backfill - what do you like?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by nukeman, Feb 24, 2010.

  1. nukeman

    nukeman Nuclear Engineer

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    I've replaced some DWV lines under my slab (2"-4"). New lines are PVC and the old stuff is CI. The backfill that was there was just the native clay soil (plus several large pieces of leftover cinder block). This was topped with plastic and a thin layer of gravel.

    If you were going to backfill, what would you use? I am not sure if it matters, but the ground water level can come above the pipes. This is a walkout basement and water has come a few inches above the pipes while the trench has been open during heavy rains or after a good snow melt.

    House is about 40 years old. I'll be having the inspection and doing the pressure test (water) early next week and will then be ready to backfill.

    Thanks.
     
  2. hj

    hj Moderator & Master Plumber Staff Member

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    we just replace the native soil.
     
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  4. jc60618

    jc60618 DIY Junior Member

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    We use 3/4 stone
     
  5. cacher_chick

    cacher_chick Test, Don't Guess!

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    Clay absorbs water, causing it to expand and contract as the moisture content changes- it's not a good base.
    3/4 stone is a good choice because it is self-draining and won't hold water.
     
  6. ballvalve

    ballvalve General Engineering Contractor

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    3/4 crushed rock for plastic pipe backfill??? You guys kidding? Maybe tucked away under a house for a french drain or septic, but against all norm and specs for a pipe under any sort of traffic or heavy fill. Fill sand, to local specs and sometimes fine pea gravel. 3/4 crush is sabotage, not backfill.

    Tamped and rock free native soil would certainly be better than 3/4 stone.
     
    Last edited: Feb 24, 2010
  7. nukeman

    nukeman Nuclear Engineer

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    Thanks for the info, guys. My only concern on using something other than native soil would be that I am only replacing about 5' of the 4" CI main. The rest is still completly surrounded with clay. If I filled this section with something else, perhaps I might have issues with this section moving more or less than the rest. Perhaps the safest for this situation would be a bed of sand + tamped native soil.

    I tried to talk to the plumbing inspector to see what he wanted to see, but I got his voicemail and he hasn't called back yet.

    One other question (was going to ask the inspector this also): For those of you who test DWV with water instead of air, does your inspector like to see the system filled before he arrives? I assume so in order to save time. I'll also try again to get a hold of the inspector and see what he wants to see.

    I have to give 100% respect to all of the plumbers out there. :) Cutting/breaking concrete, digging up and replacing the lines, and getting everything back together and properly laid out was not an easy task (or a clean one). If I wasn't so determined to do it myself, I think the phonebook and the checkbook would have came out in short order. :)

    Everything should now be properly vented and up to code. I got the vent up through the 1st floor and now I am trying to snake it up through the 2nd floor. I got the needed test balls yesterday, so I should be abouit ready to test.

    Thanks!
     
  8. cacher_chick

    cacher_chick Test, Don't Guess!

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    Don't know where you read crushed? Around here when we order stone it is a given that we want washed (round) stone. If we want crushed we specify crushed.
    We use sand for bedding pipe, but fill before pouring a floor is always stone.
     
  9. ballvalve

    ballvalve General Engineering Contractor

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    looks like he asked what to use for backfill. Backfill in this area is on top and around the pipe. Round or crushed, its a no-go. You cannot compact round river bed washed rock anyway - you need to mix it with fines and then we call it class 1 or 2 road base.

    For septics we use 1 to 2" screened river rock, but if you want a road and you have a slope [and here 16 to 25% is common] you would specify 100% crushed 3/4" -

    If you want to get into details, I'll call my brother in Kenosha who inspects all backfill and compaction on city sewer and water installs. Got another in Weyawega with the great cheese.
     
    Last edited: Feb 25, 2010
  10. cacher_chick

    cacher_chick Test, Don't Guess!

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    Our main concern is drainage.

    I'd like to see you try to compact wet clay in a basement trench full of water. :p

    I worked in Kenosha for several years- who's your brother?
     
  11. ballvalve

    ballvalve General Engineering Contractor

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    What did you do in Kenosha? Maybe you worked for Michels pipeline....?
     
  12. nukeman

    nukeman Nuclear Engineer

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    Yep. The water has been a bit of a pain. I've had to spread this work out quite a bit just to avoid working in too much mud or under water. I have to base my inspection time based on weather too. Hard to check for leaks if the pipes are under water. :)
     
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