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Thread: How are tunnels under slabs backfilled?

  1. #1
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    How are tunnels under slabs backfilled?

    There are plumbing companies that advocate tunneling under the concrete slabs of houses to replace pipes rather than jackhammering them from the top. How do these companies backfill their tunnels after the repairs are complete?

    Perhaps many companies that do jackhammering don't properly backfill. To do that would involve having the proper sand or gravel bedding under the pipes and under the new concrete. However, its hard to imagine getting those details correct when backfilling a tunnel. Can a person put down a layer of sand or gravel and compact it when backfilling sideways?

  2. #2
    I've not seen companies that actually tunnel under a slab to replace pipe; as you say, it would be difficult to backfill. I suppose you could pump a thin concrete grout into the tunnel.

    What I have seen are the companies that replace piping in place. The dig a small hole at each end of the pipe to be replaced, and then fish a cable through the old pipe. The cable is used to pull a small hydraulic ram through the old pipe. The ram pulls a new flexible pipe behind it. As the ram is pulled through, it expands to shatter the old pipe and make clearance for the new pipe. Then it contracts again, is pulled forward a little and then repeats the expansion/contraction/pull forward cycle until it gets all the way to the other hole. Supposedly available for small water or gas pipes all the way up to sewer lines.

  3. #3
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    Quote Originally Posted by Paul F Franklin View Post
    I've not seen companies that actually tunnel under a slab to replace pipe; as you say, it would be difficult to backfill. I suppose you could pump a thin concrete grout into the tunnel.......
    Yes, something like....

    Google flowable fill.

    I have used it a few times. Once when raising a sunken sidewalk, once when leveling washed out dirt from the slab under a stand alone garage, and the biggest job was an exterior stairway at one of our schools.
    A stairway with a rise of about 30’ set into a hill at the corner of the building. All the downspouts led to a medium sized culvert near the top of the stairway that was supposed to divert the water to the bottom of the hill through pipes. The pipe failed sometime within the last 40 years and the water was pooling under the stairway landing, then percolating under the steps to the bottom, eventually creating a tunnel under the steps. We opened up the culvert, reinforced the dirt on the sides of the steps and started pouring flowable fill in. We had to run in shifts as hydraulic pressure would blow out the sides after about 10 feet rise.
    Comments made here are my own and, according to my children, do not reflect the opinions of any other person... anywhere, anytime.

  4. #4
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    Any horizontal boring installations I've seen just make a hole big enough for the pipe & the pipe is the 'backfill'.

  5. #5
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    Yes, something like....

    Google flowable fill.
    Yeah - around here they use a type of that. A mix of concrete and foam. The same stuff they pump under (into?) a slab to level it. Geo-Cell is one brand name.
    My granddad always said, :As one door closes, another opens".
    Wonderful man, terrible cabinet maker...

  6. #6
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    How do they or how should they? Best answer has been given, flowable fill. Different types, some are just a very lean somewhat sloppy cement/sand mix. Any concrete supplier can provide it. That is provided it is needed. Many forms of boring do not leave any voids that need filling.
    Last edited by Ole Anderson; 05-11-2019 at 6:48 PM.
    NOW you tell me...

  7. #7
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    Quote Originally Posted by Ole Anderson View Post
    Best answer has been given, flowable fill.
    If tunnels this extensive
    http://www.masterfixplumbing.com/pro...-for-pvc-pipe/
    are filled with flowable fill then what happens if a repair to under-the-slab plumbing must be made in the future? Can flowable fill be dug out with hand tools?

  8. #8
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    Quote Originally Posted by Stephen Tashiro View Post
    If tunnels this extensive
    http://www.masterfixplumbing.com/pro...-for-pvc-pipe/
    are filled with flowable fill then what happens if a repair to under-the-slab plumbing must be made in the future? Can flowable fill be dug out with hand tools?
    Usually yes. The stuff we used was a like 31 times less cement than normal concrete. Mostly sand and small aggregate. Has almost no structural value, just takes up space. When the stuff I used for the garage slab cured it was just slightly more solid than your typical sand castle. Vertical edges had to be covered with soil.

    edit: I mixed my own for the first two examples in my posts above, and the school district had it delivered for the steps. The amount needed to fill a tunnel as extensive as your link will surely be supplied by a concrete company as Ole Anderson said. Tell them you want excavatable fill and they will mix accordingly.

    Just checked internet, Virginia Ready-Mix Assoc. says excavatable fill shall not exceed 50psi (maximum limit, they make it as low as 30psi). Compare that to a regular six bag mix at 5000psi.
    Last edited by Charlie Velasquez; 05-12-2019 at 12:03 PM.
    Comments made here are my own and, according to my children, do not reflect the opinions of any other person... anywhere, anytime.

  9. #9
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    Pipe bursting is the method used when they replaced the gas line to my Mom's house. Pulled a cable from end to end tehn pulled a hydraulic head that bursts the pipe out enough to drag a plastic pipe behind. They did this without turning off the pilot lights in the house.
    Bill D.

  10. #10
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    "Has almost no structural value, just takes up space."

    I'm curious why you would even refill the space if there is not structural value to support the slab. Seems like you would just block off the entrance and leave the excavation void for future access.

  11. #11
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    A few months ago, the plumbers hand dug under my slab and in about 5' to hook a new construction sewer line to the existing one under the two story house. They then put the same dirt back in the hole, and packed it as well as they could by hand, and put the concrete sidewalk next to the house back in.

    When I enquired about how well it would work, their only answer was that it was the way they always did it. The inspector signed it off without question, before the sidewalk was replaced.

    I have my doubts, but the inspector never even asked about how they did it. I guess it is what it is. Weird, because this inspector is really picky.
    Rick Potter

    DIY journeyman,
    FWW wannabe.
    AKA Village Idiot.

  12. #12
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    Quote Originally Posted by julian abram View Post
    I'm curious why you would even refill the space if there is not structural value to support the slab. Seems like you would just block off the entrance and leave the excavation void for future access.
    In the plumbing repair tunnels in the link I gave, the dirt removed did offer structural support. The tunnels have no shoring to hold the sides in place. If the tunnel wasn't filled, the slab might eventually crack or collapse.

    I wonder if the tunnellers move refrigerators and pianos to one side before they dig.

  13. i recently watched a gas company crew run a line under a driveway, yard and side walk to a house. They had a horizontal drill that made a hole just big enough for the gas pipe. One of the fellows told me they have a way to direct the direction of the drill head so it doesn't go in just a straight line. They had a hole in a street and the drill was on a cart that they set perpendicular to the curb. About 30 minutes later the drill operator shut down the drill and told the supervisor that he hit the house foundation. Sure enough, they dug down about 18 inches and there was their drill.

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