Retaining wall on rocky ground


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Old 05-21-14, 06:57 AM
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Retaining wall on hard rock soil

Hi!

I bought a Duplex last summer, my very first house, and I need advice for a retaining wall.

Here is my problem, there is a shed behind the house that is about 3 feet over the ground on one side. See picture: https://www.dropbox.com/s/xy5bbncv7y...2017.29.03.jpg

The soil is really rocky, only 3 to 6 inch of soil before rock. There is a big rock under the shed and the "hill" in front of it is also a big rock covered by grass.

Like you see, it is not really convenient right now, the stairs stop about 2 feet over the ground! I would like to fix that.

One option is to build a retaining wall to fill the hollow in front/under the steps, but I can't dig more than 3 inches and it's a bit sloppy so how can I make a solid footing in those conditions?

The other option would be to build a deck in front of the shed, but I would like a retaining wall

[EDIT] I live in Quebec so we have winter/freeze

PS: Sorry for my english, I speak french

Thanks!
 
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Old 05-21-14, 10:50 AM
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First I think you need to adjust your thinking about digging and what is hard. Sloppy and stone aren't normally things that go together so I'm assuming you don't have bedrock 6" down but do hit rocks in the soil. Digging will be difficult. If you want a retaining wall you will have to do some serious digging. I'm sure it can be done but it will probably be digging that is more difficult than you are accustomed to. Everything about a retaining wall is hard and very heavy. You have to get your mind in the right place for moving tons of earth, stone and retaining wall material. It is not rocket science but it is repetitive, brute force hard work.

Here is a link (1.6 meg .pdf) for a retaining wall system I use often. It will give you an idea of how deep and wide you'll have to dig for a retaining wall. Be warned... the blocks are heavy. Much heavier than "happy homeowner" decorative blocks you see at home centers. The Compac blocks I like to use are 95 pounds (43 kilo) each.
 
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Old 05-21-14, 11:34 AM
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Hi,

Thanks for the answer, I'm 99% certain that I have a bed rock 6" down. I've tried to dig at many places and I always hit rocks, and I tried to jump on the shovel (I'm about 210 pounds). I know that they Dynamited to build the basement of the house.

If I really have a bed rock 6" down, what's my options? Maybe I should make a concrete base with some reinforcing rods anchored in the rock?
 

Last edited by JPelletier; 05-21-14 at 12:55 PM.
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Old 05-21-14, 04:20 PM
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Typically, shovels are not the correct tools for moving and dislodging rocks. But if your bedrock is really that close to the surface, it's a moot point. I suggest you do one of two things (to keep from killing yourself, fighting bedrock):

1. Dig down to the bedrock over the entire area where you want the steps. Clean it meticulously, preferable with a power washer. Build some forms, anchoring them to the bedrock using a nailgun. Make sure the new steps' riser heights are the same as the existing, meaning the lower of the two will be taller to compensate for the topsoil depth. Then pour concrete steps right on top of the exposed bedrock. Anchoring the steps with dowels probably won't be necessary, if the bedrock surface is rough and irregular enough.

2. An easier approach would be to construct the steps on a bed of shallow gravel, placed where you've scraped off 3" of lawn and topsoil. Leave a 1" gap between new and existing steps, to compensate for differential movement that may take place when the new steps frost-heave but the existing ones don't (if they are anchored to the bedrock).

And when you're done with step construction, install a hand railing to prevent someone from falling over the edge. Ouch hurts.
 
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Old 05-21-14, 05:27 PM
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I've digged a square and i confirm the bed rock. Will have to confirm by digging the entire area of the wall
https://www.dropbox.com/s/5y17ihf7wz...2018.40.00.jpg
 
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Old 05-23-14, 07:06 AM
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BridgeMan45, you are talking about "steps" and existing "steps" but I don't understand, there is nothing actuallly and it's a wall not steps... maybe it's my level of English here which affected my understanding.

From what I see from my digging, I have about 3 1/2 of lawn / topsoil over the bed rock. Is it good engouh to put just enough 0-3/4" gravel over the bedrock (about 1" to 2") to level the ground and start my wall over that?
 
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Old 05-23-14, 11:11 AM
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In the first picture you posted, those long, coral-colored things are the steps. The white thing is the shed. I thought you wanted to build (3) new steps at the bottom of the existing 2, such that you didn't have such a large drop-off to the ground. Sorry for my mistake.

But I don't understand where or how building a retaining wall will alleviate the problem of having such a large drop-off. Unless you build it perpendicular to the near end of the steps, and then bring in a few tons of backfill material to slope a more gradual grade out into yard from the shed. And forget my earlier gravel suggestion--that only applies to a free-standing step unit. You want any retaining wall you build to be anchored to a footing, or in your case, the bedrock, to prevent it from shifting or even tipping over (assuming the plan is a reinforced concrete retaining wall). Use rebar, doweled in every 12" or so, using either epoxy or a neat, Portland cement slurry to do the anchoring. A dry-stack wall might be easier to build, but much more expensive than CIP concrete, and it still would need a reasonably level base before the first course of manufactured rock gets placed.
 
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Old 05-23-14, 11:50 AM
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Exactly, in fact I will remove the coral-colored steps, than build a wall perpendicular to the front of the shed and probably aligned with the side of the shed, than turn 90 degrees and continue under the shed doors. I will bring a few tons (6-7 tons) of material, probably 0-3/4" to fill it. After that, I may put back the steps or build a wood ramp.

I want to use blocks like this for my wall:
Walls | Split Face Universal* Slope Block
 
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Old 05-23-14, 02:53 PM
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I did a cursory review of that product's installation instructions. The individual units appear to be too light in weight to be very stable when large lateral loads are applied against the back face of the wall, and the plastic clips used to provide vertical offsets also look somewhat flimsy. But I don't have much experience building dry-stack walls, so my opinion shouldn't be considered gospel. Some of the other posters with wall-building experience will hopefully be along shortly.

You might want to touch base with the manufacturer's technical people for specific advice for your situation. Such walls function best when built on a compacted but flexible base, something your bedrock won't allow. It's important that the first course be perfectly level, and there may be a minimum thickness of gravel recommended between the first course and bedrock.
 
 

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