I am a Geotechnical Engineer and a Geologist and do Soils Engineering for a living. In additon I have bulit six ponds on my farm, none of which had a core trench, or leaked very much. Typically I only use core trenches where clay is in short supply.
If you are on good clay, the most important aspect of embankment construction is to use clay that has the moisture content adjusted to a couple percent wet of optimum. Then scarify the the surface of the native soil and insure that you get good compaction of the lifts. Typically you should be looking for 90% of modiefied Proctor. You get alot better sealing with the clay slightly wet, than at optimum or on the dry side.
