Wood Retaining Wall

15 Apr

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This is a project I did a few weeks ago to use up wood from an old maple tree we had to take down last winter. The process is fairly simple, just needs a fair amount of sweat equity and the logs to do it with. But sometimes it’s easier to show with pictures than explain. So, if I can figure out how to get the pics attached, I’ll do that. If not, you’ll have to make do with my description, lol.

So, what do you need to do this… First, you need wood- I prefer whole logs, with the bark intact so they don’t just rot right away. Length depends both on what you have and how high of a wall you want. This area had a pretty good slope so I had the arborist cut the logs to three foot sections (some of them are closer to four feet though, taller than I really wanted, and a bitch to move at that size.) Tool wise you need something to dig with- I used a pick axe to dig the initial trench and a square shovel to actually move the dirt, but any kind of shovel will work just as well. A pick will make any kind of hard soil easier to break up, but all you really need is a shovel. That’s it! Oh, and maybe about a gallon of water, and some aspirin, lol. Unless you are in godlike good shape, you’ll be sweating buckets, and quite possibly sore the next day. This is a good workout.

Anyway, so the first thing you need to do is decide where you want the log wall to be, and dig a trench. So, how wide and deep do you need to dig? That depends on the size of logs you are using, and to some extent the soil you have. You want these stable enough they don’t fall over once you backfill with compost or soil on the uphill side. My soil here is fairly typical Northwest heavy silt, which compacts like clay for the under soil, with a few years of decent mulch making nice humus on top. This is a fairly stable soil that once compacted, doesn’t move around a whole lot. If you have sandier soil, you’ll need to dig deeper to keep the logs stable. But my basic rule of thumb is you want about a fourth of the height of the log underground for stability. In this case, I dug down about 10-12″ for most of the trench, since the logs I was using were mostly so tall. For width, you want to be careful not to go too wide. Look at the widest logs you have and don’t go much wider than a couple of inches wider than that. It’s easier to widen the trench for one or two logs, than to back fill a ton (and is more stable in the long run too), so a skinnier trench is better than one that is too wide. I prefer to use the pick for the rough digging, then move the dirt uphill with the shovel. It’s fairly easy to roughly estimate the width you need by placing the shovel on top of the logs and kind of eyeball using the width of the shovel to tell how wide you need to go.

Once you have decided where and how big to make your trench, I find it easiest to dig a short bit, say four feet or so, with the pick axe, then use a shovel to move the dirt UP hill (assuming it’s on a slope, or inside the circle of logs if you are doing a raised bed). Once you have the trench dug down properly, drop in the first log. Make sure it is standing at the height you want, make whatever adjustments you need to, then drop in the second one. Again, assess to make sure it’s the height you want, is standing more or less straight up, is as tight as you can get it to the first one, and looks ok. Depending on how big the logs are, you may need to extend your trench now, but hopefully you can fit at least one more in before having to dig again. Now the dirt you loosen can be used to help backfill the logs you have just put in place. This is why I don’t like to dig the whole trench at once. If you do a section at a time, you can use the dirt you are displacing nearly immediately rather than having to store it somewhere, then move it again.

So, keep going, alternating digging a short section and placing logs upright in the trench. Backfill as you go and you will be surprised how quickly this can go.

Aesthetics do play a part in this. I like the knots and branch stubs showing, so you will notice that I have carefully positioned those so you can see them, and the knotty ones are more or less evenly spaced. Also, unless you specify the logs be cut even heights/lengths, you often get what I did, which is a range of log lengths from relatively short to omg long. In this case, there was almost two feet of difference in log height (length? lol) between the tallest and shortest, and since it was an old tree, quite a bit of difference in the width of the logs too. I chose with these to mix both height and width a bit for most of the wall. A few shorter wider logs between taller and skinnier logs mixed in with the larger ones will give me ‘windows’ to plant seething to spill through the opening, and both hide and soften the almost severely upright architecture of the logs. Smaller diameter logs can also be pushed back and front to give your wall a little more variation so the front of the logs isn’t all poker straight, again giving more variation in the wall and more opportunities to plant things in the odd nook or cranny.

A few notes on backfilling- make sure when you are done, to carefully backfill the trench both in front and behind the logs. On the front side (downslope, or on the outside of a raised bed), remember to put a little dirt in, then tamp it down good before adding more dirt till its flush with the surrounding soil. It’s very important to tamp this front edge properly, or your logs will fall over and your wall will come tumbling down. I like to tamp a little on the uphill or inside edge as well, especially if you are doing a raised bed, but on a slope like I have shown in the pictures it isn’t as important. But the downhill side definitely needs to be tamped properly to remain stable.

Once your wall is in place, backfill with good soil, plant and mulch. Then you can sit back and enjoy for a few years the fruits of your labors. The kind of wood you use, and your particular climate will decide how long these logs will function as a retaining wall. This is maple, and in the Seattle area’s wet winter climate I don’t expect these to remain for more than five to ten years functionally, but in the meantime I can plant to stabilize the slope and provide the long term stability the area needs. Or just assume another tree will provide the wood to do it all over again, lol. If I had cedar or madrone to use, or maybe an old oak, those woods will last a little longer. Alder or cottonwood would likely rot almost immediately, and might not even be worth the effort to dig a trench like this. It all depends on what you want out of it.

In this case, I’ve planted on both the up and downhill side all along the wall, several sword fern babies. These will help grab the soil and stabilize the slope long after the logs have rotted away. I’ve also planted a few huckleberries to take advantage of the rotting wood, since they like that. Next fall, some salal and low Oregon grape may also join these, but for now the rest of the bed is planted for hummingbirds mostly- fuchsia, Ribes sanguineum, Stachys cooleyae (which is already there… One of the challenges of this project was to NOT trample all the Stachys, or the seedling Trillium that popped up in here a few years ago.)

These wood retaining walls may not be a permanent solution for a slope like this, but I like the way they look, especially in a shaded garden with lots of natives. Over time, ant colonies will burrow into the wood, who may attract wood peckers and other wildlife to them. The wood itself will slowly release it’s nutrients back into the soil in a very natural way, enriching the plantings around it. The wood will also tend, as it starts to decompose, to act like a sponge, holding moisture and gradually releasing that moisture into the landscape in drier weather. This is a great way to use an excess of wood you may have, and at the same time add visual interest and not a little habitat value to your garden.

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