Posted: 21/10/2014 at 16:14
An important factor is the compaction of the soil. If your soil is loose and crumbly, then no amount of posts driven into the ground is going to support the required depth of soil.
I built some raised beds a few years ago. Our soil is light and sandy (no good for gripping posts) and full of stones that constantly send the posts out of true. So I concreted them in place (1 in each corner and 1 in the centre of each of the long sides for the largest beds).
Because it's a sloping site facing north, I wanted to bring the north-facing side of each bed up to the horizontal to get more sun. As a result, the north-facing sides are between 350mm and 400mm high, while the south-facing sides are 200mm
For 2 smaller beds (1.8m x 1.2m), I used 4 posts, 1 in each corner, while 2 larger beds (3.6m x 1.2m) have 1 in each corner, plus 1 in the centre of each of the long sides.
The timber I used was all 50mm thick, width either 150mm or 200mm. Screws were Timber-Tite, size 6.5 x 80mm and 6.5 x 145mm. Like most timber, some of the boards have 'dished' across their width, but the screws have held everything together very securely. Scaffolding boards are thinner (usually 38mm), so would need more supports to prevent bowing.