Get1fiddle,
I was thinking exactly the same thing -- black dirt but no clay. The core trench and the front side of the dam should be well compacted clay.
Ewest sounds like he is suggesting putting in the emergency spillway over the top of the principal spillway pipe. I would be very surprised if an engineer would recommend this. It looks like your site would have an opportunity to put the emergency spillway around the left side of the dam. In my discussions with engineers, they prefer to have the emergency spillway as far away from the dam as possible.
Bill
