Retaining wall advice needed
 

The slope of the ravine behind my house is eroding and I need to stop it. The current plan is to build a 3 foot high by 30 foot long retaining wall near the bottom, fill it with soil, and plant some kind of soil binding plant on it (crown vetch maybe?).
The problem is the wall. I just had a contractor come by who wants to drive a number 3" x 8' foot steel pipes 5 feet into the ground and put a 3/8 steel wall up against them. That would last a long time, but I'm not real keen on the idea. The terrain and accessibility make a poured concrete wall or anything that requires a foundation out of the question.
Treated wood that will last any length of time is illegal to use so near the creek.
Anybody have any other ideas?

http://forums.pelicanparts.com/uploa...1584025833.jpg

Lookng down the slope. It's a about a 45 to 50 degree pitch.

http://forums.pelicanparts.com/uploa...1584025833.jpg

This is a cross section of the slope. Black is the soil level, red is the wall, green is fill, blue is the creek. The foundation of the slope is limestone of varying strengths. Some is kind of soft and punky and some is hard as ... well, a rock.

How about using ground netting that has grass seed embedded in it?

^^^^ Very good suggestion. Grass would tend to stabilize the soil while providing a cover for water to run over, reducing erosion. Another possibility would be to build a barrier at (near) the top of the slope to collect runoff and channel it down through a drain pipe. Maybe both.

could gabion baskets be utilised?

see the gabion option on this list / link

http://www.trailstobuild.com/Articles/BC%20Trail%20Standards/appendix.htm

I'd go with some sort of erosion preventative ground cover as FB mentions. I'd guess someone at the soil and water conservation service could help you with a plan to prevent erosion.

Interlocking retaining wall? Doesn't require a concrete footing and can be built quite high:

Walls | Interlock Concrete Products

That is step 2. The first step it to get some support for the hill above the eroded area.

I had that done on my slope to create a terrace but hoo wee it was a lot of work. Those suckers are heavy and getting them down that slope is tough.

My brother faced a similar problem only the hill was coming down on his house.
He went with the locking block wall. The concrete had nothing to do with the wall construction. It was put in for parking space.

http://forums.pelicanparts.com/uploa...1584028767.JPG

Yes, it's a lot of work. He has a really good crew helping get his place ready. The wall will get two more courses of block on top.

WD - in your picture, how close to the big tree image left does the creek get when spring runoff is high?

It's kind of hard to determine from your picture and diagram just what kind of erosion is happening, but it seems to be surface erosion caused by water/runoff. Stabilizing of the slope at the base will not address that issue unless you change the angle of the slope by back fill, as you originally proposed.

Looking at the diagram, I assume the upper portion (above the green) is stable with no surface erosion but the lower portion has a much greater slope angle and that's where you are losing surface soil. If that's correct, then changing the slope angle by retaining and filling will work, along with planting erosion resistant vegetation and directing runoff around or under the bottom slope.

If you are getting bedrock slippage with fissures and deep cracks appearing due to soil movement (as opposed to surface erosion), that's a more serious issue and would require solid retaining at the foot of the slope. Interlocking concrete blocks are widely used to support hillsides of various heights and would work well in your situation. However, in some situations a concrete footing is necessary before the blocks are installed so that may complicate that choice if you decide to go that route.

This reminds me of all the cliffs along Hwy 1 that keep falling. A math prof I had a long time ago said all the hills along the coast are trying to get to a 35* slope and anything 'we' do to prevent that is just pissing against the wind.
Driving up Hwy 1 or 101 and looking at the hillsides, he was right as most appear to be at a 35* slope where they are 'natural' and not man made.

can they pump concrete in for a wall?

requirements vary by locality (e.g. rainfall amounts & freezing depth)

here, you have to back fill everything with crushed rock for drainage (wall above is illegal here) - that made me give up on putting one in my backyard (as every thing has to be hand carried down a good sized hill)

A retaining wall concentrates most of the load in one spot vs spreading it over the slope so keep that in mind.

The biggest mistake people make when they put in retaining walls is to not add drainage behind the wall.
Water is extremely heavy so you don't want to create a dam.

Drains at the bottom of the wall, and a layer of stone behind the wall from base to near the top, otherwise they dont last as long as they should.
A fabric barrier behind the stone to keep the dirt from migrating into it.

so, do you need drainage in all parts of the US?

I thought it was just for us rain forest dwellers...

Where is most of the water from, is the water from your back yard causing the problem? If so create a drain and run down the slope with pipe. Lots of shrubs?

I don't wish to create a new problem, but Cape Ivy stops surface erosion but can spread a little more than you wanted. I like the stuff myself.

And it's cheap.

Another idea is to plant a whole lot of Willow saplings on the slope.

That law may be outdated. idk

Treated lumber is no longer cyanide-based.

It's mostly copper-based these days by law:
http://www.usfenceguide.com/wood-fence/
"Most pressure treated wood is treated with a chemical to preserve the wood and protect it from rot and insects. The most common chemicals used to treat pressure treated wood include: Alkaline Copper Quaternary (ACQ), Copper Azole (CA), and, the most recent, Micronized Copper Quaternary (MCQ)."

Wood lumber is crap these days....

Steel posts will probably last ten times longer than wood posts especially in a wet environment.
Steel cross beams will also add to that shear strength from forces uphill.
Fill hollow posts with concrete and rebar.
The post foundations sitting in a drainage base is also needed to avoid further sinking or shifting.

Offhand the contractor's suggestion seems like a good idea to me actually.

(but 5' down is not nearly enough! and that loose hill soil will eventually shift)

The wall needs weep holes near the surface.
And it needs to be angled to allow the OVERWHELMING force of subterranean uphill ground water/dirt to divert and flow downhill.

Engineer for strength, but provide for overwhelming factors.

That wall looks too "vertical". Usually the blocks are stacked sloping TOWARD the grade for strength. Also, every foot of vertical stacking, there is a horizontal mesh that is attached securely to the blocks.

Second type is a wall that uses vertical piers or caissons. The depth of the piers depends on the height of the wall, and potential for wind loading. We built a retaining wall with caissons spaced two feet apart into a cmu block wall and on top of a cantilevered footing that went around an electrical transformer. It was a clever engineering solution at the time that solved several problems at once.

Uh, it does slope towards the hill. Just not that noticeable in that pic because of the angle of the shot.

http://forums.pelicanparts.com/uploa...1584049581.jpg