Basement Floor - Settling Under Slab

mt_42

I'm usually a decking/reno guy, but I figured I might be able to get some help on this side of the boards with a problem that I'm having at my own place.

We bought a house last year - New construction, just over a year old - and while starting to prep the basement for finishing, I noticed that there was a large area (roughly 10 x 10) of the poured floor that had a hollow sound when you walked/stomped on it. From being on-site and watching while the house was built, I know that there was a fairly good base of crushed gravel put down before they poured the final floor so I'm guessing that things have just settled after the brutal winter that we had last year. There are also a number of cracks throughout the floor but none of them are very wide and certainly not anything that I'd normally be worried about.

The question is if there is anything that I can/should do before I put down the new subfloor? Would there be any benefit to trying to backfill by drilling a small hole and injecting something into the cavity under the slab or is that just asking for problems? Is this type of issue something to even bother worrying about?

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Mike

JonM

I doubt you have anything to worry about, unless you have a spring or a stream running under your house......the slab is sitting on top of the footings and should be 4 inches thick.

AtlRemodeling

Was there any backfill under the basement slab or was it dug done to virgin soil?

I looked at a similar situation for a client that had a 6' subwall on the downhill side of the basement slab. The builder had simply dumped in dirt, "compacted" the fill with a skidsteer, then went about prepping and pouring the basement slab. In that situation, the entire center section of the slab was settling along with the basement walls and subsequently the main floor and 2nd floor. Turned into a real nightmare scenario.

The warranty company ended up buying the house after about a year or so of legal wrangling. We did not end up doing anything except the initial investigation then turned it over to the structural engineers.

I certainly hope you are not facing that problem.

mt_42

The best I can tell, the lot was dug out and footings poured on undisturbed soil (which as I found when drilling for the deck footings this summer is some of the hardest/ugliest clay I've seen in awhile). After the walls were poured, it looks like they backfilled with about 4-6 inches of crushed stone and then poured the floor from there.

We haven't had any water penetration issues within the first year and based on a 4" thick floor, I'm not expecting any structural issues (especially once the subfloor is in and the loads are even further spread out). The primary concern was that if there was something that should be done, it's definitely a lot easier to do now vs. once the project is finished.

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Mike

Chris Johnson

Your gravel was spread out below the slab but not compacted, it has settled slightly. You can hear a hollow noise but the gap and settlement is so small don't worry about it, no damage will happen from that.

It is a common problem you and a majority of your neighbours have...you just don't know it yet.

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Chris

painter213

On one note, If it does start to pose you problems, there is a process that uses foam to stabilize and even raise a slab back to level. They inject the foam under the concrete by drilling holes through the concrete and uses a special fitting where there mix block connects. Here is a link to a site: http://www.raise-rite.com/residential/concrete

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Benny Abbott
Abbott Consulting and Coating Inspections
E-Mail: babbott@abbottcoatinginspections.com
Phone: (205-717-0292)

genecarp

[quote=mt_42;526979]The best I can tell, the lot was dug out and footings poured on undisturbed soil (which as I found when drilling for the deck footings this summer is some of the hardest/ugliest clay I've seen in awhile).


You are able to determine the if soil is virgin or not by sight? the only way i am really certain of terrafirma, is if i am there for the dig. G

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genecarp

Man, that link is cool, never heard of that crazy stuff... G

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mt_42

Thanks for the responses, everyone. I figured that's what you would all say but it's nice to have it confirmed by the experts (and it helps settle a fight I was having with my paranoid wife about it...lol)...Now I just need to find the time between jobs to actually get down there and start working...

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Mike

sawyerEd

Looked at the raise-rite site. I know a contractor in Vernon BC that rebuilt new foundation walls around his existing house with helical piers screwed into the clay. The screws came in different sizes according to the expected loads corner, midspan or decks and were engineered. Single story house and the inspector was quite happy.