How to build garden bed behind retaining wall?

[nboucher]

 Hi all

I am looking to build a garden bed in this section between a wooden retaining wall and a stone retaining wall.

My plan is to concrete in stumps at the orange crosses under my string line and the secure some treated sleepers to those concreted stump posts.

Ill dig small channels up the hill to ensure the sleepers go deep in the soil at the top of the slope.

I will put in a drain at the lowest point that feeds into the ag drain behind the stone wall and cover with gravel and landscape mesh and then fill in the garden bed with soil. The soil will ideally be level at just below the lowest point of the wooden retaining wall.

Is there anything i should be careful of during this project?

[MitchellMc]

Welcome to the Bunnings Workshop community @nboucher. It's terrific to have you join us, and many thanks for your question about creating a garden bed.

Your plan sounds well thought out, and I can’t see any major issues with it. The key things to keep in mind are proper drainage and ensuring stability of the structure over time. It's great that you've already planned to install a drain that connects to the ag drain behind the stone wall—that will help prevent water buildup, which could otherwise put pressure on both retaining walls.

When you dig the small channels for the sleepers, ensure that they are deep enough and well-anchored into the soil to prevent shifting. As you're concreting the stumps, make sure they’re spaced correctly and are set deep enough into the ground to provide long-term support for the sleepers.

One more thing to consider is how the soil in the garden bed will interact with the retaining walls over time. Since you’re building between two walls, ensuring adequate drainage is essential to avoid waterlogging or excessive pressure. Using landscape mesh and gravel, as you've described, will help keep the soil in place and manage water movement, so you're on the right track.

The only other thing I'd point out is that the soil in that area looks to be of poor quality and predominantly clay, so I'd encourage you to remove a substantial amount of it and replace it with a higher-quality mix. This will allow better drainage and be beneficial to your plants.

Please let me know if you have any questions.

Mitchell
 

[nboucher]

Thanks Mitchell 

Really appreciate the reply.

How deep would you recommend the channels to sink sleepers in to be secure? I was planning to dig down about 200mm to accomodate one sleeper and ensuring about stumps were concreted about 300mm below the lowest points of the sleepers. Would that be secure enough?

Do you think the number of stumps i have would be sufficient or would you recommend a few more?

Kind regards 

Nick

[MitchellMc]

Provided the sleepers are fixed to the stumps securely, there's probably no need to have them buried too far in the ground @nboucher. At the top of the slope, they might be buried to full height and at the lower side, they might be entirely exposed. 

The depth the stumps need to be in the ground is governed by their overall height. Typically you'd want 1/3 of the stump in the ground. You'd normally aim for one stump every 1200mm.

Mitchell