How to add drainage for a retaining wall with no nearby drains?

[Cam2]

Greetings, I'm looking to build a small wooden retaining wall at the back of my house to level off up a small section of lawn which I intend to turn into a series of raised vegetable beds.

The retaining wall will be 9.5m in length and 0.4m in height. I'm planning to use 200 x 50mm 2.4m Sleeper H4 Treated Pine Sienna MicroPro and Jack 50 x 750mm Galvanised Steel Retain It Joiner Post Sleeper Upright. 

In all my research it has mentioned the importance of drainage and laying agi pipe to do this. My issue is I don't have any nearby drains to connect this pipe to. Are there any options for how I could resolve this drainage issue? Or is the wall small enough not to need it? Thanks for any suggestions. 

[JacobZ]

Hi @Cam2,

A warm welcome to the Bunnings Workshop community and thank you for your question.

With a retaining wall of this height, the need for drainage is still there, but it is not so dire. This is because water accumulating in the soil behind a retaining wall creates hydrostatic pressure, which massively adds to the weight pushing against the retaining wall. With taller retaining walls, this pressure can absolutely cause catastrophic failure, but with a 400mm high retaining wall, the risk is greatly reduced.

It is still worthwhile adding some kind of drainage behind the retaining wall, but it is unlikely to cause you massive issues.

At a minimum, I would suggest adding a layer of drainage gravel with a slight slope at least 200mm wide and deep wrapped in geotextile membrane to have the water flow away from the wall. Ideally, you would add an agi-pipe along this drainage channel and have it plumbed into a stormwater outlet, but if this is not available, the drainage trench can discharge into the ground, but there is a risk of erosion.

If you can upload a photo showing what is to the left of your second image, I may be able to offer a better solution. Once I can see what is to the left of this area, I will happily assist further.

Let me know what you think and if you have further questions, please don't hesitate to ask. 

Jacob

[Cam2]

Hi Jacob, thank you for this information it is very helpful. He is the photo you requested of what is to the left of the previous photos. Thanks so much for your help with this.

[JacobZ · Accepted solution]

Hi @Cam2,

Thank you for adding that photo.

As you have a nice lawn area next to where you intend to put your retaining wall, you could extend your drainage ditch out into this area and run it into a soak well. This would not only manage the water, but it will also be beneficial to the lawn.

A soak well is a vertical column that a drainage channel feeds into that allows water to easily fall to a lower level where it can dissipate into the soil.

There are products like this Novaplas Drainwell DIY Soakwell With Geo Cloth which can be dug into the soil, wrapped in the provided geo-cloth and then filled with drainage gravel. You would then run your drainage channel into the soak well and backfill over everything. This will make it so the water dissipates into the soil at a lower level meaning there is much less risk of erosion.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob