I'm building a few sitting benches for my eagle project. They will be placed in a nature preserve but I can't seem to find a way of preventing the benches from sinking. Keep in mind that the people at the preserve plan on moving the benches around even after I am finished with the project. The terrain of the preserve is mostly grass, mud and some mulch trails. To give you a better idea, the preserve is about an hour northwest of Chicago. Any ideas?