# Forum Home Renovation Retaining Walls  Post spacing- retaining wall

## JDub

Hi, 
Im building a small treated pine retaining wall (I know, I know :Rolleyes:  , its for behind a shed, out of sight, so I cant justify building in brick etc) 
Only 400mm high at the most and about 10m long, using 200x75 sleepers......   given Im using 75 thick sleepers at what distance can I space the posts?  Did a search but couldnt find much helpful info. 
Cheers

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## JDub

Oh and is it also true I only need to bury as much as the post in the ground as is out..... ie for my application the post holes need be 400 deep (600 seems overkill to me given the small height of the wall??)  
Cheers

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## andrew076

Burying the posts as far into the ground as they stick out is a general retaining wall rule.  
I built a similar wall (14000 long, 200x100 all around, 400 high + 100 capping piece) and spaced the posts at 2400 so we could drop the standard 2400 lengths in behind.  I founded my posts about 800 into the ground and each footing was about 600 square. 
It's been in about 18 months and is still ok.

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## JDub

Thanks for the reply Andrew, just found this as well.....    http://www.hyne.com.au/downloads/dih...ning%20wall%22

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## Eastie

JD - attached is the tradac link (which I see you already found  :Biggrin: ) 
Brick walls are for brickies and mud monkeys - this is a wood-work forum!  :Biggrin:   
Anyhow, following is a rare insight into how I was taught to build timber walls and why - if anyone differs I'm happy to hear your method. 
Attached are a couple of pics of one I'm building and finishing off at the moment. This one is using 200x75 F7 H5 (H4 is typically fine) as that's the spec's in the engineering comps that I have for the heights I'm dealing with.  
When it comes to protecting the wall from groundwater (given you're dealing with typical hard to dig soils) have a serious think about using doubled over black builders plastic on the back of the wall instead of geotex (and go under the agi pipe, not over it). The filtration nazis and geo tech grade 3 engineers can have their post about filter socks and undies, what I know is what has proven to work for me and a lot of other chipy's. Cut the plastic off just before finished ground level or slip it between the top two sleepers. Combine this with descent drainage pipe and 1/4 minus screenings or scoria backfill and you shouldn't get water seepage and discoloration through your sleepers that leads to premature sleeper failure. For larger walls, areas with high rainfall or surface water problems a surface drain is also a good idea and can be set in mortar mix as shown below.  
A 45 degree 7-10mm chamfer on the exposed edge of the sleepers has nothing at all to do with engineering design, it simply makes sleeper walls look a lot neater as it hides the typical imperfections of sleepers not butting together neatly. After all they are not engineered LVL or biscuit joined... 
If you are going to lean your posts into the embankment (1:10) a simple way of ensuring each post is the same angle is clamping a bit wood onto your level as shown below and placing the clamp head on top of each post when installing the post in concrete (sit the clamp head on top of the post, not the timber block). How do you determing 1:10? - it's a trade secret that most building surveyors can't work out. 
For length of sleepers I tend to stick to 2m when using treated pine and walls above 300mm - it is more sound and increased cost is marginal in you buy 2.4 m lengths. It also allows dud ends to be cut out of the sleepers. If cutting the sleepers have a serious think about priming the ends with some "reseal". For a small wall without drainage problems don't worry to much about burying the lower sleepers as shown below (if you do, make sure you mark the height of your concrete footing on each post to suit the post embedment depth - this generally means a deeper footing to compensate).  Most geo-engineers and surveyors don't understand this design or going to such lengths - probably because they never have to warranty their work against defect.   Keep in mind you may have to dig some of the soil out and part burry the bottom row of sleepers if the gound is not level along the length of the wall and you want the wall level.  
If you use galvanised steel posts (which I would use on anything structural near a building) here's a couple of tricks:  
1) dig all your footings to roughly the required depth then (take off your blouse &) dig a bit further (I normally bell out the bottom of the holes a little also). I then usually put a bag of rapid set in the bottom of each hole (you could use gravel or the like). If you use rapid set ignore the instructions on the bag for this step and mix it in a bucket with a fork to a fairly sloppy mix so it self levels - get it in the hole ASAP and level it if required. Run a string centre line above the holes to the exact height (and position) you want the wall. Measure the depth of each hole & take 20-30mm off this measurement and you have the length of steel post for each hole - if using timber posts cut them to measured length. I've seen blokes digging and back-filling for hours trying to get holes to an exact depth - it's for mugs.  
2) tip 2 was mentioned above but for the sake of grinding steel or a crap finish I thouight it should be restated: if using gal posts get them cut 20-30mm shorter than the length you measured (again it makes the wall look neat) 
The reason for digging a bit deeper is three-fold. Firstly it lets you take your frock off and act like a bloke who doesn't give a toss if he's got to dig just a little deeper to make a good job of it despite the fact that: a) it will take an extra hour or two; and b) no-one will ever see your work. And the second Firstly, for metal posts it is to install a decent foundation pad and to get yor levels. And Firstly again it is to allow for another bag of rapid set in the hole to set the post in the required position with your level and string line/dumpy/laser line before filling the required depth with a structural concrete mix (follow the instructions on the bag this time). If you missed the point, each of these points is equally important. 
What I've gone through is the method I was taught that delivers proven results that stand the most improtant test of all - warranty insurance and time.  
Hope some of these tips help make your job a bit easier. 
Cheers,
Eastie.  :Smilie:

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## Eastie

I forgot to mention the most important bit:  
each hole deserves a stubby as does each post set in concrete  :Biggrin:

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## Bleedin Thumb

Well done Eastie,
Sound advice on timber wall construction. :Wink:  
You should also add a note about Zone of Influence ie; the area from the base of the wall on a 45 deg angle to the top should not have any load applied to it. Therefore if you are building a double tier wall (as in your photo) you must ensure the top wall is not too close to the bottom wall. By following your advise about excavating the subgrade on the 45 deg angle you are forced to observe this rule.
PS this rule applies to all retaining walls, not just timber.
PPS I dont know if I agree with the steel posts being shorter- just personal taste. :Rolleyes:   :Smilie:

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## Eastie

To true - keep in mind the design of the wall I'm building is _engineered_ (note the spacing of the posts is only 1.5m on the lower wall, bottom sleeper on the upper wall are part buried, 150-180kg concrete per hole, etc) This isn't your standard garden setup that you'll find a design for on the internet. My comments are in relation to small walls of the type in question so it's a good point you raise about soil stability and zones of influence. When you're dealing with walls with structures/footings on the retained slope (house /carport / shed etc) then you're dealing with an engineered design which must take this, the surcharge angle and stable zone (typically 25 degrees from the base of the wall) into account.  
In relation to steel being shorter - the other reason is shrinkage. Although treated pine posts don't shrink much (some of the green stuff tends to go the other way and swell) The timber I use is _seasoned_ treated F7 which I've found tends to shrink. If each sleeper shrinks 1-2mm over a few years, that's up to 10mm drop in a 1m high wall. Cut your steel to exact length and it may end up proud, which is not how I'd like to have a job end up.  
Cheers,
Eastie  :Smilie:

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## JDub

Fantastic posts Easite, thankyou, greenie coming your way  :Biggrin:   
As with your wall picutred above I am using 200x75 sleepers and I was going to just use timber posts cut from the same sleepers. 
I will now look into the gal posts (but only have a couple of days though to find em though  :Wink: ) I need this wall up on the weekend, are the posts pricey? I assume you would just get them through your local buildiers yard...... 
Cheers
Joel

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## Eastie

The cost of gal H section is around $42m cut to size with the cut ends cold gal'd. Compare this to H7 treated pine posts and steel is about double. For a 400mm wall out of sight I would't hesitate to use timber posts - just don't cut the end going into the ground as you'll ruin the integrity of the CCA treatment. If you can get it H5 is the way to go (if the cost isn't over the top compared to H4 and steel), but it's not readily available. Even in the big smoke I have to place an order with the treatment plant for H5. The difference between H4 and H5 treatment is the ratio's and penetration of the treatment chemicals and the textbook lifespan. H4 is the most common on the market as it's these easiest to produce and typically lasts in real life if installed correctly.

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## Bleedin Thumb

> Well done Eastie,
> Sound advice on timber wall construction. 
> You should also add a note about Zone of Influence ie; the area from the base of the wall on a 45 deg angle to the top should not have any load applied to it. Therefore if you are building a double tier wall (as in your photo) you must ensure the top wall is not too close to the bottom wall. By following your advise about excavating the subgrade on the 45 deg angle you are forced to observe this rule.
> PS this rule applies to all retaining walls, not just timber.
> PPS I dont know if I agree with the steel posts being shorter- just personal taste.

  After visiting the www.hyne.com.au site I notice the 45 deg that I stated is a bit conservative. If someone could post a table with the differing angles per soil types it would be much appreciated. :Smilie:

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## JDub

Cheers Eastie, 
The timber is being delivered today via the local timber yard, nearly everyone around these parts (that I have seen anyway) only stock H4. 
Thanks again, I will put only uncut ends into the ground and any cuts I do have to make I will hit with some reseal. 
Now to start diggin some post holes:mad:  :Rolleyes:   Blouse off  :Wink:   
J

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## JDub

Job done, (apart from drainage and backfilling) but that wont take long.  :Rolleyes:   
I couldnt find 'reseal' at all, the two places I went to: one had no idea, the other said all related products have been taken off the market due to health concerns.... not sure if he was just telling me porky pies cause they didnt stock it or not.   
Anyway, Just made sure all uncut ends went into the ground. Turned out pretty well... will post some pics soon, now ready for the slab and then the new shed!!!  Sweet  :Cool:

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## JDub

A couple of pics are in my "Shed - WIP thread" http://www.woodworkforums.ubeaut.com...t=34677&page=4

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## Bleedin Thumb

> I couldnt find 'reseal' at all, the two places I went to: one had no idea, the other said all related products have been taken off the market due to health concerns.... not sure if he was just telling me porky pies cause they didnt stock it or not.

  Hi JD 
I,m glad it worked out. I find it interesting about the reseal because I have always maintained that the banning of CCA was stupid because if we now have to use LOSP and spread chemicals over every cut, we are exposing ourselves to greater danger than before.
I haven't heard about reseal being restricted or withdrawn... sounds like porkies.. but you never know!:confused:

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