Gravel under concrete footing
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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