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Old 06-11-2008, 09:46 AM   #1
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Building a shed, need help with pouring slab / fill


Hello Everyone,

I'm planning on submitting the permit application for building a shed in the next couple days and need some clarification on the best way to start the project in regards to the concrete.

The building is going to be a 2 floor, gambrel style building 16 x 20. The Spot the shed is going to be built happens to be the lowest point of the property in both directions. it's not severe by any means but it does collect water in the spring after the winter thaw. I can elevate the area with a couple truckloads of fill and force the water off the property into the road and back woods with a couple feet of elevation.

As far as I can tell, there is no problem with ground water. The water is all surface collection and from what I can tell, can easily be diverted with some fill.

Because of the size, the town requires a formal foundation of some kind. Basically anything except building directly on the ground. I can pour a perimiter foundation, a full slab or use sonnet tubes.

Now for my questions:

Fill seems to be by best bet to straighten out the water issues but once I do that, what are my options for a foundation? I'm guessing after disturbing the earth and filling sonnet tubes would not be the best idea.

My thoughts are fill, then crushed stone over the top of the fill, build slab forms, pour concrete, remove forms then backfill and slope to cover exposed crushed stone?

Does this sound right? What take-off should I use for my slope and how often should I be conpacting the fill? With this design do I need footings on the slab? If so how deep?

The wood portion of this project I can handle.. the concrete isn't very forgiving and I dont want to screw it up. You only get one shot that it and if it doesn't work right, it's going to be a nightmare to straighten out!

Thanks for any suggestions anyone can offer.

Regards,
Scott

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Old 06-11-2008, 11:53 AM   #2
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Building a shed, need help with pouring slab / fill


Fill dirt is a bad idea. You won't get it compacted enough to adequately serve as bearing soil under the slab and footing...Unless you own a bulldozer. Fill dirt could of course be used to build berms around the building to redirect the flow of water.

A better base would be AB3, or compactable road base (very small gravel with fly ash). You'll need to install it in layers, and rent a compactor to compact a few inches at a time.

You need to find a way to deal with the water. Can you install some french drains at the low spot and divert the water elsewhere?

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Old 06-11-2008, 12:52 PM   #3
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Building a shed, need help with pouring slab / fill


I'm sorry, I didn't mean actual dirt... I have used this 'pak' substane sold by the local gravel yards to repair dirt driveways, etc. It compacts pretty solid. I believe it's a mix of stone dust and procesed gravel. That what I would be using. I'll ask them if it's equivelant to AB3. As far as the drains go... if I were to build this up a little, even a couple feet, it would now be higher than it's immediate surroundings so the new low spots would be to either side of the built-up earth as it tapers off to natural grade. I figured crushed stone in these new low spots to direct the water down and away from the shed?
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Old 06-11-2008, 02:48 PM   #4
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Building a shed, need help with pouring slab / fill


I wouldn't pour a slab on top of fill. Even if you compact it, it will still settle and crack your slab. Since you have a spring thaw, it's obvious that you have a freeze. You need to get your foundation below the frost line for your area.You could set sonotubes in a grid and pour them, then back fill between them and pour a slab on top locking all of the piers together. The slab would bridge the fill, but you'd need to make it thick enough to support itself plus any load you impose on it.
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Old 06-11-2008, 04:10 PM   #5
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Building a shed, need help with pouring slab / fill


Hi Maintenance 6,

So your suggesting I scrape the grade (remove any topsoil, set the sonotubes, pour, backfill (3/4" crushed stone?), build a form for the slab, then lay down crushed stone and pour the slab over the sonotubes?

Should the sonotubes extend up into slab, I'm assuming so. If so, how far? 1" from the surface? more, less?

You also mention I should make sure it's thick enough to support itself and whatever I put on it. How thick is thick enough? It's only going to be for storage... 4'wheelers, household storage, motorcycles, etc. I know this is vague but I dont even know where to start with thickness.

Lastly, how close should the sonet tubes be? From the sounds of it, what I'm doing is pouring the slab onto the ground and it'll harden and be suspended by the sonotubes.




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I wouldn't pour a slab on top of fill. Even if you compact it, it will still settle and crack your slab. Since you have a spring thaw, it's obvious that you have a freeze. You need to get your foundation below the frost line for your area.You could set sonotubes in a grid and pour them, then back fill between them and pour a slab on top locking all of the piers together. The slab would bridge the fill, but you'd need to make it thick enough to support itself plus any load you impose on it.
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Old 06-12-2008, 06:19 AM   #6
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Building a shed, need help with pouring slab / fill


That's pretty much what I'm thinking. The piers would rest on undisturbed earth and support the slab. I would run rebar vertically in the piers and let a few inches stick out the top to lock in to the slab. I'm not an engineer so I'm hesitant to tell you how thick to make the slab or how many piers to place. After the piers are in, I see no reason why you couldn't back fill with gravel, tamping as you go and top it off with 6-8 inches of clean stone. Make the stone level with the tops of the piers and then form and pour the slab. The alternative is to pour some footers, lay up a block wall and some block piers, fill it in and pour your slab on top. You'll need rebar in the slab either way

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