You heard it right. 4200 truck loads of clay. Luckily, the formation where we found clay with a low PI was only 2 miles away. Even that was rather expensive.
The lake is 35 acres and had holes to 40 foot depth. We had to regrade the bottom, spread and pack 2' or clay 6" at a time over 17 acres from 205'mean sea level to 227 man sea level. Took 4 months and several hundreds of thousands of dollars. The original engineering firm and the developer handled the cost.

Mike