How to build self-bracing retaining wall & garden bed combo?

Question

I'm planning to diy landscaping my backyard with retaining wall and garden beds, I've consulted a lot with Gemini but I'm not pro enough to see if its giving me bad advice.

​A landscaper originally suggested building a new retaining wall directly against our existing boundary fence line (which sits on top of an existing concrete retaining wall). I am uncomfortable adding lateral soil weight to that boundary structure, so my solution is to move the new retaining wall 500mm away from the fence, and turning the gap into a raised planter box, leaving 50mm air gap to the fence.

The details: One side of the garden bed is facing the lawn and acting as the retaining wall,

Total 9.6m length, two row of sleepers (200x50mm) total of 400mm height. Post between 1.2m lenght of the retainer (total of 9 posts - orange color). Im thinking of using 200x50x800mm as the post (same material as the retaining wall, 400m deep with concrete footing)

The other side of the garden bed is facing a fence. Since this fence is also sitting on a concrete retaining wall, I'm not digging post on this side of the garden bed, and will be leaving 50mm gap to prevent adding weight to the fence. Instead, I'll put connecting timber between the main retaining wall and this side (light green color) for each post. I'll also add a timber connecting the sleeper timbers (yellow).

Total width of the garden bed 500mm (with 400mm internal)

Please advice what's wrong with this, and if it's diy'able (from a novice woodworker/cabinet maker background pov)

Thank you all!

Entire backyard is 13.5m

RenoMike · Answer

Hi @Redimp

I like the excel diagrams! From my understanding (please correct me if I'm wrong) the lawn is raised, I assume by 400mm, compared to the top of the existing retaining wall. The raised garden bed will act as a new retaining wall, but the back of the bed is taking the weight of the lawn and the front of the bed facing the fence just has the weight of the soil in the garden bed?

I am not a structural engineer or a landscaper so these are just general comments from what I learnt making my project. How high is the existing retaining wall? If you make a retaining wall >1m in height then you will need some kind of approval / building permit. If the existing retaining wall is <1m, but then you add another essentially on top, then you have to consider the total height of both structures. To avoid this you need to set the new retaining wall back from the old. You'd need to check specifics but I worked out that I had to set my new retaining wall (60cm) back 90cm from the existing retaining wall (90cm high) to avoid having them count as one structure. This is a critical point, you do not want to find that the council has a problem with your structure especially when it comes time to sell your property.

For the plan you described, I did not use wooden posts so I am unsure what requirements there are as to the size of the post. Most of your posts seem to be 50cm in depth which seems on the small size to me, so please check whether this is sufficient. To state the obvious, make sure you use treated wood and also use metal screws appropriate for use in treated wood. I would also suggest painting the inside of the wood for the raised bed with protectant (the details are in my post) to further add longevity to your structure.

For the posts, as @MitchellMc said the rule is depth of post below ground = height of post above ground. My reading though said an additional 10cm below the bottom of the post to make the concrete footing, so for a 60cm height wall my holes were 70cm deep with the post set 60cm down on top of 10cm concrete. The diameter of the post hole is the other element, which for your purpose would be 30cm unless I'm mistaken.

One other thing: I read that a critical element of a retaining wall is the weight of water behind it. If you are adding a garden bed on top of an existing retaining wall then you will be regularly adding water. I'd make sure you are satisfied that the drainage behind the existing retaining wall is adequate, otherwise you may wish to supplement this drainage with ag pipe etc before you build on top of it.

Hope that helps, I look forward to seeing updates!

Cheers

Mike

MitchellMc · Answer

Hi @Redimp,

I would actually look at having the additional posts on the inside of the garden bed rather than between the garden bed and the existing retaining wall.

That would give you your 50mm air gap, then the 50mm sleeper, with the post located on the inside face of the sleeper. This arrangement leaves around 100mm between the rear of the sleeper and the existing retaining wall, giving you a bit more room for the post hole and concrete footing. If you place the posts on the retaining wall side of the sleeper, which would normally be preferable structurally, you'll likely find there isn't enough room to comfortably excavate and concrete the footings.

If you decide to increase the retaining wall height to 600mm (three sleepers), I would typically be looking at around 600mm of post embedment as a starting point. However, I'd strongly recommend running that past your landscaper before proceeding. The required footing depth can vary significantly depending on the soil conditions in your area, particularly if you have reactive clay soils, where deeper footings may be required.

Mitchell

Redimp · Answer

Thank you Mitchell. Just question on the additional post, since that would need to be dug and cemented on the side the fence, wouldn't that create an issue to the existing retaining wall? Also, since I'm planning to leave a 50mm air gap, do I need to adjust the rear of the garden 50mm to retain the air gap after putting the post, or would it be okay to just leave the post touching the fence?

Last one, if i extend the retainer height to 600mm (3 sleepers) can 1M (400mm deep) post, or would I need to also increase the depth to 600mm (for total of 1.2M post)

Thanks!