Originally Posted by shoe59
they use crushed rock/gravel to pack down... I don't know the process though.
Once you are at this point, you may as well concrete.
But if you are determined to not use concrete, first, avoid any river rock no matter how small, they never pack in.
You can use mostly anything, but always remember the smaller the stone, the better the compression. If you are going to cover with carpet, you could use limestone, but the powder really gets everywhere so I would avoid it. Depending on cost in the area, Millings, which is ground up Asphalt, packs quite well, is not too messy, and can usually be purchased very inexpensive, especially if the local state is tearing out any streets/highways.
Still, by the time you frame the area, add and compress the stone/filler, lay the carpet, you may as well have just filled it with concrete and be done with it.