double retaining wall bed 50'x5'

KraBOctober 12, 2012

I am working on a design and there are two retaining walls 50 feet long, 5 feet apart and each about 5 feet high. There is a bed in between the two walls. The wall is made of sandstone, it has fallen down before and been rebuilt. I hope the second rebuild was solid.

I am trying to figure out what plants would be good here. I was wondering if this spot would be naturally moist or naturally dry? At first I was thinking dry because the walls would make the water runoff quicker but maybe my logic is flawed. Now I am leaning toward wet because the water would run into the first retaining wall then go under it causing the bed in between the walls to be wet.

It faces east is in full sun and has sandy loam.

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deviant-deziner(Oh zone)

Got Engineering ?

    Bookmark   October 12, 2012 at 12:01PM
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Select plants for a dry environment.

With only a 5ft distance between 5ft walls, the forces acting on the walls will be similar to those for a single 10ft wall. Few walls are built to withstand the force of saturated soil. It's much cheaper to build a wall plus drainage than to build one capable of retaining wet soil.

If it turns out the soil is wet and you plant "wet" plants, it wont matter because the wall will fall down. And it could be that a wall that would have stood, if you design wet and the plants are heavily watered, will then fail.

    Bookmark   October 13, 2012 at 11:53AM
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Could you add a picture. Some details are not clear. Thanks.

    Bookmark   October 13, 2012 at 5:53PM
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bahia(SF Bay Area)

There are more variables here than information to make any informed/useful advice. First off, I'd hope the walls were designed and rebuilt with proper engineering to both accomodate drainage as well as ground forces which are inclined to push them over. Assuming that those issues were addressed properly, the aspect of the slope, the soil type existing on site, the typical depth to water table would all be factors influencing soil moisture content. The type of surrounding native plants, whether they are moisture lovers or more drought resistant within your region would also tend to indicate what the existing soil moisture conditions are/were. You don't mention whether the backfill between walls is existing soil or imported gravel and compacted fill soils, etc.

I'm also curious whether you've been hired as a landscaper to design/plant this, or are the owner of the site. The lack of background leads me to assume you may not have all the information yourself necessary to approach this project. I don't know how liability issues sort themselves out in your part of the country, but I wouldn't enter into this sort of project without knowing the walls were properly designed for strength and drainage and won't fail. Here in California, if those walls fail, you could also be dragged into a lawsuit just for having worked on the plantings. Covering yourself for liability to replace dead plants could be the least of your problems if you don't know more of the background on these walls before you spec and install new plantings.

    Bookmark   October 14, 2012 at 12:21AM
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Thank you for the responses. I added a pic from google map, not the best but it gives you an idea. There are now no plants in the bed.

I asked and the wall was rebuild by a professional. So if it doesn't hold up It will be on them. Plus the client is family, so I won't get into any legal trouble. The backfill appears to be soil from onsite, which is sandy loam. There are drains coming out of the walls. I counted 2, one per wall. Could be more, I guess that is something important I should have paid attention too.

I am being hired to design and install this. Like I said the client is family, so liability isn't a problem. But this is definitely a learning experience. I had never thought of all that stuff before.

    Bookmark   October 15, 2012 at 9:00PM
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From your original written description, I first envisioned a situation completely different from what you have. This is better. It doesn't sound like the longevity of the wall is your concern. From the distance we're seeing it in the photo--it appears normal.

First, where (in the country) is this wall located? I think your original question about wet vs. dry will end up being moot. I don't think you'll find any substantial difference in the planting soil moisture content whether it's above, between or below the wall. After a rain there might be some differences, but they won't last unless there is something freaky going on. Water tends to collect at the back of a wall, but if there are drain holes (a must) it tends to leak away, too. I wouldn't assume there is something unusual unless evidence shows its face. If you have the opportunity to turn over a few spade-fulls of earth while you're on the site, you could make a cursory comparison to see the level of consistency between one area and the next. The existing planting seems to be growing more or less uniformly along the bed's length.

Have you had an opportunity to establish design goals? Or have the clients expressed specific desires? If you'll want help along those lines, you might add more views of the area.

    Bookmark   October 16, 2012 at 2:35AM
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bahia(SF Bay Area)

It's not really clear from the photo which house is the project in question, as the glimpse of the house on the left appears to have the only newer wall that is approximately 5 feet tall, while there isn't any sign of there being two 5 foot tall walls. The house on the right seems to have existing lower walls that have been there some time.

I also had been imagining something else entirely, and if the house on the left is the project, it appears that the lawn above has been leveled off and filled in at the back of the existing wall in the photo. The materials also would appear to be one of those engineered interlocking blocks with a sandstone color, rather than real sandstone? If so, the wall is probably more stable if it was installed per directions with backfill against the wall for drainage and drain lines. If not, and you should ask about this from your client/relatives, there could well be issues.

It looks like you might be located somewhere in the midwest, and I'd want to know if the lawn gets irrigation and if so likely to drain off more regularly into the planter areas, or if drainage is primarily an effect of heavy rains. If you've got the time to investigate soil moisture immediately after a heavy rain, and then check again after a week or so of no rain, I think you'd have a better idea of your planting conditions. This presumes that your planter isn't going to be irrigated.

    Bookmark   October 16, 2012 at 1:15PM
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I am in Ohio. When I did my initial investigation I only dug down a few inches. Turns out there is more clay backfill than I had thought. Lesson learned.

It is the house on the right. Sorry the pic is a bit confusing. When I made my first post I was thinking the walls were 5' high, they are only about 3' high. Should have made a note, lesson learned lol.

There is no irrigation. And I checked, there are only 2 drains coming out of the walls. One per wall. In Ohio we use crushed limestone for 'rock fill', not sure if that is the right word, like behind walls and under walkways. So ph near the wall could be wacky. Could this be a problem?

They want low maintenance and that is about it so I have free reign over the design aspect. I was thinking grasses, autumn joy sedum, russian sage, lavander and candytuft.

    Bookmark   October 16, 2012 at 8:25PM
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