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New support for existing slab-on-grade with 4" void under due to soil settlement

New support for existing slab-on-grade with 4" void under due to soil settlement

New support for existing slab-on-grade with 4" void under due to soil settlement

(OP)

An existing slab-on-grade (s.o.g.) basement parking in an old office building has a 300 ft2± area where there is a 1" to 4" deep void directly under it. This was found by GPR and verified by a number of cores. Rod penetration indicates that the soil below the s.o.g. is very soft (#5 bar can easily be pushed in about a foot). The s.o.g. is 5"± thick and reinforced with 6"x6" 6/6 welded steel wire mesh located near the bottom of the slab. There is 3" thick layer of gravel on top of the soil, but of course that dropped down with the soil. The cost of breaking out the s.o.g, removing loose soil, replacing with 0.7 MPa controlled density fill and placing new s.o.g. is about $40,000.

Question:

Is there a more economical and/or faster way of dealing with this? Can helical piles be used in this application? The clear headroom is about 9 feet. The installed cost of each helical pile i, I believe about $3000±.

If so, then:

a) how would the load get from the slab into the helical pile -- what is the detail at the top of the helical pile?
b) what sort of spacing should the pile supports be installed at?
c) what sort of calculation could be done to show that the s.o.g. can span between the supports and safely resist a 2000 pound wheel loads plus the slab self weight? Would checking it as a catenary be acceptable, given that if it fails the slab does not ave far to fall and wold not fall onto anyone?

Any other ideas on how to make the s.o.g. safe?

RE: New support for existing slab-on-grade with 4" void under due to soil settlement

Other Ideas: Not sure of your location but have you talked to the Uretek type folks who can pressure inject foam under the slab? It can also compress down bad soil (to a degree I think). What you describe may be beyond that, but it's worth reaching out to a company that does that sort of thing to ask.

A)I don't think I'd do helicals. And 9' May not be enough clearance for them, it would be close.
B)I don't think I'd do piles.
C)Flexure calculation, shear calculation and two way shear. No for catenary since it would be concrete in tension with just mesh, among other reasons.

I really think foam stuff would be your best bet if it's offered in your area.

RE: New support for existing slab-on-grade with 4" void under due to soil settlement

X2 for Nick's recommendations on foam and problems with the pile option. I did something similar last fall.

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.

RE: New support for existing slab-on-grade with 4" void under due to soil settlement

(OP)
Thank you both, Njlutzwe and Kooyk.

Just so I am clear, do you mean that you would inject the 4" space directly under the s.o.g. so that it is filled, but would do nothing more? Would the wet sandy soil under that be capable of supporting the slab-on-grade and wheel loads? I am very doubtful that it could. Also, wouldn't the wet sandy soil continue to gradually settle from its own weight, just as it has in the past?


I am not sure why so negative about the mesh being the reinforcement. Isn't that what was done (and maybe still is done) with concrete joist floor systems with 2.5" slab and mesh in the slab and joists at 30" centres (span/depth ratio = 12). A lot of that done in the 1950's and 1960's, including parking garages. Also, for modern precast double tee parking garages, the flange of the tee relies on mesh, I believe. In my current project, I would put the helical piles (or other type of support) in at about 5' centres (span/depth = 12) or closer if need be. I agree that the shear (one way and 2 way) must be shown to work, and Mr too if possible, but if Mr does not work perhaps the catenary is worth thinking about, given the special condition of not really being a life-safety issue for a s.o.g.


RE: New support for existing slab-on-grade with 4" void under due to soil settlement

I'm not negative on Mesh for general use in bending for light loads and for temp/shrinkage, but I am when using Mesh as a catenary reinforcing for an inverted concrete arch.

As for the sandy soil, etc. I'd get a geotech on board and see what needs to be done and how deep, etc. The foam folks should be able to work with you and the geotech. The reason the foam exists is for problems like this, so I assume they have some way of dealing with the bad soil too. With that said, I've only heard/seen demos of this, I haven't spec'd it myself but I haven't heard any negatives that I can recall of others using it. I do think I recommended it once, but it was only a general recommendation. I don't know what they ever did with that.

RE: New support for existing slab-on-grade with 4" void under due to soil settlement

I've used the foam products to densify soils below an existing 100 yr old masonry church. It worked wonders in that regard. It was also used to lift an existing 250 sq ft marble entrance stair back to level. The guys running the foam guns were fantastic and in the end brought the old staircase up a couple inches and brought the threshold to within an 1/8" of the existing main floor elevation. As far as I know, the fixes have performed quite well.

RE: New support for existing slab-on-grade with 4" void under due to soil settlement

(OP)
ok thanks. We of course do have a geotech on board. When last we considered this issue a few months ago, he said the only way to deal with it is to excavate the soil. I can double check with him now that the work is imminent. I would be a little uncomfortable just filling the gap and relying on the wet loose sand that one can push a #5 bar a foot into, to carry the load, but will see what geotech says.

Another idea might be to cut out a 12" wide x 30' long wide strip of the slab, and install helical piles at say 10± feet centres and then pour back the strip but making it say 12" deep and reinforce it to span the 10 feet between piles. Excavate 4"± under the edges of the existing slab so that when the concrete is poured it can extend under the existing slab and support it. What do you think of that idea?

RE: New support for existing slab-on-grade with 4" void under due to soil settlement

As jayrod mentioned, the foam stuff can actually improve the soil in addition to simply filling the gap. I had similar concerns, however, when I employed the strategy last fall. The quote below was my geotechnical engineer's response to those concerns. No doubt, there are some soil conditions that would not be suitable for foam injection.


Quote (Geo Guy)

You are correct that the existing void and future potential settlements are separate but related issues. The polymer injection can significantly reduce or eliminate settlement of fills when correctly applied by filling void spaces in the soil matrix, increasing density. This is why it’s essential that the injection program is properly defined and implemented. I hope this clarifies things and please let me know if you have any further questions.

Quote (ajk1)

Another idea might be to cut out a 12" wide x 30' long wide strip of the slab, and install helical piles at say 10± feet centres and then pour back the strip but making it say 12" deep and reinforce it to span the 10 feet between piles. Excavate 4"± under the edges of the existing slab so that when the concrete is poured it can extend under the existing slab and support it. What do you think of that idea?

I like it assuming that the existing slab can span either side of the support line. It's solves the punching shear issue at least. Given that this is a pretty small area (30'x10'?), you may wind up demolishing a substantial amount of it to complete the work.

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.

RE: New support for existing slab-on-grade with 4" void under due to soil settlement

Have you checked with slab jacking firms? They inject variable materials depending on the job. Make sure that any extincting process also has monitoring of slab elevations. You may want to bring it back up where it has settled already. Stage work usually is needed to control where the "fix" is done.

With any form of injection monitor all drain piping. Be sure that they stay open. Also monitor all slabs in the area, since it has been known to fix one area and cause damage elsewhere, as in elevator pits. I know of one job where the work was done on one end of a building and at the very other end damage took place where no one was watching.

Check the references of the contractor also, since a lot of so called slab fixers are out there and inexperienced ones can cause damage not wanted.

RE: New support for existing slab-on-grade with 4" void under due to soil settlement

(OP)

To Kootk:

Several months ago, we looked into whether injecting the soil would strengthen it, but both the geotech and the injection specialist contractor said that it cannot be done because the soil contains much more than 15% fines. In your case, you must have had a different type of soil.

To Oldestguy:

There is no visible settlement of the slab. The void was detected by GPR. The GPR was carried out to determine if there were any significant voids under the slab, because the floor above which will be repaired, will be shored down to the slab-on-grade and we did not want to shore onto a slab with large areas of void immediately below. It is standard practice of the injection contractor to monitor the pressure to detect if the material starts to flow into the weepers. However it seems to me that by the time a drop in pressure us detected, at least some of the injection material will have got into the weepers. For the reasons stated earlier in this string, I am not thinking that we will inject.

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