BillLake - I apologize for mentioning 6-mil plastic. I'm not sure where I got that.
As for the sand, it used for a couple of reasons in landfills, tankfarms, toxic waste ponds, etc. It is put over compacted clay and or hydrated bentonite. One reason for the sand is that it doesn't fracture as clay can, and it acts as a buffer for sharp objects such as rocks in the clay. In the industrial uses I mentioned above, it also provides for draining leachate if the liner should fracture - something you obviously don't need.
I can fully understand your frustration over this project. It is my belief that everyone here is just trying to keep you from spending a lot of time and money on a technique that has a lot of potential downsides.
Have you talked to Mike Otto about your dilemma? If not, give Pond Boss headquarters a call and setup a phone meeting with Mike. There is nobody with more experience with these types of issues. Plus, Mike is just a fun guy to talk with.
