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Community and Q&A

Gravel under concrete footing

Timothy Denny | Posted in Energy Efficiency and Durability on

I am trying to finalize several aspects of the foundation design for the high-performance house we’ll be building in central Vermont starting in May. Typical of the region, our house site, which has a 7% slope, is near the bottom of 600 foot ‘mountain’ and we expect it to be moderately wet (Buckland very fine sandy loam soil that is only “moderately well drained”). Our plans include a walk-out basement conditioned well enough for woodshop, so I anticipate using (A) both internal and external footing drains run to daylight, (B) a gravel pad, XPS insulation and a poly vapor barrier under the poured basement slab (XPS will continue around the perimeter of the slab and up interior walls), (C) a passive radon mitigation system and (D) both paint-on damp-proofing and a waterproof air-gap sheet on the exterior of the poured concrete walls.
While discussing our project with local excavators, several mentioned that they commonly include 4 to 6 inches of crusher run gravel UNDER the footings – properly compacted, of course – to improve forming and pouring of the footings. I know that pre-formed concrete basement wall panels and permanent wood foundations are typically placed on gravel footings, but I have not seen this detail included in descriptions of a poured foundation/basement.
My first question is whether it is OK to have the gravel layer under the footings? Second, would this gravel layer eliminate the need for the PVC water drainage pipe inside the footings? My thinking is that the water normally collected by the interior drainage pipe would instead flow under the footing and be collected by the exterior drainage system.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Timothy,
    Crusher-run gravel compacts well but does not drain very fast -- at least not as fast as 3/4-inch crushed stone (no fines).

    Talk to a soils engineer if you are worried -- but in my experience, either crusher-run gravel (compacted) or 3/4-inch crushed stone can be used under footings. So can rigid foam. An engineer may disagree.

  2. Expert Member
    Malcolm Taylor | | #2

    Timothy, as Martin says, a variety of produces are frequently used under footings. Usually they need to be approved by a geo-technical engineer. I'd consult your building inspector to see what he requires.

  3. Timothy Denny | | #3

    Thanks guys. Since it appears that using gravel under the footings will be OK, I would appreciate your thoughts about whether I could skip installing drainage pipes inside the footing based on my logic that water under the slab would equalize with that outside the footing, where the perimeter drainage system would remove it. (I would still install pipes under the slab for radon mitigation.)

  4. GBA Editor
    Martin Holladay | | #4

    Timothy,
    Some builders fill the bottom of their excavation with 4 inches of crushed stone, and then pour their footings on top of the crushed stone. If you do that, water can flow readily under the footings.

    If instead you pour your footings on either undisturbed soil or compacted crusher-run gravel, it's a good idea to include a few 4" pipes through the footings to connect your sub-slab crushed stone with the perforated pipe installed on the exterior side of the footings.

    There is a caveat, however: if your perforated pipe is connected to a solid drain pipe that leads to daylight, you need to include a check valve at the end of the pipe if you have an active radon mitigation system. If you're worried about radon, it often makes sense not to connect your sub-slab crushed stone with the perforated pipe on the exterior of your footing. When in doubt, consult a radon mitigation contractor.

  5. Rick Van Handel | | #5

    Martin hit the nail on the head. Engineers feel all warm and cuddly about 3/4" crushed stone with fines because they can get a standard proctor of material density and compaction. However, 3/4" crushed will pass very little water. We use 3/4" clean stone under footings all the time. As long as it's a small lift there are no worries. You can actually install 3/4" clear stone several feet thick with no worries about settling since it self consolidates. If you don't believe me, put 3/4" clear stone in a bucket and beat the tar out of it with a tamper. You'll break the bucket before the stone compacts. However, the one caveat is that if you have silty ground, you will need to install a typar like fabric to keep the silty ground from filling the voids between the stone. You can wrap typar along the trench sides before adding stone.

    Also, from an excavator's standpoint, we love daylighting footing drains. Add the check valve like Martin suggests and you're still good with radon mitigation.

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