Good morning Okie Writer.
Let me make an attempt to answer your question. Some also refer to the core as the keyway. One of the main reasons for the core is to provide a seal between virgin soil and the dam to prevent leaks. Numerous posts have discussed dam cores but here is one from Brettski that shows a dozer cutting the core.
http://www.pondboss.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=2;t=000108 Hopefully this will help address your question. A search on the topic should also yield more answers.
Thanks B-ski. You can peck faster than I.
