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Old 03-19-2015, 08:45 PM   #1
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build a concrete footing on a slope

I'm looking to build a retaining wall on my backyard that at its tallest point is 2' 3/4" tall. It'll be built on a slope so I can extend my existing yard out by some 350 sq ft. Once the wall is up I'll fill the new area with 4" high slabs of concrete (to extend entertaining and seating space) and build a picket fence along the end of the wall (so it isn't dangerous for the kids to fall on the slope). My questions are:

1) how deep into the ground do I need to dig to build the foundation for a wall like this on a slope? I'm in northern california, so frost won't be an issue. Also my soil is silty (appears to be a healthy mix of clay and sand). Does that mean that I just need to dig a trench as wide and deep as the foundation itself and starting building the wall from there? Or are there special considerations for the slope?

2) Once the wall is up will the concrete slabs at the top help with keeping water away from the wall? I'll naturally slope them away from the wall, so I'm hoping that'll help. Regardless, are there special considerations that I should account for knowing the extra load and surcharge of the concrete slabs and fence at the top?



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Old 03-20-2015, 06:59 AM   #2
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are you building a cmu mortared wall OR a segmented wall,,, we use versa-lok - their plant's close & we get contractor prices,,, no $ interest 4 me


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Old 03-20-2015, 09:18 AM   #3
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I'm planning on a cmu mortared wall.

For the height of the wall it seems like both do the job fine, but at the end of the day, the fact that cmu is $1/unit and the segmental wall blocks are over $7/unit it's way more cost effective for me.

Also, for a segmental block wall, since this will be built on a slope, I'd have to measure 5 feet out of the end of the wall into the slope and have that be the bottom of my trench. After doing those measurements I'm looking at an extra 3' of digging, which makes the wall go to 5' 3/4"... Then segmental block wall becomes really expensive.

That's why my question is about the footing... How deep do I need to dig for the footing? Is it the same consideration as the segmental block wall? Or is it different?

Also for a wall this big, I was planning on a 2' wide, 1' deep footing. If I need to dig more then a wider footing is probably required, but it's still cheaper I think.
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Old 03-20-2015, 12:33 PM   #4
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I don't understand why CMU mortared block wall is way more cost effective than segmental block retaining wall, which is just CMU with a key. Exactly the same material, and in my area the cost is about the same per block, assuming you get the same finish.

In any case, the segmental walls typically do not require a footing at all, unless there is something really wrong with your soils (like you are trying to build on a swamp). I am not quite sure I understand what you are backfilling with, sounds like you are planning to use old slabs of concrete. Maybe I misunderstand. In any case, with a normal segmental block retaining wall, for only two feet tall you would normally not need any additional support other than the block on tamped soil. In your case, it sounds like you plan to use standard concrete block, and you are going to mortar the blocks together. I have not built a retaining wall that way, not sure how it compares to standard segmental block with the key, but perhaps there are others on here who have done it.
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Old 03-20-2015, 12:53 PM   #5
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Thanks for the reply, Daniel.

A cinder block go for $1.54/unit and it's 8x8x16.. a segmental wall block (Allan block, etc) start at $7/unit and it's 6x6x16.. which means I'd need to get more.. so as you can see in blocks alone it would be way more expensive. Sure I'd need a footing, but a 2 foot wide, 1 foot deep footing, for my wall, would cost me $500. Anyway, this means that I could do the entire wall for a little over $1,000 with CMU, where for segmental blocks the units alone would be over $2,000 (I'd then need rock/pebble and geogrid—not cheap).

Also, please remember that my wall is on a slope, which means that I would need to burry more than one block in the soil. It's actually more like 3 feet of extra blocks. I had an Allan block professional come over to the property and he mentioned that I needed to dig an extra 3' to make up for the slope.

My question in this thread is related to that: for concrete footing, do I also need to dig 3' and THEN set the footing? or are there different considerations since it's a concrete footing?
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Old 03-20-2015, 03:33 PM   #6
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A segmented wall lets the water weep through a mortared one does not. The failure rate is lower with a segmented wall.

For a wall 3' tall those garden blocks from HD may be all you need if you put a real good base under it.

If you are pouring a footing (or even with a gravel base) you can step your footer up. You do not have to dig down to level all the way around. The guy who make their money selling the blocks tell you that is the way to do it.

The first image is of a wall at my home. This was before I capped it. There is no more than half a block buried anywhere. There are multiple steps in this wall. This was built on a compacted 6" of road fill with .5-1" of class I rock dust and the occasional 1/4" of sand. 3 years, 2 hard winters and as level as the day it was built.

The second image was a more serious wall. 12" blocks on a 24" wide 6 inch deep footer with no more than half a block buried. There was one step one block high at about the half way point.
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Old 03-20-2015, 10:04 PM   #7
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Even if the cost for a CMU wall is much cheaper, which I doubt, the significantly higher failure rate for that type of wall makes it very unattractive. Segmental block walls have a extremely low failure rate and even if they do have a problem they are very easy to repair.

You also should use the geotextile material and gravel behind either type of wall, it's certainly not something you only use for a segmental block wall.
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Old 03-21-2015, 01:35 PM   #8
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This is just something I read, but still I thought I will relay it. Block wall, you want a stable footing and on a soil that will drain. I think the picture showed 6-12" compact gravel in a soil cloth and the first block course buried under the grade. The footing was poured with horizontal rebars and verticle rebars to fix the blocks. I think this was a retaining wall 4' or higher.

Many retaining walls around here, however, are segmented wall. 3-6" gravel, bottom course about 1/2 in the ground and just glued together. No gravel backfill.
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Old 03-22-2015, 11:33 AM   #9
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and don't forget to add in the cost/time of concrete & steel for the cmu wall if it's retaining earth. Stacked grouted blocks will not hold over the long haul holding back surcharge.

Myself I'd go with the interlocking landscape blocks even if the cost might be more, quicker, easier and better looking in the end product.

Good luck!



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concrete footing , concrete wall , retaining wall

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