Retaining wall kinds

BugTerminatorFebruary 16, 2013

Hi All,

I have a 44' by 2'1/2" retaining wall that is starting to lean. It was built with 4x4 posts every 5' and the planks are 2" x 8".

I am just wondering what are the merits and problems with different kinds of retaining walls:

- retaining wall made by stacked 6" x 6" timber, anchored in ground with 4' rebar, and secured with deadman anchors (no 4"x4" posts)

- brick with mortar

- timber with posts, similar to what this wall was before, but maybe thicker planks

Are posts absolutely needed or would the deadman anchors perform the same job?

Any thoughts welcome!

Thanks,
CG

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yardvaark

I am not a fan of wood/timber retaining walls (conceding that there may be occasions where they are called for.) All other things being equal, they don't have the life expectancy of a masonry retaining wall system. Usually, it's considerably less due to the fact that wood--even treated wood--decays. Since retaining walls and their preparation tend to be expensive, it is frequently budget that determines the details of a wall beyond the basic engineering. A brick-faced wall will cost much more than a plain concrete wall. The overall layout of a wall can add a great deal of strength if such certain configurations work with the plan. If a wall is straight, it will be weaker than it's curved or stepped counterparts. Solid-concrete-filled concrete block walls can be a good, cost effective solution in the height range you are inquiring about. It's possible to face it with a variety of decorative finishes: brick, faux stone, stucco, tile, etc. There may be alternatives to walls. Not enough site information is available about your project.

    Bookmark   February 17, 2013 at 11:08AM
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marcinde(7)

In response to the specifics you asked about:

- 6x6 timber wall w/ deadmen: perfectly feasible, serviceable wall. Two years ago we came in and tore out a really extensive timber wall (it surrounded the entire backyard and stepped down the left side to the driveway). The client wanted it gone because the wall was installed in the early '90s and they were concerned that with all the shade the wall would be a rotting mess. Well, we were all surprised when a grand total of two timbers were rotten. It all depends on site conditions and quality of lumber, but with ground contact a timber wall will likely last 10 years at the least, maybe 20-25 years at the most.

- Brick w/ mortar: retaining walls that are just brick and mortar are pretty unusual these days. What's more common is to do a wall out of either poured concrete or CMUs and face it w/ brick or stone veneer, Those are great, just know that they're significantly more expensive (both labor and materials) than a timber wall

- timber w/ posts: I've never built one, and unless I get stamped drawings from a structural engineer I never will. You see the issues with the wall you have now. The thickness of the planks is irrelevant; by the sounds of it your wall is failing because all that is resisting the horizontal movement of the wall (imagine the wall with the base as a pivot point) are the post footers. That's a lot of force on a small area.

Deadmen are the way to go w/ a timber wall, and properly constructed posts are unnecessary. The link below is the guide I use for detailing walls.

Here is a link that might be useful: retaining wall specs

    Bookmark   February 17, 2013 at 1:29PM
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BugTerminator

All, thanks for the info!

I am leaning (haha) toward a timber wall so far. A whole concrete wall might be beyond my capabilities.

@marcinde: thanks for the wall specs, this is helpful. That many deadmen for a 2 1/2' tall wall would seem a bit overkill though.

With deadmen piled on top of each other, I am assuming only the deadmen in the first course above ground would be anchored with 4' rebar?

Thanks!
CG

    Bookmark   February 18, 2013 at 12:18AM
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marcinde(7)

If you read the details, the timbers are held together with 1/2" spikes, and the deadmen are spiked to a short member running parallel w/ the wall face. I've built walls both ways and spikes are way easier to deal with. Pre-drill a 3/8" hole to make the spike's passage a little easier, pound it in, move on to the next one. No cutting rebar or any of that.

You can do whatever you're comfortable with in terms of # of deadmen I guess, just keep in mind that the way these walls work. What's keeping the face of the wall from tilting forward is the fact that the deamen, tied into the wall face as they are, act as a big cantilever. It's the same reason we use geotextile fabric between courses of a segmental wall, or a wide concrete footer under a concrete wall. Physics!

It's a 44' long wall, what'll it add to do it right - a couple hundred bucks? Cheap insurance, especially because if you cut corners and need to replace the wall down the road you're unlikely to successfully pry the timbers apart and reuse them.

    Bookmark   February 18, 2013 at 8:17AM
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pls8xx

The three usual considerations are cost, appearance, and service life.

The things that most influence the selection of construction type are climate, importance of appearance, and whether it will be contractor built or DIY.

Wood walls are more feasible in a dry climate, but have a short service life in a wet warm climate.

Cold climates favor a dry stack stone or SRW, avoiding the high cost of deep footers required for masonry walls.

Concrete and masonry walls are more feasible in a warm climate, especially where appearance is important.

Additional consideration should be given for soil types, the soil grade above and below the wall, and nearby buildings, drives, or other structures.

For a contractor built wall, the most common type for the area often has the most competition and the best price.

DIY walls types should match the abilities of the homeowner.

    Bookmark   February 18, 2013 at 8:19AM
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yardvaark

"That many deadmen for a 2 1/2' tall wall would seem a bit overkill though." It's not. With the cross plates, it's what works.

"A whole concrete wall might be beyond my capabilities." Another option falls between a wall & slope, depending on its details. You haven't given enough information about your project, site conditions, needs, skills and abilities in order to make further suggestions.

    Bookmark   February 18, 2013 at 9:34AM
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BugTerminator

Indeed I have not provided too many details. I need to draw sketches, but until then, here is a summary of the retaining wall:

- the wall is 44 feet long, 2 1/2' tall, and what is behind it is a bed with roses (level), 44 feet by 5 feet
- the area in front of the wall is level (currently grass)
- soil conditions in the area are mostly clay
- behind the bed for roses there is a narrow concrete path, and another similar retaining wall of similar height, also in bad condition

Another option I saw in the neighborhood: stacked concrete blocks with a setback. These seem easy to install though heavy. I'll be back with a sketch soon.

CG

    Bookmark   February 24, 2013 at 12:47AM
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