Posted: 23/07/2014 at 18:02
I think tree hugger is right, maybe use some compost activator (urine is the cheapest) cover with soil mixed with a fair amount of grit perhaps. In the spring try throwing a couple of planks down and walking on it to see how much it is going to sink. If you build up some height with gritty earth, chances are the roots of the rockgarden plants may not reach the composted material.
This type of plant grows in poor rocky soil, so try to emulate that. Small stones will increase drainage, again emulating their natural conditions.
It is going to take a lot of earth to build it up, it does seem a little perverse to take the biggest dip in the garden and make a mound out of it. But if you have set your heart on it then good luck with it