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Old 10-14-2013, 09:12 PM   #1
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How deep should I dig for 20"Hx7"W concrete wall?


Heya guys,

Title says it all...

How deep should I dig to make footing for 20"H x 7"W Poured Concrete Wall?

Thank you in advance,
almazon

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Old 10-14-2013, 09:14 PM   #2
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How deep should I dig for 20"Hx7"W concrete wall?


Below whatever your frost level is.

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Old 10-15-2013, 06:12 AM   #3
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How deep should I dig for 20"Hx7"W concrete wall?


Just wonder what that would be?
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Old 10-15-2013, 06:23 AM   #4
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How deep should I dig for 20"Hx7"W concrete wall?


http://soundfootings.com/pdf/US_Map_Frost_DepthAVG.pdf
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Old 10-15-2013, 06:25 AM   #5
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How deep should I dig for 20"Hx7"W concrete wall?




Looks like 48" would do it in upstate NY. If this is on a hill, or has a lot of moisture, you want to go deeper.
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Old 10-15-2013, 06:25 AM   #6
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How deep should I dig for 20"Hx7"W concrete wall?


If i'm not mistaken here in Illinois (frostline) would be at least four feet. So i would go 4.5 to be safe.
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Old 10-15-2013, 06:25 AM   #7
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How deep should I dig for 20"Hx7"W concrete wall?


The local building department has the correct answer.
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Old 10-15-2013, 06:33 AM   #8
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How deep should I dig for 20"Hx7"W concrete wall?


That is a heck of a footing for a 20 inch tall wall----exactly what is this being used for?
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Old 10-15-2013, 08:39 AM   #9
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How deep should I dig for 20"Hx7"W concrete wall?


Quote:
Originally Posted by oh'mike View Post
That is a heck of a footing for a 20 inch tall wall----exactly what is this being used for?
Thanks oh'mike for understanding my concern!

After leveling out my Front Yard with pavers my Fence still on slope:


So for this reason I decided to dig the Fence out, slightly move it away from sidewalk and place it on top (or inside) Poured Concrete Wall:


or:







So the question is how deep should I go for this 20 inch high and 7 Inch Wide wall?



Thanks again,
almazon

Last edited by almazon; 10-15-2013 at 08:44 AM.
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Old 10-15-2013, 10:33 AM   #10
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How deep should I dig for 20"Hx7"W concrete wall?


That's going to need a lot of shear strength, as well as for the wall itself to withstand the hydrostatic pressure of the water wanting to move downhill. You need to talk to your local building department about the project. You've definitely got to be BELOW frost depth though. 48" might not be enough though for the footing, and you've got to be sure to have the proper drainage around the wall as well. This isn't a "simple" project.
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Old 10-15-2013, 11:19 AM   #11
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How deep should I dig for 20"Hx7"W concrete wall?


Going down to the "frost depth" is really a strange concept for a landscaping application. The "frost depth" is for buildings/structures and is based on a 50 to 100 year period where may be enough heaving of frost susceptible soil to cause structural damage to a building in that long period. Many soils are not frost susceptible and will not heave.

Has anyone ever seen a highway built on a foundation below the frost depth? The simple answer is that some structures can tolerate movement and a 5' frost heave on a road can be tolerated because the thing that is important is the DIFFERENTIAL movement, so if a landscaping wall heaves upward, it moves just about as much as the remaining soil. - Technically, very slightly more, but it just needs the strength to hold together. Soil freezes from the top down.

In the photos shown, the wall is only needed for the small lateral loads and a spread footing under the wall could create problems. The entire area inside the fence is composed of interlocking concrete pavers that are not designed with regard to the "frost depth" because they move together and should never be connected to any rigid structure.

Even the common dry stacked segmental retaining walls (SRWs) are but with no concrete footing allowed and no concrete or mortar in the wall - Just 8" buried for a 4' non-engineered wall, because of the lateral shear resistance.

Why the 7" wall thickness? - Is that the width of the shovel?

Dick
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Old 10-15-2013, 12:24 PM   #12
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How deep should I dig for 20"Hx7"W concrete wall?


I thought that might be a landscape feature---I'm not a concrete guy---but we typically dug a shallow trench--about 10" or so--and set the forms---a couple of lengths of light rebar might be added to keep the wall intact if if ever cracks---
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Old 10-15-2013, 12:40 PM   #13
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How deep should I dig for 20"Hx7"W concrete wall?


I agree with concrete. No reason to go below frost for this. Good rule of thumb is go as below grade as much as you are above. Frost will cause the ground to heave but this wall will be allowed to float. much like a monolithic slab will. I can't imagine the hydrostatic pressure will be enough to cause any issues unless there is a flood. In that case I'm sure your wall will be the least of your worries.

If it were me, I would dig down 20" + 8" . the 20" will be 7" wide but the 8" below that will be 14" wide to create load distribution into the soil. I'd also run two runs of #4 rebar in the footer.

Disclaimer: If this wall will be retaining earth on your side of the fence line there will need to be drainage issues taken into consideration. Please let us know if that is the case.

Last edited by brockmiera; 10-15-2013 at 12:43 PM.
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Old 10-15-2013, 01:31 PM   #14
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How deep should I dig for 20"Hx7"W concrete wall?


Unless there is more to it than leveling the top of the fence, and given the possibility of undermining a municipal sidewalk, I would focus on the fence itself. If you are satisfied with the height at its' lowest point, which looks like only a couple inches of difference, and assuming that the posts are stable, it could be as easy as running a line along the top, cutting the tops of the high ones, loosening a few bolts, dropping it down level, and you'd be good to go. As mentioned, you would have to check with your local autorities, but I would guess that you could not install it as is atop a 20" high concrete wall, so would end up with an awful lot of cutting to shorten the whole thing. If you still wanted to break the fence up from the inside, visually speaking, you could lay however many rows of landscape blocks on nothing much more than a shallow bed of compacted gravel, just like you have for your pavers. Dick actually knows what he's talking about, so maybe this doesn't make sense, but it's my 2 cents worth.
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Old 10-15-2013, 03:44 PM   #15
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How deep should I dig for 20"Hx7"W concrete wall?


"So for this reason I decided to dig the Fence out, slightly move it away from sidewalk and place it on top (or inside) Poured Concrete Wall"

Guys, the fence will be "inside" the concrete wall, meaning that it's what's anchoring that fence. With no mention of fence posts going below frost line. With the fence merely anchored in a shallow not below frost line concrete "wall", it's going to need to be able to stand up to a winter wind pushing on it's ice filled privacy mesh. And that's why I said that the entire structure should go below frost level.

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