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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!
