What would the ducks and pigs do?
Cement of some sort might be your best option for it staying where you put it but you will still have to seal it or add sealer to the cement because its porous (then it is only water resistant, not waterproof). Cement is just a mix of limestone and clay, adding more dirt or clay may make it even more porous. Or it may become so waterlogged, it melts. Also, if you plan to have anything living in your pond, fish or plants, you will have to seal it to keep the alkali in the cement from killing everything (its a pH problem).
I think you will have to build some sort of framework to hold the cement in place like chicken wire and/or rebar.
Let us know what you do. I'm fascinated. I have a clay bottom pond - I have to keep a hose running in it or it goes dry in a day or so.
