Using wooden retaining wall with deadmen? 1

[HofOblivion]

Hi fellow engineers,

I have a project to legalize unpermitted work and questions I'd like to bounce with others. Owners have unpermitted wooden planters that doubles as retaining walls. There are two planters (see attached) with 36 inches height. Below are my concerns:

1. Due to decay, I don't think wood members should be used if in contact with the earth, even if they are wrapped in liners
2. I cannot justify retaining wall beams to horizontal brace (to deadmen) connection. I think it's unstable.
3. Even if above 2 items are fine, I cannot justify gravity load onto deadmen to support the retaining wall, because it's utilizing retained soils.

What do you think?

https://files.engineering.com/getfi...le=Wooden_Planter_&_Retaining_Wall_Detail.pdf

 

[oldestguy]

How about treated timbers, in "log cabin" configuration? It's done a lot for small retaining walls.

 

[BridgeSmith]

I don't seem to be able to open the attached file; could be my computer. Anyway, using deadmen anchors are fairly common for timber retaining walls. As long as they are placed far enough back to be out of the retained soil zone, which is generally about equal to the height (obviously, it varies depending on the soil type), it will function to anchor the wall through passive resistance of the soil between the front of the deadman and the retained soil zone.

The horizontal soil pressure on a 36" high wall is fairly small - block walls in that range are usually adequate as gravity walls with no tie-backs.

The most common fatal mistake I've seen with walls like you're describing is failing stagger the joints (provide a running bond) between the timbers in order to provide continuity along the wall.

 

[HofOblivion]

How about treated timbers, in "log cabin" configuration? It's done a lot for small retaining walls.

It's legalizing existing unpermitted work, so I don't know if they are treated. But then again, I was thinking of proposing CMU retaining wall in lieu of existing wood, so it's not beyond the realm of possibility.

As long as they are placed far enough back to be out of the retained soil zone, which is generally about equal to the height

Apologies. I uploaded a new detail. Hopefully, this one works. Your statement eases my worries. One thing to point out though is there's one stacked on top of each other following the grading profile. So the bottom one is essentially loaded 6 feet high. I might suggest moving them beyond the 1:1 line.

 

[oldestguy]

As long as the rear walls if these "cribs" are tied at times along the wall, fine. However, if the timbers are not treated as with creosote or similar, the wrapping will not do much of anything other than holding water in the wood.

 

[BridgeSmith]

Now that I see the configuration of the system, my concerns (the things I would check, anyway) would be the global failure stability of the system and the structural stability of the exposed walls.

I should say I would have a global stability check done, since I've not done one myself. Here at the DOT, the Geology section does the global stability analysis. If the sketch you posted is to scale, and the foundation soil isn't complete crap, I doubt it would be an issue, but I'm no expert. Hopefully, someone here more familiar with that type of analysis can help with that aspect.

If you don't have continuous vertical reinforcement in the wall, which would be very unusual in that type of wall, the structural stability assessment would be based on the horizontal unbraced length (the length of the planter boxes or sections of the planter boxes) and the continuity, strength and connections of the timbers. If the timbers are partially decayed, your judgment of their remaining strength will be a key aspect. Although, as I said, the soil pressures are very small; there's not much force to resist.

 

[NorthCivil]

Can't open the images either.

In my area, only retaining walls over 4'-0" need engineering. anything less isnt subject to inspection. lots of builders will get around this even more by grading the earth by building 4' walls in sequence, so none of it needs sign off.

 

[gte447f]

Good luck if you try to engineer one of these timber walls. I have never seen it done, but would be interested to know if they actually calc out.

Considering the fact that this one is already built, an obvious challenge is your question #2, verifying the connections, not just the dead man connection to the face of the wall but the connections between each course of timbers in the face of the wall.

As far as question #1, these obviously have to be built with some kind of preservative treated timbers. Creosote is a no-no these days, so that usually means pressure treated 6x6 timbers from the big box home fix-it store of your choice. This is readily available material and I can't imagine anyone building one of these out of anything else, so it should be easy enough to verify this material. I would not stake my career on it without some documentation to prove the material, but you can probably identify this material by visual inspection, especially if this wall is newly constructed.

For your question #3, the deadmen rely on passive pressure. I suppose, theoretically, they should extend beyond the retained soil zone, and I think they usually do in practice, but by how much is questionable, both theoretically and practically. In reality, couldn't these work even if the deadmen were anchored in the retained soil, kind of like a reinforced earth retaining wall like a segmental block wall (Keystone, etc)? Of course there is no testing to prove the holding power of the deadmen like there is with geogrid reinforcing for segmental walls.

 

[BridgeSmith]

For stability, these have more in common with an MSE wall (actually, more like a 'bin wall', but those are not nearly as common) than anything else, and could be analyzed similarly.

Structurally, it is a different matter. For that, the connections, continuity, and material strength has to be evaluated. If it isn't adequate, it's fairly straightforward to provide some crossties from front to back across the top. It's a little trickier for to tie them in on the bottom, likely requiring some hefty lag bolts, but still not major work.