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zendog_7a

Help with 6x6 retaining wall 18 inches over grade

zendog
9 years ago

Hi,

After doing a bunch of research I've started to build a 18" high retaining wall out of 6x6 pressure treated lumber (well, actually the first course is 4X6, but the rest will be 6X6) which will be approximately 24 feet long. Before I get too far in the build I have some questions and hoped some folks here might have some insight. I've seen enough leaning walls around that I want to make sure I don't have that happen to me in a few years.

First of all, this is for a front yard that slopes down about 24" to a sidewalk. There are masonry steps on one end where the wall will start and the other side slopes down to a driveway, where I intend to use a 45 degree turn to resolve it into the driveway area. Currently the area is grass, but I want to make this part of my garden.

What I've done so far:
So far I've dug out about 4 inches below the edge of the sidewalk, then filled in about 3 inches of gravel, compacted, leveled and put down my first course which is 4x6 pressure treated laying on the 6 inch side. The bottom course starts about an inch below the sidewalk so I'm hoping that will hold the bottom of the wall back. I've dug into the area beside the driveway to allow for the 45 degree angle (sorry not showing in the picture) and dug back beside the masonry stairs about 2 feet. Against the stairs I was intending to go up a little before I dig further back, but the final course will go back about 5 feet.

What I intend to do:
On top of the 4x6 base layer, I was going to put up 3 more courses using 6x6s. On the first course I was going to do 2 dead men (at 8 and 16 feet) and on the second course I was going to do dead men at 6, 12, and 18. I was not going to do any dead men on the top course, since that would be at ground level. I was also going to put a dog leg (like a captial L) on the back of the section beside the masonry wall and at the back end of the 45 degree wall on the second course to act like dead men - both on the course below the top course. I was also going to step the front of it back about 1/4 inch each course to give it some lean. The separate courses will be tied together with 3/8X10 galvinized spikes going into 1/4 in pre-drilled holes, and I will drive rebar through the bottom3 courses (the 4x6 plus the first 2 courses of 6x6) at 6 foot intervals and at the back of the 45 degree wall by the driveway and the section by the masonry stairs. I thought I would use 3 foot lengths of rebar, which would be going down into the ground below close to 2 feet.

In the end, there will still be a gentle graded slope from the current planting area to the wall, and all of this will be covered under mulch.

Questions:
1. Drainage - I have good drainage (through 4" sold drainpipes) from my gutters, so the only water coming from rain will be whatever falls on that part of the yard, plus a little runoff from a small shed roof over the front entry (5x12). Do I need to do the whole backfill with gravel and put in a drainpipe thing? I'd rather avoid it since that is a lot of extra work and gravel to bring in, but will do it if that is a huge risk.

2. I'd like to dig out a bit of the hill behind the wall before I back fill and put in some old rotten logs I have to make this sort of a Hugelkultur area. Then I'd fill in over the logs and with a mixture of composted leaves from my county, the existing topsoil and composted horse manure (originally had some wood shavings and straw that would have been part of what composted). But I'm wondering if anything about this approach might increase pressure on the wall. Do I potentially create a problem by not leaving the existing soil in place that is already settled and stable and just filling the front area?

Here's a picture of the first course in place.

Comments (6)

  • zendog
    Original Author
    9 years ago
    last modified: 9 years ago

    And here is a sample of the finished height. Ignore the fact that the top is a 4x6 - I was just stacking what I had to get a look.

  • marcinde
    9 years ago
    last modified: 9 years ago

    frost can heave an inch or more, even here. I'd bury a full timber against the sidewalk,

    Water is a wall's enemy. Do the gravel drainage chimney, in the grand scheme it's cheap insurance.

    I don't see the value in battering this wall (stepping it back with each course) and aesthetically I think it'll make it look like you screwed up. Google Fairfax County Typical Wall Detail for their pdf packet and use their timber wall guidelines for deadmen, etc

  • zendog
    Original Author
    9 years ago
    last modified: 9 years ago

    Marcinde, thanks for your reply. You are bumming me out on the part about needing to bury the first course deeper, but I hadn't really been thinking about frost heaves. Based on last winter, I expect you are right. I was hoping that if I had pinned the wall in location with the rebar going down a few feet, if it came up a bit it would just settle back in place. Misguided?

    I'll go ahead and put in the drainage. Certainly better to be safe if it might be an issue down the road. I'm hoping to get at least 10 years out of this without it starting to look tilted, etc.

    And I'll drop the bettering. That was good news! I hadn't really liked the look, but thought saw one wall around here that used it and that seemed to help. Though now that I look at that wall, there are no dead men and it is about 2 feet tall, so maybe I shouldn't use them as inspiration!

  • PRO
    Yardvaark
    9 years ago
    last modified: 9 years ago

    I agree with Marcinde on all counts.

  • zendog
    Original Author
    9 years ago
    last modified: 9 years ago

    Thanks all. I'll yank it up and dig everything down more to start again. Ugh, sometimes the right thing to do isn't so fun.

    I'll just keep focusing on imagining my little wall straight and true 5 years from now.

  • marcinde
    9 years ago
    last modified: 9 years ago

    trust me, it sucks now, but it's still better than the alternative.

    On the long rebar, btw - make sure you have a grinder or a sawzall handy that you can cut excess off if need be. Nothing like pounding a length of rebar into heavy clay and hitting a rock two feet down. At that point you can't go down and it's not coming back up. Voice of experience here.