6" lifts are talked about on here, and using either a sheepsfoot roller or a mulit-tired scraper that has a pan full of dirt for weight. The layers need to be packed down but also "knitted" together. You are shooting for intertwining the layers, not having layers laying on top of each other like sheets of paper.
If the clay is too dry or too wet it won't compact correctly either.
I've heard the story about making a ball of clay and throwing it against the door of the pickup. If it sticks there, it's the correct consistency.
Like Sprkplug said, tracks are made to float over soft soil, not sinking in. Not exactly the thing you want for compaction.
