Tony,
I don't know exactly what your situation is, but when you decide to core the dam there are some things to consider. Make sure the core trench is at least 3 feet into the clay subsoil base. The clay core should contain about 20% clay, not 100% pure clay as this could crack and lead to leakage problems. The core should be built in 6-inch layer increments and packed well before the next layer is put down. Pay attention to the moisture content of the clay mixture. It should be neither too wet nor too dry. Last, but not least, find a reputable contractor, one that knows how to build ponds. Here in South Dakota we have few contractors that know how to build a dam. Most only know how to dig basements, push dirt, and put in drain tile. Good luck with your project.
Steve,
