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A design for a 1.2m high retaining wall


 
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richard7761

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PostPosted: Sun Mar 27, 2011 8:59 am Reply with quote

I believe that any wall lower than 1.2m does not need to be designed by a structural engineer.

With that in mind I am proceeding to design a retaining wall no higher than 1.2m. Length 7.5m. Piers on either end.

Costs are a key issue, and I'm wanting the face to be brickwork.

I know the depth of the wall needs to be about 300mm.

I have an assumption that it would be cheaper to build a wall of concrete with the brickwork as facing. Rather than the alternative of a wall made completely of brickwork.

QUESTION: Is my assumption correct?

QUESTION: Assuming I'm to make a largely concrete wall, can I build up the wall in stages? That is, lay courses of brickwork, pour the concrete behind, lay another course of brickwork, pour concrete etc.

QUESTION: Is concrete water resistant?

By and by I'll post a picture of my final design so that folks can verify it is adequately designed.

Any suggestions of a cheaper method of construction that retains the brickwork facing will gratefully be received.

Thanks.
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richard7761

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PostPosted: Sun Mar 27, 2011 1:39 pm Reply with quote

I'm just wondering what are the options for the masonry behind the brick face.

* Brick
* Solid block
* Perforated block (fill each block in with - whatever)
* Concrete (poured in the gap behind brick face)

That is all I can think of. Off the top of my head, I'm not sure which option is cheapest.
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htgeng

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PostPosted: Mon Mar 28, 2011 8:26 am Reply with quote

richard7761 wrote:
That is all I can think of. Off the top of my head, I'm not sure which option is cheapest.

http://www.railwaysleeper.com/Richard%20Godwin%27s%20lanscaping%20project%20with%20railway%20sleepers.htm
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richard7761

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PostPosted: Mon Mar 28, 2011 11:41 am Reply with quote

Sleepers are pretty good. But I think the answer to my design lies in what materials I have to hand. I'm knocking down a wall then putting up a wall. I was going to sell the old stones from the original wall, but what I probably should do is use the old stones as masonry behind a new brick wall. All I need to buy then is some new bricks for the facing wall and some concrete and cement. But the bulk of the mass would be the old stones. The issue then is would the wall be adequate. I think probably if metal rods were incorporated into the concrete foundation and I employed gravity as well as cantilever principles.
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r896neo

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PostPosted: Mon Mar 28, 2011 10:57 pm Reply with quote

An overkill but probably quite effective solution for you would be to pour a nicewide foundation with L shaped rebar in it with the long legs sticking up into the air. Then build a skin of blockwork on the far side of this and a skin of brickwork on the front (tied together with wall ties).

Then put a sheet of mesh into the cavity resting against the embedded L's and fill with concrete. Be sure to fill it slowly, and not just shot in out of a truck, and make sure the wall is not still green.

Concrete is relatively good at blocking water when used in this way but a sheet of dpm up the back of the wall would make certain your brickwork stayed tidy. As with any retaining wall drainage is of utmost importance.
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richard7761

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PostPosted: Wed Jun 15, 2011 6:37 pm Reply with quote

This wall will be a retaining wall, about 1.2m high, 7m long. About a brick thick. If I build this wall with an inner wall being built of concrete block, (using ties and a front facing wall of regular red bricks) what kind / size of blocks should I be using? I presume something like B&Q "Standard Aerated Concrete Block 3.6N Grey 7.3kg" @ 1.22 a go. Cheaper than building the inner wall with facing bricks.
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r896neo

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PostPosted: Wed Jun 15, 2011 10:24 pm Reply with quote

definately go with a cheaper and quicker to build block on the inner face a standard or dense 4 inch concrete block will be perfect for the job.
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r896neo

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PostPosted: Wed Jun 15, 2011 10:25 pm Reply with quote

But do yourself a favour and don't buy them from b and q. 1.22 a block?? Yikes
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Nick98

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PostPosted: Thu Jun 16, 2011 11:59 am Reply with quote

I had something similar built last year at home, it was a lot longer (circa 20m) but roughly the same height with 3 pillars.

I will describe the method below but please bare in mind it is not a straightforward as it might seem as the weight of the soil the wall holds back is scary. Anything less that 'substantial' preparation will result in it being pushed over, eventually leading to collapse!

The builder did as follows:

850mm wide footings dug to 600mm deep (it would have been deeper but he was happy he had reached firm subsoil). Filled with readymix to 50mm below finished height. Steel reinforcing mesh then laid onto the wet concrete and then the final 50mm poured.

Once gone off the inner skin was 100mm concrete blocks laid on the flat but sideways (called a header bond) with a stretcher bond behind it (overall a depth of 655mm). Under the first course you need to trap the end of a roll of membrane all the way along. Something like this:

http://www.toolstation.com/shop/Hand+Tools/Landscape+Fabric/Landscape+Fabric+2m+x+50m/d10/sd3225/p39101

The next two or three courses (cant remember as it is all buried!) is also a header and a stretcher course. At the appropriate height you need to add drainage outlets, I used square section guttering downpipes cut to the correct length as they are the same size as the height of a brick used for the outer skin. In my case the builder worked out exactly where they went so he could build them in, but he said that often you leave a reasonable gap in the inner skin so you can move them around to suit the external brick face.

Once up to about 4 courses the inner skin then became just a header course (440mm deep) which continued for another four or five courses (remembering to add wall ties as you go to tie the outer skin to!).

After another 4 or 5 courses the blockwork changed again to a normal stretcher course (effectively you are making a stepped inner skin) and continued until you are about 5 brick courses below ground level. The inner skin then became a 100mm block laid on edge - just one block high.

We then backfilled between the inner skin and the membrane with a 50-75mm thickness of 20mm stone which acts as drainage and keeps the soil away from the wall. The top of the membrane is then 'sealed' by trapping under the first course of bricks that is added to the inner skin (done this was so you don't see the blockwork from behind the wall).

Once gone off you can then start to backfill burying the membrane, stone drainage, and blockwork.

The outer skin is then a case of 'normal' bricklaying until you get to the top. I then finished off with a soldier course to close the gap between inner and outer skins but you could equally use paving slabs, concrete capping or roof tiles depending on what finish you wanted to achieve.

In my case the inner skin consumed nearly 1000 concrete blocks so as I say above please don't think it is a cheap project as that alone was nearly 1k buried underground without the cost of the readymix etc.

I appreciate the above is quite complicated to understand so feel free to look on Google Streetview on postcode BA20 2HE and you will see the wall part built (inner skin only in place) as it just happened to be half done when the streetmapping was carried out in our area!!

The finished result is below:



Hope this helps - if it doesn't make sense please shout!!

Good luck with your project.

Nick


Last edited by Nick98 on Fri Jun 17, 2011 9:15 am, edited 1 time in total
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r896neo

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PostPosted: Thu Jun 16, 2011 7:50 pm Reply with quote

i don't want to nit pick but i would personally disagree with a couple of minor details in your job nick.

Firstly steel mesh in the footings has no effect on the strength of the wall and unnecessary unless the ground is slightly dubious. Steel in retaining walls needs to be tied into the found but with upstands which will be tied into the wall if using a concrete fill between block/ brick.

Also putting a dpm or liquid dpm on the back of the wall is a good idea as it will help reduce the possibility of staining of the brick by saturation. OF course your drainage must be sorted right if chosing this route for obvious dam related reasons.

Other than that i very much agree with the rest of your post and it's good to see obviously experienced people like yourself take the time to write detailed replies!
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Nick98

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PostPosted: Fri Jun 17, 2011 9:09 am Reply with quote

Quote:
Firstly steel mesh in the footings has no effect on the strength of the wall and unnecessary unless the ground is slightly dubious.

Agreed - I think he just used the steel as an extra precaution as he didn't want the footing fracturing due to the weight of the inner skin.
Quote:
Steel in retaining walls needs to be tied into the found but with upstands which will be tied into the wall if using a concrete fill between block/ brick.

Very true - the method I described/used was about as much as the height would cope with. Any higher and the builder would have used a hollow block and steel rod inner skin tied into the footings and then the blocks filled with concrete (cant think what the blocks are called but they look like a figure of eight and interlock)
Quote:
Also putting a dpm or liquid dpm on the back of the wall is a good idea as it will help reduce the possibility of staining of the brick by saturation.

Forgot that bit! We actually painted the outer face of the inner skin in a liquid DPM (black, sticky and impossible to get off your clothes!) in order to stop bleed though to the facing brick - it was a bit belt and braces as the outer face was actually built with a small cavity (circa 10mm) between inner and outer skins as I wanted to cap the top with soldier course which determined the gap but decided to dpm it in case the cavity was bridged. The outer skin also has a damp course to stop damp being drawn up into the outer skin from the ground.
Quote:
Other than that i very much agree with the rest of your post and it's good to see obviously experienced people like yourself take the time to write detailed replies!

Thank you! I wouldn't say I am experienced (well I'm not a builder) but laboured for the guy at weekends as he build it for me. I just wanted to point the original poster in the right direction as I was a bit concerned with how he was thinking of doing it.
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richard7761

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PostPosted: Fri Jun 17, 2011 2:21 pm Reply with quote

One of the things that you come across, is that some people can say if you go by best practice you will spend a fortune - unnecessarily.

I had a 1.1m stone wall in the same position that I'm putting this new brick wall I want to build. I never put in any foundations, just hammered in small garden stones, then put the first course on that. Never had any problem for twenty years. No cracking or anything, no leaning over.

Now best practice with this wall is to dig foundations 500mmm deep, make 600mm wide and fill to a depth of 275mm with concrete. Some say that is overkill, and most folks don't to that. What say? :c)
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r896neo

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PostPosted: Fri Jun 17, 2011 3:59 pm Reply with quote

In the end the difference comes down to certainty. Professionals stake their reputation and potentially their money on their work. Under engineering a project and taking a chance by not using best practises is just not an option as i will have to go back and redo the project or my reputation will suffer.

Also i would never recommend someone who may know nothing about building work to use anything other than best practises as experience can often tell you what corners may be cut but these are usually fluid situations that crop up on site, such as, 'the ground is very good we will only dig to 300mm rather than 600' But you would never recommend someone to put that found at 300mm in the first place.

Obviously though this is a diy forum and If you are a diyer on a budget then of course you can say so and get opinions on whether it is worth the risk to do it a little simpler and cheaper and i am glad for you that in your case it worked! But if it didn't then as long as you were made aware of the risks you would only have yourself to blame.
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