Retaining wall question

16 years ago

My yard is long and narrow, on a fairly steep slope that was terraced with landscape timbers by a previous owner. The timbers are 20+ years old, rotting and beginning to sag and fail. I will need to rebuild the retaining walls within a couple of years, and I'm leaning towards using interlocking concrete blocks as a do-it-yourself project. Most of the retaining walls are low, about 24", with the highest one about 36" tall.

Here's my question: I would like to have ferns and other small shade-loving plants growing on the wall (i.e., between the blocks) so rather than setting the stones tightly against each other, and with the recommended gravel and landscape fabric behind it for drainage and soil retention, can I backfill with soil only, and leave gaps between the blocks where I can plug in plants? Since the walls are fairly low and the terraces will be heavily planted I think I might be able to get away with this, without worrying too much about the walls failing or soil washing out.

Has anybody tried anything similar? Here's a winter photo of my yard showing most of the retaining walls:

And here's a view of the garden in summer:

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Comments (5)

gardengal48 (PNW Z8/9)
16 years ago
The interlocking blocks are only truly functional if they interlock - if you don't interlock them, they don't have structural integrity. So that negates the ability to leave "gaps" between them. Using natural rock to retain the terraced beds can work, however, and does allow for some spaces or crevices between the rocks where pockets of plants can be sited. But they are more expensive, very heavy if of an adequate size and it looks like access is fairly limited. How 'bout just planting at the wall edge with plants that will drape and cascade a bit? With the lushness of your summer garden, it looks like this will be a sufficiently shady location for ferns, etc.
JohnnieB
Original Author
16 years ago
At least on the ones I've looked at, each block only interlocks with the blocks above and below, rather than the ones next to each other. This means they could be spaced out with small gaps between them within a row without compromising the integrity of the wall, as long as the gaps are not too wide or numerous. I guess my main concern is about putting just soil behind the wall without the gravel and landscape fabric that are standard for this kind of construction--will it cause the wall to deteriorate, crack, crumble, etc.?
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I'll give it a shot. Yes, your wall will eventually rot out. How soon depends on the quality of the installation. If drainage was properly provided to route water away from the wood as much as possible, it will last longer than if it wasn't. No way to tell how it was installed. If the timbers are pressure treated, as they most likely are, insect and moisture damage is slowed. Since the US is a big place, and prices for labor, and sometimes materials, can vary considerably from one place to another, the only way to know the cost of replacing it is to get a local contractor or three with good references and a reasonable amount of experience to come out and give you a quote.
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It's difficult to predict the life span of your wall. A major unknown is how the fence upright supports are integrated into the wall and if they extend into the footer or below. If I understand your first post, the block cavities were originally filled with dirt. I have a hard time believing the contractor removed all that dirt and replaced it with concrete unless I saw him do it. I would probably dig down behind the wall and drill through the block to see what was in the cavities near the footer. With all the unkowns, here is my best guess. There is no steel tie from wall to footer and a major weakness is at this junction. The soil will exert a force against the wall that would over many years cause the wall to tip over. The fence will have an added force that at times will be in the same direction as that of the soil force and at other times the fence force will counter the soil force, but only in the area of the fence supports. The effect of the live load from the fence will cause hairline cracks between wall and footer beginning at the fence support and then elongating. The fluctuating forces will also lead to vertical cracks through the wall somewhere between the fence supports and the mid point between fence spans. When the cracking reaches a critical point the wall will begin to move. If the failure occurs in the spans between fence supports, it will likely be a slow failure. Failure at the fence supports in a wind storm where the wall blocks separate near the supports could be sudden. Expect the cracks. Watch for any shift or movement from vertical or any irregularity in the wall face. Be prepared to make repairs. One to two years for beginning cracks. Two to eight years to first wall movement.
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laag
16 years ago
They do interlock side to side as well because each course is staggered over the previous one. Gardengal is correct as usual.

If you do what you have described, it will be a jumble of blocks that will cetainly move about and become very messy. She also suggested using natural rocks instead. They will look fine.

If you were to build a retaining wall and not use drainage stone behind it, it will weap soil with the water between the cracks. Even if it does not move the wall, the face of the wall will always be dirty. I see these fairly often. They are very noticeable in an unpleasant way.

If budget and planting is your goal, I'd recommend following Gardengal's suggestion of making formations of rocks. The existing planting style will look great with rocks.
Brent_In_NoVA
16 years ago
At one point somebody posted about a concrete block retaining wall system that had built in planting pockets. I tried a few searches but could not come up with any hits.

- Brent
marcinde
16 years ago
I bid a job using Loffelstein blocks about 10 years ago.
http://www.kulucrete.co.za/body_loffelstein.html

Pavestone makes something along the same lines:
http://www.pavestone.com/contractor/wall_alpenstein.html

I'm not a fan of- well, anything about Pavestone, the product or the company- but I guess if you won't see the block behind the plants then it doesn't really matter.

Dave

Retaining wall question

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gardengal48 (PNW Z8/9)

JohnnieB
Original Author

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Retaining wall question

Comments (5)

gardengal48 (PNW Z8/9)

JohnnieBOriginal Author

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laag

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JohnnieB
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