Prepping for concrete - low corner

[cdaiscool]

Hello there! I am about to start working the ground to put a 12x12 concrete pad down for a shed, and I've got a couple questions. Tried to research, but either my Google-Fu isn't working or I can't find something that really looks like what I need.

Most of the ground is flat, but it is on top of a small hill. One corner is 10" lower than the other corners, starting from about the center. Even if I do a 1" drop every 4 feet for drainage, that's 7" too low. With that said, how do I rectify this? It's more or less gotta be here, much closer and the township says no permit, much further away and the HOA says no shed.

Should I just pour enough concrete to make it flat? That could be a 7" thick pad, 12x12. I'm just putting motorcycles on this, not a school bus.

Do I put gravel under it, and do it so the slab is 4" thick on the high side, ~7" thick on the low side, basically following the grade? That would mean the bottom isn't flat, can't it over time move?

Do I put in a retaining wall, and the wall of the shed lower than ground level? Or build up the low side, knowing I'm no longer on undisturbed clay?

One article I read basically suggested to make a hump in the middle, so the edges are closer to 7" but the middle is closer to 4" thick to save on yardage for the pour. Another good idea? Bad idea?

Until I found the low spot, my initial plan was 2x6s for the form, and gravel 1.5" followed by a 4" slab. I live in Michigan, the back yard is clay, and I'd like to do this right.

Thanks for the advice/help/responses!

 

[kd3pc]

two choices -

first, is to use the lower corner as your zero point, and dig/excavate to that level, but that will likely leave you with a 10" cut to back stop somehow on the other sides.

second, is to back fill the low spot to match the rest of the plot. That will require building up that corner and backstopping it some how.

You "could" just form that corner, pour some "drier" concrete in that corner, before the correctly wetting the rest of the load - to then pour the slab at 4"....the corner will probably last 4 or 5 years before deteriorating. And your HOA may not like it or how it looks. We do that type of thing all the time pouring step footers for block laying. Pour the damns with dry, then the rest of the footers.

The real question is: will the town and the HOA let you excavate this...?

 

[Shiftless]

When I did something similar for a hot tub, Also on clay soil, I filled the low corner, compacted it with a hand tamper and poured 3 inches of concrete with wire. Over the years yes, it settled over an inch in that corner.
My tub full of water weighed over a ton.
Let's hear from others about their plans.

 

[SunsetsAndFriends]

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[Dr Stan]

First, you need to remove the top soil, including grass. You do not want to pour concrete on top of organic material. Top soil is a mixture of organic and inorganic materials.

Second, when you remove the topsoil, you can level the clay, sand or rock subsoil that you have in your yard off the lowest point.

Fourth, you could do a thickened edge (you talked about a hump in the middle). This is called a bond beam or a grade beam.

Instead of gravel you may want to consider dense grade or crushed used concrete.

Around here the thickened edge is similar to a uni-pour foundation & slab.

Another approach for a foundation for a small shed is to pour concrete piers about 2 ft below grade. Make sure you form them up so the tops are all on the same plain and they are lined up sq & parallel.

Where I live one can build up to a 100 sq ft outbuilding W/O a permit as long as its not attached to a foundation. So mine looks like its just setting on piers but has 1/2" carriage bolts set in the concrete which go through the 4X4 floor joists. The 3/4" PT plywood floor covers the joists in a way that hides the carriage bolts.

This approach allowed me to build it W/O doing much dirt work as I rented a powered auger to drill the holes for the piers. I really do not agree with the zoning rule as to outbuildings as my method will reduce the likely hood of high winds or a tornado dumping the shed on another house, etc. The "code" way simply has the sheds sitting on concrete blocks & wood shims making them susceptible to blow away. Some people simply sit them on top of the ground without leveling.

 

[SunsetsAndFriends]

...

 

[cdaiscool]

two choices -

first, is to use the lower corner as your zero point, and dig/excavate to that level, but that will likely leave you with a 10" cut to back stop somehow on the other sides.

This is pretty much it. I figure if I do a 1" per 4' slope, that means I will be lowering the concrete some and can then absorb some of that drop. I'd still probably have 7" of exposed concrete on the low side.

second, is to back fill the low spot to match the rest of the plot. That will require building up that corner and backstopping it some how.

Which would then be disturbed soil, won't that just start sinking? I don't want to grade the area and wait 6 months - 1 year for it to fully settle. I'd like to get the forms set this week, couple weeks at most.

You "could" just form that corner, pour some "drier" concrete in that corner, before the correctly wetting the rest of the load - to then pour the slab at 4"....the corner will probably last 4 or 5 years before deteriorating. And your HOA may not like it or how it looks. We do that type of thing all the time pouring step footers for block laying. Pour the damns with dry, then the rest of the footers.

The real question is: will the town and the HOA let you excavate this...?

I can do basically whatever I want as far as prepping the site. HOA has approved the shed, I already have the variance. Township cares zero about the construction method if it's under 200 sq. ft, just need a zoning permit to make sure it is far enough away from property line and buildings.

First, you need to remove the top soil, including grass. You do not want to pour concrete on top of organic material. Top soil is a mixture of organic and inorganic materials.

That's the plan. I was going to dig down ~4", put 1.5 inches of gravel, tamp by hand, then do 4" of concrete, starting below the grass and stopping a couple inches above. I would have thought that since there's no heavy vehicles on this, just a backyard shed, the 1.5" would be enough. I'm planning to do a 368 sq. ft patio at the same time, maybe I'll just dig this out while I use an excavator on that.

Second, when you remove the topsoil, you can level the clay, sand or rock subsoil that you have in your yard off the lowest point.

Third, you should add a crushed stone base on top of this. You are in MI. You get snow and rain. And they freeze. The crushed stone allows moisture to drain away from the slab and reduces the chance of the slab heaving. I would put down 6" of crushed stone. And you want to rent a plate compactor and tamp the stone until it is level and hard. Add a couple inches and then run the compactor over it. Repeat until it is thick enough.

If I even it out, the low side will not have as much dug down as the high side. So if I add 6" of gravel, it will be above the low side's top soil. Wouldn't this cause problems? I guess I could dig out the hole, make it wider, then add dirt next to the concrete and reseed next to it after the pour.

Fourth, you could do a thickened edge (you talked about a hump in the middle). This is called a bond beam or a grade beam. If I were doing it for myself, I would do it, because it really isn't expensive and adds a little bit of insurance. But technically, for this size building, you wouldn't need it.

If you do a thickened edge, do a an image search for "grade beam foundation" for pictures of what it should look like. To make it easy, at the point where you level the subsoil, you place your forms These forms will be taller than the thickness of the concrete, let's say 2x12. You square the forms and level them. You want the top of the forms to be a good 6" above the surrounding grass/ground. This keeps water from flowing into your shed. (You can always go thicker by adding some 2x4 material on top of the 2x12 form, if you find that you will not he high enough to keep water out.) Then you place crushed stone inside the form, leaving the perimeter deeper than the center. Add stone and compact.

Fifth, run rebar all the way around the slab in the bond beam. You want two pieces side by side. Keep the rebar 4-6" from the outside edge of the concrete. You can place pieces of brick under the rebar to set the height. There are other techniques for doing this. Search Google. Then run rebar or wire mesh in the field of the slab. Do a Google search or YouTube search to learn more about rebar placement and sizes.

I get the sense that you haven't done this before. Do you have help pouring it?

I was planning to do fiberglass reinforced concrete, maybe 3000 psi. I did a small concrete pit in my garage for a lift, the help I had after I dug just poured it on the soil and drilled rebar into the existing slab + I dug underneath the slab a bit to key the new concrete in. I was planning to do this pour myself, possibly have a couple buddies come to help man wheelbarrows but I have the various concrete tools from a lady at work that did stuff like this in the past. I've verbally told her the plan, she thought it would work but hasn't visited the house. I could always ask the guys that came to do the garage floor to finish the concrete after I make the forms, but as long as the forms are set and stakes are plenty to prevent bowing and blow-out, shouldn't a DIY-er be able to handle this?

 

[cdaiscool]

When I say gravel, I mean 1" minus. Same stuff I'm going to use under the paver stones I'm also doing soon, and tamp it down.

I should be more specific, my apologies.

 

[cdaiscool]

Here's a quick sketch of what I'm doing. Patio base is going to be wider than the pavers, to give support. Since I was going to do 8" of 1" minus under the pavers, I'll just do the same under the shed area. Pavers are going to be about 4" above the crushed stone, so I'll do about the same for the shed in concrete. I'll make sure that overall, there's 2-3 inches of concrete above the soil, so no water can realistically get in here.

Then the low side, I'll build up dirt against the concrete after it's sat for a couple days and the concrete has hardened. This should be fine, and if that settles I can always add more there.

Even with the fiberglass reinforcement, do you guys still suggest rebar? Or should the fiberglass be enough? What PSI, should 3000 be okay? Using this for motorcycle storage, lawn mower storage. No cars or anything.

https://scontent.**.fbcdn.net/v/t31.0-8/18208985_10105019370348502_3112467156760465871_o.jpg?oh=e88426e6847ffe40b5c3edc42b0fccd7&oe=598D9395

 

[cdaiscool]

And doing a plate compactor for this all, since I was going to for the patio. 2" lifts, 2 passes then add more stone, 2 passes with compactor, add stone...

Just was hoping to do the patio after the shed, but since it'll require the same equipment, whatevs.

 

[Toomanytools?]

I would compromise on the shed dig down the high side if you can and still have drainage you can move the high side dirt to the low side 2 or 3 inches. Water it well so it settles compact it and build up with gravel. We use 5/8"s minus for driveways ,slabs. Depending on the compactor you can do 3" to 4" lifts and be fine, making sure to water well between lifts. Depending on the freeze thaw cycles where you live you might want to go with 4000 psi for exterior slab, unless your shed is heated to avoid those freeze/thaw swings. Cost for the higher psi isn't much per yard.
For that shed slab you don't really need rebar doesn't sound like a big structure and no heavy loads on it, but doing a monolithic pour were you thicken the edges to 6" for 6" width then taper to the 4" slab gives it more strength. Fiber added helps probably not needed for this but here I paid like $10/yard more so you can do it for piece of mind.

Your only pouring about 2 yards, two guys can handle that easy, get yourself a 12' plus screed to run on top of forms to rough level the concrete and a bull float to float it out.

I just finished 65 yard pour on my shop and still have another 20 yards to go I would love to do only 2.
Remember screed, bull float, wait for the water to rise up then go back into slab and finish trowel and edge.
Good Luck Jeff