Check this website of the North American Rock Garden Society and see if it is of some help for you.
A member, Jane McGary has written a book, Rock Garden Design & Construction NARGS (Timber Press) that may be available through your local library.
Good luck!
Rb

Here is a link that might be useful: Rock Gardening

i grow sedum , hens and chicks and ice plants.. in pure yellow sand.. with little or no water after they become 'established' ... in near full sun ...

IMHO .. amending the soil would be a waste of time and money ... for those listed ....

if you want to go into the very tricky world of alpines ... then that is another ball of wax completely ... within which i lost a couple hundred dollars in amendments and plants ... having failed miserably after a few years .... and giving up ...

the problem is that your title asks a very specific question about a very specific type of plant/garden .. in other words you used a term of art in using ROCK GARDEN

but you then ask about some of the most hardy, care free plants in the world ... a hen/chick will basically grow wherever it falls ...

so which is it.. a new groups of rock garden alpines.. per the first answer

or the common everyday plants that need nothing more than a drink once or twice a year???

you bigger problem is how you will make the soil damp enough to get your plants 'established' under a huge tree ... and you would never amend under a tree or you will just have the tree grow into the great soil you just spent money on ... so it all comes down to how to get enough water there ... until the plants can take off

good luck

ken

PS: my ice plants are under the kids swing set ... and get beaten down repeatedly all summer long.. and are barely containable [they thrive] ... now that is punishment.. lol ...