void Space Beneath Slab


Thanks for your input all.
Yes, the column must go here or else major modifications will need to be made to the design.

Frog_Legs, Have you dealt with issues similar to mine?

I really feel I could just do some heavy soil compaction and then fill this with some low psi concrete and move on, but am also weary to do that and then have 15000 pounds of load cause settlement issues...

I drove a long steel rod (3/4 x 3") about 3 feet down into the soil. I did find some very hard soil that I couldn't penetrate through using a 20 lb sledge hammer.

Feel free to email me at if you prefer to communicate that way.
Herynkc@gmail.com

-Corey
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Old 05-05-2017, 01:41 AM   #11
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Re: Void Space Beneath Slab


Quote:
Originally Posted by HerInc View Post
...................
I really feel I could just do some heavy soil compaction and then fill this with some low psi concrete and move on, but am also weary to do that and then have 15000 pounds of load cause settlement issues...

I drove a long steel rod (3/4 x 3") about 3 feet down into the soil. I did find some very hard soil that I couldn't penetrate through using a 20 lb sledge hammer.

................
-Corey
The age of the bungalow (1924), along with the considerable soil settlement, seems to indicate that poorly compacted and somewhat deep fill was under that part of the house. The waste plumbing has a bar-code tag on it, so that looks to have just happened. I'm wondering if that plumbing could be relocated enough to not be so directly under the needed footing.
I would get rid of what looks to be some recent fill around the plumbing out of there, and then maybe even remove some more of the older loose fill before doing your "heavy compacting" of the remaining soil for a good spread-footing.
If the plumbing cannot be relocated, I would isolate it from the concrete footing by wrapping it with some neoprene pipe insulation before the pour. That way, some later possible settling will not be as likely to damage the plumbing.

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