What drainage for a small retaining wall?

[kippo]

I'm planning on building a small red brick retaining wall at the front of my property. It will be double leaf and about 290mm high with the bottom 60mm - 70mm underground (so the wall itself will really only be 210mm high). It will sit on concrete footings and it's length is 10 meters. It will be used to retain a garden bed with a small hedge in it.

Given that this is such a short wall do I require any drainage (i.e. agi and aggregate or weep holes)? The soil in my area is also very sandy.

Also will i have any issues with a hedges root system pushing on it.

[EricL · Accepted solution]

Hello @kippo 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your small red brick retaining wall.

For a small double-leaf red brick retaining wall at just 210mm above ground level and with a total height of approximately 290mm including the buried section, the drainage requirements are generally minimal particularly given your sandy soil. Sandy soils drain exceptionally well compared to clay or loam, which means the risk of water pressure building up behind such a low wall is quite low. In most cases like yours, installing a full agi pipe and aggregate backfill is unnecessary and could be considered over-engineering. 

However, you may still choose to incorporate a few small weep holes at the base of the wall (spaced about every 1.2 to 1.5 meters) to allow for incidental moisture to escape, especially if you notice water tends to pool during heavy rain.

As for the hedge, provided you select a small to medium hedge variety with non-invasive roots such as Japanese box, lilly pilly 'Tiny Trev', or similar root pressure should not be a problem. The wall’s double-leaf construction and solid concrete footing will give it plenty of strength for this purpose. To be safe, plant your hedge 200–300mm back from the wall, and maintain a regular watering and pruning routine to encourage manageable growth above and below ground. Root pressure issues are more likely with large, woody hedge species or when the wall is much higher and subject to greater lateral force from retained soil and moisture. 

Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.

If you need further assistance, please let us know.

Eric

[Nailbag]

Hi @kippo 

Personally I wouldn't worry about it, especially since you have sandy soil. Just staple builder plastic to the inside of the sleepers. This will stop seepage through the gaps and extend the live of the sleepers. Use this one cut in half.

Nailbag

[Dave-1]

Good Evening @kippo 

Thinking here I think a little different to @EricL on that, Even tho you have sandy type soil maybe down the track the compisition may change due to what ypu do garden wise, I always think that for a minimal cost at installing point to put in drainage. With your wall being so short im "wiggling hands" type deal If you can afford an socked ag pipe or leave a gap for the weep holes all the better but you could get away with it.

Dave

[kippo]

Brilliant, thanks guys!!!