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Preparing Subgrade for Concrete-Free Slab

steve41 | Posted in General Questions on

I’m building a small addition, ~16×16, with ICF stem walls scribed/pinned to solid bedrock.  Zone 6A. The interior will be filled and have a concrete-free raised “slab”.  The bedrock varies ~6-30″ below grade.

The native soil over the bedrock was excavated to a ~2-4ft margin to enable ICF installation.  The native soil is a mix of clay/other.  My plan was to re-fill the interior of the ICF’s with the native soil, compacting in lifts as I go, until I get to about 14″ below final floor level.  I don’t know the soil type / composition other than it’s probably 30-40% clay.

Should I use the native soil or should I get it tested for composition?

The layers from bottom to top:
1. Bedrock
2. Native fill, compacted in lifts
3. Glavel, ~12″, compacted in lifts (wrapped fully in geotextile), (4″ radon piping within)
4. 1/4″minus,  1-2″, for leveling
5. 15 mil Stego vapor barrier, taped, sealed
6. Floating subfloor, Advantech, 2 layers
7. Finish flooring

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  1. kbentley57 | | #1


    To get started, you can do the jar test and get a rough idea of your soil composition. . Another helpful tool is a pocket penetrometer like this one ( to get an idea of the compressive strength after compaction.

    If it's mostly clay like, it'll compress well enough if it's slightly moist. You'll want to use a ramming tamper instead of the plat compactor for that part though. Remember that it's only the interior of the room, and need only support 40 - 50 lb / sq ft on average, which is basically nothing to even moderately stiff couch cushions, once distributed over the subfloor, fines, and glavel. In the IRC, clay soil has a bearing capacity of 1.5 Tons / square ft at worst, which is still around 20 psi. 50 lb / sq. ft. is about 0.33 psi, for comparison.

    Having said that, I think skipping even a thin slab is a poor choice, though it can be done successfully.

  2. steve41 | | #2

    Thank you kbentley, great info.

    I agree regarding the compressive strength of the subgrade- I'm not overly concerned with 50psf provided I compact reasonably well.

    My main concern is settling, either due to the subgrade prep/elements, or due to the uneven starting base, which in this case is the undulating bedrock.

    Originally I had intended to install a concrete slab. I felt the concrete-less slab option was interesting but too risky. After doing more homework on concrete-less options I finally decided to go for it. Beyond the concrete reduction, I like the DIY friendliness of it. In my particular case, I reasoned that since all of the subgrade would be fully enveloped on 4 sides, and supported by shallow solid bedrock at the bottom, that the settlement risk was extremely low. Outside of further compaction the fill can't really be displaced. But I would really like to hear from others that have experience with concrete-less "slabs".

    Why do you believe that skipping concrete is a poor choice? I still have time to make changes.

    Thanks again

  3. Expert Member
    Michael Maines | | #3

    Your position over bedrock makes your situation pretty forgiving, but if there is any chance that water can find its way under the ICFs, I would rather not use a clay mix. In my area, "bank run" gravel is usually a safe bet, or you can get more specific and order 3/4" stone with no fines or something like a gravel with "aggregate that passes through a 3" sieve but is retained by a 1/4" sieve."

    For the screed layer it would also be best to have no fines but as long as it's reasonably clean it's probably ok.

    The biggest reasons I've found to include the concrete is that contractors and building inspectors are leery of anything new and different, and more importantly, it takes extra effort to get the sub-slab prep perfect. Pouring a slab evens out irregularities, though it also requires time, money and effort, so in the end it's probably a wash. If the sub-slab prep is subject to settling, it won't matter if the slab is concrete or wood.

  4. steve41 | | #4

    Thanks Michael.

    I'll definitely consider the more safe option of bank run or similar aggregate. Though it does make me question whether the interior "hump" of undisturbed soil should be removed so that subsequent layers are on a reasonably consistent base.

    Thankfully, our code enforcement officer is receptive to this plan, including the use of Glavel in place of EPS.

    1. Expert Member
      Michael Maines | | #5

      Steve, the concern with clay is not so much bearing capacity in this case but in its tendency to swell when wet. Different types of clay behave differently when wet, so it really depends on your soil composition. You could perform your own tests or hire a soils engineer if you want to be confident about leaving the existing material in place, but if you have an excavator on site it's probably easier and cheaper to just get rid of the existing soil. That's great that your CEO is on board! Let us know how it goes. My first Glavel project is starting soon.

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