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When rebuilding a drain field, is it imperative to dig individual trenches for each "tine" of the drain pipe "fork", or would it be possible to just dig one big hole to lay all the piping down. If I can just dig one big hole, I can just rent a skid steer. If I need to dig trenches, I'll have to rent a backhoe.

How deep below grade do the pipes need to be? I live in a place where frost typically reaches 6 feet or deeper before spring melt.

How much crushed rock needs to be beneath the pipes? Is there a limit to how deep the crushed rock can be? Is there a benefit of greater field capacity with deeper gravel?

Thanks.

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Based on your rather telling but incomplete information, I have to caution you on your plans. This may have changed, I haven't had septic in 20-years. Not good & not legal, if the field hasn't been unused for 20-years. This must be fully designed, tested, approved & inspected to be proper, legal & last 25 or more years.

If a newer field has to be re-used, all soil needs to be entirely removed & replaced with a proper draining & absorption mixture. A complete & total deep excavation. The fork must be a fork & not a single long pipe & no fork tine can be any longer than 100' & each must end with an inspection vent pipe. Each tine must be separated by dirt to avoid pooling & enforce even distribution.

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    Correct: Don't do anything at all until you've confirmed it's up to code. You also may want to look into septic chambers instead of field lines. They're preferable, presuming you specific soil conditions and code allow it. – Bob Mar 21 '16 at 13:57
  • My current field is in use but beginning to fail due to previous owners planting trees too close to the edge of the field. I know I will have to assemble the drainage piping in a fork formation, but I was wondering if 4 individual trenches need dug or if it is possible to excavate a large hole big enough to assemble the entire field prior to backfill. – Jarrett Down Mar 22 '16 at 2:49
  • Ah, that makes more sense. You may still need local approval even for a repair. But, I'd agree with just digging out the 1-pipe to chop out the roots & replace any broken sections. However, remove the tree & their stumps first. Pulling out the stumps is preferred, but stump grinding usually kills everything just fine. This will make roots nicely brittle & just the 1-pipe's root access will ensure other pipes won't be interfered with by the roots. – Iggy Mar 22 '16 at 3:05
  • Oh yeah, a perc test is a must for any new system or re-using an old site. And right again, the soil itself has to be cored & tested to be approved, all over the system's area. Failing either one will kill any plans & add major expense to correct. There's a fairly tight range of acceptable soil recipes. – Iggy Mar 22 '16 at 13:10
  • Sorry, Iggy. I commented that you need a perc test... then I figured, that goes without saying, so I deleted it. I think that you responded to my deleted comment at the same time... I hate mincing words, but I suppose that it's rude to let people think that you were just talking to yourself :) – Ben Welborn Mar 22 '16 at 14:14
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Pipe depth: Plastic septic pipes should be buried no less then 12" below the maximum frost level. However, there are exceptions for metal pipe and pipes that have (electric) heaters (but just for fun, I would check out geothermal loops for a septic system). I am fairly certain that this is all in the code, but I can't cite it. I wanted to point out that you might be able to build the soil up; maybe that would be something to look into (ask your local inspector) if you need help reaching your maximum depth.

How much (crushed) gravel: As a laborer (back in the day) I was told that you are supposed to have at least 2" of gravel beneath the pipe, but again, I don't actually know if that is code. I have seen more than 2" of gravel used lot's of times (like 12 inches deep); the reasoning (I was told) was that backfilling with soil would be a problem (maybe not allowed?) because soil will settle and sink. Unfortunately, I don't know about any codes for that, either.

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