I had 4 large (1.2m x 11m) sheets of aluminium left in my backyard. Rather than cut them up for scrap I decided to use them as a foundation for some raised garden beds. So I bought 3 metres of bedding sand & covered them up. I then bought 60 CCA treated hardwood sleepers. They were 2.4m each in length so by making the beds 3.6 x 2 I had only the offcuts from the ends left over. I used these as backing pieces for the joins on the sides. I then painted them with water proofing. CCA treatment does not prevent water pentration. Also the CCA chemicals tend to leach out of the timber into the soil. As I intend to plant veges the waterproofing may slow down the leaching. I also lined the beds with lastic sheeting to help prevent the CCA chemicals getting into the soil. I then bought pavers for the surrounds, factory seconds.
It was an expensive project & time consuming. After 4 months I haven’t finished yet but that’s mostly due to my own laziness. It would have been simpler, quicker & cheaper to buy those plastic beds similar to rain water tanks. My advise to anyone thinking to biuld raised garden beds: there are simpler, cheaper, quicker & easier alternatives to sleepers.