4x4x8's treated as retaining walls...how to do?

regaldozer(6A)April 25, 2007

Looking for some advice....

My garden is being built on a hillside. I have leveled 6 beds into terraces (or steps) each bed is about 16' long, 3' wide and 12" higher than the bed below it. I purchased 4x4x8's to make the walls. I was planning on stacking them 3 high (varying the joints) and drilling through with 3' rebar (1 foot in the wood, 2 feet in the ground) will this hold up?

My other thought was to pound the rebar through the bottom (ground) pieces only and use a 6-8" spike and construction adhesive to hold the wood together....

any other suggestions or comments about the plan?

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davidandkasie(Z8 MS)

that is a lot of force for a 4x4 to hold back. you will want to use the rebar thru the full stack, not just hte bottom 4x4. and put them 18-24 inches apart max. it still may fail.

i am not a structural engineer, but if it were me i would use at least a 6x6 and probably a 8x8 to do it. i have seen 4x4's bend and fail when used as you describe.

    Bookmark   April 25, 2007 at 1:46PM
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marcinde(7)

I've posted this link here before:

http://www.fairfaxcounty.gov/dpwes/publications/retainingwall/details.pdf

All the rebar is going to do is pin the timbers together, it won't give any meaningful support.I agree with the above poster- I don't think 4x4s will be appropriate for this application if I understand your intent correctly.

Dave

    Bookmark   April 25, 2007 at 2:01PM
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davidandkasie(Z8 MS)

one other thing i meant to mention, you need to put some of the wood under the ground level. if you want 12" above ground you should have at least 1 full 4x4 under ground so you end up with 4 stacked high. pin htese together and it will help some. otherwise all you have is the rebar actually holding and it will shift, if nothing else it will bend straight out.

    Bookmark   April 25, 2007 at 2:57PM
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regaldozer(6A)

now I am depressed. the beds are already in great shape and ready to plant. I will need to kill them and do a ton of digging to do anything meaningful. the drawings show "deadmen" which go perpendicular into the hillside. How do i do those without digging trenches.
THis stinks

    Bookmark   April 25, 2007 at 3:27PM
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marcinde(7)

Man, now I feel like I rained on your parade! I'm sorry...

So are you saying that the walls are already in place? To answer your question, I've always had to dig into the hill somewhat to set deadmen. It's worth it, structurally. If you're stepping the walls back, you'll gain additional strength if you can set the wall above on the topmost deadman of the wall below, and fasten per detail.

Dave

    Bookmark   April 25, 2007 at 8:48PM
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pls8xx

regaldozer, The situation may not be all that bad. What you propose might well work. But the things that will determine the stability you have not related to us.

The wall details linked to by Dave are some good general construction ideas. If you have marginal soils that may sometimes become saturated and you are doing a 3 to 5 ft wall you better get close to what those details show.

Keep in mind that engineers that do generic plans count on a worst case scenerio. Then they throw in a 50% safety factor. For a 1 ft high wall the details are way overblown. There is a world of difference in the force behind a 2 ft wall compared to a 1 ft wall.

Taken all by itself a 1ft wood wall can stand with rebar through the wood and 18 to 20 inches in hard undesturbed subsoil where the rebar is used on 16" centers.

But in your case you are doing multiple beds up the slope. So you must take into consideration the overall stability of the slope itself.

Since your plan calls for 1 ft high then 3 ft horizonal, I assume you have a basic 3 to 1 slope. A compacted 3 to 1 slope of most soils is stable. But it is close to the point where there can be an instability. As the length of the slope increases so does the chance of a slide. Saturation of the soil in a slope robs the ground of its strength. If the total weight of the saturated soil exceeds the soil friction, then the whole slope breaks loose and slides. The slip point will be at the depth of the soil saturation. And dead men or rebar anchors won't help unless they extend into the slope to a depth well past where the soil will become saturated.

For this reason it is important to know the geology of the slope. Take a posthole digger out there and see what you have. Does the slope level off just above your top bed or does it continue on upward?

    Bookmark   April 25, 2007 at 10:59PM
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regaldozer(6A)

ahhh, a light at the end of the tunnel....corrections, answers then pictures of the site to maybe help clarify things
1) beds are 4 1/2 feet deep and about 1 foot high (not 3 feet, my mistake)
2) soil is pretty compacted ( i dug holes for roses right next to the garden and it wasn't an easy job)
3) the slope continues above the garden but it is much less stepp (it is my front lawn in fact)
4) we had a good long hard rain last night and nothing seemed to move at all.
ok pictures



    Bookmark   April 26, 2007 at 7:36AM
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pls8xx

Looks good to me. I suggest using #3 rebar on 16" centers. You will need a bit long enough to drill all timbers at once. If you just drive the rebar without drilling, as the wood tries to warp, it will split.

Note: The bottom timber must rest on firm undistrubed subsoil or you will have trouble!

Stack the timbers on boards to hold it off the ground while you drill or the bit will dull in the dirt. Drill the first hole and drive the rebar flush to the bottom timber as shown below, then move to the next hole.

After all the rebar is started, drop the timbers to the ground. Lean the wall back into the slope as shown below and drive the rebar down. As the backfill loads the wall it will come up about vertical.

You may want to do side walls at the ends. If so, do an overlap at the corner with a rebar to tie it together.

    Bookmark   April 26, 2007 at 9:14AM
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harleysilo(7 Roswell,GA)

I agree with pls8xx. You are only supporting 12" of soil. No need for deadman and the like. I would still put the construction adhesive in between runs however.

    Bookmark   April 26, 2007 at 9:20AM
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