# Footing Rebar



## AlphaPilot (Aug 6, 2012)

Frost level is 30" or so here, I am planning a footing for a hollow concrete block retaining wall, was going to do 36" deep, Teeing out the top with a 16" wide top maybe 8" thick. Should I place lateral(vertical) rebar in the trench for added stability or just have horizontal rebar on the top 8" thick 16" wide base, with vertical rebar sticking up for my hollow concrete blocks to obtain extra stability? I don't seem to see anything in our 2005 IIRC discussing this.


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## Msradell (Sep 1, 2011)

I would definitely install vertical rebar, probably on 16"centers and then grout those cells full of concrete. Don't just stick rebar into the footer for the verticals, use "L" shaped pieces and tie them (with wire) onto the horizontals in the footing.


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## AlphaPilot (Aug 6, 2012)

@Msradell

What's the best method to suspend the horizontal rebar in the footing before pour?

Also, I understand the idea with the vertical L rebar having that L makes it so the rebar is much more stable...but what is a good way to float the top of my footing with the verticals every 16"? I picture forming the 16" wide footing top with 2x8, then using a scrap 2x4 to float across the top, sawing back and forth, but hitting those verticals and trying to get everything perfectly smooth/level for my first course of blocks....


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## Daniel Holzman (Mar 10, 2009)

The design of your footer depends on the design of the retaining wall. How tall is the wall? What is the height of soil you are retaining? Is the soil dry, moist, or potentially saturated? Do you plan to install drainage in the retaining wall?

You may want to consider a segmental concrete block retaining wall. If the soil is adequately strong, you may not need any footer at all, which may make the segmental wall less expensive and faster to install than a concrete block wall.

A retaining wall needs to be designed as a total system, it is not possible to discuss the footer design without understanding the rest of the wall.


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## ddawg16 (Aug 15, 2011)

I agree with DH.

The way it is done here.....dig trench....nothing wrong with 36"....

Lay down at least 2-4 #4 rebars......one set about 2" from the bottom...second set up about 12"..

Install vert rebar consistent with the block spacing.

Pour concrete to about 16" thick.

Let dry.....

Now do your cinder block wall

The vert rebar goes up inside the blocks....about every 3 rows you run a horz rebar. (they make blocks with the hole for the rebar.

When it's all done...pour concrete down the holes of the block where the vert rebar is

Cap


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## AlphaPilot (Aug 6, 2012)

The wall will be roughly 25' long. The first 10' from the house will be maybe 8' tall, after that 10' I will begin stepping the wall down, with the farthest point out being 3' tall or so. Being cinder blocks, this will look kind of choppy, but represent something like a staircase for looks as I take each course lower and lower.

The soil will match the retaining wall height or be just a bit less. It is just clay, but in the spring it could definitely be wet. Shouldn't be saturated though is it will drain off.

I have considered a multi-tier wall system, but I want to build a deck along this area and a straight wall will work better for the patio underneath the deck.


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## Daniel Holzman (Mar 10, 2009)

You need a carefully thought out design for an eight foot tall wall with full earth retention. I DO NOT recommend a hollow concrete block wall for such a high structure, the bending forces on the wall exerted by the retained soil will be large, probably much too high for a single wythe concrete block wall to resist, regardless of how much steel you put in the block.

You need to discuss this project with your local building inspector, where I live any retaining wall higher than 4 feet tall requires a professional engineer stamp. This is to protect the homeowner against wall failure, which can be sudden and very dangerous.

Reasonable options for an eight foot tall wall include a reinforced concrete cantilever wall, a gravity retaining wall (many options for materials), or a reinforced earth wall using concrete segmental blocks with earth reinforcing. It is also possible to design a stepped retaining wall. Your design engineer should be able to discuss alternatives with you, give you a reasonable idea of the cost to construct each alternative, and recommend the best alternative based on the specific conditions at your site. Unless you have solid fundamental understanding of soil mechanics and structural wall design, I DO NOT recommend that you undertake the design yourself, it is surprisingly tricky to get the design correct, and wall failure is a bad option.


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## AlphaPilot (Aug 6, 2012)

Indeed a big undertaking. I would not be leaving the blocks hollow, intend on filling them with mortar, especially all voids with rebar. I have considered stepping up to 12" block if needed. I'm not sure how there are 12 or 12' high poured retaining walls from the 60s and 70s still standing strong. Does a poured wall have that much more strength, or did they geogrid them into the soil? I see a lot of these for retaining walls that must slope so the top doesn't look jagged like mine, they just form and pour 'em all along the slope. I may look into a multi-tiered wall. If I do this, I would not do a footing and instead just bury the first course of blocks with a gravel base. I was preferring hollow block over the decorative wider blocks because the cost is 2x or 3x as much for those interlocking or gravity retaining blocks.


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## concretemasonry (Oct 10, 2006)

Concrete block can be obtained in many different thicknesses (8", 10", 12" 14" or 16") depending on where you live.

You will need to fill some cores (those with rebar in them) to be filled. Arbitrarily filling all cores is a waste time and materials and adds little strength to the wall.

Most areas have good producers that make block with cores that align in normal running bond so construction is easier and faster.

The retaining wall block now are so common and popular in comparison to plain block because of the appearance.

Hollow block CANNOT be laid on a compacted base and a concrete footing must be used. The retaining wall block are NEVER laid on concrete and must be laid on a compacted soil base.

Usually, any wall over 4 or 5' must have a certified design for public safety and laws/codes. The retaining wall block suppliers may be able to offer approved designs and details for various different wall heights as long as you can provide some soils information.

Dick


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## AlphaPilot (Aug 6, 2012)

Good to know that extra info, thanks. I'd rather do hollow block with mortar instead of the landscape blocks multi-tiered.

I have seen the retaining wall block with the lip that systematically steps each block back stacked 6' high and asked the owner how long that's been there, he said 12+ years and it looks as strong as day 1.


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## Daniel Holzman (Mar 10, 2009)

It is absolutely possible to build very tall retaining walls (I have seen them up to 50 feet tall) using retaining wall concrete block (technical name is segmental concrete block) and soil reinforcing. Depending on the block manufacturer, and the specific size of the block, you need soil reinforcing beginning at four to six feet tall wall. There are manufacturers of massive concrete blocks which can be used without soil reinforcing up to 20 or more feet, but this approach is unsuitable for a DIYer.

The OPS should understand that you cannot see the soil reinforcing once the wall is constructed, it is typically some form of fabric or wire mesh that is buried. So you CANNOT conclude that just because you see only the block, that the wall only includes the block. Segmental block retaining wall design is pretty complicated, the good news is that virtually all manufacturers of segmental block have a design staff to work with, and often very good design manuals that have diagrams and narrative on exactly how to construct the wall so it does not fail.

If the OPS builds a retaining wall out of hollow concrete block, he is not likely to find a design manual from the block manufacturer, since hollow concrete block is not typically meant for use in retaining walls. Not saying it cannot work, just that the manufacturer is not likely to provide design guidance for use of hollow block. There is a reason the segmental blocks cost more than the hollow blocks, and it is no accident that they work better for retaining walls than hollow block. Good luck with the project, be careful.


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## AlphaPilot (Aug 6, 2012)

Daniel, you think segmental concrete blocks offer a more cost effective method than poured footing + hollow blocks? Locals don't sell them so I'd have to go to a Menards 70 miles away.


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## Daniel Holzman (Mar 10, 2009)

For the amount of block and reinforcing fabric you would need, you could probably get a truck load delivered. But understand that you need to reinforce the soil with fabric or mesh, which means you usually cannot build a segmental block wall up against an existing slope without significant slope excavation and grading. The backfill also needs to be select granular material, so if your soil is clay or silt, you need to excavate it and replace with imported fill. That alone may cost as much as the block. All of these factors would impact the design of a hollow block wall.

I have never designed a hollow block wall, not sure what a workable design would look like. I have designed dozens of segmental block walls, several cast-in-place concrete walls, and a few gravity walls, and based on my experience the segmental block is the most cost effective where it can be used. If you go with reinforced hollow block, make sure you get someone with hands on experience to design it, easy to go wrong and end up with a bowed wall or worse.


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## AlphaPilot (Aug 6, 2012)

I spent some time yesterday evening doing dirt work and scoping things out. The more I look at it, the longer I considered a tiered wall. The cost would be more but less at the same time because I wouldn't be trenching and pouring a footing, I would just be digging one course lower and using some pea gravel or sand, and laying the pretty landscape blocks.

I would need 3 tiers, maybe 36 to 40" in height for each tier, then set back would be 3 or 4' before starting the next course. I'm thinking drainage on the first two tiers is important but the last tier would not be as big of deal since it would be a shorter wall there. A tricky part to doing this is my deck supports would rest somewhere on the middle tier I believe....meaning pouring a concrete block or two for post support in the fill soil that would not be truly settled for a year or two.


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## Msradell (Sep 1, 2011)

If you need a retaining wall system that tall you definitely need some professional help designing it. It's probably going to require some deadman retainers to keep it from bowing. Clay is one of the worst kinds of soil to try to retain.


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## AlphaPilot (Aug 6, 2012)

@Msradell Ahhhh, yea...well it would be the 8' tall for 10' long and then step down to being miniscale. But point is proven that it's a big feat to do something that tall if all in one chunk, which is why I think I'll just have to bite the bullet and do tiers to save on an engineer hosing the costs out of any savings there would have been using cinder block over the more expensive interlocking block used in a multi-tier system. Seems like a six of one, half a dozen the other ordeal when adding additional costs up for one design versus the other. lain:


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## Daniel Holzman (Mar 10, 2009)

In my town, any wall over four feet tall requires an engineered design. Even if there are two tiers of four feet each, the wall would be considered 8 feet tall, and would require a stamped design. Your rules may be different, but I strongly urge you to discuss the project with your local building inspector first. If you assume that you do not need a design, then you build the wall, and find out later that you did need a stamped design, it could affect your insurance rates, or your ability to sell the house later. Or maybe it is not an issue, but do you really want to take a chance?


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## AlphaPilot (Aug 6, 2012)

@Daniel Holzman,
I don't plan on any tier being more than 4' tall. I will use geo-grid mesh mid to 3/4 way on the tiers to add extra support. I also am probably going to avoid the hollow core blocks versus the solid ones.

Depending on the block type (back lip that locks and systematically creates an angle or just the solid flat block that uses adhesive to stick) I will dig a shallow trench and lay sand for leveling, then put the first course in. That first course will be 75-100% burried.


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