I had a post a while back similar to what you are planning. We built some docks on large reserviors by pounding pvc into the bottom, setting a 4x4 inside, and then cementing them in. You don't need to drill out the dirt from the inside. Think hydraulics, there is nowhere for the dirt to go unless there is enough downward pressure to push the cement out the top. I am planning on building this type of dock when I get around to it.
