Advice on Backfill Type (Slab)

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Old 04-05-15, 09:08 PM
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Advice on Backfill Type (Slab)

I'm in the process of building a log home, and soon the concrete will be poured for the garage. The contractor and I are a little at odds over the surface preparation. When the footings were dug for the garage, the dirt was piled into the center. I say dirt, but what I see is clay. It was dug wet and now it has dried -- really really hard. The ground level needs to be raised about 18" before the gravel and concrete, and my contractor says he plans on using a BobCat to level it off and compact it. I protested this and convinced him to use a vibrating compactor (walk behind kind) rather than the BobCat, which has a static pressure on the ground of only 5 psi.

But now I'm thinking that I don't even want that clay to be any part of the soil under the concrete. I want it removed and replaced with the granular soil that was dug up for the house. The two photos I have attached show the clay and the soil I would prefer using. Can someone tell me if I'm being overly concerned about this, or should I insist on doing what I have described?

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Old 04-06-15, 03:08 AM
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You don't want "soil" in the garage for a pour. Clay is the best for that substrate. Compacting as you want is best, as a complete compacting with rolling machinery will be questionable. Where did the clay come from that is piled in the middle? Was it not part of the original excavation, or was it brought in for another purpose? What part of NC are you located in. Several types of clay in the state from sandy in the outer banks to hard rock clay in WNC. I am used to Georgia red clay which compacts quite well. Our concrete specialists will be here shortly with their take on it, so with a few answers, they will be armed.
 
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Old 04-06-15, 03:36 AM
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I'm just a painter but I wouldn't want top soil under the slab. Around here we use slate and/or gravel for fill under slabs. One of the better builders I've painted for would dump in a foot or so of fill, compact it, wet it down and then come back and repeat until the desired level was achieved.
 
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Old 04-06-15, 05:27 PM
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I thought clay expands when wet and contracts when dry (or is it vice versa). Anyway, like I said, the clay in the middle came from the footings that were dug for the garage. Right now that stuff is really hard, and I was thinking that even after compacting there would be voids left over at some depth that would collapse later. And that "soil" or dirt or whatever in my 2nd picture came from the dig from the basement. I think it's sandrock, or so I'm told. Whatever it is it seems to be fine-grained and I can see it compacting better than those hard clay clods. Can you tell by the picture? Which is best as the foundation for the garage pour?

I'm in Mocksville, NC, but I don't think the dirt in my property is typical. Everyone who sees it and gets stuck in it after a rain says it's like nothing they have seen before. Doesn't look like it in the picture but when wet that stuff is very dark mud, almost black, and slippery as grease.
 
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Old 04-07-15, 09:15 AM
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Personally I would have the contractor remove all of the clay from the inside. I have seen too many jobs halted because of wet sub-soil with too much clay. We need some more information about the footers and if you have a stem wall. These should be backfilled at the same time as you add the backfill to the area of the clay. The clay and any top soil/organic material must be removed prior to backfilling.
 
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Old 04-09-15, 06:02 PM
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looks like a clear winner to me - no one wants clay you only get 1 chance to do it right the 1st time - that time is now !

better you should have it done right than suffer expensive consequences later
 
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Old 04-09-15, 06:30 PM
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I understand removing all the top soil, but that leaves nothing but clay, here. Just curious as to what you will replace the clay with. And in reality, all you are replacing is a small amount of clay, and what you put down will be sitting on clay. Of course, I do realize not all areas are as hard as we are. Always learning.
 
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Old 04-10-15, 07:28 AM
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As long as the clay is native, undisturbed and dry, I would place 4 oz. fabric over the clay and back fill with #57 crushed stone and compact in 6" lifts. If you have a stem wall, place your backfill on the outside of the wall as you increase the backfill on the inside.
 
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Old 04-12-15, 08:11 PM
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Another vote here for removing that clay, and then building up the subgrade with something more compactable (in lifts as previously suggested), and having less cohesion. Clay is not a good material for pouring a slab over, even if proper base material is placed between the clay and concrete.

If your construction contract for the home didn't specify what was to be used instead of the clay, your contractor may bill you for what he will consider an extra cost to the contract, as agreed upon. Bite the bullet, pay for it, and know that you are doing the best you can to ensure a well-performing garage slab.
 
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