# Forum Home Renovation Retaining Walls  Is drainage needed for 400mm timber retaining wall? and other questions...

## Greggorian

Hi there,  I'm looking for some advice regarding drainage behind my 400mm (2 sleepers high) treated pine sleeper / steel upright retaining walls.  Some of the walls are tiered - ie 2 walls each 400mm high, one below the other.   Here's some pics of my sketchup design vs where the project is up to now:           
There's some more angles / pics at this link: https://www.dropbox.com/sh/cx7y9ko67..._wNFbLa?dl=0#/     I've been checking out various forums and a common thread of advice for retaining walls is to line the inside of the wall with geofabric, put socked-agpipe at the base of the wall, cover with 20mm aggregate, and wrap the geofabric back over the top of the gravel.    My questions are:   1) Do I need to do all of this if my retaining wall is only 400mm? I'm not really keen to do the extra work (and expense) if all I'm doing is completely over-engineering it.  However, I'm happy to do it if it will definitely make a difference, and will mean that I don't have to rebuild the thing.   2) I also have a query about using black plastic to line the inside of the wall (remembering that it's only 400mm high.  A number of people have suggested this to keep water off the treated pine sleepers, thus extending it's life.  Other people have said not to use plastic at all as this creates a dam effect against the wall.  I can see the wisdom in both statements.  Can I use plastic lining if I also have suitable drainage behind the wall too?   3) If agi-pipe is necessary, what would you suggest for a 400mm high wall - 50mm unsocked, or 65mm socked?   4) The area between the two walls (900mm wide) will be a paved footpath (see sketchup diagram).  Does the retaining wall that supports the footpath require drainage?     If geography / rainfall matters... I live in Blackwood, Adelaide Hills.  There's a good amount of clay around. 
Thanks for reading, If you've got some wisdom / experience, I'd love to hear from you.  Cheers, Greg.

----------


## STX550

Hi Greggorian, 
I have not long completed a retaining wall of 1.4M from 2 walls of 600mm stepped, similar to yours at 400mm. Personally I would suggest 50mm Agi-Drain Unsocked and backfilled 2/3rds with large scoria stones. Agi-Drail isn't that expensive and Scoria is cheap too. For the extra money now it would be better than a waterlogged garden that doesn't grow grass. Just find your closest storm water pipe and pop the Agi-Drain into that. Also don't forget to line the back of the walls with plastic to stop seepage through the timber and make the timber last longer. These are just a few thoughts and suggestions from a weekend DIYer.  
Cheers,

----------


## Oldsaltoz

> Hi there,  I'm looking for some advice regarding drainage behind my 400mm (2 sleepers high) treated pine sleeper / steel upright retaining walls.  Some of the walls are tiered - ie 2 walls each 400mm high, one below the other.  I've been checking out various forums and a common thread of advice for retaining walls is to line the inside of the wall with geofabric, put socked-agpipe at the base of the wall, cover with 20mm aggregate, and wrap the geofabric back over the top of the gravel.  My questions are:  1) Do I need to do all of this if my retaining wall is only 400mm? I'm not really keen to do the extra work (and expense) if all I'm doing is completely over-engineering it.  However, I'm happy to do it if it will definitely make a difference, and will mean that I don't have to rebuild the thing. Hi Greg, in short it depends on a few factors:
> The amount of moisture you can expect behind the wall is a major factor, any moisture in contact with the wall may lead to mold growth, not pretty or healthy. 
> Timber posts and high moisture = short life for posts. 
> Drainage is critical, depending on what volume of water you expect to be dealing with behind the wall. so installing some sort of drainage will help and should be included in your plans.  2) I also have a query about using black plastic to line the inside of the wall (remembering that it's only 400mm high.  A number of people have suggested this to keep water off the treated pine sleepers, thus extending it's life.  Other people have said not to use plastic at all as this creates a dam effect against the wall.  I can see the wisdom in both statements.  Can I use plastic lining if I also have suitable drainage behind the wall too? Drainage would be high on the list.  3) If agi-pipe is necessary, what would you suggest for a 400mm high wall - 50mm unsocked, or 65mm socked? In the iterst of low maintenace and confidence I would avoid the 50mm and install the geosock and gravel around the pipe. The top 100mm can be soil by the way, so the lawn contiues to the wall.  4) The area between the two walls (900mm wide) will be a paved footpath (see sketchup diagram).  Does the retaining wall that supports the footpath require drainage? Drainage should cover all areas that have the potential to retain moisture.  If geography / rainfall matters... I live in Blackwood, Adelaide Hills.  There's a good amount of clay around. So you could even have a small spring in the area, or under you garden. 
> Thanks for reading, If you've got some wisdom / experience, I'd love to hear from you.  Cheers, Greg.

  I spend most of my day waterproofing and have been at it for many years.
The standard for your wall would be waterproof the wall and install a suitable drainage system. The whole job can be done by the waterproofer and you get a full written warranty. 
Good luck and fair winds.

----------

