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Thread: Replacing cast iron under slab

  1. #1
    DIY Junior Member barasingha's Avatar
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    Default Replacing cast iron under slab

    Great forum. Longtime reader but first post. I am not a plumber but have done some plumbing.

    I have decided to replace the cast iron drains under my slab on grade house and could use some advice. All work will be done below slab, not thru floor. Instead of tying onto the old cast iron dropping below the slab, I am considering coring new holes for the new PVC to penetrate the slab for sinks/laundry. Has anyone done this? If so how did you tie into the vents? My current plan is to fill the old drain lines with grout (below slab) and connect the new PVC to the old line in the wall to tie into the existing vent. (Hope that makes sense) Suggestions? Toilets will get new plumbing to them and the tub traps will be replaced from below.

    Also, currently the laundry room p-trap is under the slab. I have a place above slab to move the p-trap to but the existing line has no vent. What are your experiences with one way vent valves?

    Thanks in advance,
    barasingha
    Last edited by barasingha; 09-09-2013 at 03:37 PM. Reason: Clarification: Slab on grade

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    Moderator & Master Plumber hj's Avatar
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    I have absolutely no idea how you plan to do what you describe, but it should be an IMMENSE task to do it without cutting the concrete floor for the horizontal pipes.
    Licensed residential and commercial plumber

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    In the Trades Tom Sawyer's Avatar
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    I'm glad I'm not the only one to have come to the same conclusion LOL
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Junior Member barasingha's Avatar
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    Quote Originally Posted by hj View Post
    I have absolutely no idea how you plan to do what you describe, but it should be an IMMENSE task to do it without cutting the concrete floor for the horizontal pipes.
    It is done by tunneling under the slab. I realize that slab on grade homes are not very common outside of the south, but this method (tunneling) is quite common here. It prevents having to bust up the floor and slab, which in my case would split the slab into four pieces. By tunneling, pipe can be replaced without disturbing the interior of the home. Common practice is to no-hub the new PVC onto the cast iron as it penetrates below the slab. Instead, I would like to core new holes thru the slab to allow the new PVC all the way to the fixture.

    And yes... it is an immense task!

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    DIY Junior Member barasingha's Avatar
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    Quote Originally Posted by hj View Post
    I have absolutely no idea how you plan to do what you describe, but it should be an IMMENSE task to do it without cutting the concrete floor for the horizontal pipes.
    Just noticed you're from Arizona. Are no under-slab plumbing problems fixed from below slab tunnels there?

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    In the Trades Tom Sawyer's Avatar
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    Quote Originally Posted by barasingha View Post
    It is done by tunneling under the slab. I realize that slab on grade homes are not very common outside of the south, but this method (tunneling) is quite common here. It prevents having to bust up the floor and slab, which in my case would split the slab into four pieces. By tunneling, pipe can be replaced without disturbing the interior of the home. Common practice is to no-hub the new PVC onto the cast iron as it penetrates below the slab. Instead, I would like to core new holes thru the slab to allow the new PVC all the way to the fixture.

    And yes... it is an immense task!

    How do you know if you got the pitch right? I done plumbed a lot of places including the south andi have never heard of tunnling to replace an entire plumbing system. Straight shots yes but there is no way to tell if your branch connections are right.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Junior Member barasingha's Avatar
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    Quote Originally Posted by Tom Sawyer View Post
    How do you know if you got the pitch right? I done plumbed a lot of places including the south andi have never heard of tunnling to replace an entire plumbing system. Straight shots yes but there is no way to tell if your branch connections are right.
    How do you mean? I will use a level to maintain pitch starting from the clean-outs and pipe hangers to suspend/support pipe under slab. As said in my first post: I am not a plumber. Because of the last ten days you might consider me a digger though. My tunnels are 4'x3' and total to 100+'. This weekend I am replacing the pipe and had questions about tying into the existing cast iron vents and using a one-way vent for the laundry. Any suggestions on these parts?

    Here you can see the old cast iron running along the left side of the tunnel.
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    Holy schneikes!

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    In the Trades Tom Sawyer's Avatar
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    Quote Originally Posted by jm66208 View Post
    Holy schneikes!
    That's it exactly!
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    Moderator & Master Plumber hj's Avatar
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    quote; Are no under-slab plumbing problems fixed from below slab tunnels there?

    You've got to be kidding., right? By the time you tunneed for EVERY cast iron line, there would be very little support for the slab and there is almost no way you will compact the fill back afterwards unless you pump concrete into the holes which will then encase your new drain lines. The Mexican drug cartels could use you along the Mexican border.
    Licensed residential and commercial plumber

  11. #11

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    Quote Originally Posted by hj View Post
    quote; The Mexican drug cartels could use you along the Mexican border.
    LOL..exactly what I was thinking!

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    Retired Defense Industry Engineer jadnashua's Avatar
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    That cannot be safe! If a so-called professional did that, OSHA would have a field day! Common or not, I would not want to think about it!

    There are concrete core drills, but many of them use a gasoline engine since it takes a fair amount of power. ANd, they generally use water to flush and lubricate the coring bit. Neither of those attributes are good for an enclosed area in a tunnel underneath a slab if you care to live much longer!
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member barasingha's Avatar
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    I think we're getting off topic. I know what I am doing with the soil, slab, fill, etc. Coring through concrete is easy for me, too.

    I could really benefit from y'alls suggestions on my plumbing questions. I started this thread avoiding the whole tunnel concept as I thought it might spin out of control; perhaps I should have framed my questions around a fictitious, sub-floor remodel senario. Most re-plumbs from tunnels like this no-hub pvc to the cast iron below the slab, I want to bring the new pvc above slab via cored holes, offset from the original cast iron. How would you tie the new pvc into the existing vents? The house was built with a under-slab p-trap for the washer, it is 12 feet from the washer near the kitchen sink branch which is where the nearest vent is. If I relocate the p-trap above slab by the washer, I assume it will need a vent of its own. Because the washer is in the garage, I was thinking of trying a one way vent. Suggestions?

    By the time you [tunneled] for EVERY cast iron line, there would be very little support
    EVERY cast iron line is exposed. The tunnel is 110' long and 3' wide or <15% of the supporting soil.
    there is almost no way you will compact the fill back afterwards
    Correct.
    unless you pump concrete into the holes which will then encase your new drain lines
    Wrong. Flowable fill is an option as you don't fill much past the grade beams and in my case all plumbing goes thru grade beams and would be above the fill level. Regardless, the fill (or spoils replacement) is only there to stabilize the tunnel walls, the slab is supported from the grade beams and in my case, ultimately, drilled piers.

    There are concrete core drills, but many of them use a gasoline engine since it takes a fair amount of power
    I use electric Hilti wet core drills. I am using 4" holes thru grade beams around the old 3" cast iron and 3" cores thru the slab for 2" PVC sink drains.

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    Forum Admin, Expert Plumber Terry's Avatar
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    At the Southcenter shopping mall in Tukwila, everything is on piers fifty feet into the ground. The dirt under the slab had sun about six inches from when it had been poured. When we plumbed for a donut shop, we cut a few holes in the slab and made tunnels like that. We hangered our pipes from the concrete slab. I don't like being in tight places, and that was pretty darn tight. I remember that I dropped into the hole, and crawled 15 feet, made a right turn, then ten feet, left turn, then fifteen feet. There was no room to turn around. I had to inch out backwards. I brought a 3 tap cord for my light, my rotohammer and sawzall. That was the last time I was ever on a job like that.

    I did have a job before that was running condensate drains between the lobby ceiling and the second floor. The ceiling I slithered on was hung by wire from the concrete structure above. There was 17" from the lowest part of the beams, with sharp nails protruding downward. If I turned my hips sidewise below the beam and I was below a nail, it would bury itself into my side. I started taking a hammer and bending the nails back. I was in there on and off for six weeks in the dark with a light that was on the end of my extension cord as I slithered around hanging pipe. Netiher the inspector or my boss ever saw the work that took me six weeks.

    This week I've been working over open floor joist above a kitchen, roughing for a new bath. I almost put my foot through the ceiling when I made a mistep, but decided to slide against the wall tearing skin rather then put my foot through the kitchen ceiling. Sometimes construction is all kinds of fun.

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    Forum Admin, Expert Plumber Terry's Avatar
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    The washer p-trap should be above the floor, and vented. The vent needs to be within five feet of the trap.

    It shouldn't be a problem hanging pipe from the concrete with hangers and rod.

    You can use no-hub type couplings to mate to the existing vents. They make a plastic to cast size. Plastic tends to be just a bit larger then galvanized for cast. Some old cast is more like copper sizing on the OD. And yes, they make copper to cast couplings that work nice for that.

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