We built our 3rd pond on our own 2 years back right at 1/2 acre. My advice is to aim for overkill on the core and dam. I have good clay but found a sand lens down about 5-7 feet that had to be cut by a solid core on 3 sides of the pond. Water coming into the pond through this lens required a bottom drain be installed during construction (valve closed at pond bottom after completion). I also built the u-shaped core and dam using good clay compacted with a sheep-foot roller in 6-8" lifts and it is at least 25 feet wide at water level everywhere. Also have over 5' of freeboard everywhere. Pond has held water like a drum since filling. Dam could be considered overkill, but I don't stress over its durability. I also have an earthen emergency spillway, but it has not yet been used. I actually built a berm on the upside to stop surface water in my small watershed from running directly into the pond (must filter down and through soil. Biggest issue so far is low fertility, but there is a treeline a ways below the dam that adds leaves each year when we get strong southern winds in Fall. Just letting the FHM and LCS forage base build now, and my emergent plant spread along with a lotus that is struggling with the low fertility. Best of luck to you! Find a pond expert, not a dirt digger.