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July 24, 2011 10:48 PM   Subscribe

What's the best way to build a concrete block retaining wall for my small backyard?

Not sure the best way to present this...

So, I've done a fair amount of research on this and I'm leaning towards a segmental type wall installation. To me, that means no mortar will be needed. This is a plus since this wall project is somewhat of a test run to see if it suits our landscaping needs as well as if the fiance can live with it aesthetically.

Does this type of install mean that I'm not going to need to do a concrete footer and that a well tamped gravel bed would be sufficient? I'm pretty sure I'm going to end up filling the inner cavities of the wall with gravel/fill rock but I'm still not sure if I'll reinforce it with rebar or metal poles. Of course I'll follow other best practices, as I understand them, regarding weep holes, backfilling with gravel and putting swales on the top. I just need to know if I'm pushing my luck with this height/material/construction method combination.

Oh, details for the scenario:

I'm looking at putting in a wall (really two independent walls cut in half by a few steps) that's about 16 feet wide and 3 (maybe 4) runs of 8 x 8 x 16 hollow block high. Soil type is non-sandy and the frost line is not an issue (northern FL) compared to other places in the US. The soil will be up to the top of the highest run of blocks and either level or ever-so-slightly sloped.

Top view of finished wall ([] represents neither orientation nor count):

[]---------- FLOWERS []STEP[] FLOWERS ------------[]

Two blocks into the slope.

Front View:


Three, potentially four, runs high. 16 feet from end to end.

The construction material type is not up for debate. Nor am I really worried about code/legal issues. If you want more information beyond the scope of the question (aka Why I'm building a wall, neighbor's shoe size, etc) just memail me and I'll get back to you, really I will.
posted by RolandOfEld to Home & Garden (5 answers total) 2 users marked this as a favorite
Response by poster: I forgot I had pictures from a previous question.

Shot 1 The wall edge will be adjacent to the fence where no solar lights or gate exists.

Shot 2 The wall will span the width of the yard, with steps in the center.

Basically the slope will go away and a terrace will take its place.
posted by RolandOfEld at 10:54 PM on July 24, 2011

Best answer: As far as a poured footer, you probably don't need that for your purposes. Tamped gravel, 3" or so of it, should be good enough. One thing to consider might be to install a french drain behind the wall to divert water around it. In that case, significantly more gravel would be needed. When I was doing this kind of work for a living, I always installed french drains when installing retaining walls.

The height shouldn't be a problem, but you should consider that one or more courses should be below the grade. This will anchor the wall and also will provide stability. Consider that when figuring out courses.

Most of the segmented blocks you get from Home Depot or such places step back naturally by an inch or two, so that it appears to actually lean into the slope. This is good, because you want to avoid building a wall that is either completely plumb or even worse, pitches away from the slope. (Seems like a no brainer, I know, but people actually do this.) The blocks only lock in one way, so there's little chance that you'll install it wrong. You might want to consider using some kind of cap stone; it ends up looking a lot more professional, and properly installed(with either glue or some mortar) locks the wall together. The mortar required for that should be minimal. You might have to cut some pieces in order to do this project properly. A cheapo circular saw with some abrasive blades or if you want o get fancy, a diamond blade, should do the trick there. Use a mask and eye protection and have a chisel and hammer ready. Don't try to cut all the way through- 3/4 should do it, then finish with the hammer and chisel.

The steps might be the biggest problem for you-remember to cut your retaining wall in on both sides of the steps or when it rains you'll have runoff. Are the steps going to be the same material? Something to consider, as comfortable steps have a definite riser height/tread length ratio and it could be difficult to achieve with just the retaining wall blocks. Plan the steps out carefully because that will help determine how to plan out the rest of the wall, its height, etc.

Check out Home Depot or Lowes for a Sunset book about hillside landscaping or building retaining walls- they usually have simple, good directions for installation.

Also, watch out for whatever that electrical box is there. You definitely don't want to stick a shovel into a high voltage wire.
posted by dave78981 at 11:44 PM on July 24, 2011 [1 favorite]

Best answer: The company who manufactures the blocks you use should have specific installation requirements (possibly online), but if I design an unspec'd modular concrete block wall I use a 6"D (six inches is standard for these systems) by 2 x (width of block) well compacted base. Make sure you use chips or road base that is angular, and not rounded pebbles as you use for drain rock. First course should be embedded below grade minimum 1/8th of total height of wall. You will need pebble/drainage aggregate* behind your wall, from the bottom to about 10" below top of wall, 12" wide. Use the road base again in the blocks, unless your manufacturer specifies otherwise.

*not the same as road base- rounded pebbles help maintain pore space.

You should be fine without mortar, especially if you use a modular system with locking pins. Most important to resist overturn is to do the base correctly, batter the wall (lean it into the slope, as dave78981 points out), and insure there is drainage behind the wall. Without those things, even a mortared wall will fail.

Agreed that you need to plan your steps carefully. If you'll be using block and capstone you'll need to use adhesive on the capstone. Sides of the steps don't need to be battered. Here's a general guide.
posted by oneirodynia at 2:45 PM on July 25, 2011

Response by poster: Thanks for the replies. Just wanted to get an idea if I was WAY off base thinking I could get away without mortaring things.

So what is the chance of doing the job with plain, not-made-to-purpose blocks. Because, unless I'm mistaken, 8x8x16 blocks don't have tapers or ridges for the battering. I'm confident I can batter things fine on my own, and I will be pinning it with rebar or some type of pole through the fill in the center of the blocks. I just didn't know if not having the certain block type was a necessity since I have easy/cheap access to some blocks already.
posted by RolandOfEld at 6:10 AM on July 26, 2011

Response by poster: Oh, and thanks for the guide on the steps, those might be tricky and I'm looking forward to them least of all.
posted by RolandOfEld at 6:11 AM on July 26, 2011

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