How to build a raised garden bed on top of existing retaining wall?

Hi,

I am looking to build a food forest on my sloped back yard (attached photo). I have a landscape designer produce a concept design for me (attached photo). The garden beds will be terraced with pathways to meander through the garden.

I have had some quote from professional landscapers and the costs are quite high. Therefore I am keen on attempting to build this myself if possible, although I have no hands-on skills and would love to learn. I understand that the garden beds, regardless of where they are terraced in the slope also act as retaining walls. I want this to last as long as possible so I want to do it right.

I want to build the first level using ACQ H4 Hardwood sleepers as the bed will be used for food; I am not sure about the thickness of the sleepers - whether it should be 50 or 75mm. It will be 200 - 400mm above the existing soil, when the final growing soil is added, it will be ~200 - 300mm above the existing soil. I was originally planning to stake the beds but I understand that doesn't add much strength against the soil, especially when wet. I am therefore planning to add galvanized steel posts to hold the sleepers. As the maximum height of the sleepers for the first bed is 400mm, I understand that the posts will also have to be the same distance below the soil.

I understand that drainage is also important in the longevity of garden beds and retaining wall so I originally wanted to add a socked agi pipe against the back of the sleepers. But I found that there is also an existing agipipe (socked) running behind the stone blocks. This agipipe is shared with the neighbors on both sides.

I have a few questions at this stage.

1. Will I still need to add another agi pipe when I build the first level bed? And will this be simply connected to the existing pipe?

2. When I dig down to install the posts, the posts will be roughly on top of the agipipe (as digging 400mm down will hit some of the pipes). Am I able to add the posts and cement while the agi pipe is there?

Thanks in advanced,

Tony

backyard 1.jpeg design 1.jpg

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Comments

Jason

Hi @Tony431,

Welcome to the Bunnings Workshop community. We trust you'll get loads of help and inspiration for all your projects around the house and garden from our wonderful members.

Let me tag some helpful members to kick off the discussion for you: @Dave-1, @Nailbag, @AlanM52 and @sjkehoe72.

One of our resident Bunnings D.I.Y. experts will also be happy to help as soon as possible.

Jason

Dave-1

Good morning @Tony431

Love retaining walls and garden beds 🙂 and love those sandstone blocks of yours! So lets step through the things you have mentioned and questions you have.

Garden beds acting as retaining walls... Mmmmm yeah nah 🙂 It will help when shrubs/tress are firmly established but the inbetween time is a no. I would putt plant based retaining walls in the same level as 100mm staked edges for their ability to hold back soil.

With the Gal steel posts the holes will needto be 600mm deep at least if not more (meausured down from the low side of the slope)

As to agpipe connection to existing shared agpipe, i would not do so. Mainly because the capacities of drainage have already been calculated presumubly and adding into it may cause issues with the neighbours drainage.

Your questions

Q1. Will I still need to add another agi pipe when I build the first level bed? And will this be simply connected to the existing pipe?

A1. Yes I would suggest to do so, I would NOT join it into your existing pipe but maybe consider where you would like any water to end up in yoru yard, maybe even joing it to some PVC pipe and ending up in a rain bed/low area that you can grow certain things in?

Q2. When I dig down to install the posts, the posts will be roughly on top of the agipipe (as digging 400mm down will hit some of the pipes). Am I able to add the posts and cement while the agi pipe is there?

A2. I would move the posts or shift the pipe, The weak point would be the short distance the posts are burried. (400mm is way to short for a retaining wall post as it wont hold back the weight of the soil)

So I have other ideas then the Gal post plus timber 🙂 I would like to suggest our friendly "Gabion Walls" as an valid option. No posts needed, They allow free drainage, so no ag pipe needed either. They can be moved at a later date but emptying the cage and shifting it compared to a concreted in Gal post. They also hold back large amounts of soil depending onteh size of cages you use.

Some examples

Large gabion retaining wall

Landscaping front yard (not a retaining wall as such but involves a retaining wall and paths)

Landscaping Front Yard Part 2 (I built the paths first and then the cage went in)

Gabion wall bench seats This is the size id suggest for your area.

Front corner gabion wall These cages can be bought from bunnings (tho the gauge of the wire has shrunk) They are very easy to put together.

I would have a row of gabion cages Jack 80 x 40 x 40cm Rectangle Landscaper's Plant Support Garden Gabion across the back fence so the base of cage sits as close to the posts base as possible. The cage sits on compacted earth, no footing needed.

I would have another row midway across the yard, with a break in it for some steps or ramp depending on what you feel like. I prefer a slow ramp tomake it easy to garden with a trollley. Tho if steps you want then,

Gabion retaing wall and gabion steps

There are smaller cages available at Bunnings or you can make your own cages as I have. They really arnt hard and save so much money. I used excess broken up concrete, but have seen all sorts of fill used.All you need is to face the cages with the nice rocks and then fill behind.

Any questions, just let us know 🙂

Dave

EricL

Hello @Tony431

I believe @Dave-1 has provided excellent recommendations in regards to your raised garden plans. My only suggestion is to speak to your plumber in regards to the linking of the agi pipe to your storm water drain. The new agi drains must be independent of your existing one so that there will no chance of an overflowing issue.

If you need further assistance, please let us know.

Eric

Tony431

https://www.workshop.bunnings.com.au/discussion/comment/53036#Comment_53036

Hi Dave,

The Gabion walls look interesting and will definitely be a lot easier to setup. I was set on the timber because I liked the neutral look that timber gives off. However, I could add sleepers in front of later, to give the garden more of a timber garden bed look. If there are thinner gabion walls, I could potentially add the sleepers as a cap as well as I liked the look of timber garden beds capped.

Thanks for your recommendation regarding the posts' location. The agipipe cannot be shifted as it runs across both Neighbour's property. As it cannot be added above the existing agipipe, I will have to move the posts back, leaving a small space of soil between the rock bed and the front of the sleepers. If I was making the first level 1 sleeper high and I created an enclose bed, could I use the rear sleeper wall (in image) as the retaining wall so that I would ensure appropriate depth and steel posts (in image, red lines) to hold back the large amount of soil from the higher end? Whilst the Front could use some stakes or rebar through the wood to hold it down? I'm a bit confused at this point - I see alot of online photos sloped yards terraced a metal posts or concrete to hold the beds down, but simply some rebar or wooden stakes hammered into the ground to hold the bed in position (see example image) - Will these builds not last very long?

Regarding your comment on the depth. Is that a universal rule for galvanized steel posts to be 600mm deep, regardless of height? I had though that the height above ground is the same as height below ground. So if I were planning a 1 sleeper high (200mm) front of garden bed, I would still need to dig 600mm for the post to be cemented?

Regards,

Tony

online image.jpg

backyard 1 - Copy.jpeg

Nailbag

https://www.workshop.bunnings.com.au/discussion/comment/88711#Comment_88711

Hi @Tony431

Unlike a straight line retaining wall, if you are building boxed raised garden beds albeit tiered, I don't believe you will need any gal posts as each box has both return sleeper connections and next level to hold them in place. Basically the same as there sample photo by DIY designs.

As for answering the depth of a gal retaining post, depth in ground is based on depth out of the ground. So, 600mm above ground then it's 600mm in the ground etc.

Nailbag

Tony431

https://www.workshop.bunnings.com.au/discussion/comment/110057#Comment_110057

Thanks for your reply,

Will I still need some sort of stake to ensure the garden beds dont move too much?

As for the length, if I am following what my landscape designer gave me, the first garden bed would be over 9m long. Will the wood warp, even if its 1 sleeper high?

Tony

EricL

https://www.workshop.bunnings.com.au/discussion/comment/122384#Comment_122384

Hello @Tony431

I would err on the side of caution and use a heavy-duty support stake to prevent the sleeper from warping even if it just one sleeper high. Perhaps use the sleeper itself as a post as seen in this guide How To Build Retaining Wall. The last thing you want to happen is to see the sleepers bowing due to pressure from the soil inside the garden.

Eric

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/130321#Comment_130321

Good Morning @Tony431

There are thinner gabion baskets but then you wont have enough weight within the basket to hold back the amount of soil you want to, It will wont to topple or slide. The weight of the basket is what makes them so good to use.

The depth of posts into the ground, I look at it this way. You know when you hammer a tomato stake in the ground. Its solid at first, but by the end of teh season you can generally wiggle it enoughto be able to remove it. With your gal posts you mention wanting to use, same deal, if the gal post is only in 400mm, it may feel solid but the top 200mm of soil is what id class as "soft" soil so its not really going to have the strength to hold back a large weight.

600mm is usually what I head for as a starting poing of any foundation hole. The 600mm is measured from the first part of soil that is undisturbed. Otherwise you have a post that looks strong but will have to go back and fix the lean in 6-12months.

Timber will warp/bend. The more supports you have the easier it will be to keep straight.

Looking at your diagram, why dont you go for a wider flat part the same level as the rocks then a taller retaining wall that will be able to handle the deeper posts? So if you do a simple bend and stretch to reach something across a table, that ill give you a fair feel on how wide you may want the garden bed. You could then potentially make the bottom bed wider and the top thinner.

Its worth the effort and extra cost putting in deeper holes and more posts. Worth doing well, worth doing once 🙂

Dave

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