Filling void under concrete

[8870]

Hello,

I recently bought a property that has a 60'x64' pole barn with a floating concrete slab that has several woodchuck holes going underneath the backside of it. The woodchuck problem has been taken care of and now I am looking to fill the holes. The concrete in the barn is approximately 6" thick and has not sunken or cracked, but judging by the amount of dirt outside, there is quite a void under there somewhere. I was looking into mudjacking or polyurethane foam jacking to fill the tunnels as I plan to use this building as a shop and am uncomfortable with the possibility of the floor caving sometime in the future. I was wondering if anyone had any experience using Great Stuff or other canned spray foam for this application? My spitball idea was digging around the outside of the rear of the building and attaching a form board to to the 6x6 poles, backfilling, then drilling holes and injecting as much foam as I can. I can go to Home Depot and buy a cart full of foam a lot cheaper than getting a company out there to do it. Being that I am not trying to raise or level the slab, just to stabilize it, it seems like this should work. I want this done once and done right, but would like to save some cash if I can do it myself. Does anyone have any suggestions or experience with something like this? Any help would be appreciated.

Thanks, Roy

 

[240sxguy]

I have a similar problem. I'm curious to hear what suggestions you will get.

 

[Shiftless]

How will you know where in your nearly 4000 sq. ft. floor to drill your 6 inch deep holes to inject the canned spray foam?

Is there any rebar in that slab?

 

[RPH]

I wonder about the foam post hole pouches. If one can get bag activated and pushed into the void.
CAUTION! DO NOT EXCEED THE 15
SECONDS MIXING TIME AS THE MATERIAL IN THE
POUCH WILL EXPAND CAUSING THE POUCH TO BURST
AND SPRAY CONTENTS, RESULTING IN POSSIBLE
ILY INJURY OR PROPERTY DAMAGE.
That could be helpful!

 

[8870]

How will you know where in your nearly 4000 sq. ft. floor to drill your 6 inch deep holes to inject the canned spray foam?

Is there any rebar in that slab?

Not sure if there would be rebar in the slab or not. I was going to try to find a long bore scope to use and see if I could tell how deep the burrows go and get a little better idea of what I am dealing with.

 

[Shiftless]

That’s a good idea

 

[Flail]

How about tapping on the concrete with a steel bar to “sound out” the hollows?

 

[ozyborn]

I would use the bore scope. Then, most likely a wet concrete slurry to fill.

 

[Shiftless]

You could pipe in the wet concrete slurry through the tunnels the woodchucks so graciously dug under your slab. Maybe some 3 inch plastic pipe and with some kind of huge funnel held up high at the outside end. Use heat to bend a nice sweeping ell to keep clogging to a minimum.

 

[readhead]

It seems like you are trying to fix a problem you don’t have. Even if there are a lot of single tunnels I don’t see that causing a problem with the concrete. A 6” slab can easily bridge over a small tunnel.
If you do try to fill them with something how will you know if you have been successful?

 

[8870]

It seems like you are trying to fix a problem you don’t have. Even if there are a lot of single tunnels I don’t see that causing a problem with the concrete. A 6” slab can easily bridge over a small tunnel.
If you do try to fill them with something how will you know if you have been successful?

From the experiences I have had with groundhogs, the hole usually leads into a few good size chambers, and these are the areas that I would have a concern with. I have seen a few older barns in my area that have had the floors cave due to groundhogs. I suppose that there would be no realistic way to know if I had filled everything, but if I inject as much material as the hole will take, its better than nothing under there in my opinion

 

[Ricky Joe]

Keep in mind that concrete parking garages are six inches, and effectively are over a void each level. Also, multi-story buildings have four inches of concrete, though usually over steel.

 

[Steve in UT]

Pour a 4" cap with lots of rebar and be done.

 

[quickfarms]

Use ground penetrating radar to find the voids then you can drill into them and fill them

 

[TurnipTruck]

I have found voids under large slabs by dribbling a basketball and listening for hollow sounds.

 

[BombShelter]

I thought parking garages were something like 8"-12" thick Spancrete with another 6" poured on top of that?

I see a lot of normal attached garage floors up here with voids under them. Just from experience I'd say they hold out around 50 years and then after that you see large sections start to settle. Typically you'll see long cracks develop and you can hear that hollow thud when you hold something like rebar vertical and let it drop a few inches.

I usually recommend the mud-jackers, the guys using the thick rocky slurry that looks like wet concrete but isn't. From the outside you might also be able to find a waterproofer using a bentonite slurry, if he can find the holes and "inject" them. I'm not a big fan of foams, especially home-store solutions, they're expensive and don't last a long time, maybe 3+ years.

 

[rsanter]

I don’t think you can DIY with the foam
I doubt it will reach where you need it to.

I think the only DIY you may be able to get done is a thin concrete slurry pumped in with a hose reaching as far as you can and using vibration to get as much air out as possible

 

[scooterbum46]

I want to know how you "solved" the ground hog problem... (Shhh - don't tell Strouty)

 

[pcmeiners]

You want to do it once and be done with the hollow beneath the slab. Not going to happen. Unless your soil below the slab was completely remove down about 4 feet, replaced with compactable materials with NO organic materials in it, and compacted correctly. This is rarely done, and at a high cost.
So for 50 plus years your soil will continue to settle, how much depends on your original soil condition and workmanship.
That said either a cement slurry or foam would need to be injected over the next 50 years a number of times.
I have the same issue, I plan to inject Great stuff foam in the center of the garage floor, raise the floor an inch past level, I figure that will be good for the remaining slab's life , as my floor has been down 30 years. There are U-Tube video's of this procedure.

 

[Fav Onefour]

It might be possible to do a slight DIY with foam.
I had a project that required boring under concrete steps near the curb. One side was open for the work, but the other had limited access. That meant working from one side and trying to backfill and tamp through the entire length.
It helped that I knew where the cavity was located in my case. I still had to reach six feet with the foam.
I found that 3/16" I.D. polyethylene line fit nicely on to the stem of those great stuff cans. It was cheap too. I picked up a 25' roll with the thought of having one good shot per can.
I taped the polyethylene line to a stiff snake line so I could shove it all the way into the hole. I also taped it to the can stem to prevent blowout. The foam fed through the long line pretty well. I can say better than I expected.

I know it has been awhile @8870 , but did you try snaking under the slab to figure out how far and how much they dug? I bet you could run spray foam with a hose in there quite a distance. The biggest challenge would be finding the back end so you could start filling from that point.

 

[Vtor]

Most concrete mix plants can make very watery blend of grout to fill gaps such as that you speak of.

 

[Big Bad Dad]

Most concrete mix plants can make very watery blend of grout to fill gaps such as that you speak of.

They call it flowable fill in my area.

 

[Walkers]

I just excavated the opening a bit and then used a pogo stick pneumatic tamper. Start by tamping the empty hole, add some fil and tamp, etc. I was able to get just about all the excavated dirt back in.

 

[cannuck]

Don't go with foam because you can't be sure you are getting it to full depth and once it starts to react you have no control over expansion. "Flowable fill" is top of the list, but if you have a concrete shop that really knows there stuff ask for PFCC (Preformed Foam Cellular Concrete = "foamcrete") as it can be blended to be VERY good flow and can penetrate deep into the cavities as it will remain fluid until full and set. If foam expands only part way down, it will block off access to caverns.

 

[HoosierBuddy]

If it was me, I'd be concerned too. Once the ground hogs are gone, will the snakes move in?

We had an issue at work with a water leak under a slab washing out the crushed stone underneath and eventually the slab dropped several inches. We hired a company that fixes foundations and slabs and they came in, drilled and injected a product that filled the voids and lifted the slab back into place. So....It can be done....but I don't know what product they use for that. Whatever it is....it'd likely work to fill your voids. Maybe someone else knows.

Phil

 

[b-boy]

I had this problem. It was caused by the plumber that ran water/conduit to my pole barn. He dug out the gravel under the slab so he could run the lines up into the building. He didn't completely back fill the gravel. I had him back twice to fix the problem, but he never really did anything. My concern was cold air getting under there and freezing my water line and critters getting under there and gnawing on stuff. I used many cans of spray foam to fill the voids, then backfilled with dirt. It's held up for 3 years so far.

 

[Dig Doug]

So if have a Slab with a bunch of holes it it how are you going to patch all the holes that you fill with foam?

how bad are the holes? Maybe dig up a small section to get an idea how bad the situation actually is re compact sub grade repour concrete

1-
the expansion foam will travel the path with the least resistance, unless you have a long hose to get deep down in each hole your wasting your time! Plus you don’t know for sure how much hole is filled

2-
you could do the foam and possibly get the woodchucks back and they would just eat
/ chew / dig the foam in the existing hole

3-
you’ll never get all the voids filled correctly! And you’ll have a false sense of security thinking the slab is supported w/ a solid Sub grade

best way

and the for sure way - is to break out and re compact the Sub grade or really bad areas and bring in some 3/4 rock, DG, class 2 base compact then repour the concrete.

once the holes are gone you know they won’t be back!

 

[slimpickins]

Deleted

 

[engineer2]

I have the same problem with my front porch. It's a concrete slab attached to the foundation with rebar. It is mostly hollow underneath from the ground settling. Chipmunks love living under it. I like the idea of injecting flowable concrete.

 

[John McA]

I wonder about the foam post hole pouches. If one can get bag activated and pushed into the void.
CAUTION! DO NOT EXCEED THE 15
SECONDS MIXING TIME AS THE MATERIAL IN THE
POUCH WILL EXPAND CAUSING THE POUCH TO BURST
AND SPRAY CONTENTS, RESULTING IN POSSIBLE
ILY INJURY OR PROPERTY DAMAGE.
That could be helpful!

Has anyone seen these pop?
Asking for a friend.