Gravel under concrete pad

[marrt]

I've roughed out a site for a pole barn to store my tractor and for a shop. The lower side of the site is about 7 inches lower than the upper side (side 2 in the attached picture is 7 inches lower than side 1). I'm thinking of using gravel to make up the difference when I go to pour the pad. Assuming 3" of gravel under the upper side, this means the lower side would have 10" of #57 gravel under the pad. Do you see any issues with this approach? Or should I just take the time and completely level the site before applying the pad gravel?

 

[EddieWalker]

Gravel will work, but it wouldn't be my first choice. There are two issues with gravel for fill material that you should consider. One is the price. It's very expensive compared to dirt or sand. The picture looks like you have some already, so the expense might not be an issue for you. The second issue is compaction. Getting gravel compacted takes allot of effort, and even with years of expereience, not all the pros get it right all the time. If it's not compacted properly, the concrete on it will shift, move and crack. If you are in an area that freezes, you also risk water getting into the gravel and expanding when it freezes. This is very, VERY bad.

Sand is probably the perfect base material for building on. It is the only material that self compacts. It's easy to work with, and in most areas, fairly cheap. Here, it's about the same price as clean fill dirt.

Dirt is also good for building up a pad, but will require compaction before you can pour concrete on it, or you will have issues with the concrete cracking as the dirt settles under it.

Eddie

 

[shaley]

I'd use "crusher run" vs #57. Crusher will pack much better than 57. It doesn't matter what you use for a sub base as long as it is compacted to a suitable load bearing value for the pad. Gravel will offer better frost heave than earth so maybe a small course of compacted earth and then gravel. If you have that typical NOVA clay it should compact great.
What does the local building code say for gravel base thickness, 3" sounds thin. Are you pouring a turndown monolithic slab?

 

[marrt]

I'm not sure about the price of sand around here. I'll check. I used 3 truck loads of dirt originally. "Most" of the organics were out but I still had some issues. Then I converted to crusher run as shaley mentioned. Crusher run is good stuff if you have a compactor handy. I used a compactor for everything of course. Twice, I had 22 ton dump trucks over most of the fill area with no tracks left. However, I did have a big problem in one area that has about 2 feet of fill. After 4 days of rain, this area would not compact when I started working it again. Amazingly, you could literally feel the softness of the area with your foot. I took a backhoe, dug down a good 4 feet by 10 feet and filled the entire hole with crusher run, compacting between shifts. I "feels" solid now. I hope it holds.

Regarding my original question, the upper side of the pad area is indeed clay. This area also has lots of rocks and it very hard to dig with a tractor. However, if I reduced this area by three inches, and moved the spoils to the lower part of the pad, I would be about level across the pad. This was my original intention. However, counting my time plus wear and tear on the equipement, the cost to level the pad by cutting down the upper area, vs buying and spreading new material probably isn't much different at this point. Code calls for a minimum of 3" of gravel, but I was intending to go 5+ anyway. Also, will probably go with a 5" monolithic turndown slab and use rebar instead of wire across the slab. And finally, I the structure will be heated.

 

[varmint]

You mention clay, and that could be a concern, especially if its expansive and gets wet- it'll potentially cause trouble, even without freezing. I failed to see where you're located, but if you are heating, then I strongly urge you to lay 2" Dow board or the equal at least for 24" horz. all around your perimeter. It'll pay for itself in no time, even without fuel cost increases, and your feet will thank you for the extra warmth. Stone or crusher run should be easily compacted enough to surpport a slab if it's only 5-7" deep and you've driven over it with equipment. Don't forget your shrinkage control joints.

 

[crbr]

I'd use "crusher run" vs #57. Crusher will pack much better than 57. It doesn't matter what you use for a sub base as long as it is compacted to a suitable load bearing value for the pad. Gravel will offer better frost heave than earth so maybe a small course of compacted earth and then gravel. If you have that typical NOVA clay it should compact great.
What does the local building code say for gravel base thickness, 3" sounds thin. Are you pouring a turndown monolithic slab?

That's the ticket... crusher run.

I agree with Shaley on all points here, not that he needs my approval, ( ) but this is the ticket.

Proper compaction of the clay or soil is required by lift segments of about 8" per lift / then compact.

Your site looks OK without re-leveling. No need to unearth what is already there. Just get a cood compaction and use rebar in the pad, but that is for another thread. Looks like a good setup for a nice placement.

 

[jimmysisson]

If you have the height, add 1" of styrofoam (XPS) under the whole slab. It cuts down on condensation on those humid days when it's cool at night (cool slab attracts airborn moisture). Our northeast climate is different from yours but reducing heat loss through the slab will help your heat bill, too. Maybe $.55 a square foot or so.
Jim

 

[WVBill]

martt,

Snce you're going to heat the building, consider using a frost protected shallow foundation. This is basically an insulated monolithic slab pretty much as the others have described. The advantage is that with a regular monolithic turned down slab your perimeter footings will need to go down below the frost line (24" in NoVA as I recall from when I lived there). With a FPSF you only need a 12" footing in NoVA.

See document at this link on FPSF.

DESIGN GUIDE FOR FROST-PROTECTED SHALLOW FOUNDATIONS

WVBill

 

[Ford850]

I've roughed out a site for a pole barn to store my tractor and for a shop. The lower side of the site is about 7 inches lower than the upper side (side 2 in the attached picture is 7 inches lower than side 1). I'm thinking of using gravel to make up the difference when I go to pour the pad. Assuming 3" of gravel under the upper side, this means the lower side would have 10" of #57 gravel under the pad. Do you see any issues with this approach? Or should I just take the time and completely level the site before applying the pad gravel?

57's would be my choice. They natuarlly fill an area and settle the least of any of the other suggestions. They will be the easiest to tamp or compact, with little compaction after you spread the layer. They also do not pull or draw moisture like sand, bank run, or "fines" do.

 

[shaley]

Hey Bill, Thanks for that link. That's a great designers guide.