RE: backyard tracks
Another good way if clay is not there is to use a mix of concrete on the face of the jump so it always stays smooth and clean. Seen this done on several tracks and everyone seems to agree that its a good way to go. I havent tried it yet but plan to as soon as I move to our new house and I start to work out my track.. One thing I dont like about clay is that when it rains it make a mess..lol Could just be me up in Montana but I think I'm going to go with soil dirt and make the face with a thin layer of concrete.
Also found that if you put a level of river rock down then dirt and use a plate wacker to pack it seems to hold up more. The river rock is free and makes an nice under surface so your dirt dont settle and shift as much from natural causes.. Works good to with building up burms and jumps. Makes a solid layer under the dirt/clay that dont shift if packed good.