I did something like you are planning, however, I only dug down one foot in a 15m x 7m garden, removed the clay-like soil and replaced with a topsoil/sand/peat/wood chip/whatever I could find mixed with some of the clay. It is on a very gentle slope, almost unnoticeable. I also did it all the way around the house for the flower beds. I did that five years ago and have never had a problem with roots, rot, drowning, sloppiness or ponding. Unless you expect several inches of rain over a short period, or your clay is absolutely not permeable, I'd go for it. if its the raised area behind the hammock (with cinder/concrete blocks), you'll be fine. If things get ugly (ponding), put a 1/2 inch drain pipe in the block.
removing the soil is backbreaking work and you will have to have somewhere to put the removed soil.
