Replacing cast iron under slab

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by barasingha, Sep 9, 2013.

  1. barasingha

    barasingha New Member

    Messages:
    7
    Location:
    Texas
    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: Sep 9, 2013
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,615
    Location:
    Cave Creek, Arizona
    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.
  3. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,172
    Location:
    Maine
    I'm glad I'm not the only one to have come to the same conclusion LOL
  4. barasingha

    barasingha New Member

    Messages:
    7
    Location:
    Texas
    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!
  5. barasingha

    barasingha New Member

    Messages:
    7
    Location:
    Texas
    Just noticed you're from Arizona. Are no under-slab plumbing problems fixed from below slab tunnels there?
  6. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,172
    Location:
    Maine

    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.
  7. barasingha

    barasingha New Member

    Messages:
    7
    Location:
    Texas
    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.
    image.jpg
  8. jm66208

    jm66208 Member

    Messages:
    99
    Location:
    KC
    Holy schneikes!
  9. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,172
    Location:
    Maine
    That's it exactly!
  10. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,615
    Location:
    Cave Creek, Arizona
    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.
  11. jm66208

    jm66208 Member

    Messages:
    99
    Location:
    KC
    LOL..exactly what I was thinking!
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,995
    Location:
    New England
    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!
  13. barasingha

    barasingha New Member

    Messages:
    7
    Location:
    Texas
    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?

    EVERY cast iron line is exposed. The tunnel is 110' long and 3' wide or <15% of the supporting soil.
    Correct.
    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.

    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.
  14. Terry

    Terry Administrator Staff Member

    Messages:
    15,000
    Location:
    Bothell, Washington
    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.
  15. Terry

    Terry Administrator Staff Member

    Messages:
    15,000
    Location:
    Bothell, Washington
    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.
  16. barasingha

    barasingha New Member

    Messages:
    7
    Location:
    Texas
    Thanks for the response Terry.

    Would an air admittance valve work here or do you recommend going all the way to the roof? My concern is the reliability of the one way valves. What have your experiences been with them?


    Here is what I was thinking and the reason for my post. For all of my sinks, the cast iron comes thru the slab into a wall behind the sink, the sink drains into this pipe and then the pipe continues up thru the roof for the vent. To avoid removing the cabinets and opening the walls and supporting the old cast iron vent, I was contemplating coring the new pvc pipe thru the slab and into the cabinet below the sink. Could I then attach this new pvc to the wall where the sink currently drains for the vent, or must I open the wall and connect the new pvc directly to the cast iron in the wall after cutting and supporting it. I was thinking I could fill the old cast iron pipe with grout below the sink tee, plumb the sink into the new pvc, and attach the pvc to the cast iron sink tee for the vent. This way I would eliminate all wall, and cabinet demo. My concerns are the size of the vent connection. Currently the cast iron is 3" from below slab to the roof with a 1 1/2" tee for the sink drain. This is the only vent for the bathroom--both bathrooms are like this, each with a 3" vent. If I connect the new to old thru the existing 1 1/2" tee will the vent be big enough to support the toilet, tub and sink?

    Thanks.
  17. barasingha

    barasingha New Member

    Messages:
    7
    Location:
    Texas
    Thanks for the feedback. My re-plumb is now complete. Before I completely retire this thread I thought I'd post an action picture of one of the cores in progress.

    I also replaced both toilets with the Toto Drake (round) and have no complaints thus far.

    photo.jpg

    All that's left is to plug the hole,
    Bye now - Barasingha
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