My friend and I building a wooden berm for our local trails. Everything had already been pre-cut in my garage. This berm was also mostly funded by donations from local cyclists.
Mark C Here in South Texas our soil consists of a lot of sand. The berm doesn't settle a lot to be an issue. If you have a settling issue you could use 1ftx1ft sections of plywood to set the 4x4s on.
This soil was a mixture of sand and clay. Sand actually settles quite nicely, but if you're afraid of it sinking, widen the base by screwing on a section of plywood. Bury that in the ground so it stays stable.
