Clay is "expansive" - it swells when it get wet and holds the water, expanding even more when it freezes. It is lousy base material for any kind of paving. Sorry, but you should remove all clay (and topsoil) and backfill with a material (crushed rock, sharp sand) that will pack. If you don't have access to a vibratory compacter, compact by hand and foot, soak with water to pack more, and screed off flat.

I poured 72 feet using the cobblestone walkmaker mold and did very little prep. My soil is well drained, I would say it has some sand in it, definately not clay. It looks exactly like it did last year after a very cold winter for Mass., down to -20 degrees. I used a fiber reinforced concrete by the way.