How deep should my footing be?

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Old 03-16-15, 01:44 PM
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How deep should my footing be?

I'm looking to build a retaining wall on my slopped backyard. I'll be using cinder blocks (with mortar to join them together and rebar for added stability).

(added info: at the lowest point the wall is going to be close to 4 feet tall, not counting how deep I have to dig and how much of the wall needs to be underground)

Some questions:

1) Footing or no footing? Can I start setting the cinder blocks directly on base gravel, with rebar vertically every 3-4 feet for added stability, and filling them up with mortar as I go? Or do I need footing?

2) If I need footing how deep into the ground should my footing be? Not how deep the footing itself should be (I'm thinking I'll go with a 1' footing—feel free to comment on that), but rather how deep into the ground do I need to dig my trench in order to set the footing?

I know that for gravity walls on slopped yards I need to measure 5' out of where I plan my wall to be and dig the difference. What about for cinder block walls, is it the same? Or the added strength and stability of rebar+mortar, or footing+rebar+mortar, makes it so I don't have to dig as much?

Hope the info I provided is sufficient to get help and answers from. Let me know if you need me to try and sketch the design of the wall for added context.

Thanks
 
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Old 03-16-15, 01:49 PM
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Welcome to the forums.

Two schools of thought:
1. Concrete solid wall = footing.
2. Segmental block wall = no footing

# 2 is the more common choice.

Where are you located/what's the frost line in your area?
 
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Old 03-16-15, 01:54 PM
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Thanks for the quick reply stickshift!

Follow-ups:
1) Do offset cinder blocks (with mortar and rebar) fall under your #2? I looked into allan block and other interlocking block options, but cinder blocks seem to be the most cost effective option. Sure I wouldn't want to risk having a faulty retaining wall because I didn't want to spend more money, that's not it. I just want to know if this is achievable with cinder blocks and if it falls under your #2 / no footing required
2) I'm located in Northern California, specifically in Hayward, CA
 
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Old 03-16-15, 02:08 PM
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No, your idea is category 1 and you would need a footer for that.

Sorry, I can see from my wording how you would think otherwise - because you're using mortar to join the blocks, you're building a solid wall instead of a segmental wall.
 
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Old 03-16-15, 02:36 PM
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Got it!

From a very brief and basic research online it seems like in Hayward the air-freezing index is between 0-1000, which puts me at a "footing depth" of 12-24 inches. I assume that means that digging a 1 to 2 feet trench would be sufficient to build the footing on. But what about on a slopped yard, in what way does it change that? Or as long as I reach undisturbed soil it doesn't?
 
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Old 03-16-15, 02:38 PM
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Speaking as an engineer, with more than 40 years of inspection experience, I would vote for going with a loose block wall, no footing option. That's looking at the big picture, taking into account the initial construction effort, materials cost, and long-term performance of the finished product.

Conversely, I think I can easily count on one hand the number of well-performing, mortared block walls I've ever encountered over the years. The usual failure points are cracking mortar joints--once it happens, there is no practical fix, short of rebuilding the entire wall (in the cracked mortar areas).
 
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Old 03-16-15, 02:43 PM
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Thanks BridgeMan45

When you say "loose block wall" do you mean specifically interlocking blocks for gravity wall? I don't particularly enjoy the look of a gravity wall, with the setback that it creates. If not, can you point me to some time of blocks that offer a non-setback surface?
 
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Old 03-16-15, 03:11 PM
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Do not use "loose" block walls!! The Segmental Retaining Wall units (SWRs) are different have some features molded into the to units to provide shear resistance and even lock in horizontal soil reinforcement that may be necessary for walls over 4-5' or so. - These type of walls are classified as "flexible" walls that must be laid on a level compacted base (never concrete) for installation and performance. The "rigid" walls must be laid on a concrete footing and be tied to it. The major SWR block suppliers have site on line that give excellent detained installations for the lower walls.

Both types of walls work and have different advantages. Even a unmortared interlocking SRW wall 53 miles long can be between 4' and 20' high continuous with variable heights and inside and outside curves. The rigid walls can be higher, but changes/angles can be a challenge.

Both types should be engineered if they are over 4'-5' high (retained soil height) because Mother Nature and moisture cause huge forces that can get your neighbor's yard in yours or your yard elsewhere.

Dick
 
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Old 03-16-15, 07:05 PM
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My mistake, for using the term "loose" to mean the opposite of mortared CMUs.
When describing structural rigidity, my lazy brain has always considered loose to be synonymous with flexible.

To answer the OP's question regarding SRWs that have vertical faces instead of offsets (leaning into the backfill), I don't know of any. Dick, how about you?
 
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Old 03-16-15, 08:39 PM
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I recall one or two units from a couple of the 4 major SRW licensing companies that could have a vertical face, but they were thicker (maybe 2" to 4") since they were still gravity walls.

The "batter" does wonders structurally, although the batter, non-mortared joints and no concrete footings are the major reasons internationally and in the U.S. - The square footage of the systems are globally about the same as poured concrete wall and far more than precast walls.

All of the developer companies of SWRs are headquartered in MSP, MN so I got to see a lot of testing and understand some of ins and outs of the product approvals by municipalities, counties and the U.S. highway departments. The testing of full walls has been extensive. - At times, I wondered if they delivered test results for approvals by the truck load.

Dick
 
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Old 03-17-15, 11:34 AM
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Thanks for the replies here... This is great insight, but I'd like us to get back to my original questions, if that's okay.

One of them has been answered: if i choose to go with cinder blocks and concrete wall I do need a footing.

My second question is: how deep should I dig to build my footing.

Name:  retainingWall-SWRmeasurements.jpg
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This drawing is an okay visualization of what I'm trying to accomplish. Sorry for the proportions being off, but hopefully it'll get the point across. Anyway, as you can see I'm building this on a slope. For a wall with SWR I would have to measure 5 feet out on the end of the wall and see how deep my trench would need to be... as illustrated in the drawing attached.

My question is, if I don't go with SWRs, but rather a concrete wall with cinder blocks, do I also need a 3' deep trench, or the considerations are different here for a footing?
 
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Old 03-17-15, 01:30 PM
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It would be good to know there you are from for a climatic reference.

In warmer climates, with minimum frost, the minimum depth of the bottom of a footing would be similar to the local code requirement for buildings, although the model codes usually cover remote structures.

In colder climates, a poured concrete wall or concrete block retaining wall may be required to be about as deep as a footing for a building, but more sensible officials recognize the a lesser depth depending on the soil because of the different use and application. The codes for building frost depth is based on the worst winter in about 100 years since it is a long term life-safety issue, while retaining wall is not as critical.

The different wall types (rigid and flexible) and necessary depth are the reason SWR walls are by far the most common residential retaining walls in the world (up to about 10'). If the wall is attached to a structure like house, everything can be different in cold climates.

Dick
 
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