I can't get the soil to compact sideways.

[MiamiCuse]

In order to replace a section of cast iron sewer pipe below grade, I had to cut open my concrete slab. The cut is about 10 feet long and two feet wide. I was fairly certain the pipe was directly underneath so I started to dig, and dig, and dig. I dug about four feet of soil and still no pipe, so I started to dig sideways, and eventually found the pipe about 18" off from where I thought it would be. It means the pipe is not directly under the slab cut, but 18" inside of it. I had to dig a "cave"about 2' inside and under the slab to work on replacing and repairing the pipe, and I have completed the repair.

Now I have a big hole, about 10 feet long, 4 feet wide (2 feet is cut open 2 feet is "tunneled inward", and 4 feet deep). Since I am in South Florida my soil is mostly sand. I pushed the sand back into the big hole but it is difficult to fill up the tunneled part. I used a 4x4 to pact it sideways as much as I can, and when I filled up the hole, I still have about three cubic feet of soil left. I then used a garden hose to wet the soil to compact it., and as much as I compacted the soil sideways under the "tunnel", as soon as I point the garden hose at it. the sand quiclkly erode away and I can feel the hose with water a foot or more into the tunnel, indicating there are cavities and air pockets. So I dug the sand back out to a depth of 8" or so, and pushed the soil sideways again. I repeated this process a few times to try and compact the sand in the tunnel, but it does not seem to be packed enough like it was. Should I worry about this? Will there be any issues with that part of the slab unsupported? Are there any tricks to get it really packed?

Thanks in advance,

MC

[NGDirect]

I'd say eventually your going to have problems with the slab sinking and cracking in that area if it isn't supported by well compacted soil. Personally I'd do it right and go ahead and cut the extra 2 ft of slab so I could back fill properly and then replace the 4x10 foot section. Unless you are planning to mix your own concrete, you will probably pay for a minimum delivery of 1 cubic yard even if you don't use that much, and your current opening is well under a cubic yard.

[Dean Hoffman]

How about renting an air or hydraulic tamper?

[mm]

If it didn't do it quickly, a water hose would still erode anything you just put there, maybe just slowly.

Unless you have a sink hole, can't you just fill the hole to the top and use the garden hose to get it to flow into the empty spaces, by gravity. Now it seems like gravity is allowing it to flow into some other empty space that would be filled by dirt if you filled the hole more.

I don't know. How thick is it? Any apparent cracks? Any non-apparent cracks?

[hrhofmann]

Slab under the house, slab for a driveway, what kind of slab?????

[DD_BobK]

Why are is there a garden hose involved in this process? I'm confused.

An unsupported area of slab will likely crack.

"Sideways" compaction has never worked for me. I've cracked thin sections of concrete the three times I attempted it. :( In hindsight I should have removed the concrete, placed & compacted the soil and replaced the concrete. Sounds like more work but in reality "doing it right the first time" would have been overall less work.

I would suggest:

1) cutting slab back to excavation edge, then fill with sand (soil) up depth that would give you new slab thickness comparable to old slab

2) consider the use of "flowable fill" (Google it)

cheers Bob

[MiamiCuse]

I cannot cut additional width because there is a wall right above that line. Not load bearing wall but it's a major hassle to remove that wall just to cut floor.

[MiamiCuse]

Slab under an interior portion of the house, 4" thick.

[HeyBub]

Don't use sand?

Maybe layers of wire and gunnite?

Clay?

[norminn]

Never having done that kind of work, I can only guess :o) A "retaining wall" of some sort will hold the sand, at least temporarily. Fashion a retaining wall of block or heavy scrap wood along the length of the cavity that is under the slab, leaving open at the top to allow more filling. Fill and tamp as well as you can...block might work better for this, as you can work a layer at a time. Fill the ends, working toward the middle. Wet it a bit to help settle, but don't flood it. When it is as compact as you can make it, make final fill with concrete so it can be packed in. Then fill in the rest of the trench. Could even pay a contractor for an estimate to do it...might find out a better method. Is the trench in living area?

[krw]

Why not compact it as best you can under the slab portion, say 6", below the slab and then fill the last 6" with more concrete, making sure to flow it under the slab? Concrete will flow better than your sandy soil. Perhaps form it out a foot or so from this wall so the amount of concrete needed would be more reasonable. Perhaps you could put some rebar in the new concrete so *it* doesn't crack. If it doesn't, the slab above it won't.

This would give a little head-room to work at compacting the base and add a

*little* support to the slab at the same time if the compaction isn't perfect. Just a thought...

[Colbyt]

If it were my house I would discard the sand and use the small gravel that is traditionally used to fill a block framed foundation before the concrete is poured for the slab. This gravel settles into position and does not shift or compact after that, same stuff is used around sewer pipes for the same reason. I suspect using that you can fill all but the last inch or two.

I also like the suggestion of working a little slightly looser concrete back into the space as it is poured. Even weak watered down concrete is better than a void.

Keep in mind I have never done this but it should work.

[MiamiCuse]

The trench is inside the perimeter of the house. It is in a bathroom, but the pipe is slightly over in an adjacent bed room. See the image below of the trench dimension and where the pipe is. The trench is 10' long and 2' wide and 3' deep.

If I put up a wall I can fill up gradually but again when I get to near the top, the last 12" it becomes impossible to compact. I tried to show in the image the way I backfilled the trench, trying to pile up higher on the inside and used a 2x4 to push horizontally when it gets to the top. Still not compacted enough though.

I am not sure concrete would float easily into the gap. Unless there is some way of injecting it with pressure.

Thanks,

MC

[WW]

[aemeijers]

A bag of portland cement mixed with the sand, and get it just wet enough to be packable? You wanna use long rubber gloves to do the packing, and a set of knee pads. HF has 'good enough' ones real cheap.