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gravel and slab on soft soil? - Forno Bravo Forum: The Wood-Fired Oven Community


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gravel and slab on soft soil?

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  • gravel and slab on soft soil?

    I am re-siting my oven (due to dubious planning on my part) a few feet from an existing slab to a soil area that was filled and tamped 10 years ago. However, after digging one foot I can push a piece of re-bar down about 3 feet in some places.

    Is this one of those "piers with feet -then a reinforced slab" situations, or is a floating slab on a couple of feet of gravel or concrete blend a possibility?

    The oven will be a 36" Pompeii.


  • #2
    Re: gravel and slab on soft soil?

    Is it in a low spot that has been wet or holds water? Is that typical soil stability in your area or is this spot different than surrounding soil?
    My Progress:


    • #3
      Re: gravel and slab on soft soil?

      "However, after digging one foot I can push a piece of re-bar down about 3 feet in some places. "

      I don't know exactly how hard you can push, but this sounds to me like very weak soil conditions.

      Placing a slab on grade would likely give you some serious settlement problems. I would be tempted to continue to dig a test hole (rent a hand held posthole digger) down to firm ground. If this firm ground is in the order of 4 feet or so (or is uneven, i.e 3 feet at one corner, 5 feet at another), I would opt for a pile/pier type construction with a suspended slab.

      If you can get some local expertize this would be good. It is worth fussing around at this stage.
      Last edited by Neil2; 06-26-2012, 03:38 PM.


      • #4
        Re: gravel and slab on soft soil?

        Thanks for the replies. A bar cannot be pushed down so easily in typical soil conditions here (glacial drift). The specific area was beneath a garden bed and has suffered little traffic since fill leveled the space. It was formerly a hillside. I probably have firm ground at 4-5 feet. I'll check tomorrow.

        If so, is the procedure to pour four footed piers with re-bar, then gravel up to the tops of the piers, then a reinforced slab tied to the piers?


        • #5
          Re: gravel and slab on soft soil?

          I am not an expert on frost conditions living where we do not get more than a couple of minor freezes a year, but my understanding is that you have two options:

          Dig down below frost level, which could be 4' in NY, pour a footing and then build stem walls to finish grade and pour a suspended slab.

          Excavate below frost level, fill with compacted sharp gravel and build a floating slab.

          A piered structure would have to be engineered which is normally beyond the scope of an oven. There are other designs using insulation and floating slabs with less fill, but they also have to be engineered.


          • #6
            Re: gravel and slab on soft soil?

            Don't go for the slab on fill dirt. If you get unlucky enough to get half on the fill and half on undisturbed soil you will have problems that won't quit. I have run across this a lot especially with homes on basements. You would need to dig down 10' just to find something solid. You can dig deep piers down to something solid is one option but once you get past the 4' mark and your still on fill the piers will be a real challenge. One thing you can do if your fill is deep is get some 10' sections of galvanized pipe and drive them like a fence post into your loose soil. Drive them hard until they find something solid and won't go any more. If they don't go all the way in cut them at a level that will allow you to incorporate them into the slab.

            The other way its to move the oven to solid ground. I would take this option if possible.


            • #7
              Re: gravel and slab on soft soil?

              Three piers not four. (Four is statically indeterminate).

              See my build for details:


              As Tscarborough notes, the reinforcing transition from the piers to the suspended slab is not something normally done in oven builds, but if you are competent in placing and bending rebar and in concrete pours this is an option.
              Last edited by Neil2; 06-27-2012, 11:57 AM.


              • #8
                Re: gravel and slab on soft soil?

                Thanks for all the sage advice. I researched and read about some of the possible solutions. ( I am trained in geophysics, but not an engineer). None seemed achievable for a site beyond the reach of machinery. I can move the oven site to a 50 year-old, 6 " reinforced slab. I know that the concrete is very strong, and reinforced, because I had to cut some pieces out a few years ago.

                I need to level the slab a bit, so I am researching steel and concrete options for leveling, now.



                • #9
                  Re: gravel and slab on soft soil?

                  Is there any reason you can't simply use self leveling compound. Under the oven itself you could probably work out an inch or two just with mortar joints on the block.


                  • #10
                    Re: gravel and slab on soft soil?

                    I was checking the compressive strength of the leveling compounds. My local masonry supplier thought a portland mix with fine sand would be okay as a leveler.


                    • #11
                      Re: gravel and slab on soft soil?

                      How much out of level is it? For the area of the base itself, 1" +/- can be taken care of with the mortar bed.


                      • #12
                        Re: gravel and slab on soft soil?

                        It is out about 0.375"/foot across the width of the base (a little more than 1.5" total). I think a wet slab and a portland mix with cold water spread in the early morning should be strong and durable enough.


                        • #13
                          Re: gravel and slab on soft soil?

                          Really it isn't. First you can't featheredge portland/sand, and second anything over an inch really needs a larger aggregate. They do make polymer modified repair mortars for that application, and while a little pricey they will last, especially in a freeze-thaw wear application such as you have.


                          • #14
                            Re: gravel and slab on soft soil?

                            That almost sounds like a drainage slope to me; a bit over 1/4 inch per foot, but close enough to be intentional. I might suggest that you don't level the slab, fix half of the slope with the block mortar bed, and make up the rest with mortar joints on the stand. Then for the "finish floor" outside the stand, set some cheap slate and float out the drop over the area of the slab.


                            • #15
                              Re: gravel and slab on soft soil?

                              What he said.