We have 50 feet of retaining wall to install. It's tallest retaining point is about 24 inches. The wall will retain 24 inches for about 25 feet, then it will retain about 18 inches for another 25 feet.
We don't want to build a concrete block wall. We plan to use wood or railroad ties that we can install ourselves.
Location is Central California - no heavy frost or other weather-related issues.
Couple of questions.
1. We think that the vertical posts should be 4x4s or 6x6s. The horizontal retaining boards should be 2x6s. Either using redwood untreated or the preservative-green treated wood.
Do the materials sound correct?
2. How far into the ground should we sink the vertical posts? We would like to install without cement.
3. How far should the vertical posts be from each other?
4. If we use redwood, should it be treated with a preservative also?
Sounds like your choice of materials should work out okay. I would probably set the poles 40-48 inches deep, (with cement you might get by a little shallower 36-40 inches), and set the posts no more that 6 feet apart.
18 and 24 inches high is not real tall for a retaining wall, and I doubt you would have to tie back into the hill, but you might want to in several places, just in case.
Will your wall be straight or curved? Remember, that a curved wall adds strength.
Wow - I'm surprised that we have to go in 4' to support 2' of dirt on a shallow slope.
It's starting to sound like quite a bit of material and labor for this little retaining wall. Would you think that I might be better off (and possibly even cheaper) going cinderblock in cement and rebar?
When a bank of dirt falls, usually the top 1/3rd or so starts to slide first. (The weight of that material on the pile holds the bottom part of the pile in place. ) After the top part is gone, then the lower material will begin to move. The wall works by being strong enough to remain in place against the weight of the moving dirt. You can accomplish this through weight of materials, increasing the base, or added bracing.
The taller the pile of material, the more weight will be moving when the top part begins to slide. The reason I would choose 4' is to ensure that the wall will stay where I put it when that top 1/3rd starts to move. The materials you have chosen are not individually very heavy, so I would opt for more below the ground in order to try to keep the wall vertical. (Remember that rainfall, lawn mowers and just people walking around can add to the weight of the material trying to move.)
There are several other ways to strengthen the wall that would not require as much depth on the poles. You can add a curve, or a "v" shaped pattern like VVVVVV. This makes the wall stronger by making the base wider. You could also slope the wall so it leans back slightly at the top. (If you look at the cement work on overpasses, it is often sloped this way.) Again, this makes the base wider.
When I built my own wall, I used stone. The wall is thicker at the bottom and thinner at the top. It also is curved and leans slightly backward at the top. It is about 5' high. I helped a buddy build his walls using old railroad ties. We stacked his ties to about 4-5 feet and used "T" shaped anchors made of more ties to run back into the bank. Each tie was lagged to the ones beside, above and below it. We then drove stakes into the ground for added support. His wall goes straight up and is the same thickness top to bottom.
Both walls are a minimum 8" thick and have been up more than 3 years with no movement. I would think the block would have more weight and therefore would hold better. You would just need a good footing. Personally I would prefer the look of the treated lumber, and it seems like cost would be about the same.
You might try getting a few boards and building one section of wall. Also, get a few blocks and try drystacking them. (Just stack them up without mortar). See how both hold up and next spring figure out what you'd like to do. That gives you a few months to see how the walls hold up and which you like the looks of. Make sure you get through winter and the rainy season before making a choice.
Here is a picture of the site. The walkway is mostly dug out. The retained dirt is highest on 'this' side of the picture (closest to us) and it is about 24 inches at the tallest point. The lowest retained dirt is about 18 inches at the tallest point.
I think we will need to build a wall that is somewhat higher than the 24/18 inches.
The length of the wall is about 50 feet, give or take.
They make a 2x6 T&G treated board that is used for skirt boards on pole barns. Some of those might work for you. They would lock together to make one solid wall.
Don't forget to consider your drainage there. Looks like the bank will tend to move water toward the building. May need a french drain to keep you dry in a heavy/continual rain.
I'm in Northern CA and the terrain you're working with looks pretty familiar to me. I've got two walls that we put up. One is about the same size as what you're talking about (75' long, about 24" in height) and the other is about 30' long and 3' in height.
I've wrestled with the same issues you're dealing with. I have a pretty large wall that needs to be built behind my house that I've avoided doing for years because I'm pretty sure I need to do a concrete block wall. It is far more expensive than a wood wall, but for something heavy duty I think you kind of have to bite the bullet and not try to get by with something that will fail in a few years.
But your wall looks like it could go either way. You aren't holding back a steep slope and it appears to be pretty stable. What you really want to do is make sure you provide excellent drainage behind it, though.
As far as post depth is concerned, with the hard soil we have here I drive my posts to a depth equal to 75% - 100% the height of the wall and fill with concrete. When I get 2 or three feet down I'm chopping rock out with a big mauler. If you have softer soil, you'll need to go deeper or use "deadmen" - beams dug into the slope that are tied in to the posts.
Make sure that if you do a wood wall that you put the boards on the inside of the posts. I tried to secure them to the outside for one of my walls and it wasn't a good idea. If it was much more than the oversized planter box that it is it would have failed by now.
You also might consider using 2x10 or 2x12 boards. I've got a hodge podge of sizes and it seems that the larger boards hold the shape of the wall better. Somewhat more expensive, though. Unless you are worried about chemical content, pressure treated will be less expensive and will probably last as long or longer than redwood.
But you might carefully consider whether an interlocking block wall would be a better idea. I have seen several walls built of those interlocking blocks that are pretty darn large, and many that are about the size you're considering that seem to work really well. They're available in a huge array of sizes. Not cheap - I'm not sure if the cost is less than doing a cinder block wall or not.
Keep us posted - I'm curious to see what you decide to do and how it turns out.
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