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Raised garden bed retaining wall for slope

Building a Reputation
Building a Reputation


A raised garden bed built with treated hardwood and Retain It posts, and coated with decking oil, with drainage installed.





The project


I was looking to do some outdoor landscaping this summer break and planned to add 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 wasn't really useable because of it and we wanted to box out the slope with a retaining wall to put ornamental plants.




The dimensions we wanted were 4.8m wide, 600mm high, and roughly 500-600mm deep from the back fence.




This was how I envisaged it.




We purchased Bunnings 200 x 50mm 2.4m CCA treated hardwood and Retain It posts to construct the wall. We 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 Cabot's 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 plan to reseal the surfaces every year or so. For anyone else interested in CCA research, I did find this interesting study showing that sealant on CCA wood is highly effective preventing leaching. 


For anyone digging into super hard red earth clay, ditch the shovel and get a fencing bar. It 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 garden bed/retaining wall and the concrete retaining wall. I ended up modifying a 450mm Retain It post to complete the section.




I used a 10mm cobalt 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 I drilled two holes with a 10mm masonry drill bit into the concrete sleepers and secured the posts with two 75 x 10mm Dynabolts.




Our neighbours on both sides are higher than us and naturally water would travel to our property, and we didn't want it collecting behind the wall. I dug a trench around the perimeter directly behind the lowest sleeper. Then laid with gravel, then the Ag-pipe, and then filled with gravel. This backfill was around 200mm deep and enveloped in Geo fabric. 


As we already have a stormwater drain in front of the retaining wall, I used 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. And I used a Vinidex 100 x 90mm PVC Stormwater Draincoil Adaptor for attaching both Ag-pipes to the PVC.  




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, I used the 100mm to 90mm reducer to connect the 90mm PVC run to the wall, which connected to a Vinidex 100 x 900mm PVC draincoil adapter. That 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 Geo fabric and drainage gravel.




We decided to replace the grass in front of the retaining wall with pavers as well. So it ended up being a long nine days of getting muddy. 






I used two sheets of Geo fabric above tampered blue metal 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 fenceline. 




We're 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.




Before and after






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