Flowable fill or 911?

[Bammer25]

Had a client that had a basement wall badly failing. We built some false walls and rebuilt the wall. No issues

As the contractor was waterproofing the wall he figured he would go ahead and make sure the rest of that wall (not failing) had gravel/drain pipe. Well now the garage slab is pouring out gravel and he is super nervous about doing anything (rightfully so). I am heading out there this afternoon to try to come up with a solution. I told him pumping flowable fill may be the best option as you will never get anything compacted back up on there. Here’s a picture. Thoughts?

 

[XR250]

That's a tough one. The lateral pressure of the flowable fill may blow-out the adjacent foundation wall.

 

[Bammer25]

The basement wall in that area looked fine. Maybe some shoring is what I was thinking and pump.

 

[XR250]

It may look fine now but how about when you add over 100 psf/ft of lateral pressure to it?

 

[Bammer25]

Only other option would be demo entire garage slab and start over

 

[XR250]

Backfill with some crusher run and pack it under the slab and footing as you go.

 

[Bammer25]

Just seems like you’d never get it even close to compacted under there

 

[XR250]

It is a pretty common practice around here.

 

[jayrod12]

You're able to compact granular underneath an existing slab?

That's news to me.

 

[SWComposites]

Can you build some sort of dam / foundation wall on the outside of the garage slab to stop the material flow and close out the gap, then cut a hole in the garage floor to inject fill?

 

[XR250]

You're able to compact granular underneath an existing slab?

That's news to me.

Crusher Run (ABC) is not granular backfill as it has a lot of fines in it and can be compacted
Just shove it under there and keep hitting it with the flat end of a digging bar.

 

[Canuck65]

I would consider pumping some flowable fill to support the garage floor. It will need to be done carefully to ensure the adjacent basement wall is not overloaded when the concrete fill is still plastic. The basement foundation wall could be temporarily shored while the flowable fill is being placed, or the rate of flowable fill can be controlled, i.e. add first lift and wait until set before adding 2nd lift, etc... Much less economical this way, but it won't overload the existing basement wall.

 

[jayrod12]

Interesting, never heard of that being done before. Wouldn't expect it to be as good as flowable fill. You could get the mud-jacking contractor's to come pump mud into the void pretty cheaply in my area.

 

[dauwerda]

Looks like the void is less than a foot high, I would have zero worries about pressure caused by flowable fill.
I'm not sure if it's a concern or not, but I know underpinning is often finished off with drypack grout as just straight concrete will shrink while curing and not provide positive contact/support. Just using flowable fill here might have a similar issue where a small gap is created and existing foundation and slab would have to settle some to make positive contact (probably small enough this wouldn't be an issue, but worth considering).

 

[TLHS]

The joys of pea gravel and other stuff people try to say requires zero compaction. It has a terrible friction angle and just flows if you run into the side of the original excavation

My two second thoughts are that this is now basically a voided slab, so do what we do for that.

Close up a bunch of the side, but leave at least a panel there that you can use to observe.
Drill injection and vent holes into the slab
Pressure grout or flowable fill into there until you're happy
Maybe to a GPR afterwards to search for voids and correct as necessary.

I'm also not at all worried about lateral pressure from flowable fill in this situation.