1. 11th Sep 2010, 04:06 PM #1

    Default Ag pipe- gravel under or not?

    I have read conflicting opinions here and there on this forum about whether or not there should be gravel under an ag pipe drain.

    I would be interested in informed, experienced opinions on the topic.

    My uninformed, inexperienced first take on it would be no. If you have gravel under it, any water that finds its way to the ag will just drain out the bottom into the gravel. This may still work as the water will then travel through the gravel and come out the other end of the trench. But it seems the ag pipe serves no purpose in this case.

    On the other hand if you have the bottom say 1/3 of the pipe sitting on the clay or other soil with no air gaps, then hydrostatic pressure will force water out of the clay (which is under pressure due to the weight of water and clay above it) and into the ag pipe (which is at atmospheric pressure as it opens to the air at one end). Water will make its way in but not back out because it will be fighting against a pressure differential.

    With gravel you would not have this pressure differential because the air gaps in the gravel would be close to atmospheric pressure.

    Look at it this way, imagine you have a pile of wet clothes and you put a collander on them and step on it (=provide pressure). The water will seep up into the collander. Now put a layer of gravel between the clothes and the collander. Step on it and nothing happens. Pour some water into the collander and it will sink straight through.

    That would be my take on it but as I said I would value input from those with actual practical experience.

  2. 11th Sep 2010, 08:46 PM #2


    Hi russell2pi,

    The prime reason for having some gravel under the agg pipe is two fold.
    1. It helps reduce the amount of small particles getting into the lower section of the drain and restricting the flow over time.

    2. the added space under the agg pipe provided by the extra gravel leaves room for particles to fall out.

    Also note the top of the agg pipe should be below the top edge of the footing and against the side of it.

    Hope this helps.

    Good luck. title=Smilie

  3. 12th Sep 2010, 01:21 PM #3


    Plumbers were around long before Jesus was a carpenter

  4. 12th Sep 2010, 05:48 PM #4

    Default Gravel talk -

    It depends a bit on the purpose of the ag pipe and the nature of the soil. If you are trying to remove excess ground water, the gravel provides a larger cross-sectional area for the water to seep into. You are correct in saying that the water will seep through the gravel as well as along the pipe.

    In my experience, most ag pipes are laid with gravel around them.

    Where they are used as a sullage drain (ie as the dishcarge from a septic tank) then it is essential to have gravel. Reln drain is a better option in this case as it provides a larger air space under ground and is not going to block up as quickly as Ag pipes.

    Alles gute!title=Biggrin

  5. 14th Sep 2010, 03:21 PM #5


    I pondered this point too when I was installing my ag pipe, so many conflicting theories out there.

    The trench was 800 deep with about 400mm of gravel on top. Decided not to use Geo Fabric on my second trench though as the majority reckons they just clog up with silt ??

    I ended up placing 50mm ( using 10mm gravel) underneath the ag pipe and after a big rain the water pours out of the gravel and NOT the ag pipe hhehe

    any it works

    Id be interested to hear which way you went

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