I have very poor soil, and it is too rocky to dig, if I use planks to hold imported soil how deep must the

As far as I know 60cm depth is enough for nearly any type of planting. This may come to a suprise to many people.

You see after 60cm depth, the soil becomes too compact and lacks oxygen, therefore worms and other microbs cannot thrive at that depth, the soil is compact and lifeless, so it's very seldom you get roots penetrating this depth, although some trees do send there roots down to depths of up to 2 to 4 meters however these are just one off roots and not the entire rooting system, and you only want it for vegetables right, so 60cm depth would probably be actually too much so I'd aim for a depth of at least half that anyway, maybe a little more, between 60cm and 30cm I'd imagine would be sufficient for a vegatable crop.

Hope this helps you.

That all sounds like good advice. Hope you get some yummy veg next year. Welcome to GOY.