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Community and Q&A

Below Grade Monlithic Slab

CamWalker | Posted in Green Building Techniques on

Hi everyone,

I have had great success with monolithic slabs in the past, and I’d like to use one again on a current project I’m working on, but I’ve not seen anything regarding doing one below grade The site is rather hilly, and whilst there is a high and flattish spot, it just doesn’t speak to me nor the client, for a variety of architecture, poetic reasons. I would much rather see the house set into the hill slightly a way down and further from the neighbors an entrance framing the better views and taking better advantage of solar gain and such. It would also suit the topography rather well. One end of the house would be 3′ -4′ below grade, and the other at grade. 

For all the reasons why monolithic frost protected slabs are great, I’d like to do it on this project, but does anybody have any resources for doing one below grade? Or am I being ridiculous? 

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  1. Expert Member


    You could use a monolithic slab with integral footings, and pour stem-walls on top of it to retain grade on the high side.

    I suspect you will need that attachment engineered to resist lateral forces, and you will also need to pay close attention to how the joint between them is detailed, as it will be vulnerable to water infiltration.

    What going that route precludes is insulating on the interior, as you could with conventional stem-walls and a slab.

    One way round these complications it to detach the retaining walls from the house by flattening the site another three feet or so, and building an independent retaining wall on the high side. Then you are free to use the standard FPSF slab.

  2. CamWalker | | #2


    Thank you. I had an idea that the retaining wall (concrete block for affordability and aesthtics with white limewashing on the interior) would be wrapped with at least 4" of eps, if not 6", going down to the bottom of the integrated haunched footing with a typical ick out. On top of that and its kickout I would use EPDM and good channel drains.

    I do like your indepentant retaining wall idea though.

    Furthermore, dependign on the location exactly, there migth actually be a slope in the other direction, can you have a very long and deep haunch? or a retaing wall, back filled and then a monolithic slab on top? Now I ask myself why?

    1. Expert Member
      MALCOLM TAYLOR | | #4


      "Now I ask myself why?

      I think that's what Akos is getting at.

  3. Expert Member
    Akos | | #3

    This has the feel of a square peg in a round hole.

    If you are excavating that much on the site, the additional cost of digging for a standard or frost protected shallow foundation will be noise. This also moves you to a prescriptive foundation design which saves engineering fees.

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