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Old 03-30-2019, 02:12 AM   #1
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Appropriate dirt for grading?


What makes for a good fill dirt? Especially for what's going under the slab, can it just be any type of dirt? Does it have to be subsoil? Is this also true of what's under a driveway or what's used for slopes and swales?
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Old 03-30-2019, 02:22 AM   #2
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Also, is it common to be able to find fill dirt for free? Even if I need 500 CY?
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Old 03-30-2019, 04:55 AM   #3
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Re: Appropriate dirt for grading?


There are certain types of soil you do not want under the house, that is best judged by a geo tech engineer while the hole is being dug out.

500 CY, you may need a permit for that.
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Old 03-30-2019, 05:17 AM   #4
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Re: Appropriate dirt for grading?


Locally we try to use slate rock or gravel to build up under a slab. Whatever you use you need to make sure it will compact well before you pour the concrete.
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Old 03-30-2019, 08:07 AM   #5
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Re: Appropriate dirt for grading?


Quote:
Originally Posted by farmerjohn1324 View Post
What makes for a good fill dirt? Especially for what's going under the slab, can it just be any type of dirt? Does it have to be subsoil? Is this also true of what's under a driveway or what's used for slopes and swales?

Also, is it common to be able to find fill dirt for free? Even if I need 500 CY?
Ayuh,..... Where on the planet are you,..??

Around here, our soils are pretty much bedrock limestone, sub-clay, 'n top-clay,....

For under a building, We use dug limestone, with abit of the sub-clay to fill voids, 'n help compaction,......
The key bein' enough stone for support, 'n just enough soils to hold things together,......
Old hyway dig out spoils is Perfect, as no soils are necessary,....
So long as any pavement chunks, concrete or asphalt, is busted into chunks less than a foot big, dependin' on the depth of the fill,.....

Under driveways, 'n parkin' lots, We box it out anywhere from 8" for a light duty driveway, to several feet for a heavy use parkin' lot/ road,....
Once boxed out, if it's softish, frabic is put down, then heavy crushed stone, followed by finer, 'n finer stone, til the top couple inches is 3/4" minus,.....
Compacted every few inches along the way,.....
This is another place to dump hyway spoils, in deeper fills, because of the chunk sizes,.....

Slopes, 'n swales are dug outa whatever is there, then covered with a couple inches of top dirt to support grass growth,.....

As far as Free Fill,....... Ya, it's Easy, if there's a dig goin' on fairly near where you wanta fill,......
It has to be within' a couple of miles, 'n closer than another waste site fill,.......
It comes down to the dollars, 'n cents of movin' the stuff, 'n the nature of the spoils,.....
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Old 03-30-2019, 09:59 AM   #6
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Re: Appropriate dirt for grading?


You really don't want to fill a "topsoil" under the gravel base of concrete flatwork, because what makes it topsoil is the amount of organic material in it which will compress/create voids after your initial compaction.

What I see for free (from DIYers on Craigslist) is easy to come by, just requires 2 digs (one into truck, one out of truck). Who knows what you'll get in the mix. A jobsite usually doesn't want to mess with a bucket for 1.0yard at a time, so you'd probably have to pay for trucking it and make sure you can fit a tandem if that's what they're hauling.
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Old 04-02-2019, 11:12 AM   #7
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Re: Appropriate dirt for grading?


@farmerjohn1324 , where are you? That could be important. As @Nealtw points out, some places make a big deal over permits.

Here in California, it's super easy to get fill dirt because for some crazed, inexplicable reason, landfills have refused to take it any more. (Well, maybe too much trash from our 40+ million people?) So, Craigslist is full of ads for free dirt, sometimes really really large amounts, sometimes with delivery because they just want to get rid of the stuff.

(What can I say? It's the Land of La La and home.)

I'm not an engineer, but based on my limited experience, you'll want heavy dirt under your slab, that also drains well. @3onthetree 's advice appears sound to me, because I work with a LOT of rotten organic stuff that just disappears sometimes.

If you're in his 'hood, @Bondo 's advice is also worth checking out, too.

You want something that will give "dimensional stability" and drain well. If you're in a place where drainage is bad that's a big deal, trust me. A buddy with a "swimming slab" in wet clay cried in an ocean of beer about the cracks it got.

So, let us know what you do, and contribute to the body of knowledge.
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Old 04-10-2019, 11:45 AM   #8
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Re: Appropriate dirt for grading?


Under a slab you want gravel that is compacted and the inches needed depends on the drainage and the thickness of the slab. Contractors often use 3 inches when there should be 5 inches.



Also be sure whoever does the rebar grid that it is elevated above the surface of the ground. That is what the 2x2x2 blocks of concrete with wires that you find at Home Depot are used for. Rebar at the bottom of a slab provides no added strength.



With a swale I would have the concrete follow the contour of it as much as possible. You could use a culvert but simply moves the problem from one place to another spot.
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