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tomtery 07-05-2012 04:51 PM

orientation of 4x6 posts in retaining wall
I am building a 20 foot, 3 foot high retaining wall. I will be using 4x6 posts and 2x12s. I have a simple question - which side of the 4x6 should be parallel to the 2x12 - the 3.5" side or the 5.5" side. I have seen it done both ways.



Thunder Chicken 07-05-2012 05:06 PM

I'd put the short span parallel to the 2' by 12's. Generally you want to put the deepest dimension of the beam in the direction of the load that you want to support.

Having said that, 3 ft is a lot of soil to hold back, especially after a rain. I'm not sure what your plan is but 2 by 12 sounds a bit thin. Have you thought about using ties with deadmen to form the wall? It's a pretty common and strong method to build short retaining walls.

joecaption 07-05-2012 05:59 PM

2 X 12's are not below ground rated. Read the tag right on the lumber.
You would be a whole lot better off with at least 4 X 6's 6 X 6's would be even better.
I would not use railroad ties, the reason is there to short so there's going to be lots of seams, they were removed in the first place because they were nearing the end of there expected life span.
Without a drainage system, using rebar or long spikes, off setting each row, tie backs, silt screen and the support beams at least 4' in the ground your plans not going to work.

tomtery 07-05-2012 07:10 PM

thanks for the responses. I didn't give all the details - at maximum the wall is 3 feet, but quickly tapers to 1.5 feet - the wall is at the edge of a small hill, and is actually two walls at right angles. i plan to sink the posts 4 feet into the ground, have drainage pipe, gravel, and geotex against the wall, and compact the fill in layers.

i live in california and have had two contractors bid the job. both spec'd 2x12's, and i have had a 2x12 wall on the property for 10+ years now with no apparent degradation. however your comments are worthwhile, and i will now investigate using 4x6 timbers. thanks!


Daniel Holzman 07-05-2012 08:34 PM

You may want to consider building the wall using concrete segmental block. In my experience, it is often less expensive than wood, it lasts essentially forever if properly installed, and you can get a wide variety of block types that look very nice.

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