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We are ready to backfill the foundation for our crawl-space addition. I’ve heard that it’s a good idea to use a layer of gravel at the bottom of the trench before backfilling a foundation. But I also know that the current thinking is, when planting flowerpots for example, that you get better drainage of the water through the pot and out the bottom if there is just one continuous material (the soil) in the pot, and then a sort of “suction” happens which draws the water down through the soil. We cannot have a system of drain pipes to move water away from the foundation because of the roots of some valuable trees in the area, but we seem to have good natural drainage. Is gravel at the bottom of the trench really of any use? We will use plastic sheeting under the slab, and do the proper grading and roof gutters.
Any thoughts on this, and should I draw any comparisons between flowerpot soil and foundation backfill?
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Carole and Doug:
French Drains like the one you described can work without pipe. A ditch excavated and tamped, then filled with a deep layer of gravel and topped with fill will act as a conduit for water. It won't last as long as a properly constructed drainage system (it will get clogged with silt and dirt), but you should be able to get five or ten years out of it. More if you place poly at the bottom and a landscape fabric on top.
The problem is that the water has to go somewhere, and unless you provide for some exit for the drainage, all you will accomplish is to puddle the water in the french drain, and place hydrostatic pressure on the outside of the foundation wall/footing. I suppose you could lose the poly. This would have the effect of letting the water pass through the gravel and into the soil, where hopefully it will be disbursed by natural drainage.
I really don't no much about planting flowers, so the analogy is lost on me. The purpose of gravel is to provide loose fill, by which water can easily drain through and be collected or disbursed by the layers underneath it. Compacted soil retains water. That is why under all streets, driveways, slabs, drains, etc there is a fairly thick level of gravel to disburse the water and keep the slab dry.
*Without drain tile, you might be better off with compacted, less permeable soil instead of making an easy path for water to get against the foundation wall.As for the flower pots: Fill one flower pot with soil, another with small stones. Pour water in both and see which one has water comming out the bottom first.
*Thanks guys.Ryan, to clarify about the flowerpot theory- if you put gravel for drainage in the bottom of one pot, then fill it the rest of the way up with soil, and have another pot with only soil, then place them both on the ground and water them, the soil in the pot with only soil will be dryer more quickly because there is a continuous route for the water to be drawn out the bottom which is in contact with the ground- something I learned in horticulture school. I just wondered if the same principal might be true with backfilling a foundation, since it also has to do with water moving through soil...