Liquidsquid, We did not add any rock on the bottom of the liner. I'm a little foggy on the details, but I think we concluded that if the bottom floated up a little bit from seepage that would be less likely to pull the liner out of the trench than if we had put material on the bottom to weight it down.
Esshup, I think we ended up with about 18" blanket of clay/bentonite. It was difficult to compact due to some of the area having relatively steep sides. The slope was made up of rubble, which was like a honeycomb and made it difficult to pinpoint the source of the leaks. The actual path of the water could follow the rocks 15' or more feet from where the sinkhole would appear.
Initially we had aboue 30 sink-holes. On recommendation of a geotech we'd patch the holes with a mixture of reject gravel and a type of unrefined bentonite/clay from a local bentonite mine (50/50), then fill it back up. The idea of the including the gravel was to provide some structure and to help fill in the fissures. Each time the number of sink holes would decrease and I'd think we were making progress until there was only one "problem" area. Unfortunately, we could never get that area to stop taking material, and the hole would move around on us each time we thought we had it.
There were some pretty depressing times, but we stuck with it, used the liner and now have a beautiful pond. In the end it was worth it.
