when i built my lake the contractor told me i needed aleast 12" of clay with 18" being best.i had a leak 1 time just like you did,finished dam and that night got 4 inches of rain.we packed the dam at 11:00 o'clock at night with track hoe.contractor did not think it would hold up but next morning everything was fine.i have found it is best if they fill up slowly over time,this seems to get the dirt to settle better.