How will one back fill dirt when between the bottom of slab and footing?

I need to have someone come dig a tunnel to access a "Y" connection in the sewer line that needed to be replaced. Access from above is too problematic and messy.
The quote I got is $187 per foot of digging. They will need to dig down several feet beyond the footing, then dig a horizontal tunnel six feet over to the location of the pipe. Because we are in Miami and near the beach, the soil is all sandy, I guess when they tunnel, the compacted sand above the tunnel will fall? This means more dirt will be removed from the tunnel. Now when they finished repairing or replacing the drain line, they told me they can just throw the dirt back into the tunnel and all will be ok.
I wonder how you will throw the dirt back into the tunnel to fill the space below the slab and above the footing? Can this really be done? Does it not require some sort of a sand/dirt pump? Even if this is doable it will not be compacted right?
Will this cause the line to be "suspended" and not resting on dirt and will that cause future failure - possibly the weight of water going through may create a back pitch? Will the foundation settle as a result of this dig?
Thanks for any comments,
O
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they spray the dirt with dry nitorgen before completely backfilling,,, you know out of a welding type tank, kind of blow most of the dirt back in. Then it expands to fill the hole as it gains moisture. Problem might be the residual nitrogen in the soil causes fungus and various weed seeds to grow uncontrollably sometimes and your house might get stinky.

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You are safer cutting through the floor, either way talk to their insurance broker for verifying full coverage, liability and workers comp. Holes can fill in, fail. It will remain a hole underneath never to be solid again. It will be very hard to re support the repaired area. Talk to your building inspector.
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Unfortunately cutting the floor is more difficult because of several reasons:
- they estimated $2000 to open up the 8" concrete floor, $187 per foot to dig down, $1000 to fix the drain from above, and then they leave. How it is up to me to pay additional to repair the damage by refilling the concrete floor, redo the tiles (no spare tile) and redo kitchen cabinets (area of excavation is right where I have cabinets), and my wife strongly objects the "messy" inside excavation.
- outside tunneling in is $187 per foot so if it is 15 feet total up across and down it is $2800, and is cheaper than the floor opening option.
I don't know, I am going crazy.
O
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Those prices to cut and repair sound real high, get bids.
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I watched my next-door neighbor with this problem -- Palm Beach County, with similar sandy soil. The crew came in, dug the tunnel, did the repair work, put the soil back into their tunneled area, cleaned up and left. That was almost 10 years ago, and no subsequent problems. Also, I had to have my slab cut to install new plumbing, and it's a mess -- plus if you're worried about a void under the slab with a tunnel, shouldn't you also be worried about weakening the slab if you have to slice into it?
It seems to me that tunnel repairs to plumbing must not be unusual in an area like south Florida where many homes are on slabs and the soil is easy to work. As long as you've hired a licensed company that's been around for a few years you shouldn't have a problem.
I understand the rationale for getting bids, but then you've got to worry about whether the low bidder isn't omitting something (workman's comp coverage? insurance?) or using unqualified labor. I suggest you find a qualified, licensed company, ask them some intelligent questions, let them do the work, and not obsess over it too much.
And let us know how it works out.

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Cutting the floor is problematic compared with a void under the floor after the repair...
I would not even consider tunneling.
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Are you saying cutting the floor is worst or tunneling is worst?
O
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"O",
Have you or the contractor considered that this tunneling might go down far enough to hit water? I know when I lived in Jupiter (FL), the water table was only 2.5-3 feet below the floor level of our house.
I have a feeling that if the tunnel went down to the level of the water table, things could get (even more) complicated real fast.
Eric Law

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I asked them that. They told me the soil will get moist four to five feet deep, but not soaking wet.
O

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