When is 18" too deep?

If you know you have a great deal of clay soil but you haven't done a quick infiltration test you'll never know how long it will take for rainwater will filter through your rain garden and into the ground.

We advocate shallow gardens with minimal amendments. Unless you have experience with clay keep it shallow and wide.

No rain garden should be deep. The difference between the garden rim and the top of the garden bed should be no more than 5" (with mulch). 

If you go deep get an engineer. Otherwise let the plants do their job and go shallow.