How to build a retaining wall with 3m sleepers?

[Yangaz]

Hi all,

I am working on a small area in my yard (3.2m x 3.8m) to install a stepped retaining wall to level out surfaces at both levels. Refer to the pictures below. I plan to use the retain-it metal posts to hold in the 3 x (50mm x 150mm) timber sleepers making the wall 450mm high, the pieces are 3m in length.

1. Are 50mm thick sleepers too thin to hold back the soil? should I go to 75mm?

2. Should I be cutting the 3m lengths into two 1.5m lengths and have a middle post, concerned the 3m sleepers with no middle post may bow over time? I will install an ag line to drain water buildup in front of the retaining wall to the open area as I don't have a connection to stormwater for this small area.

Thank You

[JacobZ]

Hi @Yangaz,

Thank you for your question about using 3-metre sleepers to build a retaining wall.

There is certainly some potential that 3m sleepers can bow over time and you might like to consider going the extra mile now when the work is still being done to prevent this from happening.

The ag line will certainly help to mitigate this issue as the most weight the sleepers will need to hold back will occur when the soil becomes sodden. Adding this drainage will move the water away from the retaining wall and lessen the load applied to the retaining wall.

Moving to a 75mm sleeper, or even a hardwood sleeper will lower the potential for bowing over time, but as it appears like you've already purchased your sleepers, I'd suggest cutting the sleepers you have to 1.5m and adding the mid-post. 

It is very unlikely that the sleepers will bow over 1.5m so this would be the most cost-effective way for you to ensure a nice straight retaining wall for years to come.

Let me know if you have any further questions.

Jacob

[Yangaz]

Thanks Jacob, so is using 50mm thick sleepers 450mm high (3 pieces) okay?

[JacobZ]

Hi @Yangaz,

There are no worries at all assuming the posts are well anchored with at least 2 bags of concrete each.
 

Let me know if you've got any other questions.

Jacob

[Yangaz]

Thanks, what would you do for the short end of the area? i.e. 1.4m where it slopes down slightly? any good ideas?

[EricL]

Hello @Yangaz 

If one side of the retaining wall is lower than the other side, my best advice is to make sure to compensate for the low area. Just keep in mind that the top part of the retaining wall must be all at the same level. 

That could possibly mean that you'll need to use a longer post, or the soil area will need to be adjusted in order for the wall to be uniform.

If you need further assistance, please let us know.

Eric

[Yangaz]

Thank you, 

Regarding drainage, I intend to install an ag line using some spare 100mm socked slotted draincoil which I can use on the front of the retaining wall. I've read that some use a fabric blanket before pouring blue metal to avoid dirt and debris getting into the ag line. Given the ag-line has a sock already is this fabric still required?

Regards

[JacobZ]

Hi @Yangaz,

The geotextile membrane is more to keep the soil from getting in between the stone which will limit the water's ability to flow freely between the stones and find its way into the ag line.

It is not 100% necessary but is a fairly simple way to prevent unnecessary maintenance.

I would suggest it is worth the relatively small expense to save yourself a lot of work in the future.

Let me know if you have any further questions.

Jacob

[Yangaz]

Thanks, Jacob,

Here is the progress shot below. Please see the area circled in red. Unfortunately, I cannot connect this back into an existing drainage point but will distribute the water from the retaining wall into a larger area. Do I still need to tie the fabric ends of the slotted black pipe?

Also, what would you recommend for a good surface where the dirt is? I was considering 40mm blue metal but maybe something more aesthetically pleasing?

Thank You 

[JacobZ]

Hi @Yangaz,

It's coming along nicely, thank you for sharing the update.

You might find that the soil erodes a bit where you discharge the ag line, so a good amount of drainage gravel at the end is a must. Also, make sure the ag line falls towards the end you want it to discharge, otherwise it will just sit in the ag line and not drain away as intended. 

You should definitely tie the sock over the ends, this will allow water to pass through the sock but will stop dirt and debris getting in.

I actually like the look of blue metal, but that may just be me. There are certainly other options you could use, maybe something like these Iceberg Decorative Stones or Black Decorative Pebble Mix will work for you.

Let me know what you think and if you have any further questions.

Jacob