Thanks for all the replies. I know it will be quite a chore to make this come out the right way.
Robinson, Pressure grouting sounds like a great idea. Certainly the grout would be able to find it's way into all the cracks and crevices. I'll have to get some folks out here to take a look and see what that might cost. My biggest problem would be getting the grout to the pond. When I mentioned using rubble and concrete I was thinking that it would provide support for the clay liner above. I'm afraid if I pack clay into the hole without something to support it that water will eventually find it's way through and set up a slow trickle into the hole, gradually eroding a larger passage through the liner.
Cecil, If you'll volunteer to crawl down the hole to do a little spelunking to check it out, I'll split any future profits with you! But just in case the commercial cave business doesn't pan out, I have a bucket of grout I need you to carry in with you.
Meadowlark, Thanks for the well wishes, for sure this hole will be covered deep with Sumner County's finest.
