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General => General Discussion => Topic started by: DannyG on January 18, 2020, 06:21:12 AM
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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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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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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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If you are doing footings for the blocks maybe reinforce with some rio or mesh. Clay will shrink and expand with the seasons.
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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.
(https://www.rockwoodlandscape.com/wp-content/uploads/2019/08/retaining-walls-2.jpg)
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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.
(https://www.rockwoodlandscape.com/wp-content/uploads/2019/08/retaining-walls-2.jpg)
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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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.
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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/ (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.
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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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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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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:
What he said :cup: :cup:
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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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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 (https://www.bunnings.com.au/everhard-1200m-x-50m-roll-drain-matting_p4770240)
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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 (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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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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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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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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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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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 (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 (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 (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 -
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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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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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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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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 (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(https://uploads.tapatalk-cdn.com/20200119/5b33ca6e2247f7daed69cd18277ae2a7.jpg)
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Do you have to get an engineers certificate for every job you do over 1m ?
Builder or owner does.
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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(https://uploads.tapatalk-cdn.com/20200119/5b33ca6e2247f7daed69cd18277ae2a7.jpg)
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.
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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?
No, no and no!!!
Any silt small enough will just run through the fabric, into.your aggregate backfill and eventually out the aggy drain.
If you have enough groundwater or rainfall to see water pouring out of the aggy drain or outlets, you're in trouble!!! You will barely notice any, if at all.
Again, maybe a very small amount, but if done right, then no. That's why you have at least 300mm of aggregate between the back of the wall and the soil. Water will fall down to the aggy drain before it makes it way across to the wall.
:cheers:
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I wish I had of taken photos of the fountain of water that would come pouring out of rock retaining wall out the back of a house that we rented in Coolum a few years ago.
There was 3 spots along a 10m long wall that water would shoot out of the wall like a tap. ::)
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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.
Trouble with that diagram is it doesn't comply with the Aus Standards for retaining walls. But in reality if the op is just doing sleepers and/or blocks/rocks then it's probably likely to get seepage anyway. The only real way to stop that would be a besser block solid filled wall that has 3 coats of waterproofing membrane & the 408 drainage cell and the use varying grades of backfill, didn't think he would be going to that much trouble.
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Thanks very much for the advise.
I am going to build both walls the traditional way that is being described here in regards to drainage behind them.
As for the treated pine sleeper wall, I did consider the gal posts that the sleepers slide into as I feel they are the best option. However I already have some sleepers that are twisted and bowed slightly so I want to use them by cutting them to length and using them as posts. And even though no one really see's this particular wall, we do prefer that look.
I know the posts will eventually rot away but I have a feeling we will be gone before that happens ;)
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Just don't forget the golden rule of thumb >:D The posts/uprights must into the ground at least as much as out of the ground :cheers:
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Thanks very much for the advise.
I am going to build both walls the traditional way that is being described here in regards to drainage behind them.
As for the treated pine sleeper wall, I did consider the gal posts that the sleepers slide into as I feel they are the best option. However I already have some sleepers that are twisted and bowed slightly so I want to use them by cutting them to length and using them as posts. And even though no one really see's this particular wall, we do prefer that look.
I know the posts will eventually rot away but I have a feeling we will be gone before that happens ;)
1200mm centres for pine posts.
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We live on the side of a hill so we dug into the hill mean making the ground level usable. This means the back and both sides of the house have a retaining wall with the highest point being 3.2m and gradually steps down following the contour of the land. it's a block wall and for a wall of this height it needed huge foundations, bottom 5 rows of blocks are 300 series blocks and after that are 200 series blocks. Can't remember the size but the high sections have extra thick steel in every block hole and anything below 2m just had the normal steel dropped down every hole.
Then a pump and 3 trucks of concrete to core fill the wall. After that we put 2 coats of waterproofing membrane on the back, just normal agg pipe of the bottom and while the engineer said I could back fill the 1st metre with recycled concrete and then use some sort of coal ash for the balance to save money, I opted for 60T of recycled concrete and backfilled to half a metre from the top all around. For the extra $400 it saves spending tens of thousands fixing it is 20 or 30 years time.
Seeing this wall is off the house by a metre it doesn't have to be 100% waterproof, so I drilled a couple of holes about a 800mm from the bottom as an extra pressure release measure. While I've only seen these holes in action twice, it has meant there was a lot of water sitting behind the wall judging by how it was squirting out.
After a lot of rain like Brisbane is currently receiving, my wall will weep water down to the drain for about a week.
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No issues with it? We’ve had engineers specs which specifically prohibit recycled concrete backfill. There are issues with high alkalinity, fines, and it compacts well - good for roadbase, not so good for drainage. I have 30t of it in my backyard and have had no issues with it in 10 years. I love the stuff.
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9 years on and no problems so far as it drains really well. Only down side is the grass on top dries out very fast is summer due to lack of moisture.
Good chance the rules have changed in since building the wall, but I have a certificate from the engineer signing it off. Also the excavator driver who carried it behind the wall said he has put thousands of tons of this stuff behind retaining walls over the years.
But I do remember the builder saying when you put as much backfill (gravel or recycled concrete) behind a wall like we did (1 metre wide up to 3M high) it takes the sideways pressure off the wall but instead makes downwards weight, but this weight helps stop any movement of the foundations.
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9 years on and no problems so far as it drains really well. Only down side is the grass on top dries out very fast is summer due to lack of moisture.
Good chance the rules have changed in since building the wall, but I have a certificate from the engineer signing it off. Also the excavator driver who carried it behind the wall said he has put thousands of tons of this stuff behind retaining walls over the years.
But I do remember the builder saying when you put as much backfill (gravel or recycled concrete) behind a wall like we did (1 metre wide up to 3M high) it takes the sideways pressure off the wall but instead makes downwards weight, but this weight helps stop any movement of the foundations.
A lot has to do with how they recycle it too I think. They wash and open grade it now so not much fines mixed in. The early stuff we used to get from pinkenba was like it was rolled in toothpaste. That is the nasty gear apparently.
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One wall down and now the larger one to go.
The next one is going to built using normal retain wall blocks. It is 25 mtrs long and will be around 800-1000mm high as it turns out.
I have a really out there question to see what you blokes reckon............how would you go using plastic drink bottles as drainage fill behind a retaining wall?? They last 100's of years, they are light, they fill out quickly and easily?? What are your thoughts?
The reason I ask is because the next larger wall will require a massive amount of back fill. Due to some large rocks being in the way and the excavator bloke already digging out the area I will require around 25 cubic mtrs of fill. And I dont want to wheel barrow 3 truck loads of gravel nor do I really want to use a machine and chop up all my lawn if I can help it lol
Here are some pics of the smaller hidden wall I just did behind the shed.
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One wall I replaced had plastic plant pots, coke cans and the odd stubby and general rubbish.
Another on at my place had strips of rubber that looked like they were the cuttings from truck tyre retreads. Seemed okay but we're a pain in the a... to clean up afterwards.
Bottles might not drain too well and might hold puddles of stagnant water.
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Thanks Im thinking some polystyrene waffles or something like that to bulk it out a little might work. >:D
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I wouldn't use bottles, I think you would have ongoing settlement issues as everything settles a bit, a bottle could easily be punctured, leading to further settlement leading to more bottles being punctured which could just keep on happening.
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When I was a brickies labourer 30+ years ago it was common practice to hide stubbies, rum cans and basically all our lunch rubbish behind retaining walls.
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You must have built my house
Cheers Glen
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When I was a brickies labourer 30+ years ago it was common practice to hide stubbies, rum cans and basically all our lunch rubbish behind retaining walls.
It still is...... ;D ;D
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I hope I’m wrong but see my first post. Once loaded up the sleepers will bow.
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I hope I’m wrong but see my first post. Once loaded up the sleepers will bow.
I did take on board your advice, in fact I had already been told that 1500 post spacings would be ideal but unfortunately due to rock in the ground this was just not possible in this instance. So 1800 it had to be.
Theres no load behind that sleeper wall. Well theres several 100 tonne of very large rocks but they have been there for 1000's of years and I dont think they will be moving towards the wall soon. The only load behind it is the drainage rock and its in place.
And the sleepers were purchased from Bunnings, they are all already twisted and bowed so a bit more wont bother me ;D
https://www.google.com/search?rlz=1C5CHFA_enAU791AU791&sxsrf=ACYBGNS0XPoOfhosvfl572M7HLd_-N3iIQ:1580944355364&q=treated+pine+sleeper+retaining+wall&tbm=isch&source=univ&sa=X&ved=2ahUKEwiN-9fVxLvnAhUoxTgGHUMhAG8QsAR6BAgJEAE&biw=1920&bih=868 (https://www.google.com/search?rlz=1C5CHFA_enAU791AU791&sxsrf=ACYBGNS0XPoOfhosvfl572M7HLd_-N3iIQ:1580944355364&q=treated+pine+sleeper+retaining+wall&tbm=isch&source=univ&sa=X&ved=2ahUKEwiN-9fVxLvnAhUoxTgGHUMhAG8QsAR6BAgJEAE&biw=1920&bih=868)