Pond Boss Forums POND CONSTRUCTION Building a dam coring question

I can't give you a qualified answer on exactly how deep it should be.

But the reason for a core trench is to keep water from flowing beneath the dam. As long as there is a continuous clay bottom you might be ok but during excavation if you run into anything other than the clay (like a rock or sand layer) water could follow this layer under the dam. So the core trench needs to go deep enough to prevent this from happening.