The depth of the core trench requires reaching an impermeable layer (usually good clay). On one of my ponds, there was a small sand lens between two layers of good clay which was very obvious when digging the pond because it was full of water which drained into the hole to such an extent that a bottom drain was needed to finish the pond. That lens needed to be cut by the core trench. Then the trench was filled in 6-8" lifts of good clay each compacted with a sheep's foot roller. Pond is as tight as drum. Dozers are typically not sufficient to compact clay.