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Thread: Septic drainfield distribution box depth?

  1. #1

    Default Septic drainfield distribution box depth?

    Hi -
    How deep must the d-box feeding a septic drainfield be buried to SAFELY allow riding mowers, etc. to be used without damaging the box?

    Long story, but I am having a disagreement with a neighbor whose drainfield is *right* behind my house (located on homeowner association common ground via an easement). Turns out their d-box was destroyed at some point long ago - probably heavy equipment when the lots were being graded. It's just been replaced but is now very near the surface. They are insisting on building something ugly with rocks and stick (looks like a gravesite!) right behind my deck to ensure nothing ever goes over their d-box again. Instead, I want them to build up the dirt mound enough to protect it and then grow grass again (like the entire surrounding area).

    Their only concern is that the d-box be protected so it doesn't get broken again. I'm sympathetic but this thing is a mess. They are open to reseeding IF I can come up with a way to convince them it's protected enough.

    Any professional advice would sure be welcomed - how deep does it have to be?

    thanks, Rob

  2. #2

    Default Distribution box depth

    More information would be helpful. You didnít indicate the current depth of the distribution box (from the surface of the ground to the top of the box) or the material from which the box is constructed. Is this information available? If the distribution box is pre-cast concrete with a concrete lid and buried under two feet or more of soil, the weight and vibration of a typical riding lawnmower shouldnít have any adverse effect on the box at all. Depending on the depth the box is buried, there may be no need to construct any sort of protective mound unless itís to keep much heavier equipment than a riding mower from being driven over it.

  3. #3


    Thanks for the reply. The new box is precast concrete with a concrete top. The top surface is almost at ground level; the backfill was mounded over the top to a depth of about 6-8" and packed down by the backhoe.

    This morning I spoke to an environmental engineer at the County, who said that regular mowers should be no problem as long as we have about 8" or more covering the box. Like you, his big concern would something like using a huge tractor towing a BushHog to mow the field (they used to do that here to mow this common area, which might have been the cause of the broken D-box in the first place!)

    My suggestion is to dump a load of topsoil to increase the depth several more inches and also allow a more gradual slope to the top of the mound, followed by seed and straw.

    Does this make sense? Do you agree with the County's expert?

    Thanks, Rob

  4. #4
    Moderator & Master Plumber hj's Avatar
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    Aug 2004
    Cave Creek, Arizona

    Default d-box

    The depth of the actual distribution box is not negotiable. Its openings have to be at the level of the pipe coming from the septic tank and also doing to the several seepage line. The only thing that change is the height of the top of the box above that point. As stated, as long as there is sufficient dirt on top of it, and given its normal 24" or more depth to the pipes, there should be more than adequate dirt at that point it should not be subject to damaging. It was more than likely damaged during construction when it had very little soil over it and trucks were driving around the construction site.

  5. #5

    Default d-box

    Yep, I do agree with the county engineer. HJ's info is also pertinent. If the distributuion box is downhill from the septic tank (sloped lot), it could be installed just below the surface and still be lower than a more deeply buried tank. Also, you didn't indicate whether or not you know if there is a pump in the tank in which case, effluent could actually be pumped uphill to the distribution box and leech field (again, depending on the topography and system design).

  6. #6


    In this case, the septic tank is lower than either the distribution box or the drainfield. They have a pump that pushes the liquid waste over 400' and slightly uphill to the D-box. From there, it's simple gravity feed into the drain lines. The new box had to be placed almost at ground level in order to properly feed the drainfield -- as was pointed out, there's no choice in it given that the drainfield already exists at a fixed depth.

    That's what led to the whole issue: The concrete top of the D-box is by necessity virtually at ground level, so I want to build up the soil to provide enough depth over it that mowers can't break the box.



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