Infill on a slabless slab
Hello,
I am looking for some advice on my infill in advance of a concrete free slab. My grade beams were poured last week and wound up being a bit higher than anticipated (4 feet tapered down to 2 feet) mostly on account of the slope of the site and the need for soil over wing insulation. As a result, I need to bring in about 90 yards of infill. I plan on bringing in bank run sand, which I can get for cheap, and compacting it in 5 inch lifts with a pretty beefy medium -duty plate compactor. I am not concerned with the drainage needs of the infill, since I have double footing drains, a French drain at the edge of my wing insulation, and a site that falls away. I am simply hoping that it compacts well. I imagine I could also do a 4-6 inch layer of 3/4 inch gravel on top of the bank run sand as a gas layer, and 3/8 on top of that as scree. Cheaper still is fdirt ill, which can be had for close to nothing, plus trucking. Thanks for any advice!
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That seems fine. Well-compacted sand can take up to about 1500 lbs/sq.ft., so plenty for non-structural areas and fine for structural footings, they just need to be a bit larger than they would be on gravel. Compacting in small lifts like that is a good idea. Wet it as you go to help it settle.
Graded sand is gas-permeable; the key is for particles to be of roughly similar size so the pores remain open, vs. bank run or gravel with a variety of sizes that will clog the pores. Sand is easy to screed and makes a fine base to put rigid insulation on top of.
Have you considered using Glavel or another expanded glass aggregate for a portion of your fill? It has higher compressive ratings than sand, it's lightweight so it's easy to move around, and it's roughly R-2/in so with 10" or so you have a high level of slab insulation. You would want a screed layer of sand or 3/8" stone over a geo-fabric and to place radon pipes down in the sand layer.
I already have my EPS foam and am on a tight budget so glavel is out. Its looking like bank run sand compacted. My horizontal perf pipe for footing drains and radon will be at the bottom of the grade beams. Do you think I should do à 4 inch layer of 3/4 inch crushed stone on top of all that compacted bank run as a gas layer that the vapor barrier will direct towards my vertical perforated pipe? I could then do a screed layer of bank run sand before I lay my EPS.
As long as the perforated pipe is in a gas-permeable layer that extends across the entire foundation, that should be fine. It just needs to intercept the gas on its upward journey. It's probably better near the vapor retarder but it should still work. If it's only in a limited area of crushed stone or graded sand then I would add a full system below your vapor retarder, if you're in a radon-risk zone. Bank run sand is likely not very vapor-permeable but it depends on the sand deposit.
Thanks Michael. I hear the logic in what you are saying. It seems best to forgo the combined footing drain/radon pipe at the base of the grade beam and move the perforated pipes up below the vapor barrier in a 6 inch layer of 3/4 inch stone that will sit on the bank run sand. I already have a footing drain on the outside of the grade beam, with 2 foot overhangs and 4 feet of wing insulation so I don't think water will be an issue. The site falls away, etc. I am in a radon zone so it seems best.
Sounds good. I just remembered that you're in Maine, correct?
yes, that's right. Waldoboro.