Bedding Prep and Tips for Below Slab Drain Pipes

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by DJL, May 13, 2014.

  1. DJL

    DJL New Member

    Messages:
    12
    Location:
    Ames, Iowa
    Hello all,

    I am in the process of moving my basement bathroom stubouts. I've got the concrete busted up and trenches dug, but I'm trying to figure out the best technique for preparing the area for the PVC pipe. My inspector said to use beds of sand, adding 1/4" slope to the sand then lay the pipe on top of that. Some books I've been reading mostly gloss over this and suggest to use wooden stakes and vinyl strapping to support the pipes and establish the slope. The books then completely ignore how to prepare the soil underneath the pipes.

    I'm considering going the sand route, but I'm not sure what to do. Basically, I'm considering doing these steps:

    1) apply the sand to the pipe trench, wet it down, tamp it, establish 1/4" slope for toilet/shower/lav back to the main drain line.

    2) Glue the pipe, test it, have it inspected.

    3) Add the soil I excavated back a couple inches at a time, compacting each layer of added soil, burying the piping completely

    4) Add two or three inches of gravel above that, even to the bottom of the slab, wet the gravel, tamp it

    5) Patch the concrete slab.

    Is this at all right? Any tips or suggested changes that you would make to this process? By the way, the soil is all clay. Thanks.
  2. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,250
    Location:
    Land of Cheese
    If you have walked across a beach, you should understand that sand does not compact well as it will not stay in place. After removing the stone from under the slab I run the tamper over the underlaying subsoil and bed the lowest few feet of pipe on the subsoil and then use sand/clean subsoil to pitch the pipe as you work away from that point. I use 2' long pieces of re-bar pounded in and overlapping in an "X" pattern over the pipe to keep it from moving side to side or floating up if the trench floods. The pipe must remain exposed for the test and inspection before completely bedding the sides of the pipe with sand and/or clean subsoil. Compact the subsoil around the pipe around the pipe and then replace the stone. If there was any insulation (foamboard) under the slab, that should be replaced before pouring concrete.
  3. DJL

    DJL New Member

    Messages:
    12
    Location:
    Ames, Iowa
    Thank you cacher_chick. With your beach comment, I think you are saying don't bother tamping the sand, because there's no need -- just use it to establish the slope?

    Thanks for the comment on rebar and backfilling the sides of the pipe with sand. I will plan to do both.

    For the layer of gravel just below the concrete, can I resuse the gravel I pulled out pretty much as is. It has some soil mixed in and many small jagged chunks of broken up concrete in it.

    Thanks again!
  4. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,250
    Location:
    Land of Cheese
    There should be no problem with what you have. We use a homemade sifter box to pull out anything bigger that 3/4" when we use a breaker on the concrete. We don't like too much loose dirt in the stone bed pack because it can clog the drain tiles system if the water level come up under the slab. This might or might not be an issue where you are working.
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