Concrete masonry retaining wall?

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Old 05-19-15, 12:58 AM
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Concrete masonry retaining wall?

Hey Guys,

I'm building a deck off the back of my house with a retaining wall separating it from the rest of the yard which is at a higher grade than the house by about 2.5-3'

I've started excavating and am buying materials to build a CMU wall with a proper re-bar reinforced footing.

I want the poured concrete wall look which is why I'm not using mortar-less stack-able concrete blocks. I'm planning on stuccoing it a dark grey. It also had 6 right angles in is which is another reason I want to use CMUs.

I've found all kinds of information about CMU walls but it's all tailored to building a home foundation. I'm starting to wonder what's necessary for my application since it's non load bearing. Should I just construct it as if though it is a proper building foundation wall?

I want to use 6" concrete block so it seem I should make the footing about 12" wide by 6" deep. I'm in Seattle so I think the frost line is 16-18". Do I just dig the soil out and pour the footing into the un-compacted soil directly, no crushed rock base? I've got 3/8" re-bar to put in the footing, two pieces, then was planning on one vertical piece per block. I figured I measure it all out and hammer the vertical re-bar down into the ground before pouring the footing. I think this is overkill, maybe I only need a piece every 3' or so...every other block perhaps?

After I assemble it should I waterproof the backside before putting in the drain fabric and gravel?

I'm attaching a (bad)drawing I made of cross section of how I think I should build it.

I'm looking forward to this project as an exercise in foundation building as I'd love to do a proper two story garage in the near future. I'm definitely interested in learning as much about best practices as I can. As well the deck will butt almost up against the wall so if the wall ever shifted it could compromise the deck and make for a nasty repair job.

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Old 05-19-15, 04:29 AM
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Your drawing didn't show any of your right angles, so we can't comment on that other than to say they will add to the stability of the wall. How do you propose to keep the wall vertical once mother nature decides to send hydraulic pressure pounding down? Generally there are deadmen installed to keep this to a minimum. Next, your deck won't be attached to this wall, correct?

As far as the size of footing and use of 6" block, I'll have to defer to our masons, so hang in there for their comments.
 
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Old 05-19-15, 04:34 AM
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It's best to waterproof the back side of the retaining wall especially if you ever paint the front side! It's hard to get a lasting paint job if moisture migrates thru the substrate from the backside. You have somewhere for the water collected in the drain tile to exit, right?
 
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Old 05-19-15, 06:39 AM
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Do not make the mistake of using 6" thick block walls even if they can resist the loads (which you probably do not know), since the selection of available shapes is more limited in most areas. - Go with 8" block walls. Building 6" 90 degree corners requires the correct units (such as 6" "L corners") to maintain "modularity" since masonry basically is on a 4" module.

You seem to have a "deck" between the house and the wall. What is going and how is it supported?

A 6" thick footing only 12" wide offers very little strength to the cantilever wall retaining wall, so it can rotate when the soil behind the wall (and upslope) gets saturated. The floating deck can then push on the sides of the house causing long term problems.

Dick
 
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Old 05-19-15, 07:42 AM
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Personally I much prefer precast, dry stack block retaining walls. They are designed specifically for structural retaining wall use and they allow some movement of the wall while allow for drainage through the wall.

I'm not a fan of decking in the area between the house and retaining wall. It's just asking for moisture and critter issues being closed off, dark and damp.
 
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Old 05-19-15, 09:10 AM
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I think the previous posters are pretty much summed up the comments I was going to post about your plan. The once and they didn't mention is that you don't want to pound the vertical rebar into the ground because that will give it a place to start resting and the rust will migrate upward.

Also, can you give us a sketch of the top view of your plan so we can see if the right angles will provide sufficient support or if you will need dead mananchors. I'm also not familiar with soil types in Washington state, what kind of soils are you dealing with? Soil type makes a big difference when designing a retaining wall.
 
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Old 05-19-15, 09:14 AM
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Thanks for all the comments so far, very helpful. Here's a top view of the wall plan. I want to add a step up to the yard grade. This shows the angles. From what I'm hearing it sounds like...

-I should use 8" block, and build the footing bigger, maybe 16" wide x 10" deep?
-I was planning on anchoring the deck to the house and then digging post holes in front of the retaining wall and not connecting it to the wall. Perhaps I should give it a few inches of space.
-I was thinking of crushed rock under the deck for drainage purposes.
-There will only be a couple inched beneath the 2x6's supporting the deck so hopefully no critters will take residence or maybe that's their perfect habitat. I could bury wire mesh along the sides of the deck stapled to the floor joists.
-I will waterproof the backside of the wall and perhaps the front-side after the stucco is applied.

How many cores of the CMU's should I fill with concrete, just the ones with re-bar in them?

Thanks again for all the comments. I'd love to figure out how to make this work and perhaps get somewhat of a go ahead nod from the expert masons out there. If that doesn't work maybe someone has a suggestion of a stack-able block that makes nice corners. I'll compromise if I'm asking for trouble.

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Old 05-19-15, 10:36 AM
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johnnygrace -

The cast retaining wall block that are commonly available have a built in set-back of about 1" per course, so vertical wall are not practical. The problems incurred with the 6 corners put the slope into all the walls because the wall faces are not vertical. The standard CMUs can and should be laid with a vertical wall face, so corners are not a problem. - Not all CMU producers make 6" "L corner units" and suggest using different combinations of 6" units to make a 90 degree corner, so that is why 8" CMUs are better even with smaller CMU producers. The cost difference is minimal for such a small project.

When it comes to grouting, just fill the block cores with rebar. Filling the other cores is not beneficial, but the cores are a good place to get rid of excess waste grout. The structural effects of filling empty cores are minimal.

Dick
 
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Old 05-19-15, 12:29 PM
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I'm not sure what type of soil we have here so trying to figure that out. I've attached a photo of the site and the excavation I've done so far. I still have a lot of digging to do but doing it by hand so... I think I'm beginning to have a grasp on the project in terms of block size, waterproofing, drainage, re-bar in cores, only fill those cores with cement (still wondering exactly how far apart vertical re-bar should be spaced)

My big question still is about the footing. It seems that knowing soil type, etc is critical and from looking at diagrams on the internet there doesn't seem to be a "right" answer. The diagrams from San Diego obviously look vastly different from the ones in MN.


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Old 05-25-15, 11:07 AM
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Thanks for the comments so far. Im almost done excavating and im checking to see if anyone has an opinion about footing size in relation to the bends in my wall. I found some resources online calling for large footings fot cantaleiver type retaining walls but with all my corners i'm wondering if a 15" wide x 8" deep footing is sufficient. Im pouring the concrete myself because i dont want my driveway destroyed so trying not to go overboard on the footing.
 
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Old 05-25-15, 01:24 PM
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Footing size/depth depends a lot on both location and soil type so it's best to find out locally what is required or used in your area.
 
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Old 05-25-15, 02:40 PM
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Hi Mark
I did research into the Seattle building codes and I can't find any differentiation between foundation or retaining wall footers. A 15" wide x 8" deep footing would be sufficient for a two story house in Seattle. I found some stuff online about recommending footings that were half as wide as the wall is tall. This would be way overkill for the require foundation footing code. I'm just uncertain about the distinction between foundation footings and retaining wall footings. It seems like they would be similar except in an instance where the retaining wall was straight without any bends.
 
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Old 05-29-15, 01:10 PM
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thought I would update with my progress. Feel free to jump in with any suggestions along the way.

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Old 05-30-15, 06:42 AM
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What is the little box for? If it is to save on concrete then you will be spending more time and money removing the box after the concrete sets. Make sure you use release oil on the forms.
 
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Old 03-27-16, 12:20 PM
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finished the wall now time for the deck

So the wall has been finished for a while. Here are some photos of the process and completed wall. I also put a sliding door in and am getting ready for the deck.

I know some of you said not to put the deck between the retaining wall and the house but obviously that's not an option as that was the whole point in creating this space. I'm not entirely convinced attaching the deck to the house and the wall isn't a good idea. I'm on the fence and trying to decide between that or floating the deck between the house and the wall. Attaching it the the house and wall would be the way easier option. I will say we've had one of the wettest winters in a while here in Seattle and the wall hasn't moved the slightest. I guess if I floated the deck I could leave a gap around the whole thing and then if the wall began to move it would be obvious and I could tear it out before it pushed the deck into the house and compromised the foundation. That seems like a real hypothetical to me though.

Any thoughts?

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Concrete being poured

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Vertical rebar

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Stacking blocks

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Bond beam 2nd course from bottom

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Wall built waiting for grout in rebar voids and top bond beam

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Grout all poured

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First coat of stucco

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Drainage, however it terminates into drain tile filled french drain holes, didn't seem necessary to run it out much further. I did put a clean out in so I could look in there if we got a huge amount of rain to see if water was backing up.

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Done, kinda, and the fence going up. The final surface of the wall is a product called Raeco Skim Wall - sandable stucco like product to give me a smooth surface. I waterproofed behind the wall on the stucco where it wasn't skim coated.

Attachment 64391
Finally today with the sliding door put in ready for the deck! It's been a lot of work and looking forward to getting that deck in place for the summer.
 
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Old 03-27-16, 03:05 PM
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It came out looking very nice!
 
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