Cheap(er) retaining wall idea

wisconsin_cur · Posted: Fri Dec 14, 2007 6:51 am Post subject: Cheap(er) retaining wall idea

I own 10 acres of hill. The only good place to garden is along some retaining walls that will need to be replaced soon. But looking up the hill directly behind the house I began to wonder:

The scene:
a hillside eroded down to the subsoil (horizon b) that currently grows only the most hardy resileint weeds. The soil is primarily sand and rock. I have added some organic matter in the three years that we have owned the property (used large mounds of rabbit manure to grow squash there) has allowed a few new weed species to inhabit the space.

The proposal:
a) install 6' steel t-post every 16" four feet deep along the contour of the hill.
b) along the line of t-posts dig down ~6" and place gravel.
c) level gravel
d) use the t-posts as anchors, placing concrete cinder blocks through the t-post to build a retaining wall, alternating the seam every other row. Every cinder block should have one of its "eyes" passing through a t-post.

e) as I go along fill the remaining space in the open area of the cinder block with gravel to create a tighter connection between the t-post and the cinder block.

I was thinking I shouldn't go much higher than three cinder blocks (24") and maybe only 64" out from the hill (horizontally from native hillside to edge of the retaining wall).

Are there any professionals (engineers, permaculturists or the like) who can tell me if this would work? Any educated and experienced laypersons with an opinion?

I haven't done a formal analysis of the cost per square foot of garden space but my intuition is that it would be cheaper than the mass produced retaining blocks at the building supply store.

thanks in advance,
cur
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edit: I removed my attempt at illustration since it did not translate well when viewed.
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Heineken · Posted: Fri Dec 14, 2007 9:08 am Post subject: Re: Cheap(er) retaining wall idea

Overall seems like a good idea to me!

Consider using concrete mix instead of gravel to fill the blocks, Cur. A bit more work, but the result would be much stronger and more stable.

The higher the wall, the wider the horizontal gardening space will be, but the more vulnerable the wall will be to sagging or collapse. A max of three courses of blocks sounds about right as a compromise.

Make sure you and the soil are up to driving those metal posts 4 feet deep! I don't know if that's possible; the deepest I've ever driven a metal post with a maul is a bit over 2 feet, since I start running into rocks. I think 2 feet would be plenty deep; you could then adjust your other parameters accordingly. I think metal posts are available in 5-foot lengths; maybe try to drive them 2 1/2 feet deep.

I'd drive the posts as I built the wall along and not in advance! Otherwise, if your measurements wander a bit, your posts and the holes within the blocks might conflict.

How long a wall are you planning?
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MrBill · Expert Joined: Sep 15, 2005 Posts: 5215 Location: Eurasia

oldstyle

I like the idea, but I would have a concern about it's strength during a heavy rainstorm. You describe a soil of mostly rock and a slope on a hill. During heavy rains, I imagine the water flows rather quickly down the hill (that old rock dont soak up too much in my experience) and I would be concerned about the weight and force of moving water hitting against the retaining wall.

Perhaps you could add a few 4" drilled pvc channels that run perpendicular to the retaining wall for a few feet with light gravel above. That would give the water a reasonable way to route around the wall instead of needing to push against it.

So, using that approach, you would drive your T posts every 16" as you said. I agree that deeper is better. Then dig 10" or so channels every 16" between the Tposts running perpendicular. Place 6' of 4" pvc pipes in these channels pointing downslope. The pvc pipes would have holes drilled in it every couple of inches to allow water in. Gravel (or concrete) the base course in around the pvc. Put the cinderblocks in going up to whatever height you need. Finally, backfill the area your retaining and add a course gravel above the pvc to allow the rain to percolate through into the drainage pipe.

I am not a structural engineer, so this is just an opinion.

Madpaddy · Expert Joined: Jun 25, 2004 Posts: 1983

WC,
I'ld love to see a picture so I could comment more definitively but the last comment about water hitting the retaining wall is certainly a message that you need to take on board.

large pic changed to a link by Wisconsin_cur

In this diagram, the bitmen coating forces the water down along the inside of the wall where it is taken away by a drain.
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Heineken

IMO, drains are not necessary, because he is not mortaring the blocks together.
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Madpaddy · Expert Joined: Jun 25, 2004 Posts: 1983

Maybe so but it can't hurt and it might be a useful way of collecting water. The important thing for any retaining wall is the width of the footings which should be at least 3 times the thickness of the wall. From what I gather of WCs proposal he will have no footings whatsoever. Over time his wall will bulge and buckle.
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Heineken

Well, nothing lasts forever, Madpaddy. As POers know all too well!

However, I think those metal posts spaced 16 inches OC will provide tremendous staying power. The wall is going to be a low one, which also favors Cur's design. There won't be enormous lateral pressure against it as would be the case for a taller retaining wall.

I say keep it simple.

I do think he should use concrete and not gravel as fill for the block centers.
_________________
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---Abused, abandoned hunting dog

"Things have entered a stage where the only change that is possible is for things to get worse."
---Me and my brother

wisconsin_cur · Posted: Fri Dec 14, 2007 8:32 pm Post subject: Re: Cheap(er) retaining wall idea

Thanks for all of the great feed back. I do think I will incorporate a lot of it... esp the use of concrete instead of gravel and the setting of the posts as I go a long instead of placing them all at once.

I was thinking of experimenting this year with a shorter wall (~20 foot) and see how it does. Ultimitely I would like to have the whole hillside under this kind of cultivation but first I would need to take down some trees.... and convince the bride.

I will try to get a picture of the hillside up in a few days.
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Heineken · Posted: Fri Dec 14, 2007 9:59 pm Post subject: Re: Cheap(er) retaining wall idea

Good luck with this, Cur. It would be great to see before/after pics!

To fill in behind the wall, I wish I could send you some of the soil I excavated during my dam-and-pond project. I have a huge pile of it and it's as rich as worm castings!
_________________
"Actually, humans died out long ago."
---Abused, abandoned hunting dog

"Things have entered a stage where the only change that is possible is for things to get worse."
---Me and my brother

wisconsin_cur · Posted: Sat Dec 15, 2007 2:30 am Post subject: Re: Cheap(er) retaining wall idea

Concerning some of the other questions:

unfortunately the only tractor I have is my legs, back and arms. That is, in part, the allure of the design that I offered. All of the constitute pieces can be man powered. Not that i am looking forward to walking hundreds of blocks up that hill but I can.

The subsoil that is now, unfortunately, bare to the sky is mostly sand. Water does seem to soak in pretty fast. There is some ground cover, but not much, and I've never had any gullies form from erosion during heavy rain. I think it perculates rather quickly down into the bedrock/water table.

I probably will put in some drain tile as a bit of insurance. I will just have to do some thinking about where the tile drains to.
_________________
"Against stupidity the gods themselves contend in vain."
-Friedrich von Schiller

"A waste is a terrible thing to mind."
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pedalling_faster · Intermediate Crude Joined: Dec 10, 2005 Posts: 751

how about using some incredibly tenacious
fast growing plant as part of the wall ? like
blackberries.

plant them one season, train the vines
over the higher ground, hack them back
as need be.

the plants do most of the work !
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strider3700 · Posted: Sat Dec 15, 2007 1:49 pm Post subject: Re: Cheap(er) retaining wall idea

I have a permanent war against blackberries. Never ever plant them anywhere that you want to do anything with. I strongly recommend against them for retaining walls. They do work good to hold hills inplace if thats all you're trying to do.
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kuidaskassikaeb · Posted: Mon Dec 17, 2007 4:00 pm Post subject: Re: Cheap(er) retaining wall idea

It’s been twenty years since I’ve done this, and I am not a PE . but I can work you through what a PE would do.

First any PE would say build it, a catastrophic failure isn’t. Your heights are more like raised beds than retaining walls. So just go for it.

There are a couple of failures that you normally calculate for.

The first is shearing or sliding of the entire mass as a unit down the hill. The slope was 20.5 degrees from the data provided. The soil mass of per foot, assuming 120lb/ft3, is 641 pounds per foot behind the wall. A typical friction coefficient of soil would be 0.2. The forces resolve to be, using the old incline plane as: 241 pounds per foot down the hill, opposed by 120 pounds per foot of friction up the hill.. Since each T bar is spaced at 1.333 feet this is a force of 160 pounds per T bar. They should have no trouble holding this.

A second failure mode is tipping over due to torques acting to tip the wall. The torque is calculated as you would water pressure torque, density times depth times gravity equals force. The centroid of the force is at 1/3 the height. So the tipping moment (torque) from the soil is 160 foot pounds, or 240 pounds acting at 8 inches above the bottom of the wall. These are per foot also. Each T bar will then have to be able to resist a bending moment of say 215 foot pounds. I think they could do this. The problem is, would the soil hold them in place for this.

Your design is innovative enough that the tipping calculation is hard to make. Typically the walls are built like bookends with material under the soil, which would have to be lifted to tip, or with very wide bases to resist tipping. In your design, the soil the bar is driven into, resists tipping and it is difficult determine if this is a problem, and impossible to calculate without doing soil tests that are probably more expensive than your wall. I would probably drive a bar in 4 feet, grab the top and pull like hell down-slope (try to pull with 100 pounds of force at a height of 2 feet, down hill). If the T bar moves it won’t work.

The last thing that an engineer would worry about is the soil. Like any foundation the wall must be built on a sound footing, the soil must be competent to carry the weight you are adding to it. This is also difficult to know without testing, and typically people would dig down to some extent. The quick answer is that sand and gravel are okay, while clay if it is not drained, is not, so once again you are probably okay.

wisconsin_cur · Posted: Mon Dec 24, 2007 3:39 pm Post subject: Re: Cheap(er) retaining wall idea

As promised a photo of the hill in question:

a view from the side. the area with the weeds poking up through the snow (whatever they are they are the only thing that will grow there without amendment). Is the area that I would want to convert to a retaining wall/raised bed.

Thanks for the ideas. If any more come to mind let me know.
_________________
"Against stupidity the gods themselves contend in vain."
-Friedrich von Schiller

"A waste is a terrible thing to mind."
-Mama Cur