Author Topic: Retaining Wall Help  (Read 1707 times)

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Online DannyG

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Retaining Wall Help
« on: January 18, 2020, 06:21:12 AM »
I am about to build two retaining walls. One of them is using 200x75mm Sleepers and the other is your usual Retaining Wall blocks.

What I am getting mixed information about is the method used for drainage behind the wall.

I was going to fill behind the wall with drainage rock and use a fabric between the rock and the back filled dirt. However some people say dont use the fabric as it will clog up over time and end stopping water from going through it. Some people say definitely use it to stop the silt from the dirt filling up the back filled drainage rocks and running out the front of the wall. Other people say make sure you use a ag pipe at the bottom of the rocks to allow the water to run out. Others say dont bother as the water will run out the front of the wall way before it makes its way down to the ag drain.

Does anyone on here do this sort of thing for a living and can give me a definite answer??

Both walls have very dense clay behind them. The clay is battered down and holding itself in place no problems so the wall is more decorative than anything else but there is a fair amount of silty material gathered at the bottom of the batter from the rain so that will end up behind the wall.
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Offline gronk

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Re: Retaining Wall Help
« Reply #1 on: January 18, 2020, 07:35:11 AM »
Any pics ?

In a perfect world, behind the wall (timber or blocks ) you would have an open drain of cement, leading to an area of rock to filter/slow the water down and then to where ever it goes.
I would have the fabric against the wall.....that way if the water was to try and force itself thru the wall instead of flowing down the drain, it would at least be fairly clean.
Say the wall was 4 blocks high, how hard would it to have a cement ( or 1/4 minus/cement mix ) battered up to approx 1 block high, creating a "hard" drain, which is still filled with rocks ?
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Offline HEM19X

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Re: Retaining Wall Help
« Reply #2 on: January 18, 2020, 08:27:51 AM »
As Gronk says... Fabric against the wall plus Ag line at the bottom to drain away excess water - assuming you have a low point to drain the water to.
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Offline tedota

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Re: Retaining Wall Help
« Reply #3 on: January 18, 2020, 09:05:52 AM »
If you are doing footings for the blocks maybe reinforce with some rio or mesh. Clay will shrink and expand with the seasons.

Offline tryagain

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Re: Retaining Wall Help
« Reply #4 on: January 18, 2020, 09:36:01 AM »
Fabric behind the gravel or don't worry about it, the idea of it is to stop the drainage material getting clogged up, the small amount of flow the geotextile fabric is likely to have through it means it's not really going to clog up in the lifetime of the wall.

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Offline Spada

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Re: Retaining Wall Help
« Reply #5 on: January 18, 2020, 12:58:40 PM »
Fabric behind the gravel or don't worry about it, the idea of it is to stop the drainage material getting clogged up, the small amount of flow the geotextile fabric is likely to have through it means it's not really going to clog up in the lifetime of the wall.



I'll second Tryagains thoughts. His attached picture is the prefered method. To add to it, the Ag pipe drain should have an outlet at least 1 in 20m. This lets the moisture out and the air in to allow the gravel to drain and dry out. For mine, I just tee'd off the ag line and punched a 90mm hole through the wall in strategic locations. The mesh strainer keeps any crawlies out.
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Offline archer63

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Re: Retaining Wall Help
« Reply #6 on: January 18, 2020, 01:36:34 PM »
I concur with tryagain.

I recently had to rebuild our pool fence and wood sleeper retaining wall...23meters x 800mm ish high.
 I replaced the entire retaining section with the larger hollow retaining wall bricks. Once my base was level, I put in a layer of river sand, then laid one long layer of crushed rock inside and behind bricks about 150mm wide. Then placed a length of black drainage pipe behind the first layer. Then next layer of bricks went on and followed the same procedure up until the last layer where I backfilled with topsoil.

I believe that there is a requirement that you must cement in the first layer if the wall is going to be above 900 ?

I got all the info I needed off YouTube and the brick makers website.

Edit: I forgot to mention that I also wrapped the drainage pipe in drainage cloth as well.

« Last Edit: January 19, 2020, 07:34:02 PM by archer63 »

Offline rags

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Re: Retaining Wall Help
« Reply #7 on: January 18, 2020, 02:10:36 PM »
My take, ditch the timber sleepers for at least concrete sleepers and galvanised post or blocks all round.
I agree with tryagains diagram and the geo fabric between drainage  material and the back fill.
Finally I would consider replacing the 90 mm agg drain with a product like in the link below but if you use the agg drain get one with a sock over it. Finally the idea of “ relief pipes through the wall like suggested by spada is a sound one however if you don’t want to have water running out the front of wall then you may reconsider the idea.
https://atlantiscorporation.com.au/flo-log-trench-drainage/

In fact you can get away without using gravel in some circumstances when using the Atlantis product but I would still use it.
« Last Edit: January 18, 2020, 02:12:58 PM by rags »

Offline Troopy_03

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Re: Retaining Wall Help
« Reply #8 on: January 19, 2020, 08:20:15 AM »
If it's over 1M high, or within 1.5M of a boundary, the engineer will sort that out for you. ;-)
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Offline brickiematt

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Re: Retaining Wall Help
« Reply #9 on: January 19, 2020, 08:27:20 AM »
Yep, the image tryagain posted is how it's done. I also use 20mm conduit out the front of the wall as per Spada's pic, but that is not absolutely necessary. Depends on the aesthetics you want.
If you're laying the blocks with mortar, put some waterproofer in the mortar. Also a roll on waterproof compound on the back of the wall with a layer of corflute between the wall and your drainage aggregate.
This is how I build them, often up to 2 or 3m high. Do not underestimate how much pressure builds up behind the wall, especially if the water can't drain away, so pay particular attention to your drainage and waterproofing ;D
 :cheers:

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Offline xcvator

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Re: Retaining Wall Help
« Reply #10 on: January 19, 2020, 08:30:03 AM »
Yep, the image tryagain posted is how it's done. I also use 20mm conduit out the front of the wall as per Spada's pic, but that is not absolutely necessary. Depends on the aesthetics you want.
If you're laying the blocks with mortar, put some waterproofer in the mortar. Also a roll on waterproof compound on the back of the wall with a layer of corflute between the wall and your drainage aggregate.
This is how I build them, often up to 2 or 3m high. Do not underestimate how much pressure builds up behind the wall, especially if the water can't drain away, so pay particular attention to your drainage and waterproofing ;D
 :cheers:


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Re: Retaining Wall Help
« Reply #11 on: January 19, 2020, 09:01:20 AM »
Thanks very much for the advice.

Just to clarify a couple of things.

The filter fabric won’t get clogged up over time with silt off the clay?

Do you ever see water running out of those ag pipes and/or outlets you have coming out the front of your wall?

If the wall isn’t sealed, meaning there are gaps in the sleepers or the blocks are not mortared, won’t the water come out the gaps way before it gets down to the ag drain?

By the way the sleeper wall is up behind my garage where no one ever sees it, so I wanted a cheap and easy method to stop the clay silt running down the batter into my pit drain. It is only around 12mtrs long and I’ll do it 5 sleepers high, so 1 mtr high.

The larger block wall is approx 25mtrs long and I’ll also do it around 800-1000mm high.

I’ll try and get some photos later.

Thanks again.
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Online DannyG

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Re: Retaining Wall Help
« Reply #12 on: January 19, 2020, 09:09:53 AM »
One more question, is this stuff ok? Won’t clog up with clay silt?

https://www.bunnings.com.au/everhard-1200m-x-50m-roll-drain-matting_p4770240
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Offline gronk

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Re: Retaining Wall Help
« Reply #13 on: January 19, 2020, 09:33:24 AM »
One more question, is this stuff ok? Won’t clog up with clay silt?

https://www.bunnings.com.au/everhard-1200m-x-50m-roll-drain-matting_p4770240

All the matting stuff will clog up with silt......how long it takes is anyones guess.
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Offline gronk

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Re: Retaining Wall Help
« Reply #14 on: January 19, 2020, 09:35:46 AM »

This is how I build them, often up to 2 or 3m high.
 :cheers:

Do you have to get an engineers certificate for every job you do over 1m ?
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Offline rockrat

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Re: Retaining Wall Help
« Reply #15 on: January 19, 2020, 09:54:28 AM »
Do you have to get an engineers certificate for every job you do over 1m ?
Have been looking at similar issues in Brisbane my understanding is that a wall over 1m requires building approval which means both engineering sign off and building certification.

Even if below 1m and there is a surcharge load, eg building wall, concrete slab, etc, acting on the wall, you also need approval. You may also need approval if you are putting a wall on a boundary line.


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Offline #jonesy

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Re: Retaining Wall Help
« Reply #16 on: January 19, 2020, 10:34:52 AM »
Do use gal steel uprights for the sleeper wall.  That way in the future if you need to replace the wall there's one less job. Also give you a straight wall without the uprights in the way.
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Offline xcvator

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Re: Retaining Wall Help
« Reply #17 on: January 19, 2020, 11:24:01 AM »
Do you have to get an engineers certificate for every job you do over 1m ?

The owners always had to get the documentation, and the building inspectors/engineers did their inspections at the required stages. Biggest 1 I did was 3.5 metres high, that was fun back filling.
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Offline Spada

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Re: Retaining Wall Help
« Reply #18 on: January 19, 2020, 12:31:18 PM »
One more question, is this stuff ok? Won’t clog up with clay silt?

https://www.bunnings.com.au/everhard-1200m-x-50m-roll-drain-matting_p4770240


Thats the one I used when I built this - http://www.myswag.org/index.php?topic=41000.msg1003604#msg1003604

The blocks are 200mm deep, and I did the footing trench at 400mm with a fairly dry concrete mix for the base layer. Have at least 1/2 of the bottom course below ground at the base. I part core filled the base layer with cement as I went, and most of the rest of the blocks with 10mm ag stone, the corners and stair sections were core filled with cement.

Just a tip for the sleeper wall, timber rots over time. I originally used treated pine sleeper for a low wall behind the shed as it was cheaper. 15 years later they'd rotted out and I had to completely replace the wall, so there is merit in using gal posts as Jonesy says. For the second attempt behind the shed, I went with Adbri Versalock blocks (https://bricksblockspaversonline.com.au/collections/retaining-walls/products/adbri-masonry-versaloc-standard-block-200mm-series-p713) at about $10 a metre per course (plus corefill) they weren't much dearer than sleepers.

Here's some more picks if it helps -
« Last Edit: January 19, 2020, 12:39:41 PM by Spada »
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Offline Spada

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Re: Retaining Wall Help
« Reply #19 on: January 19, 2020, 12:37:43 PM »
and a couple of "in progress" pictures of the house wall. The cloth is against the dirt face with the gap between the block and the cloth filled with 10mm ag stone. I used the same stone to core fill the blocks.
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Offline tryagain

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Re: Retaining Wall Help
« Reply #20 on: January 19, 2020, 02:15:43 PM »
No need to waterproof, in regards to brickiematt's waterproofing comments, they  are applicable to brick/block walls, but not necessary for sleeper/non moartered block walls.

I am not going to claim that it's impossible, but I have never heard of geofabric actually clogging up, (I almost accidentally used some bentonite impregnated stuff behind a wall when I was working in Fiji which wouldn't have been good though) It can only really get "clogged" with the same soil that is already compacted in the profile so I can't see how it can really get worse than the soil the water is coming through to get there in the first place.

In reality, what you are doing is pretty small scale, you can't go to far wrong.

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Offline ronmac

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Re: Retaining Wall Help
« Reply #21 on: January 19, 2020, 02:35:57 PM »
I'm with tryagain on this one, In 30 years of forming up and pouring 2 to 3 meter retaining walls and back filling if your'e worried about this back fill with 300mm of 6mm gravel or recycled concrete. Do not compact it down just water it and every 300mm of soil over the top of that water again. Let it compact naturally. cheers Ron.
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Re: Retaining Wall Help
« Reply #22 on: January 19, 2020, 03:20:54 PM »
One more question, is this stuff ok? Won’t clog up with clay silt?

https://www.bunnings.com.au/everhard-1200m-x-50m-roll-drain-matting_p4770240
This would be better. It Newton 408 (Google it), it comprises a drainage cell as well as polyester fabric. The dimples will promote drainage. You can get it from Bayset, though don't think you'd have one in Tassie, but I know they have Melb branches. It's usually 20m x 2.0m high

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Offline brickiematt

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Re: Retaining Wall Help
« Reply #23 on: January 19, 2020, 04:15:44 PM »
Do you have to get an engineers certificate for every job you do over 1m ?

Builder or owner does.

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Offline tryagain

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Re: Retaining Wall Help
« Reply #24 on: January 19, 2020, 04:27:10 PM »
This would be better. It Newton 408 (Google it), it comprises a drainage cell as well as polyester fabric. The dimples will promote drainage. You can get it from Bayset, though don't think you'd have one in Tassie, but I know they have Melb branches. It's usually 20m x 2.0m high

That good for using against a solid wall, but would be as disastrous as my bentonite impregnated geofab near disaster if used as is in the diagram shown above, instead of ensuring free drainage, it would ensure little to none.
« Last Edit: January 19, 2020, 04:34:35 PM by tryagain »