The area that could go bad is the soil layer (and gravel can go bad too), and I initially had a foot tall soil layer, no kidding, but the compost saved it. I took out more than half and added the large 1/2"-1" gravel all mixed in to keep it fresh. If I added a gravel layer on bottom of that, I don't think it would matter, beause I added a LOT of those 1/2"-1" gravel so technically it's kind of a gravel bed with soil mixed in, but having clay hunks still keeps it more soil like. What you don't want is a foot tall layer of soil and debris that you can push your hand like a lake all the way to the bottom easy. Having rocks mixed in with the soil, there's no layer that compresses. If I put thick clay in a 2 or 3 inch layer between rocks, and pushed everything down from the top, the clay would just squeeze in between the rocks. What I tried to do was make sure everything can't be lowered further by pushing down, and happened to have those larger gravel pieces. And when you add the top sand layer, go ahead and push on it, it's going to sink down, and I don't know how larger sand is going to prevent the soil from filtering up without adding a large amount.