# Forum Home Renovation Retaining Walls  Retaining wall advice

## carshi

Hello all, 
I am in the process of placing the timbers into my steel sections for my retaining wall. 
A friend of mine, who also built a wall spaced at 2.4m apart, placed a sleeper midway behind the wall to add extra strength and rigidity to the wall.  He used  galvanized coach bolts with washer and nuts.  His wall looks great and is very straight after 3 years.  Do you think this is necessary to prevent bowing?  Any recommendations on wedging material to using behind the wall in between the steel?  Those green triangle wedges can add extra cost. 
Also, with regards to giving the wall some colour he used a product called intergrain in a merbau colour, it looks fantastic, 
see Intergrain - NaturalStain 
Anyone had any experience with this product or should I just stick with my inital choice of Dulux Weathershiled as it is more durable? 
Any advice is appreciated.  See photo.

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## Timberking

Hey there, 
The sleeper reinforcing is a good way to keep your spans straight.  Treated Pine (which I assume you're using) tends to move a bit in the heat after the sleepers are in so packing out the back of the H and C sections behind the sleepers will stop some movement, as will backing the span with a 50/75mm Centre Support.  I would definately recommend this, with the Gal coach screws or coach bolts.  Screws will not show through to the other side, or you could bolt it if you want the feature.  You can use any sort of timber off cuts to pack out the back as long as it is stable and treated, or hardwood.  Plastic wedges will also work a treat. 
With regards to staining, we sell a lot of Intergrain stains for that purpose.  Heaps of people do it and are happy with the results (as far as we know!).  Cabots and FeastWatson also make stains for the same purpose, but the Intergrain is a bestseller.  You will need a couple of coats to acheive a nice result.  Every coat will darken the look. 
Hope any of this helps. 
-DVM10

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## carshi

Thanks for the reply Timberking.  I purchased the coach bolts today as I intend to use a backing.  I figured bolts rather than screws as they will be stronger and last longer with all the expanding an movement of timber, whereas the scews may lose their grip over time. 
With regards to the centre support I intend to embed it about 200mm into the ground and backfill with dirt.  Do you think this is necessary?

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## carshi

Hi all, 
Finished off the sleepers on the weekend. Just need to back fill with scoria.  I was very happy with the centre support as it pulled everything together nicely and straightened the timber up. I used hot dipped gal M10 bolts with a 75mm x 200mm post as a backing, I highly recommend this for a stiffer wall.  I was originally going to paint the timbers light grey, but the misses thinks it looks too light and that I should match the Ironstone gutters.  DIdnt want to go stain as I am after minimal maintenance.  Any suggestions? 
Also do you think it is necessary to paint the back of the sleepers to increase lifespan with some sort of tar?  See pic. 
Any feedback is appreciated.

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## stevoh741

> Also do you think it is necessary to paint the back of the sleepers to increase lifespan with some sort of tar?

  It won't hurt. Personally I use CN emulsion. Goes hard providing a protective coating whilst also soaking into the timber providing extra protection. Make sure you wrap the scoria in geofabric and include a socked aggi drain.

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## carshi

Thanks for the tip Stevoh.  Where can I buy CN Emulsion?  I still want to paint the front of the sleepers with Weathershield, will this be ok?  I have already used a socked aggie drain and plan to use geo.

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## stevoh741

paint the front should be fine. I get CN from the timber yard and hardwares stock it up here to. Make sure you use the emulsion on the back, not the oil. Should be a number on the can somewhere or search the web to double check the painting on the front. If not possible, I'd go for a tar paint on the back.

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## carshi

on second thoughts stevoh, if the timber is treated, is it really necessary to use the CN emulsion.  I am also using a builders concrete underlay to line the sleeper wall as a water barrier.  How about engine oil as I cant find this stuff anywhere.  Bunnings have something similiar that is recommended for untreated wood and or cut ends.

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## shauck

Is that concrete underlay a solid black plastic sheet? Don't do it. Not supposed to use anything solid. You don't want to allow a possibility for build up of water behind the wall. Too much pressure.

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## stevoh741

> on second thoughts stevoh, if the timber is treated, is it really necessary to use the CN emulsion. I am also using a builders concrete underlay to line the sleeper wall as a water barrier. How about engine oil as I cant find this stuff anywhere. Bunnings have something similiar that is recommended for untreated wood and or cut ends.

  No not necessary but I have had treated pine rot out on me in under 2 yrs (hence I only use h'wood now) so the more protection the better IMO. Engine oil is illegal to use but if you dont want CN then tar paint the option (price prob negligable to CN though). I wouldn't bother lining/blocking the wall like shauck mentioned however if you do the drainage right like i posted earlier then it shouldnt matter so much. The thing is, if you get the drainage wrong and you have sealed the back of the wall then you are promoting risk failure from hydrostatic pressure.

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## carshi

FAR OUT, didnt realise that I couldnt use the non porous black plastic.  The bloke at the sleeper joint who is apparently an expert mentioned this is the right stuff.  I have been researching this project for so long and there are so many varying opinions on what constitutes proper drainage, etc. I haven't backfilled yet (plan to do it this weekend, weather permitting  :Confused: )  I have excellent drainage with outlets every 4m or so into a line parallel to the wall which drains with plenty of fall into two pits at both ends.  The concrete underlay is already laid down and tacked to the wall.  Should I punch some holes into it?

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## shauck

I would just remove it. Your drainage and scoria will do all the work and the plastic will only give an opportunity for potential failure (wall pushed over).

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## Juz86

The black plastic is fine, the water won't have a chance to cause your wall to fail, as it will drain downwards from the plastic, through your scoria and into the AGGI drain. Alot of people (including myself) line the walls in this plastic to help extend the life of the wall through stopping the watter saturating the timber causing rot.

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## stevoh741

> The black plastic is fine, the water won't have a chance to cause your wall to fail, as it will drain downwards from the plastic, through your scoria and into the AGGI drain. Alot of people (including myself) line the walls in this plastic to help extend the life of the wall through stopping the watter saturating the timber causing rot.

  Not true, you will still get condensation between the timber and the plastic which keeps the environment wet promoting rot. IMO this is even worse. Line with geo fabric, lay the socked aggi, fill with scoria or similar, enclose with geo fabric to keep dirt out of drainage, top with clay based soil or similar that allows runoff without soaking too much throught to the drainage layer. Therefore the drainage layer is just diverting seeping groundwater which should gather at the bottom of the cut and runaway via the fall behind the wall / aggi drain. So given the drainage layer is 300 wide, technically the back of the timber shouldnt be exposed to too much wet which can easily be further prevented with a coat of CN emulsion or similar before backfilling. There is a lot more to building a wall then just shoving some timber in and chucking some black plastic behind prior to back filling.

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## carshi

Thanks again Stevoh.  You see, this is exactly my point.  There is too much varying opinion with the building of retaining walls, leading to confusion and problems down the track.  Thank God for sites like this.  I too thought the black non porous plastic behind retaining walls was the correct way of doing things as so many people use it.  Since starting this thread I have spoken with several engineers, searched the web for info and watched videos.   
I have come to the conclusion that black plastic lining a retaining wall is not the correct way.  Geo fabric lining the wall and wrapped under and over the pipe with scoria infill is the only way.  Black plastic can allow a build up of water behind a wall if: 
a) the drainage is not functioning properly or is blocked;
b) there is a down pour that exceeds the flow of the drainage 
This will in turn place excessive force on the wall and cause it to act as a dam and potentially fail. 
Black plastic will also hold water behind the wall in pockets or flatspots which are generally invevitable along a relatively level or slight slope.  The water can pool and not dry out promoting rot. 
With geofabric the water has a chance to escape and dry out.  Worse case scenario your sleepers may get a little dirty on the sruface which can be easily cleaned. 
I dont think that black plastic is necessarily taboo, as many people use it, I just think creates more problems than its worth and is not necessary.  The risks in this case outweigh the benefits.  Overall geo fabric is safer, keeps the clay and silt out of the pipe and can perform the same function as black plastic on the inside of a retaining wall for those considering use both materials.  
This video sums it up nicely:  Timber Retaining Walls - Finishing - YouTube 
My mind is now made up, period.

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## stevoh741

> Thanks again Stevoh.  You see, this is exactly my point.  There is too much varying opinion with the building of retaining walls, leading to confusion and problems down the track.  Thank God for sites like this.  I too thought the black non porous plastic behind retaining walls was the correct way of doing things as so many people use it.  Since starting this thread I have spoken with several engineers, searched the web for info and watched videos.   
> I have come to the conclusion that black plastic lining a retaining wall is not the correct way.  Geo fabric lining the wall and wrapped under and over the pipe with scoria infill is the only way.  Black plastic can allow a build up of water behind a wall if: 
> a) the drainage is not functioning properly or is blocked;
> b) there is a down pour that exceeds the flow of the drainage 
> This will in turn place excessive force on the wall and cause it to act as a dam and potentially fail. 
> Black plastic will also hold water behind the wall in pockets or flatspots which are generally invevitable along a relatively level or slight slope.  The water can pool and not dry out promoting rot. 
> With geofabric the water has a chance to escape and dry out.  Worse case scenario your sleepers may get a little dirty on the sruface which can be easily cleaned. 
> I dont think that black plastic is necessarily taboo, as many people use it, I just think creates more problems than its worth and is not necessary.  The risks in this case outweigh the benefits.  Overall geo fabric is safer, keeps the clay and silt out of the pipe and can perform the same function as black plastic on the inside of a retaining wall for those considering use both materials.  
> This video sums it up nicely:  Timber Retaining Walls - Finishing - YouTube 
> My mind is now made up, period.

  thats how it is done - anyone that says otherwise doesnt know what they're talking about. I build a lot of walls and have never had one fail yet. The only thing I disagree is that the top layer of drainage in that video was not wrapped in geo fabric. Over time the exposed drainage will clog up. I wrap the *whole* drainage layer in geofabric to avoid this and as extra protection from silting up use a "socked" aggi - 100mm IMO.

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## shauck

With mine I wrapped the aggi pipe then layed the fabric back over the clay side not the wall side. Then fiiled the space (which was big due to rain induced cave in) with scoria and then pulled the fabric back over the top before soil top up. My idea was to keep the clay from silting into the scoria. We've had some massive rains and it hasn't moved. Did get water running over the top during the last big downpour that was just surface water tho.  
I do wonder about geofabric sometimes tho. I remember when I built the retaining walls last year and hadn't finished topping up one section with soil yet. the ground behind the wall was on a slope parallel to the wall. It poured down and there was so much water running along the back of the wall towards the corner of the house, it was like a waterfall. This was because the geofabric couldn't let water in. It had no soil on it to slow it down. So my wondering is, if in torrential downpours, could it be having an adverse effect. Not letting water through quick enough.

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## carshi

I dont think Geo would caue water to block up.  I think it is a happy medium.  You wouldnt want something too porous or it will defeat the purpose.  
I have never seen it done, but why don't people put holes half way up a timber retaining wall as a precaution, like they do in stone walls (besides the obvious answer that it will look butt ugly)?

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## shauck

Do you mean like a weep hole? I think it's because with a stone wall, there is very little permeability (if concreted) but a timber retaining wall has lots of gaps, (between each wale) to let water seep through.

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