I met and enjoyed a few good learning sessions with a CD agent in an NRCS office in north-west Illinois. I came back to him a couple of times later as I inspected properties for pond-ability, even tho they weren't even in his county. At one time, during a Q&A discussion, we talked about the wisdom of cutting a core (aka; keyway/keyhole), and he related a short story about a local farmer that used his own bulldozer to build his pond. He dismissed use of a core. Shortly after it filled, the entire dam mass started to move from the water pressure. Another man-made disaster.