What kinds of surfaces can these be installed over? Can they be installed directly over dirt? Is there any special ground preparation required other than grading?

Yes, the Jamboree Playground Tiles - Designer Series can be installed directly over dirt and grading the ground is all you will need. However, we do not recommend installing these tiles directly over dirt because drainage could be an issue during rains. We recommend installing these tiles over hard surfaces like concrete, asphalt and crushed gravel.