Jwwann:
I know the $ is adding up, but if he just does the core, and the pond still leaks, what will it cost then?
I'm not inferring that good pond builders won't have problems too, but your pond problems are why we say that there are many dirt movers out there, and anybody can dig a pond, but only the true pond builders can dig a hole that will hold water.
If he says the clay will seal the pond, and he will come back on his dime if it leaks, then I'd say it's worth it.
I for durn sure would have the clay placed all the way up the sides of the "bowl" so that it ends above the emergency spillway elevation.
Here's why I say it that way:
Go to your kitchen. You probably have a double sink. Take a bowl that is slightly shorter than the divider between both sink halves. Place it in the sink. The bowl is the "bowl" of the pond. The divider between the sinks is the dam and emergency spillway. If he just covers the "bowl" of the pond up to the normal high water level, then it'd be the same as you trying to fill up the sink with the bowl in it. You'll never get it to the emergency overflow because once the bowl is full it'll leak out the drain.
