foam jacking a slab on grade adjacent to a retaining wall

[gte447f]

I have a situation where foam jacking (pumping expansive polyurethane foam under the slab to lift the slab) is being proposed to lift a slab on grade that has settled behind a retaining wall. Should I be concerned about possible lateral pressure on on the retaining wall from the foam? The wet weight of the foam? The injection pressure? The expansive pressure? My gut tells me that the pressure is minimal, but I have not been able to find any references addressing the issue.

A few additional details... The retaining wall is an 8" thick cip concrete wall 10.5 feet high. The slab is 10'x10'x12"thick bearing on backfill on the retained side of the wall. The edge of the slab opposite from the retaining wall has settled about 6-8 inches. The edge of the slab on the retaining wall side has not settled at all because it actually bears on the top of the retaining wall. One corner of the slab opposite from the retaining wall has a post on it that supports the corner of one floor and a roof above (about 100sf of each). There are no other loads on the slab except foot traffic.

 

[KootK]

Maybe check in with a dirt guy for good measure but my gut feel is the same as yours: non-issue. I've done it once without issue.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

I would imagine the injection pressure would be not much more than the weight of the slab. I guess it depends on the height of void behind the wall if it might be a problem or not.
Th mudjacksers around here are really cautious about doing garage slabs adjacent to basement walls and will take no responsibility for failure.

 

[jayrod12]

I've seen it used to lift some pretty heavy stuff, I wouldn't immediately discount the expansion pressure as negligible.

The wet weight, non-issue, even the injection pressure is likely not a problem. But if confined, then the expansion pressure could be fairly substantial.

 

[Once20036]

It sounds like you're going to be foaming the slab some distance away from the retaining wall, correct?

"The edge of the slab on the retaining wall side has not settled..."
"The edge of the slab opposite from the retaining wall has settled..."
Both of these statements are the slab on the high side of the wall, correct?
What's the horizontal distance from the wall to the area of the settled slab?

 

[msquared48]

I have seen horizontally cracked retaining walls from slabjacking at the topside of the wall.

However, using a foam rather than a flowable grout, it just feels that there will not be a problem.

Mike McCann, PE, SE (WA)

 

[gte447f]

Thanks for the responses everyone.

Once20036, yes the slab is on the high side of the retaining wall. It's a 10ftx10ft slab and one edge of it actually bears on top of the top edge of the retaining wall. The opposite edge of the slab is where the max settlement of 6-8 inches has occurred, but there is a constant slope from the high point at the retaining wall across the slab to the low point 10ft away. So yes, I guess its fair to assume the majority of the foam will be injected at the edge 10ft away and minimal foam will be injected directly adjacent to the top of the wall.

 

[KootK]

Any chance the slab laterally restrains the wall?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[gte447f]

KootK,

I can't tell if there is a mechanical connection between the slab and the wall. There is an elevated wood deck/walkway that frames into the edge of the slab/top of the wall from the side of the wall opposite the slab on grade, but I'm not sure I trust it to brace the wall.

 

[gte447f]

An update... I have written a condition assessment report (not a final design repair document/drawing) in which I have recommended using helical pile underpinning (2 piles on the slab edge opposite of the retaining wall) and under slab injection with poly foam. I've included a warning/disclaimer that, although significant lateral pressures are not anticipated, the contractor should monitor injection pressure, foam volume, and wall movement, and discontinue foam injection if there is any sign of unanticipated changes. Not iron clad, I know, but I'm not terribly worried about it. I suppose I should just recommend to temporarily brace the wall, but I'm sure that will be viewed as unnecessary overkill.

 

[KootK]

I was going to suggest the very same approach. While we're all justifiably worried about liability, the most important thing is usually simply for smart people to do smart things. What necessitates the screw piles in addition to the foam? Perimeter bearing wall settlement?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[gte447f]

Kootk... I specified the piles because, presumably the reason for this slab settlement is consolidation of poorly compacted backfill behind the retaining wall. The wall is 11ft high, so I am concerned that there could potentially be additional settlement in the future, and the foam at the surface will not do anything to address that possibility. Also, there is a post supporting structural loads located at one corner of the slab, and I inherently don't trust slab jacking near unconfined edges of a slab, especially if line loads or point loads on the edge of the slab.

 

[gte447f]

KootK, I meant to add that I think the screw piles alone would work without the foam injection. I am wary of using piles to jack a 4 inch slab, but this slab is 12 inches thick (for some reason). You could use only 2 piles to jack the side that has settled and then the slab would easily span 10 feet from the piles to the top of the retaining wall without any need for the foam injection. I guess the foam will do a nice job of filling the void under the slab that would otherwise be there after the slab is raised back to level.

 

[KootK]

Thanks for the clarification gte447F. Was just curious about the approach. makes sense.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[oldestguy]

Was surface water infiltration possibly involved with the backfill settlement? If so, maybe the foam will tend to direct that away from the treated area.