As it is a raised garden, then drainage should not be a problem. Firstly i would break up the surface of the clay with a mattock or a fork to around 6", and add gypsum generously. You could also put blood and bone, manure, compost, whatever, down before topping with topsoil. 6" will not be enough as it will most likely compact after it is put down. I suggest you look into putting down layers of newspaper, compost, lucerne hay and straw, then just put in some topsoil (generously) where you are planting. This is called a "no dig" garden, is inexpensive and easy on the back. Herre is a link for you

Here is a link that might be useful: No Dig garden

g'day lychas,

many thanks sparaxis, that was going to be my suggestion as well go for 'no_dig', 'no-weed' raised garden beds too easy.

len

Here is a link that might be useful: lens garden page

I avoid bringing in soil where ever possible. Is this a new house block? Is that why you have all clay?

Before you do anything test for pH; before any new soil is brought in, it is essential to improve the existing stuff as much as possible, otherwise it will come back to haunt you in the future. I would recommend adding as much organic matter as possible to the clay to make it more workable.

I have seen really hard sticky, clay soils transform into excellent garden beds with patience and organic matter. In fact that is what a lot of my soil is now. You can also get a compost mix from landscape suppliers that will improve your clay soil no end.

A lot of Australian clays will improve a lot if given lots of Gypsum. Here is an extract from Angus Stewart's bit on clay soils from Gardening Australia.
"The next test is a gypsum requirement test to determine how to stop that dispersion.

"To do this get some clean jars. Put some soil in clean water and shake it until it goes milky. Then stand it for about five or 10 minutes. If it doesn't clear in the water, chances are it is going to respond to gypsum.

"Do the test adding some gypsum. See whether the gypsum makes the clay flocculate, or clump together."

"Where the soil falls apart you need to add organic matter. Red or your yellow clays are more likely to slake and fall apart, whereas lots of organic matter and dark, black coloured clay are more likely to stay together." According to Simon, organic matter represents the glue.

When using gypsum on a new clay start with about 200 grams per square metre. That's about two cupfuls. Dig that in, and do the gypsum test again. "If it settles out in the jar within five or 10 minutes you don't need any more," Simon says.

SoÂ.clay doesn't necessarily have to be a bad thing. With some simple testing and the right sort of treatment, itÂs possible to turn a nasty, heavy clay into a beautiful crumbly top soil that will grow virtually anything. The secret is to throw away the green waste bin and put every bit of organic matter back into your soil.
To quote Angus again "Clay often gets bad press in gardening books. But clay soils can be the best in the garden, depending on the type of clay and how itÂs managed."

I reckon adding six inches of top soil should be avoided at all costs.

may be yes, cheer !

Here is a link that might be useful: http://www.g3unique.com