While I'm no expert, I think your slope needs to be more a more gradual 3/1 on both sides. The soil doesn't look like good clay either and the top of the dam should be wider. If you have a drain pipe leave it open for a year or so before you fill the pond for things to dry out and firm up. Rock line your emergency spillway and let it dry up good before it is used. The emergency spillway should be a long gradual slope and installed against undesturbed soil where the core off the dam meets it. Pack dry, Pack dry. I think he just didn't compact it well or use a gradual slope.
