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Gravel Under Footings, Thickened Slabs, Any Concrete Bearing Item

Discussion in 'Residential Foundation Codes' started by Hank, Jul 21, 2011.

  1. Hank

    Hank Member

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    Does anyone know where a good discussion about gravel under concrete might be found other than elastic discussions on pavement composite sections.
     
  2. fatboy

    fatboy Administrator

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    Sorry, can't help you out, but welcome to the forum!
     
  3. KZQuixote

    KZQuixote Sawhorse

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    "Gravel Under Footings, Thickened Slabs, Any Concrete Bearing Item"

    As long as it's not feet deep and loose is a good thing.

    Bill
     
  4. knightj

    knightj Member

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    Gravel under footings

    Here were I am, we have exspansive soils so most enginers don't want gravel under the footings unless its very fine and tightly compacted because of the possiable accumulation of water.
     
  5. khsmith55

    khsmith55 Bronze Member

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    Agree with knightj, we have the same problem with expansive soils here. The Geotechnical Engineers express the same concerns and also recommend eliminating the gravel under slabs on grade and pour the slabs directly on compacted earth.
     
  6. Hank

    Hank Member

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    A little more specific question on the post

    Thanks for the comments and understand the expansive soils considerations, etc. To be a little more specific, one person has insisted that a thickened slab under an interior bearing wall NOT have gravel below it. That person's logic is that since the exterior turndown footings are on dirt without gravel there could be differential settlement. From everything I have read, the stone is actually extending the bottom of the concrete to the soil because the stone is so much stiffer than the soil and has a considerably higher bearing capacity than typical well-compacted engineered fills or virgin soils suitable for 2,000 psf or so bearing. So, my conclusion is that stone under the thickened slab for an interior bearing wall is better than without it because the load is actually being spread even further due to the angle of repose of the soil and the differential settlement related to a few inches of gravel versus no gravel is negligible and should be ignored. So, the bottom line question here is should the gravel under a thickened slab under an interior bearing wall be there or not? Personally, I think not but I have not been able to find any discussions anywhere on the web dealig with this specific item. One final observation is that if you remove the gravel, the capillary stop action provided by the gravel is removed. (PS - most of the reading was in the ACI Manual of Concrete Practice 2011 in several sections but 360R-10 specifically deals with slabs on ground.)
     
  7. Hank

    Hank Member

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    Sorry I didn't proof the above post. Personally, I think the gravel SHOULD be below the thickened slab.
     
  8. globe trekker

    globe trekker Sawhorse

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    Hank,

    Welcome to The Building Codes Forum! :)

    I am not an engineer, nor do I know much about the various soil

    mechanics and the such. We rely on our city engineer to come

    up with compliant solutions to answers such as yours.

    With that said though, I am listing 2 separate links to

    "Engineer's Forums" for you ( and others ). Check `em out!

    http://www.eng-tips.com/threadcategory.cfm?lev2=8

    http://www.engineerforum.net/

    Hope this helps!

    .
     
  9. Mark K

    Mark K Platinum Member

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    While individual pieces of gravel may be stiffer than soil the gravel is not typically compacted enough to prevent further change in volume of the gravel. Depending on the type of gravel it may not be feasible to compact it.

    I would have no problem with omitting the gravel under the thickened slabs under interior walls.

    See the ACI recommendations for where you need a capillary break and where not.
     
  10. peach

    peach Sawhorse

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    all footings for bearing walls need to be bearing uniformly; whatever the SBC is for the exterior footings needs to be the same for interior bearing walls. How that is achieved is up to the designer.
     
  11. ICE

    ICE Sawhorse

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    I had this happen: I can't remember dimensions other than to say everything was large. I do remember that the footing steel was #8 and there was a bunch of it. The footing was over excavated 3' and gravel was placed in that 3'. At the footing inspection I found the steel resting on the gravel. The contractor was perplexed and assured me that the steel had been 3" clear of the gravel and he had a special inspectors report to prove it. I couldn't approve the work so they had me come back a few days later. That time most of the steel was an inch above the gravel. The steel was supported on dobies and there was a railway about 25' away. A train passed by and I watched the gravel dance and the steel sinking.

    They had to line up backhoes and lift the steel with chains during the pour. I wonder if they got the designer to pay for the chains.
     
    #11 ICE, Jul 24, 2011
    Last edited by a moderator: Jul 24, 2011
  12. peach

    peach Sawhorse

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    permanent wood foundations require gravel, not concrete or soil for the footings - every situation is a little different, but all footings need to bear on the soil/gravel/bedrock/ whatever is supporting the structure equally, or it's going to settle at different rates. All buildings are going to settle some, the problem comes when one portion settles faster or slower than the rest.. that's when cracks happen.
     
  13. Mark K

    Mark K Platinum Member

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    ICE

    You may have solved the problem of reinforcing cover but I would suspect that over time as the 3' of gravel consolidated over time that the footing would continue to settle. One of the limitation of the IRC is that it doesn't deal well with these sort of situations.
     
  14. ICE

    ICE Sawhorse

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    This was under the IBC about ten years ago. I certainly wouldn't discount the settlement issue that you raised but I made everybody aware of the situation and nobody threw a flag.
     
  15. Sifu

    Sifu Gold Member

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    Globe trekker, I went to engineerforum.net, it was full of what appears to be online pharmceutical sales/scams. I found what looked like a few engineer related topics but not many. Have you used this forum in the past and it was legit?
     
  16. gbhammer

    gbhammer Platinum Member

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    Slurry lots of slurry
     
  17. ICE

    ICE Sawhorse

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    A respected engineer in these parts that concentrates on swimming pools, recommends 6" of 3/4" rock under flat work around a pool. His take on it is that as the soil expands, it will push into voids between the rocks rather than lift the deck. Along with that, he requires a 6" wide, 12" deep perimeter footing and any trenches beneath are to be compacted.
     
  18. steveray

    steveray Sawhorse

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    If you fill it...you engineer it.....correct?
     

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