How to add a raised garden bed in sloped backyard?

[asharkey]

Hello everyone and happy holidays

Looking to do some outdoor landscaping this summer break and would really appreciate some 2nd opinions on our plan for adding a small raised garden bed/retaining wall to our back yard.

Our yard is small to begin with and there is a decent slope that backs up to our back fence.  the area is not really useable because of it and we want to box out the slope with a retaining wall to put ornamental plants.

Dimensions of the wall be 4.8m wide, 600mm high, and roughly 500-600mm deep from the back fence.

Eventually we’ll remove all the grass directly in front of the garden bed and replace with rock and pavers for a seated patio area.

We've purchased Bunnings h4 CCA hardwood sleepers and retain-it posts to construct the wall. We'll seal the facade of the sleepers with Cabot's Natural Decking Oil.  And seal off with Bondall Bitumen the back of the sleepers, any bottom wood touching soil, and the cut ends.

I have a few questions about planning in regards to drainage.   Since our neighbours on both sides are higher than us naturally water is going to want to travel to our property, and we don't want that collecting behind the wall.   I’m planning to dig a trench around the perimeter directly behind the lowest sleeper.  Fill with gravel, lay ag pipe, and fill will gravel.  This backfill would be around 200mm deep and enveloped in geo fabric.  As we already have a storm water drain in front of the retaining wall, I’d like to then branch off that drain with PVC in the ground and then with a T joint connect to the ag pipe behind the wall.

Does this sound like a good plan?  Anything I’m missing?  And is this overkill for such a small garden bed?

Another question is because the back fence is tiered as it’s on a slope, there will be a portion that would be roughly 200mm below the garden bed.  If we want to fill the entire planter bed for a flat even look, this part of the colour bond fence would be underground.  Is this okay?

Thanks in advance for any advice!

[MitchellMc]

Welcome to the Bunnings Workshop community @asharkey. It's great to have you join us, and many thanks for your question about building a raised garden bed.

That sounds like a plan, and I don't think you are missing anything. It's not overkill and good insurance that you'll have a solid build for many years.

If possible, I'd prefer you to construct a rear wall to the garden bed and not have the soil up against the fence.

Please let me know if you have any questions.

Mitchell
 

[asharkey]

Thanks for the quick reply @MitchellMc and confirming my game plan!

i was thinking that about the rear wall.  However I wouldn’t be able to go the whole length as the concrete footings for the neighbours fence would stop the wall the whole length. 

what if I were to put a barrier, not load bearing, substrate in between the two.  Even just marine grade ply, to seperate the dirt from color bond?

also any recommendations on adapting my 100mm stormwater drain to the ag pipe?

Was thinking of using 90mm pvc pipe to run the length from the drain to the wall, and a 100-90mm reducer to adapt to a t joint at the stormwater pipe. However, Can’t seem to find a 100mm T joint to replace at the stormwater drain, and 90mm T joint for attaching both ag pipes to the PVC.  Happy to hear alternative suggestions as well.  

[asharkey]

Actually would this work for adapting the current 100mm pipe to make a pvc T extension?

[MitchellMc]

Marine ply will rot away within a year due to the moisture @asharkey. It's not a permanent solution. What about some garden edging? 

Are you sure that's a 100mm pipe, or is the fitting connecting it to the pit 100mm? Stormwater is normally 90mm, and you use this Vinidex 100 x 90mm PVC Stormwater Draincoil Adaptor to connect to it.

Mitchell

[asharkey]

Good idea with the garden edging!

And yup it's 100mm pipe.  Wanting to reduce it to 90mm past the joint since that's more commonly available.  

[MitchellMc]

Hi @asharkey,

This Holman 100 x 90mm PVC Storm Socket Reducer will allow you to convert the 100mm to 90mm stormwater. 

Mitchell

[asharkey]

Thought I post some finished photos! We decided to replace the grass in front of the retaining wall with pavers as well. So it ended up being a long 9 days of getting muddy!  But stoked with the results.   Thanks for all the advice @MitchellMc and the local crew at the Ballina Bunnings for the many tips along the way

For the wood sleepers we used 200x50mm 2.4m CCA treated hardwood.  Wanted something that would withstand the harsh, wet, and humid coastal weather of the Northern Rivers. We sealed the back and bottoms with two coats of bondall black bitumen and the tops and faces we sealed with two coats of cabotts natural decking oil.  The garden bed won't be used for food.  But with the level of sealing and backfill I'm not too concerned with any CCA leaching into the soil or through the wood.  We'll plan to reseal the surfaces every year or so.  Anyone else interetested in CCA research I did find this interesting study showing that sealant on CCA wood is highly effective preventing leaching.

https://preservedwood.org/portals/0/documents/archive/CoatingsTechline.pdf

And some progress along the way...

for anyone digging into super hard red earth clay, ditch the shovel and get a fencing bar.  would have been impossible without it.  hard work though!

Completing this small section was tricky as a 600mm concrete retaining wall with fence footing was already on the property line.   and there was a 250mm gap between my sleeper gardenbed/retaining wall and the concrete retaining wall.  I ended up modifying a 450mm retain-it post to complete the section.  Used a 10mm carbide steel drill bit to drill two holes in the retain-it post (didn't struggle getting through the post).  the concrete sleeper already had two pre-drilled holes.  then drilled two holes with a 10mm masonry drill bit into the concrete sleepers and secured the posts with two 75mmx10mm dynabolts.

Using a hacksaw and a few pilot drill holes, I cut out and accessed the stormwater drain and used the snap-on 100mm Tee junction to connect the retaining wall ag pipe to the the 100mm drain pipe.  From the stormwater drain pipe, used the 100mm to 90mm reducer to connect the 90mm pvc run to the wall which connected to a Vinidex 100x900mm PVC draincoil adapter which connected to an Ag Pipe 100mm T-Piece. I then cut in two pieces from one 8m length of socked and slotted ag pipe which connected to the tee.  I enveloped the backfill behind the wall with Geofabric and drainage gravel.

used 2 sheets of geofabric above tampered blue dust and below the pavers

filled the gap with 20mm gold aggregate 

and used h4 treated dividers with hardwood stakes for the paver boundaries and the ground level garden bed along the fence line

         

[Akanksha]

Absolutely terrific work, @asharkey! The raised garden bed looks lovely and good call on adding those pavers. They have really elevated the look of your backyard and turned it into a wonderful area for you to relax and entertain it. 

Many thanks for coming back to update us on your project and also for sharing your research on CCA leaching. I'm sure our members will appreciate all those details.

It's a pleasure having you in our community. We look forward to seeing what you tackle next.

Akanksha

[EricL]

Hi @asharkey 

I totally agree with @Akanksha, that is terrific work! What an awesome transformation from an unusable spot to a raised garden bed. The addition of pavers and pebbles really gives it a fantastic look.

Thank you so much for sharing such a wonderful project.

Eric