I had the same problem, as we dug out clay for the dam ran into a rock ledge on one side of pond. It came out about 150 feet on one side.
After the frst fill-up pond leaked about 1 inch a day. So i drained pond brought in a pc300 and rekeyed the the whole 360 foot dam as far down as it would dig. We also added about 8 feet to the widith of the whole dam. Let it refill and it leaked 1 1/2 inches a day. So once again i drained it down. Brought back in the pc300, and put a hammer on it and dug rock, rock , and more rock at the inside base of the dam. I belive we went deeper the the two previous keyways and packed it back with clay. Pond has refilled and still has a small leak of about 1/4 inch a day that drains out about 200 feet below dam.