Retainig wall on sloping bedrock

[mcormie]

Hello all,

I would like to build some retaining walls around the perimeter of my backyard. The problem lies in the fact that the property line follows a large rock hill, and can be at steep angles at times.


Questions:

What is the maximum angle for a form before concrete will slough downhill?

Should I epoxy rebar into the bedrock to pin the wall in place?

Should I pressure wash / scrub the rock for a good joint?

What about expansion joints in long (>100 ft) retaining walls?

Thanks in advance for everyones input.
Matt

[Rich]

I don't think your big problem will be the steepness of the slope - but how you brace your formwork. You'll have to place your forms to whatever the rock is and at a thickness suitable for what you are building. Then your formwork will have to be braced like nothing has been braced before to keep them on the slope of the rock without movement during placement :)
Placing rebar into the rock is an excellent idea. Probably not going to help much by pressure washing. I would say maximum length would be 20' for expansion joints. These joints can be as simple as a piece of chamfer strip placed right into the formwork.

[mcormie]

Rich,

Thanks for your input. I was wondering how I might go about keeping the framework in place. I'll try and describe what I was thinking.

I had thought perhaps to have epoxied rebar almost as high as the wall will be, and wire remesh and form ties to the rebar to hold everything in place.

Another problem I see is getting the base of the form to contour the rock closely enough to prevent seepage underneath.

Thanks again,
Matt

[Rich]

Epoxying the rebar into the rock is great - but one you get any height the rebar will be like a wet noodle so it's not going to give you much stability. You're going to have to stabilize the top of the wall also - not sure what the ground close to the wall is. If it's earth and not rock you can easily place a formstake and then put 2 - 2x4's against it.. one angled to the top of the form and one angle to the bottom of the form. Do this for the visible face or the face that you want to see when it's stripped. The ties will hold the other form to whatever the thickness of the ties you have.
The best way to get the formwork to contour is scribe it to the contour :) A thought that just came to mind (disclaimer: I've never done this before and have no idea if it would work :) ) put the form in place and then use expanding foam to fill in the gap at the bottom and then shave off the interior side in line with the formwork.

[mcormie]

Unfortunately there is no earth around the site to stake into, just bedrock. Sounds like I probably don't want to exceed 12-18" wall height with just the central rebar support.

I like the expanding foam idea to prevent seepage under the form.

[Rich]

If you're just going to go 12-18" high I would think the rebar would be able to hold it in place.