Questions about building a pond in clay?

I am going to build a pond in some clay soil. I have built ponds in the past in sandy/rocky soil, and I was wondering if you do it any different. Normally I would clear it if need be, then dig the core. I then start digging the pond out and building the dam. I generally take the core down to solid rock and then start the dam, by filling it in 6 to 8 inch lifts and packing.



My questions are how deep to take the core?



Do I need to pack the bottom and sides of the pond?



Thanks