For placing big rocks on steep banks so they don't roll, get a bunch of smaller rocks embedded into the bank and excavate a bit to cradle the larger rock. Regardless the larger rock will eventually slide downward over time on a clay bottom. It is best to put something towards the top of the dam instead, not on the slope.
If you have the thickness of liner and are not worries about breaking the seal, use rebar or concrete rebar 3' spikes driven in to prevent slippage, cradle with smaller rocks, then roll in the big-un slowly.
With that said, I did all of the above by hand. Managed to roll a 500lb+ rock into a garden trailer which groaned
(used physics as my friend). Had everything prepped to accept the stone... and promptly rolled it too fast into position. Went right up and over my cradle which I thought was robust enough. The large rock just squashed the smaller ones into the clay, and now I have structure on the bottom. I could see it the other day about 12' down.
I really would love to have a tractor... but until I win the lotto, everything I do is on a budget.
