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Floating slab OK in Northeast?

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  • Floating slab OK in Northeast?

    I know this has been discussed before, but I'm not sure there was a consensus.

    I've started excavating for an L-shaped foundation for a Casa 90 oven at the corner and work space and small stone bench at the left and right (sketch attached). Our soil here is hard clay. I am leaning toward a 5" reinforced slab on 5" of crushed stone, maybe with thickened edges. I know that if I try to dig or drill to the frostline I'll hit large boulders and it will be a huge undertaking but I'm a little nervous about having the thing float.

    This weekend, I cut the adjacent patio to make room for the oven. The patio is 4" thick, with no reinforcing (looks like the truck showed up before they had the mesh raised above the stone bed and they just poured the slab on top instead of embedding the meash), on only a few inches of fine crushed stone, yet it's been quite stable. This makes me a little less nervous about floating.

    Any thoughts?

  • #2
    frost heave

    Originally posted by Alan
    This makes me a little less nervous about floating.

    Any thoughts?
    I think I'm about the only one here who has done the excavate to frost line bit. I'm doing it because i'm building a two story masonry chimney as an addition to an existing structure. It's a LOT of work. I think for a free standing oven, a slab floating on crushed stone is fine.

    The theory here is that the coarse crushed stone draws ground water away from the slab, and frost heave is not a problem. If you have serious clay, you may want to run some drain tiles from the stone bed to a lower location to make sure things stay well drained.

    Good luck.
    My geodesic oven project: part 1, part 2


    • #3
      Originally posted by dmun
      I think I'm about the only one here who has done the excavate to frost line bit.
      Don't sell me short. I dug 4 cassions down to frost line then poured a wall on top of those. The wall goes down 1 foot and the caissons 2 more feet below that for a total depth of 3 feet. Why? I am not friends made fun of me...Of course when I spoke with a friend who is an engineer, he said I should have 5 cassions!

      We do have really heavy clay, but no boulders underground.

      You can always mud-jack it if it heaves.

      My Oven Thread:


      • #4
        All I need - two people who did all that work to make me feel lazy!

        The hard reality for me is that I either rent a backhoe or dig for weeks. My chances of digging cassion holes around here without hitting boulders is low. Doing some more poking around on the net, I see that there are very cold climates in which buldings are commonly placed on floating slabs, well-reinforced with thickened edges, so I think that's what I'll do.

        So I might be nervous about it but most likely I'll leave it floating but make it really strong. The extra thickness might justify a concrete truck.

        Thanks for your perspectives.


        • #5
          Don't give me too much credit, I rented the mini-excavator:
          My Oven Thread:


          • #6


            I live in Ontario, and the ground around here is almost exactly what you describe: topsoil over hard clay with glacial rocks imbedded. I rented a post hole auger and drilled as far as I could into the clay, then used sono tubes (8 inch) with rebar in the centre, tied to the mesh above. I have a lot of ground water here, so I added a lot of crushed stone and drain tiles around the perimeter.

            I've had exactly zero problems with frost heave, and here code is four feet down.

            It's true that in the Canadian Arctic they do build on true floating slabs, but that's on permafrost, and that you cannot defeat, no matter what you do.

            "Made are tools, and born are hands"--William Blake, 1757-1827


            • #7
              Frost heave is a complicated phenominon, and much study has been made of it. The shape of footings is almost as important as their mass. Here is a proprietory system of flared footings that is engineered to stay put. The idea is that footings should be bigger at the bottom than at the top: making "ice cream cone" shaped footings can actually make the problem worse than just having a floating slab.

              Here in New Jersey, I once dug a trench to supply electrical power to my studio in January. Winter long freezes are extremely uncommon here.

              For frost heave to occur, you need three things:

              1) Fine soil
              2) Moisture
              3) Freezing temperature

              Slab on gravel systems work by removing the "fines" and the moisture from the system.
              My geodesic oven project: part 1, part 2


              • #8
                just my thoughts

                Although i am still in the planning stages i have some set ideas on pouring my slab. Since i live in Northeast Pennsylvania winters here can be harsh at times and heaving can and will occur. A reenforced slab that heaves from freeze and thaw in all reality will not crack. However there is a good chance that once it does heave it won't return to level. My plan is to build a gable enclosure with a tile exterior that will even cover the block stand. I am striving for no movement. I'll excavate deep enough for my slab enabling myself to use 2 x 10 forms. In each corner I'll dig a hole using a rented auger for placement of 6 inch tubes. Rebar will be bent to extend from the bottom of one tube; extending to a height thats halfway into the slab then accross and to the bottom of the next tube.This will be done for all 4 sides. Rebar will then be placed on top of this and tied topped with a mesh grid. The 2 x 10 forms will allow me to have 4 inches of gravel and a concrete slab that is 5 and 1/4 inches thick. Hopefully the result will be a slab with no movement and no cracking. A sound and level foundation will just make each progressive step that much easier and result in an oven that will hopefully last a lifetime